CA1196710A - Variable air volume system controls - Google Patents
Variable air volume system controlsInfo
- Publication number
- CA1196710A CA1196710A CA000419624A CA419624A CA1196710A CA 1196710 A CA1196710 A CA 1196710A CA 000419624 A CA000419624 A CA 000419624A CA 419624 A CA419624 A CA 419624A CA 1196710 A CA1196710 A CA 1196710A
- Authority
- CA
- Canada
- Prior art keywords
- air
- fan
- signal
- control signal
- flow
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F24—HEATING; RANGES; VENTILATING
- F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
- F24F11/00—Control or safety arrangements
- F24F11/0001—Control or safety arrangements for ventilation
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F24—HEATING; RANGES; VENTILATING
- F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
- F24F11/00—Control or safety arrangements
- F24F11/70—Control systems characterised by their outputs; Constructional details thereof
- F24F11/72—Control systems characterised by their outputs; Constructional details thereof for controlling the supply of treated air, e.g. its pressure
- F24F11/74—Control systems characterised by their outputs; Constructional details thereof for controlling the supply of treated air, e.g. its pressure for controlling air flow rate or air velocity
- F24F11/77—Control systems characterised by their outputs; Constructional details thereof for controlling the supply of treated air, e.g. its pressure for controlling air flow rate or air velocity by controlling the speed of ventilators
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F24—HEATING; RANGES; VENTILATING
- F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
- F24F11/00—Control or safety arrangements
- F24F11/30—Control or safety arrangements for purposes related to the operation of the system, e.g. for safety or monitoring
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F24—HEATING; RANGES; VENTILATING
- F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
- F24F11/00—Control or safety arrangements
- F24F11/0001—Control or safety arrangements for ventilation
- F24F2011/0002—Control or safety arrangements for ventilation for admittance of outside air
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F24—HEATING; RANGES; VENTILATING
- F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
- F24F2110/00—Control inputs relating to air properties
- F24F2110/10—Temperature
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F24—HEATING; RANGES; VENTILATING
- F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
- F24F2110/00—Control inputs relating to air properties
- F24F2110/10—Temperature
- F24F2110/12—Temperature of the outside air
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
- Y02B30/00—Energy efficient heating, ventilation or air conditioning [HVAC]
- Y02B30/70—Efficient control or regulation technologies, e.g. for control of refrigerant flow, motor or heating
Landscapes
- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Combustion & Propulsion (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Fluid Mechanics (AREA)
- Air Conditioning Control Device (AREA)
Abstract
ABSTRACT
A method of controlling the air flow output of a selected variable inlet vane fan comprising the steps of: generating a flow control signal in a controller which is programmed to selectively provide a first signal which is proportional to the increasing response plot of air flow with respect to vane position when the vanes of the selected fan are opening or a second signal which is proportional to the decreasing plot of the air flow with respect to vane position when the inlet vanes of the selected fan are closing, detecting the direction of response required to meet the demand variation as between a requirement for increased air flow and a requirement for decreased air flow, activating the controller to provide said flow control signal in the form of said first signal when a requirement for increased flow is detected and to provide said flow control signal in the form of said second signal when a requirement for decreased flow is detected, transmitting said flow control signal to said fan to control the position of the inlet vanes of the fan as required in use.
A method of controlling the air flow output of a selected variable inlet vane fan comprising the steps of: generating a flow control signal in a controller which is programmed to selectively provide a first signal which is proportional to the increasing response plot of air flow with respect to vane position when the vanes of the selected fan are opening or a second signal which is proportional to the decreasing plot of the air flow with respect to vane position when the inlet vanes of the selected fan are closing, detecting the direction of response required to meet the demand variation as between a requirement for increased air flow and a requirement for decreased air flow, activating the controller to provide said flow control signal in the form of said first signal when a requirement for increased flow is detected and to provide said flow control signal in the form of said second signal when a requirement for decreased flow is detected, transmitting said flow control signal to said fan to control the position of the inlet vanes of the fan as required in use.
Description
~36~
This invention relates to improvements in air-conditioning system controls.
In particular, this invention relates to improvements in the control of the output oE a variable inlet vane fan.
The present invention also relates to the positioning oE the sensing probe in an air-conditioning system.
~ n addition, the present inven-tion provides improvements in the stability of a control system of an air-conditioning system.
PRIOR ART
For many years, variable inlet vanes have been a commonly used device for controlling the output of a fan such as the fans commonly used in air-conditioning systems. The use of variable inlet vane fans in air-conditioning systems has increased substantially since the popuiarity of variable air volume air-conditioning systems has increased.
In controlling the movement of the variable inlet vanes in order to adjust the fan output capacity, it has been found that while the air flow is a function of the position of the variable inlet vane, the actual air flow varies depending upon whether the vanes are opening or closing. This difference in air flow is as a result of hysteresis losses which are particularly due to the linkage mechanisms required to control the movemen-t o~
the inlet vanes. The hysteresis is more pronounced on double inlet fans than on single inlet fans mainly due to the more ' -'''F
extensive linkages required for the former. This hysteresis would normally de-stabilize a conventional inlet vane con-trol system. A conventional control system normally cycles about a set point and the hysteresis which is inherent in the operation of variable inlet vanes of a fan results in an unstable control 1 oop .
To prevent the system from cycling about it's control or set point, the conventional practice has been to introduce restrictors into the sensing loop of a pneumatic system or damping circuits in an electrical or electronic control system.
The purpose of these control system modifications are to provide system damping. These systems do not address the problem correctly and therefore do not provide the required result. The only efEective remedy presently in use is to decrease the control system sensitivity, i.e. loop gain, to such an extent that the control system cannot differentiate between the two extreme sensed conditions. The disadvantage of this solution is that the sensitivity is so low that the control system cannot follow changes in fan system conditions and therefore the control system is slow and sluggish and in some cases may be inoperative.
I find that I can improve that stability of the air conditioning system of the present invention in circumstances where the sensed variable which is usually the static pressure, is not steady enough for conventional control. If the sensed variable is not steady enough for control, I provide a filter with a long-time constant. rhis filter serves -to ~ilter out transients and to allow actual load changes, heavily filtered, to pass from the signal transmitter to the controller. The time constant should be adjustable and should normally be in the range of 0 to 200 seconds. The time constant may then be adjus-ted on site to be a good compromise between the desired system response and the control system stability.
Alternatively, I may substitute an integrator for the filter described in the preceding paragraph. In control systesm requiring fast response times, cycling occurs at a specific frequency. This frequency varies with the setting of -the sensitivity or gain ad~ustments. If the control system is ~et for the desired sensitivity and impermissable and excessive cycling occurs, substituting an integ~ator for a filter would be helpful. The integrator is inserted into the circuit between the transmi-tter and the con-troller. As the transmitter senses the system cycling, the integrator will act on the output. As long as the cycling is of a set frequency, the integrator will integrate the fluxuations out. The integral of a symmetrical periodic function is 0 and consequently, the integrator output will consist of the steady value only as sensed by the transmitter. It is important that the frequency of the cycling be measured on the job si~e and adjusted into the integrator.
The conditions under which air flow takes place in an air conditioning duct is subject to the following equation:
dP = Clq ~ C2q ~ C3q EQrJATIO~ 1 Where C1, C2, C3 are constants q = air flow in suitable uni-ts dp = pressure drop oE duct system in suitable units EQUATION 1 breaks up the pressure drop into components due to totally turbulent flow depicted by the q2 term; due to laminar ~low depicted ql term; and a component independent of flow depicted by the q term. (Note q = 1). Experience indicates that in the range of operation of air conditioning systems, the laminar flow component is negligible. If we assume this term to be zero, then Equation l-simplifies to the following:
dP = Clq -~ C3q EQUATION ~
The characteristic curves of a typical fan are well known and are provided by fan manufacturers. If a fan is a-ttached to an air conditioning system consisting of filters, coils, ducts, terminal boxes, dampers, etc., air flow will take place at the point on the characteristic curve where the fan and the system curves intersect. If the system is a constant volume system, there would be one fan curve depicting the head-flow condition at a particular constant speed. However, there are an infinite number of system curves between a minimum and maximum Elow. Each time a damper changes position, resulting in different pressure in the duct, flow would be affected. Since ~ ~3ti7~3 air flow can only take place at the point of intersection of the fan curve and the system curve, if the duct pressure changes, this would have the effect oE moving away Erom -the point of intersection. Since it is known that this cannot happen, the condition must intersect. The only way this can happen is with a new system curve intersecting the fan curve at the new flow-pressure conditions. Hence, there can be an infinite number of system curves.
If the fan is to vary capacity, there would likewise be an infinite number oE fan characteristic curves between minimum and maximum capacity; one for each speed. The fan system operation is somewhere in the region defined by the minimum capacity-flow and the maximum capacity-flow which is determined by the system condition. That is to say the minimum capacity flow is that which is achieved when the system is clean and the maximum capacity-flow is that which is achieved when the system is dirty. Dirty, in this context, refers to the clogging of coils, filters, or other system components which result in a higher pressure drop across them on the air side.
I have found that in order to keep the fan-system operation steady, stable and accurate, it is important to locate the tap of the static pressure sensing probe as short a distance downstream of the fan as is practical and to locate the pressure sensing device as close as possible to the tap. Ideally, the tap would be placed at the fan discharge flange, however, this is ~9~JJ~
impractical since turbulence is too great at this point leading to erroneous readings. Effectively, the same result may be obtained if the tap of the sensing device is located at any point in the duct system upstream of the first branch line.
PreEerably, the tap oE the sensing device is located within the fan room and preferably less than 20 duct dia~neters downstream of the discharge fan Elange. PreEerably, the sensing device is located adjacent to the tap to eliminate control system time lags and phase shifts associated with long sensing lines.
Traditional control -theory states that a control system should react fast and it is generally taken that the faster the reaction, the better. While this leads to good accurate control, it is detrimental in air conditioning fan systems since there are other control loops downstream of the fan, i.e. the terminal boxes which discharge air into the space which is under control of a room thermostat. The whole air conditioning system can become unstable if the duct air pressure is caused to vary considerably. This instability can cause undesirable temperature changes in the space and provide a generally poorly operating air conditioning system. Considerable difficulty has, however, been experienced in attempting to maintain the duct pressure stable and constant.
Most large air conditioning systems have a supply fan and a return air fan. The supply fan is sized to deliver the required air quantity at a pressure estimated to overcome losses r~
of the filters, dampers, coils, ducts and other restrictors.
This means that the design fan pressure selected is normally high, being about 4 to 6 inches WG (1 - 1.4 kPa) at the design flow for medium pressure systems. The only purpose of the return Ean is to overcome the duct losses of large duct systems.
Consequently, re-turn fans are selected to operate at a pressure typically ranging from 0.5 to 2O0 inches WG (0.125 - 0.5 kPa).
Traditionally, the capacity control of the return fan has been a problem. Early systems slaved the return air fan off the supply air fan static pressure controller so that both fans tract. The problem with this approach is that due to the vastly different pressure selections of the fans, their capacity changes versus linear control signal inputs are vastly different, i.e., their curves have different slopes and sometimes are not linear.
Therefore, this matching of capacities only occured at the point of calibration. This problem manifested itself in pressure changes in the space as well as poor temperature regulati~n.
The pressure changes were sometimes so bad that in large buildings lobby doors could not be opened or kept closed.
In an attempt to overcome this problem, some engineers have controlled the capacity of the re-turn air fan from a space static pressure controller. This offered only limited improvement and actually introduced problems of its own. First of all, the space pressure controller had to control a very low value, usually around 0.25 inches WG (62 kPa~. It is difficult to find a controller that can control space static pressure reliably. Usually, the opening of the door admitting a blast of air or the wind genera-ted by a person running the sensor would cause the controller to take corrective action even though none was required. The second problem was the location oE the sensor.
If the sensor is located in the front lobby of a large building, the pressurization problem would still exist in the back lobby and vice ~Jersa. If two sensors were used, the questions arises which should be used or should the signals be combined to provide an average. These proposals have not provided a satisfactory solution to this problem.
I have found that these difficulties can be overcome by providing a method of capacity control of the return air fan that matches the capacity of -the supply fan.
A further difficulty which is experienced in attempting to air condition large buildings is that known as the "stack effect". The stack effect is caused by temperature differences between the constant temperature space and the changing outside air temperature. As the outside temperature drops, the density of the cold air increases, causing it to fall to the ground causing an increase in this air pressure. Since the air temperature in the building is kept constant, its pressure may be considered constant. This higher density air then forces its way inside the building, entering through cracks around the doors anc.
windows, which are recognized by the inhabitants of the building as drafts. Sometimes the pressure difference is so great that the building doors cannot be opened except with great difficulty.
The exac-t opposite occurs in summer. In this case, the outside air temperature is higher than inside tempera-ture, so that the air has a tendency to flow to the outside through the building cracks. This problem is not noticeable since i-t does not create uncomEortable "drafts" to the inhabitants. One noticeable effect, however, is that the building doors may stay open due to the higher internal pressure and the presence of a whistling noise around tall shaft access doors like stairwells or elevator shafts. The escape of conditioned air does, however, represent a loss in efficiency.
I have found that these pressurization problems can be solved by providing a stack effect compensator. A manual stack effect compensator may be in the form of a potentiometer wired to the return fan offset. In effect, operating the potentiometer would offset the tracking of the return fan by an amount equal to half of the rotation of the potentiometer. At 50%/rotation, the AFC would track the return fan as if the SEC was not in the circuit. Rotating i-t to one extreme would decrease the return fan tracking ran~e and rotating it to the other extreme would increase the return Ean tracking range. This could result in the return of less air than would be delivered by the supply fan resultiny in more air being available to pressurize the building, as may be required in winter, to compensate for the higher outside air pressure. This would eliminate the door operating problem and whistling around the building shafts. Rotating it in the other extreme would increase the amount of air being returned~ This would reduce the amount of air exfiltrating from the building and remove the door operating problems and the whistling around the building shafts.
As previously, indicated that stack effect is caused by a temperature differential between the constant building temperature and the variable outside air temperature, I further propose that an automatic stack effect compensator be provided.
The automatic stack effect compensator consists of an outside air tempera-ture sensor supp~ying an adjustable range offset to the AFC in the sa~e manner as the manual potentiometer described above. This stack effect compensator has the ability to automatically adjust the tracking range of the re-turn fan to minimize space pressure fluctuations.
Difficulty is frequently experienced in attempting to provide adequate air conditioning in buildings which have supplementary exhaust systems such as laboratories which have fume hood exhaust fans. These exhaust fans are normally under the control of the occupant and may be started and stopped in any order and they may be of different capacities. The total air exhausted by these exhaust fans may represent a substantial, if not total, percentage of the air supplied by the air supply fan.
Since a variable air volume system supplies a variable amount of i7~
Dl 7 - d~ 4 6 1--1 air depending on space demand, and the fume hoods are made to be started and stopped and random, the space pressure control becomes a formidable problem. I have developed a simple and effective solution to this problem. The basic problem is to keep the total air exhausted and the return air about the same as -the air supplied by the supply fan. There may be a slight offset either negative or positive depending on whether it is desirable to keep the space negative or positive with respect to the outside. I provide a potentiometer for each exhaust fan. If all of these potentiometers are wired in series and fed from a constant current source, and the potentiometer value is adjusted to be analogous to the air exhausted by the fan, then if that fan were to be started by the occupant, an auxiliary contact would short out that resistance, a voltage change proportional to the air exhausted by the fan is obtained. The voltage can be used to provide an offset to the return fan tracking circuit in the same manner that the stack effect compensator does. The net effect is that the return fan speed can be decreased or increase by an amount corresponding to the air exhausted by the exhaust fans.
The present invention seeks to overcome the disadvantages of the prior art described above and provides a control signal which takes account of the inlet vane hysteresis.
According to one aspect of the present invention, there is provided a method of controlling the air flow output of a selected variable inlet vane fan comprising the steps of:
D17-~461-1 a) generating a flow control signal in a controller which is programmed to selectively provide a first signal which is proportional to the increasing response plot of air Elow with respect to vane position when -the vanes of the selected fan are opening or a second signal which is proportional to the decreasing plot of the air flow with respect to vane position when the inlet vanes of the selected fan are closing, b) detecting the direction of response required to meet the demand variation as between a requirement for increased air flow and a requirement for decreased air flow, c) activating the con-troller to provide said flow control signal in the form of said first signal when a requirement for increased flow is detected and to provide said flow control signal in the form of said second signal when a requirement for decreased flow is detected, d) transmitting said full control signal to said fan to control the position of the inlet vanes of the fan as required in use.
According -to one aspect of the present invention, there is provided in a variable air volume air conditioning system having a variable air volume fan located to discharge air into a duct leading to a plurality of branch lines, the improvement of pressure sensing means communicating with said duct upstream of said branch line.
;7~
According to a further ~spect of the p.resent invention, there is provided in a variable air volume air-conditioning system in which a controller, detecting the direction of response required to meet the demand variation as between a requiremen-t Eor increased air flow and a requirement for decreased air flow, Figure 1 is a diagramatic illustration of a building having an air conditioning system of the type of the present invention.
Figure 2 is a block diagram illustrating a control system suitable for use in association with the fans illustrated in F.igure 1.
Figure 3 is a diagram illustrating a variable inlet vane controller for controlling the supply fan of the air conditioning system of Figure 2.
Figure 4 is a diagram illustrating a variable inlet van controller for controlling the return air fan of Figure 2.
Figure 5 is a graphic illustration of the variable inlet vane response curves of a typical variable inlet vane fan.
Figure 6 is a diagram illustrating a response curve of a typical control system.
Figure 7 is a diagram of a system Eor generating an offset signal to the controller when air exhaust systems are provided in the space to be conditioned.
~9~
With reference to Figure 1 of the drawings, -the reference numeral 10 refers generally to a building in the form of a multi-storey structure which has a fan room 12 from which conditioned air is discharged through a duct 14 from which a plurality of branch ducts 16 extend at various levels through the building.
As shown in Figure 2 of the drawings, a variable inlet vane supply fan 18 is connected to the duct 14. A constant speed motor 20 is connected to the fan 18 and is powered from a suitable power source. An inlet vane operator 22 is connected to the vanes 24 and operates to move the inlet vanes to and fro between an open and a closed position. In addition, a return air fan 19 is connected to the return air duct lS. A constant spaced motor 21 is connected to the fan 19 and is powered from a suitable power source.
An inle-t vane operator 23 is connected to the vanes 25 and operated to move the inlet vanes to and fro between an open and a closed position.
A sensing tap 26 is located in the supply duct 14.
Preferably, the sensing tap 26 is located at a distance L from the discharge end of the supply fan 18 which is less than 20 D wherein D
is the largest duct diameter downstream of the fan.
The location of the sensing tap 26 is of considerable importance. In order to keep the fan system opera-tion steady, stable and accurate, it is more important to keep the fan from operating in its unstable range than it is to satisfy the duct 1~
system requirements for good overall control. I have been able to achieve this objecti~e by locating the static pressure sensing tap 26 as short a distance downstream from the Ean as is prac-tical.
Ideally, the static pressure sensing probe 26 would be placed at the supply fan discharge flange, however, this is impractical since turbulence is too great at this point leading to eroneous readings.
Effectively, the same result may be obtained by locating the static pressure sensing probe 26 within about 10 to 20 duct diameters downstream from the fan discharge. It will be noted, however, that the static pressure sensing probe may be located atany position of convenience along the discharge duct upstream of the first branch line 16. The location of the static pressure sensing probe in this manner is contrary to the conventional practice in the air-conditioning industry. Conventional thinking is that it is the space which is to be conditioned that must be satisfied and therefore the sensor should be located about 2/3rd's of the way along the longest duct run. While it may be difficult to argue against this reasoning from an intuitive point oE view, in practice it is virtually impossible to determine where such a location may be found. The duct systems usually run from the fan discharge down one or more shafts in a building with multiple take-oEfs on each floor with the result that it is difficult, if not impossible, to determine where the optimum location for the pressure sensing probe is within the building. The main reason for selecting the 2/3 location, however, relates to the control system stability and is a v compromise between satisfying the longest duct run and the practicality of identifying such a location. It has been generally believed that iE the sensing point or tap is sufficiently far away from the fan, the problems associated with the usual inlet vane controls do not show up. The main problem, however, is the hysteresis losses in the operation of the vanes of the fan with -the result that locating the sensor in the location previously considered to be the optimum location, does not solve the principal problem~
HYSTERE~;IS LOSSES
In Figure 5 of the drawings, the curve 32 serves to illustrate the airflo~ output of a typical variable inlet vane fan as the position of the inlet vanes move from a closed position to an open position and a curve 34 serves to illustrate the air flow of the same fan as the inlet vanes move from a fully open position to a fully closed position.
From Figure 5 of the ~rawings, it will be seen that for a fan inlet vane position Y, air flow in the amount Ql or Q2 may be obtained depending upon whether the inlet vanes are opening or closing. The difference between these two values results from the hysteresis of the control mechanism required to adjust the position of the vanes. This hysteresis is more pronounced on double inlet fans than on single inlet fans, mainly due to more extensive linkages required for the former. This hysteresis serves to de-stabilize any existing control system used for controlling the position of the inlet vanes of a variable inlet vane fan. If, for example~ it is desired to con-trol the air flow at the value Qc illustrated in Figure 5 of the drawings, it will be apparent that this air flow may be obtained with the inlet vane position YCl if the vanes are opening or in the position YCh if the vanes are closing. In view of the fact that a conventional controller system normally cycles about a set point, the addition of hysteresis results in an unstable control loop.
VARIABLE INLET VANE CONTROLLER
The variable inlet vane controller 30 (Fig.2) receives a signal from the sensor 26 through a line 28 which may be in the form of a plastic tube or the like which conveys a pneumatic signal to the controller 30. The controller 30 generates an output signal 32 which is directed to the vane mo-tor operator 22 which is operable to open and close the vanes 24 as required in use.
The variable inlet vane controller 30 will now be described with reference to Figure 3 of the drawings.
As shown in Figure 3 of the drawings, the line 28 communicates with a transducer 40 which serves to convert the pneumatic signal to an electrical signal. The electrical signal is fed to a signal conditioner 42 which provides a signal having an output of the order oE O to 5 volts or O -to 10 volts. This signal is fed to an adjustable filter ~4. The Eilter 44 is ~ ~967~
adapted to overdampen the signal which it receives from the conditioner 42 so that its output signal will change progressively to a steady state over an extended time period to ensure that a substantially stable air pressure is maintained in the air duct system 14 in use. This overdamping feature will be described hereinaEter with reference to Figure 6 of the drawings in more detail. The outpu-t signal from the filter 44 is directed to a controller ~6 which communicates through line 48 with a driver 50 and through line 52 with a response de-tector 54. If the response detector 54 detects an increase signal, the feedback loop provides a feedback signal to the controller through line 56 and if the response detector 54 detects a decrease signal, this is communicated -to the con-troller through the feedback line 58.
The controller 46 is programmed to generate a first signal output which is proportional to the increasing response plot of air flow with respect to vane position when the inlet vanes of the selected fan are opening or to generate a second signal output which is proportional to the decreasing response plot of air flow with respect to vane position when the inlet vanes of the selected fan are closing. The detector signals provided through the feedback lines 56 or 58 serve to activate the controller 4~
to generate the first or second control signal. The value of the Eirst or second control signal is determined by the controller which compares the signal output from the filter 44 with the Dl7-4461-l curves 32 and 34 (Fig.3) and a set point signal which is provided by the set point control 60.
As shown in Figure 3, a line 202 is taken from the line 28 after tile filter 44. As shown in Figure 2, this line leads to a second controller 200 which serve to control -this operation of the inlet vanes 25 of the return air inle-t vane operator 23 through line 201.
METHOD OF OPERATION OF VARIABLE INLET VANE CONTROLLE~
In use a drop in pressure in the air duct 14 will signal a requirement for increased air supply and this signal will be transmitted from the detector 26 throu~h the line 2~ to the controller 30. The controller 30 is operable as previously described to determine whether or not the signal is indicative of a demand for increased air flow or decreased air flow. If as previously indicated, a requirement for increased air flow is detected, the controller determines the vane position requ.ired to obtain the required air flow with reference to the curv 32 and gener~tes a first signal output to the driver 50. If, on the other hand, -the response detector 54 had detected a requirement for decreased air flow, the controller 46 would cletermine the required vane position with reEerence to the curve 34. The output of the driver 50 is conveyed to the vane control motor 22 which moves the vanes 24 to the position required in order to provide the desired air flow.
Various modifications of the controller of Figure 4 will be apparent to those skilled in the art. For example, an integrator 62 ma~ be used in place of the filter 44 or an integrator 64 may be used in addition to the filter 44. IE the integra-tor 6~ is used in addition to the filter 44, it is arranged in series with the filter 44 and may be located before or after the Eilter 44. The integrator 62 or 64 may serve the same function as the filter for periodic disturbances, that is to say the integrator 62 and 64 may serve to integrate out ~luctuations in the signal prior to transmission of the signal to the controller.
RETURN AIR FAN CONTROL
As previously indicated, the input signal to the return air fan controller 200 is the signal convened by the line 202 from the controller 30. It will be understood that the return air fan inlet vane characteristics are considerably different to those of the supply fan with the result that before passing the signal in the line 202 to the contxoller 246, it is necessary to transpose or condition this signal be means oE a conditioner 20 so -that it varies in accordance with the output curve of the return air fan rather than the output curve of the supply air fan. This signal is transmitted through the line 228 to provide the set point signal of the controller 246. The controller 246 driver 250 and response detector 254 operate in the same manner as the corresponding cornponents of controller 30 and serve to provide a feedback signal to the controller 246. Thus, i-t will be seen that the output signal of the controller 200 which is directed to the inlet vane operator 23 through line 201 serves to control the operation of the return air fan in a manner which will serve to insure that the operation of the return air fan closely matches the operation o~ the supply air fan.
OFFSET TRACKING OF THE RETURN AIR FAN
In order to overcome the difficulties associated with the "stack effect" previously described, I provide a manual stack effect compensator potentiometer 270. This signal is conditioned by a signal conditioning or translating device 272 and is then fed to the controller 246 through the line 274. The signal generated by the stack effect compensator is received by the controller 24~ and serves to offset the tracking of the return fan by an amount equal to half the rotation of the potentiometer.
A 50~ rotation of the AFC would track the return fan as if the special effect compensator was not in the circuit. Rotating it to one extreme would decrease the return fan tracking range and rotating it in the other extreme would increase the return fan tracking range. This would result in the return oE less air than would be delivered by the supply Ean resulting in more air being available to pressurize the building, in winter, to compensate Eor the higher outside air pressure. This would eliminate the door operating problem and whistling around the building shafts.
Rotating it in the other extreme would increase the amount of air Dl7-4461-l being returned. This would reduce the problem oE air exfiltrating from the building and would eliminate the door operating and whistling problems.
The stack efEect problem is caused by a temperature difference between the constant building temperature and the variable outside air temperature. This problem is overcome by providing an automatic stack efEect compensator. This device is illustrated in Figure 4 oE the drawings and includes an outside air tempera-ture sensor 280 which provides a signal which is conditioned by a signal conditioner 282 which has a range adjustment potentiometer 284 and a 0 set potentiometer 286. The output signal from the signal conditioner 282 may be Eed to the conditioner 272 through line 288 in the same manner as the signal of the manual potentiometer. This signal has the ability to automatically adjust the tracking range of the return fan to minimize space pressure fluctuations.
A further signal may be used for the purpuses of oEfsetting the controller 246. This signal is supplied through line 290. This signal can be used to compensate for air which is exhausted from the air space which is to be conditioned such as by way of the fume hood exhaust Eans which are widely used in laboratories. A typical Eume hood control system is illustrated in Figure 7 oE the drawings wherein an auxiliary contact 301, 302, 303, 304, 305 and 306 is provided and a line which communicates with the fume hood e~haust fan motor starter of each of six different fume hoods. Potentiometers 310, 312, 313, 314, 315 and 316 are provided, one associated with each Eume hood motor. The poten-tiometers 11~ to 116 are wired in series and fed from a constant current source 320. The potentiometer values are adjusted to be analogous to the air exhausted by the fan with the result that when the fan is started by the occupant, the auxiliary contacts associated with the fan short out that resistance and a volt changed proportional to the air exhausted by the fan is obtained. The signal translator 322 translates this signal to a signal which can be ied through line 290 to the signal conditioner 27~. It will ~ appararent that by this system, it is possible to keep the total air exhausted and the returned air about the same as the air supplied by the supply fan. There may be a slight offset either negative or positive depending on whether it is desirable to keep the space negative or positive with respect to the outside. This system is particularly well suited for use in controlling serious air control problems which plague laboratory air supply systems. It will be understood that this system is not restricted to fume hood exhaust systems but is applicable to any system in which the air is discharged from the space which is to be conditioned by way of a separate exhaust system.
OVER-DAMPENING OF CONTROL SIGNAL
According to general control theory, the optimum control system is one that is op-timally dampened, i.e. a system wherein the output amplitude cycling fails within a decay envelope within approximately 3 excursions. In this regard, reference is made to Figure 5 of the drawings which illustrates optimal damping of the amplitude of the conven-tional control signal illustrated by the line 70. The signal 70 is normally dampened within the decay envelope illustrated by the lines 72 and 74 within approximately 3 excursions.
In air-conditioning systems, any changes that take place in the system loading are due to the sun shining in windows, due to people moving into and out of the space to be conditioned or due -to lights and heat producing equipment such as computer terminals or the like being turned on or off. If, for example, a computer is running and producing heat, when it is turned off, it ~unctions as a heating element. The residual heat reduces very slowly. In addition, there is the thermal mass of the building structure that dampens all air-conditioning load changes. By the time the terminal air conditioning boxes collectively react to these load changes, and they should collectively react slowly, sometimes minutes will pass.
Frequently the load changes in one part of a system are cancelled by load changes in another part of the system which are nearly identical in magnitude but opposite in direction. For example, the load resulting from the movement oE direct sunlight around a building in the course of a day can result in changes in the load applied to various parts of a building while the total load may remain substantially the same over the full day.
Conventionally, control system response times are measured in very small fractions of a second, frequently in milli or micro seconds. ~owever, air-conditioning load changes take place in much longer time periods as for e~ample, periods as long as ten minutes or more. When these changes take place, the control system must be sensitive enough to sense them and to react accordingly. I have recognized these characteristics and for this reason I provide a control system in which the control signal is over-dampened, thereby to increase the controller sensitivity. Over-dampening is achieved by the filter 4~ or integraters 62 and 64 previously described so as to generate a signal to the controller 46 which follows the curve 76 illustrated in Figure 5 oF the drawings.
~ he slow system response which I am able to generate is important for other reasons. Terminal boxes are connected to the fan duct which are usually under the control of a room thermostat. Such a thermostat will allow a typical box to pass just the correct amount oE air to the space to satisfy the load.
The correct amount oE air depends on the pressure upstream oE the box remainincJ constant. If the fan capacity control system which -tries to keep this pressure constant is "live" and allowed to cycle, causing the duct pressure to vary, the box will allow more or less air to pass to the space causing temperature ~ . J
fluctuations.These fluctuations would be sensed by the thermostat which would try to correct them. The net result of this scenario is that the two con-trol systems fight each other and the whole air-conditioning system would be unstable. The over-dampening of the fan capacity controls helps to prevent this from occurring and greatly assists in providing stable controls.
As previously indicated in Figure 2 oE the drawings, the sensed variable which is detected by the detector 26 is the s-tatic pressure. Generally, this static pressure is no-t steady enough for control purposes and the filter 44 which has a long time constant serves to filter out transients and to allow only actual load changes heavily filtered to pass to the controller 46. The time constant is adjustable with the adjustment providing a range of 0 to 200 seconds. The time constant may therefore be adjusted on-site to be a good compromise between the desired system response time and control system stability.
As previously indicated, integrators o2 or 64 may be substituted fGr or used in conjunction with the filter 44. In control systems requiring fast response times, cycling occurs a-t a specific frequency. This frequency varies with the damping ra-tio of the sys-tem. If the control system is set for the desired sensitivity and impermissable and excessive cycling occurs, substituting an integrator for a filter may help to overcome this difficulty. The integrator must be inserted into the circuit as illustrated in Figure 3 of the drawings between D17-44~1-1 the signal conditioner 42 and the controller 46. As the transmitter senses the system cycling, the integrator will act on the output. As long as the cycling is of a se-t frequency, the integrator will integrate the fluctuations out. The integral of a symetrical periodic Eunc-tion is 0. Therefore, -the integrator output will consist of a steady value only as sensed by the transmitter. It is important that the frequency of the cycling be measured on the job site and adjusted into the integra-tor. An alternate position of the integrator is in the Eeedback loop, as shown by integrator 16 in Figure 3. This device may be used with or without filter 44 and integrators 62 and 64.
Controller 30 may also be constructed with a micro-processor replacing some or all signal processing, conditioning, filtering, integrating, controlling, feedback, set point and detecting functions by means of software algori-thms contained in the micro-processor's memory.
Various modifications of the present invention will be apparent to those skilled in the art. It will be apparent that the air conditioning system oE the present invention may be used to advantage in a single storey building in which case it serves to overcome the difficulties experienced with lateral rather than vertical air disturbances.
This invention relates to improvements in air-conditioning system controls.
In particular, this invention relates to improvements in the control of the output oE a variable inlet vane fan.
The present invention also relates to the positioning oE the sensing probe in an air-conditioning system.
~ n addition, the present inven-tion provides improvements in the stability of a control system of an air-conditioning system.
PRIOR ART
For many years, variable inlet vanes have been a commonly used device for controlling the output of a fan such as the fans commonly used in air-conditioning systems. The use of variable inlet vane fans in air-conditioning systems has increased substantially since the popuiarity of variable air volume air-conditioning systems has increased.
In controlling the movement of the variable inlet vanes in order to adjust the fan output capacity, it has been found that while the air flow is a function of the position of the variable inlet vane, the actual air flow varies depending upon whether the vanes are opening or closing. This difference in air flow is as a result of hysteresis losses which are particularly due to the linkage mechanisms required to control the movemen-t o~
the inlet vanes. The hysteresis is more pronounced on double inlet fans than on single inlet fans mainly due to the more ' -'''F
extensive linkages required for the former. This hysteresis would normally de-stabilize a conventional inlet vane con-trol system. A conventional control system normally cycles about a set point and the hysteresis which is inherent in the operation of variable inlet vanes of a fan results in an unstable control 1 oop .
To prevent the system from cycling about it's control or set point, the conventional practice has been to introduce restrictors into the sensing loop of a pneumatic system or damping circuits in an electrical or electronic control system.
The purpose of these control system modifications are to provide system damping. These systems do not address the problem correctly and therefore do not provide the required result. The only efEective remedy presently in use is to decrease the control system sensitivity, i.e. loop gain, to such an extent that the control system cannot differentiate between the two extreme sensed conditions. The disadvantage of this solution is that the sensitivity is so low that the control system cannot follow changes in fan system conditions and therefore the control system is slow and sluggish and in some cases may be inoperative.
I find that I can improve that stability of the air conditioning system of the present invention in circumstances where the sensed variable which is usually the static pressure, is not steady enough for conventional control. If the sensed variable is not steady enough for control, I provide a filter with a long-time constant. rhis filter serves -to ~ilter out transients and to allow actual load changes, heavily filtered, to pass from the signal transmitter to the controller. The time constant should be adjustable and should normally be in the range of 0 to 200 seconds. The time constant may then be adjus-ted on site to be a good compromise between the desired system response and the control system stability.
Alternatively, I may substitute an integrator for the filter described in the preceding paragraph. In control systesm requiring fast response times, cycling occurs at a specific frequency. This frequency varies with the setting of -the sensitivity or gain ad~ustments. If the control system is ~et for the desired sensitivity and impermissable and excessive cycling occurs, substituting an integ~ator for a filter would be helpful. The integrator is inserted into the circuit between the transmi-tter and the con-troller. As the transmitter senses the system cycling, the integrator will act on the output. As long as the cycling is of a set frequency, the integrator will integrate the fluxuations out. The integral of a symmetrical periodic function is 0 and consequently, the integrator output will consist of the steady value only as sensed by the transmitter. It is important that the frequency of the cycling be measured on the job si~e and adjusted into the integrator.
The conditions under which air flow takes place in an air conditioning duct is subject to the following equation:
dP = Clq ~ C2q ~ C3q EQrJATIO~ 1 Where C1, C2, C3 are constants q = air flow in suitable uni-ts dp = pressure drop oE duct system in suitable units EQUATION 1 breaks up the pressure drop into components due to totally turbulent flow depicted by the q2 term; due to laminar ~low depicted ql term; and a component independent of flow depicted by the q term. (Note q = 1). Experience indicates that in the range of operation of air conditioning systems, the laminar flow component is negligible. If we assume this term to be zero, then Equation l-simplifies to the following:
dP = Clq -~ C3q EQUATION ~
The characteristic curves of a typical fan are well known and are provided by fan manufacturers. If a fan is a-ttached to an air conditioning system consisting of filters, coils, ducts, terminal boxes, dampers, etc., air flow will take place at the point on the characteristic curve where the fan and the system curves intersect. If the system is a constant volume system, there would be one fan curve depicting the head-flow condition at a particular constant speed. However, there are an infinite number of system curves between a minimum and maximum Elow. Each time a damper changes position, resulting in different pressure in the duct, flow would be affected. Since ~ ~3ti7~3 air flow can only take place at the point of intersection of the fan curve and the system curve, if the duct pressure changes, this would have the effect oE moving away Erom -the point of intersection. Since it is known that this cannot happen, the condition must intersect. The only way this can happen is with a new system curve intersecting the fan curve at the new flow-pressure conditions. Hence, there can be an infinite number of system curves.
If the fan is to vary capacity, there would likewise be an infinite number oE fan characteristic curves between minimum and maximum capacity; one for each speed. The fan system operation is somewhere in the region defined by the minimum capacity-flow and the maximum capacity-flow which is determined by the system condition. That is to say the minimum capacity flow is that which is achieved when the system is clean and the maximum capacity-flow is that which is achieved when the system is dirty. Dirty, in this context, refers to the clogging of coils, filters, or other system components which result in a higher pressure drop across them on the air side.
I have found that in order to keep the fan-system operation steady, stable and accurate, it is important to locate the tap of the static pressure sensing probe as short a distance downstream of the fan as is practical and to locate the pressure sensing device as close as possible to the tap. Ideally, the tap would be placed at the fan discharge flange, however, this is ~9~JJ~
impractical since turbulence is too great at this point leading to erroneous readings. Effectively, the same result may be obtained if the tap of the sensing device is located at any point in the duct system upstream of the first branch line.
PreEerably, the tap oE the sensing device is located within the fan room and preferably less than 20 duct dia~neters downstream of the discharge fan Elange. PreEerably, the sensing device is located adjacent to the tap to eliminate control system time lags and phase shifts associated with long sensing lines.
Traditional control -theory states that a control system should react fast and it is generally taken that the faster the reaction, the better. While this leads to good accurate control, it is detrimental in air conditioning fan systems since there are other control loops downstream of the fan, i.e. the terminal boxes which discharge air into the space which is under control of a room thermostat. The whole air conditioning system can become unstable if the duct air pressure is caused to vary considerably. This instability can cause undesirable temperature changes in the space and provide a generally poorly operating air conditioning system. Considerable difficulty has, however, been experienced in attempting to maintain the duct pressure stable and constant.
Most large air conditioning systems have a supply fan and a return air fan. The supply fan is sized to deliver the required air quantity at a pressure estimated to overcome losses r~
of the filters, dampers, coils, ducts and other restrictors.
This means that the design fan pressure selected is normally high, being about 4 to 6 inches WG (1 - 1.4 kPa) at the design flow for medium pressure systems. The only purpose of the return Ean is to overcome the duct losses of large duct systems.
Consequently, re-turn fans are selected to operate at a pressure typically ranging from 0.5 to 2O0 inches WG (0.125 - 0.5 kPa).
Traditionally, the capacity control of the return fan has been a problem. Early systems slaved the return air fan off the supply air fan static pressure controller so that both fans tract. The problem with this approach is that due to the vastly different pressure selections of the fans, their capacity changes versus linear control signal inputs are vastly different, i.e., their curves have different slopes and sometimes are not linear.
Therefore, this matching of capacities only occured at the point of calibration. This problem manifested itself in pressure changes in the space as well as poor temperature regulati~n.
The pressure changes were sometimes so bad that in large buildings lobby doors could not be opened or kept closed.
In an attempt to overcome this problem, some engineers have controlled the capacity of the re-turn air fan from a space static pressure controller. This offered only limited improvement and actually introduced problems of its own. First of all, the space pressure controller had to control a very low value, usually around 0.25 inches WG (62 kPa~. It is difficult to find a controller that can control space static pressure reliably. Usually, the opening of the door admitting a blast of air or the wind genera-ted by a person running the sensor would cause the controller to take corrective action even though none was required. The second problem was the location oE the sensor.
If the sensor is located in the front lobby of a large building, the pressurization problem would still exist in the back lobby and vice ~Jersa. If two sensors were used, the questions arises which should be used or should the signals be combined to provide an average. These proposals have not provided a satisfactory solution to this problem.
I have found that these difficulties can be overcome by providing a method of capacity control of the return air fan that matches the capacity of -the supply fan.
A further difficulty which is experienced in attempting to air condition large buildings is that known as the "stack effect". The stack effect is caused by temperature differences between the constant temperature space and the changing outside air temperature. As the outside temperature drops, the density of the cold air increases, causing it to fall to the ground causing an increase in this air pressure. Since the air temperature in the building is kept constant, its pressure may be considered constant. This higher density air then forces its way inside the building, entering through cracks around the doors anc.
windows, which are recognized by the inhabitants of the building as drafts. Sometimes the pressure difference is so great that the building doors cannot be opened except with great difficulty.
The exac-t opposite occurs in summer. In this case, the outside air temperature is higher than inside tempera-ture, so that the air has a tendency to flow to the outside through the building cracks. This problem is not noticeable since i-t does not create uncomEortable "drafts" to the inhabitants. One noticeable effect, however, is that the building doors may stay open due to the higher internal pressure and the presence of a whistling noise around tall shaft access doors like stairwells or elevator shafts. The escape of conditioned air does, however, represent a loss in efficiency.
I have found that these pressurization problems can be solved by providing a stack effect compensator. A manual stack effect compensator may be in the form of a potentiometer wired to the return fan offset. In effect, operating the potentiometer would offset the tracking of the return fan by an amount equal to half of the rotation of the potentiometer. At 50%/rotation, the AFC would track the return fan as if the SEC was not in the circuit. Rotating i-t to one extreme would decrease the return fan tracking ran~e and rotating it to the other extreme would increase the return Ean tracking range. This could result in the return of less air than would be delivered by the supply fan resultiny in more air being available to pressurize the building, as may be required in winter, to compensate for the higher outside air pressure. This would eliminate the door operating problem and whistling around the building shafts. Rotating it in the other extreme would increase the amount of air being returned~ This would reduce the amount of air exfiltrating from the building and remove the door operating problems and the whistling around the building shafts.
As previously, indicated that stack effect is caused by a temperature differential between the constant building temperature and the variable outside air temperature, I further propose that an automatic stack effect compensator be provided.
The automatic stack effect compensator consists of an outside air tempera-ture sensor supp~ying an adjustable range offset to the AFC in the sa~e manner as the manual potentiometer described above. This stack effect compensator has the ability to automatically adjust the tracking range of the re-turn fan to minimize space pressure fluctuations.
Difficulty is frequently experienced in attempting to provide adequate air conditioning in buildings which have supplementary exhaust systems such as laboratories which have fume hood exhaust fans. These exhaust fans are normally under the control of the occupant and may be started and stopped in any order and they may be of different capacities. The total air exhausted by these exhaust fans may represent a substantial, if not total, percentage of the air supplied by the air supply fan.
Since a variable air volume system supplies a variable amount of i7~
Dl 7 - d~ 4 6 1--1 air depending on space demand, and the fume hoods are made to be started and stopped and random, the space pressure control becomes a formidable problem. I have developed a simple and effective solution to this problem. The basic problem is to keep the total air exhausted and the return air about the same as -the air supplied by the supply fan. There may be a slight offset either negative or positive depending on whether it is desirable to keep the space negative or positive with respect to the outside. I provide a potentiometer for each exhaust fan. If all of these potentiometers are wired in series and fed from a constant current source, and the potentiometer value is adjusted to be analogous to the air exhausted by the fan, then if that fan were to be started by the occupant, an auxiliary contact would short out that resistance, a voltage change proportional to the air exhausted by the fan is obtained. The voltage can be used to provide an offset to the return fan tracking circuit in the same manner that the stack effect compensator does. The net effect is that the return fan speed can be decreased or increase by an amount corresponding to the air exhausted by the exhaust fans.
The present invention seeks to overcome the disadvantages of the prior art described above and provides a control signal which takes account of the inlet vane hysteresis.
According to one aspect of the present invention, there is provided a method of controlling the air flow output of a selected variable inlet vane fan comprising the steps of:
D17-~461-1 a) generating a flow control signal in a controller which is programmed to selectively provide a first signal which is proportional to the increasing response plot of air Elow with respect to vane position when -the vanes of the selected fan are opening or a second signal which is proportional to the decreasing plot of the air flow with respect to vane position when the inlet vanes of the selected fan are closing, b) detecting the direction of response required to meet the demand variation as between a requirement for increased air flow and a requirement for decreased air flow, c) activating the con-troller to provide said flow control signal in the form of said first signal when a requirement for increased flow is detected and to provide said flow control signal in the form of said second signal when a requirement for decreased flow is detected, d) transmitting said full control signal to said fan to control the position of the inlet vanes of the fan as required in use.
According -to one aspect of the present invention, there is provided in a variable air volume air conditioning system having a variable air volume fan located to discharge air into a duct leading to a plurality of branch lines, the improvement of pressure sensing means communicating with said duct upstream of said branch line.
;7~
According to a further ~spect of the p.resent invention, there is provided in a variable air volume air-conditioning system in which a controller, detecting the direction of response required to meet the demand variation as between a requiremen-t Eor increased air flow and a requirement for decreased air flow, Figure 1 is a diagramatic illustration of a building having an air conditioning system of the type of the present invention.
Figure 2 is a block diagram illustrating a control system suitable for use in association with the fans illustrated in F.igure 1.
Figure 3 is a diagram illustrating a variable inlet vane controller for controlling the supply fan of the air conditioning system of Figure 2.
Figure 4 is a diagram illustrating a variable inlet van controller for controlling the return air fan of Figure 2.
Figure 5 is a graphic illustration of the variable inlet vane response curves of a typical variable inlet vane fan.
Figure 6 is a diagram illustrating a response curve of a typical control system.
Figure 7 is a diagram of a system Eor generating an offset signal to the controller when air exhaust systems are provided in the space to be conditioned.
~9~
With reference to Figure 1 of the drawings, -the reference numeral 10 refers generally to a building in the form of a multi-storey structure which has a fan room 12 from which conditioned air is discharged through a duct 14 from which a plurality of branch ducts 16 extend at various levels through the building.
As shown in Figure 2 of the drawings, a variable inlet vane supply fan 18 is connected to the duct 14. A constant speed motor 20 is connected to the fan 18 and is powered from a suitable power source. An inlet vane operator 22 is connected to the vanes 24 and operates to move the inlet vanes to and fro between an open and a closed position. In addition, a return air fan 19 is connected to the return air duct lS. A constant spaced motor 21 is connected to the fan 19 and is powered from a suitable power source.
An inle-t vane operator 23 is connected to the vanes 25 and operated to move the inlet vanes to and fro between an open and a closed position.
A sensing tap 26 is located in the supply duct 14.
Preferably, the sensing tap 26 is located at a distance L from the discharge end of the supply fan 18 which is less than 20 D wherein D
is the largest duct diameter downstream of the fan.
The location of the sensing tap 26 is of considerable importance. In order to keep the fan system opera-tion steady, stable and accurate, it is more important to keep the fan from operating in its unstable range than it is to satisfy the duct 1~
system requirements for good overall control. I have been able to achieve this objecti~e by locating the static pressure sensing tap 26 as short a distance downstream from the Ean as is prac-tical.
Ideally, the static pressure sensing probe 26 would be placed at the supply fan discharge flange, however, this is impractical since turbulence is too great at this point leading to eroneous readings.
Effectively, the same result may be obtained by locating the static pressure sensing probe 26 within about 10 to 20 duct diameters downstream from the fan discharge. It will be noted, however, that the static pressure sensing probe may be located atany position of convenience along the discharge duct upstream of the first branch line 16. The location of the static pressure sensing probe in this manner is contrary to the conventional practice in the air-conditioning industry. Conventional thinking is that it is the space which is to be conditioned that must be satisfied and therefore the sensor should be located about 2/3rd's of the way along the longest duct run. While it may be difficult to argue against this reasoning from an intuitive point oE view, in practice it is virtually impossible to determine where such a location may be found. The duct systems usually run from the fan discharge down one or more shafts in a building with multiple take-oEfs on each floor with the result that it is difficult, if not impossible, to determine where the optimum location for the pressure sensing probe is within the building. The main reason for selecting the 2/3 location, however, relates to the control system stability and is a v compromise between satisfying the longest duct run and the practicality of identifying such a location. It has been generally believed that iE the sensing point or tap is sufficiently far away from the fan, the problems associated with the usual inlet vane controls do not show up. The main problem, however, is the hysteresis losses in the operation of the vanes of the fan with -the result that locating the sensor in the location previously considered to be the optimum location, does not solve the principal problem~
HYSTERE~;IS LOSSES
In Figure 5 of the drawings, the curve 32 serves to illustrate the airflo~ output of a typical variable inlet vane fan as the position of the inlet vanes move from a closed position to an open position and a curve 34 serves to illustrate the air flow of the same fan as the inlet vanes move from a fully open position to a fully closed position.
From Figure 5 of the ~rawings, it will be seen that for a fan inlet vane position Y, air flow in the amount Ql or Q2 may be obtained depending upon whether the inlet vanes are opening or closing. The difference between these two values results from the hysteresis of the control mechanism required to adjust the position of the vanes. This hysteresis is more pronounced on double inlet fans than on single inlet fans, mainly due to more extensive linkages required for the former. This hysteresis serves to de-stabilize any existing control system used for controlling the position of the inlet vanes of a variable inlet vane fan. If, for example~ it is desired to con-trol the air flow at the value Qc illustrated in Figure 5 of the drawings, it will be apparent that this air flow may be obtained with the inlet vane position YCl if the vanes are opening or in the position YCh if the vanes are closing. In view of the fact that a conventional controller system normally cycles about a set point, the addition of hysteresis results in an unstable control loop.
VARIABLE INLET VANE CONTROLLER
The variable inlet vane controller 30 (Fig.2) receives a signal from the sensor 26 through a line 28 which may be in the form of a plastic tube or the like which conveys a pneumatic signal to the controller 30. The controller 30 generates an output signal 32 which is directed to the vane mo-tor operator 22 which is operable to open and close the vanes 24 as required in use.
The variable inlet vane controller 30 will now be described with reference to Figure 3 of the drawings.
As shown in Figure 3 of the drawings, the line 28 communicates with a transducer 40 which serves to convert the pneumatic signal to an electrical signal. The electrical signal is fed to a signal conditioner 42 which provides a signal having an output of the order oE O to 5 volts or O -to 10 volts. This signal is fed to an adjustable filter ~4. The Eilter 44 is ~ ~967~
adapted to overdampen the signal which it receives from the conditioner 42 so that its output signal will change progressively to a steady state over an extended time period to ensure that a substantially stable air pressure is maintained in the air duct system 14 in use. This overdamping feature will be described hereinaEter with reference to Figure 6 of the drawings in more detail. The outpu-t signal from the filter 44 is directed to a controller ~6 which communicates through line 48 with a driver 50 and through line 52 with a response de-tector 54. If the response detector 54 detects an increase signal, the feedback loop provides a feedback signal to the controller through line 56 and if the response detector 54 detects a decrease signal, this is communicated -to the con-troller through the feedback line 58.
The controller 46 is programmed to generate a first signal output which is proportional to the increasing response plot of air flow with respect to vane position when the inlet vanes of the selected fan are opening or to generate a second signal output which is proportional to the decreasing response plot of air flow with respect to vane position when the inlet vanes of the selected fan are closing. The detector signals provided through the feedback lines 56 or 58 serve to activate the controller 4~
to generate the first or second control signal. The value of the Eirst or second control signal is determined by the controller which compares the signal output from the filter 44 with the Dl7-4461-l curves 32 and 34 (Fig.3) and a set point signal which is provided by the set point control 60.
As shown in Figure 3, a line 202 is taken from the line 28 after tile filter 44. As shown in Figure 2, this line leads to a second controller 200 which serve to control -this operation of the inlet vanes 25 of the return air inle-t vane operator 23 through line 201.
METHOD OF OPERATION OF VARIABLE INLET VANE CONTROLLE~
In use a drop in pressure in the air duct 14 will signal a requirement for increased air supply and this signal will be transmitted from the detector 26 throu~h the line 2~ to the controller 30. The controller 30 is operable as previously described to determine whether or not the signal is indicative of a demand for increased air flow or decreased air flow. If as previously indicated, a requirement for increased air flow is detected, the controller determines the vane position requ.ired to obtain the required air flow with reference to the curv 32 and gener~tes a first signal output to the driver 50. If, on the other hand, -the response detector 54 had detected a requirement for decreased air flow, the controller 46 would cletermine the required vane position with reEerence to the curve 34. The output of the driver 50 is conveyed to the vane control motor 22 which moves the vanes 24 to the position required in order to provide the desired air flow.
Various modifications of the controller of Figure 4 will be apparent to those skilled in the art. For example, an integrator 62 ma~ be used in place of the filter 44 or an integrator 64 may be used in addition to the filter 44. IE the integra-tor 6~ is used in addition to the filter 44, it is arranged in series with the filter 44 and may be located before or after the Eilter 44. The integrator 62 or 64 may serve the same function as the filter for periodic disturbances, that is to say the integrator 62 and 64 may serve to integrate out ~luctuations in the signal prior to transmission of the signal to the controller.
RETURN AIR FAN CONTROL
As previously indicated, the input signal to the return air fan controller 200 is the signal convened by the line 202 from the controller 30. It will be understood that the return air fan inlet vane characteristics are considerably different to those of the supply fan with the result that before passing the signal in the line 202 to the contxoller 246, it is necessary to transpose or condition this signal be means oE a conditioner 20 so -that it varies in accordance with the output curve of the return air fan rather than the output curve of the supply air fan. This signal is transmitted through the line 228 to provide the set point signal of the controller 246. The controller 246 driver 250 and response detector 254 operate in the same manner as the corresponding cornponents of controller 30 and serve to provide a feedback signal to the controller 246. Thus, i-t will be seen that the output signal of the controller 200 which is directed to the inlet vane operator 23 through line 201 serves to control the operation of the return air fan in a manner which will serve to insure that the operation of the return air fan closely matches the operation o~ the supply air fan.
OFFSET TRACKING OF THE RETURN AIR FAN
In order to overcome the difficulties associated with the "stack effect" previously described, I provide a manual stack effect compensator potentiometer 270. This signal is conditioned by a signal conditioning or translating device 272 and is then fed to the controller 246 through the line 274. The signal generated by the stack effect compensator is received by the controller 24~ and serves to offset the tracking of the return fan by an amount equal to half the rotation of the potentiometer.
A 50~ rotation of the AFC would track the return fan as if the special effect compensator was not in the circuit. Rotating it to one extreme would decrease the return fan tracking range and rotating it in the other extreme would increase the return fan tracking range. This would result in the return oE less air than would be delivered by the supply Ean resulting in more air being available to pressurize the building, in winter, to compensate Eor the higher outside air pressure. This would eliminate the door operating problem and whistling around the building shafts.
Rotating it in the other extreme would increase the amount of air Dl7-4461-l being returned. This would reduce the problem oE air exfiltrating from the building and would eliminate the door operating and whistling problems.
The stack efEect problem is caused by a temperature difference between the constant building temperature and the variable outside air temperature. This problem is overcome by providing an automatic stack efEect compensator. This device is illustrated in Figure 4 oE the drawings and includes an outside air tempera-ture sensor 280 which provides a signal which is conditioned by a signal conditioner 282 which has a range adjustment potentiometer 284 and a 0 set potentiometer 286. The output signal from the signal conditioner 282 may be Eed to the conditioner 272 through line 288 in the same manner as the signal of the manual potentiometer. This signal has the ability to automatically adjust the tracking range of the return fan to minimize space pressure fluctuations.
A further signal may be used for the purpuses of oEfsetting the controller 246. This signal is supplied through line 290. This signal can be used to compensate for air which is exhausted from the air space which is to be conditioned such as by way of the fume hood exhaust Eans which are widely used in laboratories. A typical Eume hood control system is illustrated in Figure 7 oE the drawings wherein an auxiliary contact 301, 302, 303, 304, 305 and 306 is provided and a line which communicates with the fume hood e~haust fan motor starter of each of six different fume hoods. Potentiometers 310, 312, 313, 314, 315 and 316 are provided, one associated with each Eume hood motor. The poten-tiometers 11~ to 116 are wired in series and fed from a constant current source 320. The potentiometer values are adjusted to be analogous to the air exhausted by the fan with the result that when the fan is started by the occupant, the auxiliary contacts associated with the fan short out that resistance and a volt changed proportional to the air exhausted by the fan is obtained. The signal translator 322 translates this signal to a signal which can be ied through line 290 to the signal conditioner 27~. It will ~ appararent that by this system, it is possible to keep the total air exhausted and the returned air about the same as the air supplied by the supply fan. There may be a slight offset either negative or positive depending on whether it is desirable to keep the space negative or positive with respect to the outside. This system is particularly well suited for use in controlling serious air control problems which plague laboratory air supply systems. It will be understood that this system is not restricted to fume hood exhaust systems but is applicable to any system in which the air is discharged from the space which is to be conditioned by way of a separate exhaust system.
OVER-DAMPENING OF CONTROL SIGNAL
According to general control theory, the optimum control system is one that is op-timally dampened, i.e. a system wherein the output amplitude cycling fails within a decay envelope within approximately 3 excursions. In this regard, reference is made to Figure 5 of the drawings which illustrates optimal damping of the amplitude of the conven-tional control signal illustrated by the line 70. The signal 70 is normally dampened within the decay envelope illustrated by the lines 72 and 74 within approximately 3 excursions.
In air-conditioning systems, any changes that take place in the system loading are due to the sun shining in windows, due to people moving into and out of the space to be conditioned or due -to lights and heat producing equipment such as computer terminals or the like being turned on or off. If, for example, a computer is running and producing heat, when it is turned off, it ~unctions as a heating element. The residual heat reduces very slowly. In addition, there is the thermal mass of the building structure that dampens all air-conditioning load changes. By the time the terminal air conditioning boxes collectively react to these load changes, and they should collectively react slowly, sometimes minutes will pass.
Frequently the load changes in one part of a system are cancelled by load changes in another part of the system which are nearly identical in magnitude but opposite in direction. For example, the load resulting from the movement oE direct sunlight around a building in the course of a day can result in changes in the load applied to various parts of a building while the total load may remain substantially the same over the full day.
Conventionally, control system response times are measured in very small fractions of a second, frequently in milli or micro seconds. ~owever, air-conditioning load changes take place in much longer time periods as for e~ample, periods as long as ten minutes or more. When these changes take place, the control system must be sensitive enough to sense them and to react accordingly. I have recognized these characteristics and for this reason I provide a control system in which the control signal is over-dampened, thereby to increase the controller sensitivity. Over-dampening is achieved by the filter 4~ or integraters 62 and 64 previously described so as to generate a signal to the controller 46 which follows the curve 76 illustrated in Figure 5 oF the drawings.
~ he slow system response which I am able to generate is important for other reasons. Terminal boxes are connected to the fan duct which are usually under the control of a room thermostat. Such a thermostat will allow a typical box to pass just the correct amount oE air to the space to satisfy the load.
The correct amount oE air depends on the pressure upstream oE the box remainincJ constant. If the fan capacity control system which -tries to keep this pressure constant is "live" and allowed to cycle, causing the duct pressure to vary, the box will allow more or less air to pass to the space causing temperature ~ . J
fluctuations.These fluctuations would be sensed by the thermostat which would try to correct them. The net result of this scenario is that the two con-trol systems fight each other and the whole air-conditioning system would be unstable. The over-dampening of the fan capacity controls helps to prevent this from occurring and greatly assists in providing stable controls.
As previously indicated in Figure 2 oE the drawings, the sensed variable which is detected by the detector 26 is the s-tatic pressure. Generally, this static pressure is no-t steady enough for control purposes and the filter 44 which has a long time constant serves to filter out transients and to allow only actual load changes heavily filtered to pass to the controller 46. The time constant is adjustable with the adjustment providing a range of 0 to 200 seconds. The time constant may therefore be adjusted on-site to be a good compromise between the desired system response time and control system stability.
As previously indicated, integrators o2 or 64 may be substituted fGr or used in conjunction with the filter 44. In control systems requiring fast response times, cycling occurs a-t a specific frequency. This frequency varies with the damping ra-tio of the sys-tem. If the control system is set for the desired sensitivity and impermissable and excessive cycling occurs, substituting an integrator for a filter may help to overcome this difficulty. The integrator must be inserted into the circuit as illustrated in Figure 3 of the drawings between D17-44~1-1 the signal conditioner 42 and the controller 46. As the transmitter senses the system cycling, the integrator will act on the output. As long as the cycling is of a se-t frequency, the integrator will integrate the fluctuations out. The integral of a symetrical periodic Eunc-tion is 0. Therefore, -the integrator output will consist of a steady value only as sensed by the transmitter. It is important that the frequency of the cycling be measured on the job site and adjusted into the integra-tor. An alternate position of the integrator is in the Eeedback loop, as shown by integrator 16 in Figure 3. This device may be used with or without filter 44 and integrators 62 and 64.
Controller 30 may also be constructed with a micro-processor replacing some or all signal processing, conditioning, filtering, integrating, controlling, feedback, set point and detecting functions by means of software algori-thms contained in the micro-processor's memory.
Various modifications of the present invention will be apparent to those skilled in the art. It will be apparent that the air conditioning system oE the present invention may be used to advantage in a single storey building in which case it serves to overcome the difficulties experienced with lateral rather than vertical air disturbances.
Claims (23)
1. A method of controlling the air flow output of an a selected variable inlet vane fan comprising the steps of:
a) generating a flow control signal in a controller which is programmed to selectively provide a first signal which is proportional to the increasing response plot of air flow with respect to vane position when the vanes of the selected fan are opening or a second signal which is proportional to the decreasing plot of the air flow with respect to vane position when the inlet vanes of the selected fan are closing, b) detecting the direction of response required to meet the demand variation as between a requirement for increased air flow and a requirement for decreased air flow, c) activating the controller to provide said flow control signal in the form of said first signal when a requirement for increased flow is detected and to provide said flow control signal in the form of said second signal when a requirement for decreased flow is detected, d) transmitting said flow control signal to said fan to control the position of the inlet vanes of the fan as required in use.
a) generating a flow control signal in a controller which is programmed to selectively provide a first signal which is proportional to the increasing response plot of air flow with respect to vane position when the vanes of the selected fan are opening or a second signal which is proportional to the decreasing plot of the air flow with respect to vane position when the inlet vanes of the selected fan are closing, b) detecting the direction of response required to meet the demand variation as between a requirement for increased air flow and a requirement for decreased air flow, c) activating the controller to provide said flow control signal in the form of said first signal when a requirement for increased flow is detected and to provide said flow control signal in the form of said second signal when a requirement for decreased flow is detected, d) transmitting said flow control signal to said fan to control the position of the inlet vanes of the fan as required in use.
2. A method of controlling the air flow of a variable inlet vane fan comprising the steps of:
a) detecting the direction of response required to meet a demand variation as between a requirement for increased air flow and a requirement for decreased air flow, b) generating a first vane control signal when a requirement for increased air flow is detected, said first vane control signal being proportional to the increasing response plot of air flow with respect to vane position when the inlet vanes are opening, c) generating a second vane control signal when a requirement for decreased air flow is detected, said second vane control signal being proportional to the decreasing response plot of air flow with respect to vane position when the inlet vanes are closing.
a) detecting the direction of response required to meet a demand variation as between a requirement for increased air flow and a requirement for decreased air flow, b) generating a first vane control signal when a requirement for increased air flow is detected, said first vane control signal being proportional to the increasing response plot of air flow with respect to vane position when the inlet vanes are opening, c) generating a second vane control signal when a requirement for decreased air flow is detected, said second vane control signal being proportional to the decreasing response plot of air flow with respect to vane position when the inlet vanes are closing.
3. A controller for controlling the operation of a variable inlet vane fan in which the vanes are moveable in an opening direction and in a closing direction comprising:
a) detector means for detecting the direction of response required to meet a demand variation as between a requirement for increased air flow and a requirement for decreased air flow, b) first vane control signal generating means operable to generate a first vane control signal when a requirement for increased air flow is detected, said first vane control signal being proportional to the increase in response plot of air flow with respect to vane position when the inlet vanes are opening, c) second vane control signal generating means operable to generate a second vane control signal when a requirement for decreased air flow is detected by said detector means, said second vane control signal being proportional to the decreased response plot of air flow with respect to vane position when the inlet vanes are closing.
a) detector means for detecting the direction of response required to meet a demand variation as between a requirement for increased air flow and a requirement for decreased air flow, b) first vane control signal generating means operable to generate a first vane control signal when a requirement for increased air flow is detected, said first vane control signal being proportional to the increase in response plot of air flow with respect to vane position when the inlet vanes are opening, c) second vane control signal generating means operable to generate a second vane control signal when a requirement for decreased air flow is detected by said detector means, said second vane control signal being proportional to the decreased response plot of air flow with respect to vane position when the inlet vanes are closing.
4. In a variable air volume air-conditioning system having a variable air volume fan located to discharge air into a duct having a generally circular cross-sectional configuration, the improvement of;
i) pressure sensing means in said duct downstream of said fan, said pressure sensing means being located within about 10 to 20 duct diameters of the discharge end of said fan, said pressure sensing means communicating with said fan to control the volume of air discharged therefrom in the proportion to the static pressure within the duct at the pressure sensing means location.
i) pressure sensing means in said duct downstream of said fan, said pressure sensing means being located within about 10 to 20 duct diameters of the discharge end of said fan, said pressure sensing means communicating with said fan to control the volume of air discharged therefrom in the proportion to the static pressure within the duct at the pressure sensing means location.
5. In the variable air volume air-conditioning system in which a controller controls the volume at which air is supplied to a space to be conditioned through an air duct system and in which a signal generator monitors a system variable and generates a control signal which is transmitted to the controller, the control signal being proportional to the required air output volume of the system and wherein the output amplitude of the control signal varies in response to changes in the system variable, the improvement wherein;
i) the output amplitude of the control signal is over-dampened to change progressively to a steady state over an extended time period sufficient to ensure that a substantially stable air pressure is maintained in the air duct system in use.
i) the output amplitude of the control signal is over-dampened to change progressively to a steady state over an extended time period sufficient to ensure that a substantially stable air pressure is maintained in the air duct system in use.
6. A variable air volume air-conditioning system as claimed in Claim 5 wherein the control signal is filtered by a filter having a long time constant, thereby to filter out transients and only allow an heavily filtered control signal to be transmitted to the controller, the heavily filtered control signal be indicative of actual load changes in the system.
7. A variable air volume air conditioning system as claimed in Claim 6 wherein the control signal is dampened to achieve a time constant in the range of 0 - 200 seconds.
8. A variable air volume air-conditioning system as claimed in Claim 6 wherein the time constant of the control signal is variable within the range of 0 - 200 seconds and the time constant is adjustable on-site to provide any required compromise between desired system response time and control system stability.
9. In a variable air volume air-conditioning system, in which a controller controls the operation of a variable air volume fan which communicates with the space to be conditioned through an air duct system and in which a signal generator monitors the system variable which varies in accordance with the load demands of the space to be conditioned, the signal generator generating an output signal which varies in response to changes in the system, the signal generator be operable to cycle at a set frequency, the improvement wherein;
i) the output signal of the signal generator is integrated to integrate out fluctuations prior to transmission of the output signal to the controller in the form of a steady value only as sensed by the signal genertor.
i) the output signal of the signal generator is integrated to integrate out fluctuations prior to transmission of the output signal to the controller in the form of a steady value only as sensed by the signal genertor.
10. A variable volume air-conditioning system as claimed in Claim 9 wherein the set frequency of the signal generator is adjustable to be adjustable at a job site in use.
11. In an air conditioning system having a variable inlet vane supply fan and a variable inlet vane return air fan, a method of controlling the capacity of the return air fan to closely match that of the supply air fan comprising the steps of:
a) generating a supply flow control signal in a controller which is programmed to selectively provide a first signal which is proportional to the increasing response plot of air flow with respect vane position when the vanes of the supply fan are opening and a second control signal which is proportional to the decreasing plot of the air flow with respect to vane position when the inlet vanes of the supply fan are closing, b) detecting the direction of response required to meet the demand variation as between a requirement for increased air flow and a requirement for decreased air flow, c) activating the controller to provide said supply fan control signal in the form of said first signal when a requirement for an increased flow is detected and to provide said flow control signal in the form of a second signal when a requirement for a decreased flow signal is detected, d) transmitting said supply fan control signal to said supply fan to control the position of the inlet vanes of said supply fan as required in use, e) generating a return fan flow control signal in a second controller which is programmed to selectively provide a third signal which is proportional to the increasing plot of air flow with respect to vane position when the vanes of the return air fan are opening and a fourth signal which is proportional to the decreasing plot of the air flow with respect to vane position when the inlet vanes of the return fan are closing, f) detecting the direction of response required to meet the demand variation as between a requirement for increased air flow and a requirement for decreased air flow of the return fan, g) activating said second controller to provide said return fan control signal in the form of said third signal when the requirement for increased flow is detected and to provide said flow control signal in the form of said fourth signal when the requirement for decreased flow is detected, h) transmitting said return fan flow control signal to said return fan to control the position of the inlet vanes of the return fan as required in use.
a) generating a supply flow control signal in a controller which is programmed to selectively provide a first signal which is proportional to the increasing response plot of air flow with respect vane position when the vanes of the supply fan are opening and a second control signal which is proportional to the decreasing plot of the air flow with respect to vane position when the inlet vanes of the supply fan are closing, b) detecting the direction of response required to meet the demand variation as between a requirement for increased air flow and a requirement for decreased air flow, c) activating the controller to provide said supply fan control signal in the form of said first signal when a requirement for an increased flow is detected and to provide said flow control signal in the form of a second signal when a requirement for a decreased flow signal is detected, d) transmitting said supply fan control signal to said supply fan to control the position of the inlet vanes of said supply fan as required in use, e) generating a return fan flow control signal in a second controller which is programmed to selectively provide a third signal which is proportional to the increasing plot of air flow with respect to vane position when the vanes of the return air fan are opening and a fourth signal which is proportional to the decreasing plot of the air flow with respect to vane position when the inlet vanes of the return fan are closing, f) detecting the direction of response required to meet the demand variation as between a requirement for increased air flow and a requirement for decreased air flow of the return fan, g) activating said second controller to provide said return fan control signal in the form of said third signal when the requirement for increased flow is detected and to provide said flow control signal in the form of said fourth signal when the requirement for decreased flow is detected, h) transmitting said return fan flow control signal to said return fan to control the position of the inlet vanes of the return fan as required in use.
12. A method as claimed in Claim 11, wherein the return fan flow control signal is offset with respect to the supply fan control signal thereby to substantially eliminate building stack effect.
13. A method as claimed in Claim 11, wherein an outside air temperture sensor supplies an adjustable range offset to adjust the return fan flow control signal effect offset tracking of the return fan with respect to the supply fan as a function of the outside air temperature.
14. A method as claimed in Claim 11, wherein a plurality of auxiliary exhaust fans are provided which exhaust air from the space which is to be conditioned, wherein an offset signal is generated which is proportional to the discharge capacity of the exhaust fans which are operable, said signal offsetting the tracking of the return air fan with respect to the supply air fan.
15. In an air-conditioning system having a variable speed supply fan and a variable speed return air fan, a method of controlling the capacity of the return air fan to closely match that of the supply fan comprising the steps of;
i) monitoring the supply fan speed and generating a first signal which is a measure of the supply fan capacity at the monitored speed, ii) matching the supply fan capacity with the return fan capacity to achieve any required ratio of supply fan output to return fan output and generating a second signal which is a measure of the return air fan speed required to provide the return air fan output and driving the return fan at the measured speed indicated by the second signal.
i) monitoring the supply fan speed and generating a first signal which is a measure of the supply fan capacity at the monitored speed, ii) matching the supply fan capacity with the return fan capacity to achieve any required ratio of supply fan output to return fan output and generating a second signal which is a measure of the return air fan speed required to provide the return air fan output and driving the return fan at the measured speed indicated by the second signal.
16. An air-conditioning system for conditioning an enclosed air space comprising;
i) a supply air fan adapted to supply a variable volume of air to the air space, ii) a return air fan adapted to return a variable volume of air from the air space to the supply air fan, iii) control means adapted to monitor the volume of air supplied by the supply fan and to adjust the volume of air returned by the air fan to track the volume of air supplied by the supply air fan, iv) means for offsetting the tracking of the volume of air returned by the return air fan with respect to the volume of air supplied by the supply fan whereby the air pressure within the air space may be adjusted to achieve any required balance with the external air pressure.
i) a supply air fan adapted to supply a variable volume of air to the air space, ii) a return air fan adapted to return a variable volume of air from the air space to the supply air fan, iii) control means adapted to monitor the volume of air supplied by the supply fan and to adjust the volume of air returned by the air fan to track the volume of air supplied by the supply air fan, iv) means for offsetting the tracking of the volume of air returned by the return air fan with respect to the volume of air supplied by the supply fan whereby the air pressure within the air space may be adjusted to achieve any required balance with the external air pressure.
17. An air-conditioning system as claimed in claim 16 wherein said control means generates a first signal which is proportional to the monitored value of air supplied by the air supply fan and said means for offsetting the tracking of the supply air fan by the return air fan comprises a potentiometer arranged to receive the first signal and discharge a second signal which is offset with respect to the first signal by a predetermined amount whereby offset tracking of the supply fan by the return air fan may be achieved.
18. An air-conditioning system as claimed in Claim 17 wherein the potentiometer is manually adjustable to vary the offset as required in use.
19. An air-conditioning as claimed in Claim 17 wherein the air surrounding the enclosed air space is subjected to temperature variations and wherein temperature sensing means is provided for monitoring the temperature of the surrounding air and generating an offset control signal which is indicative of the monitored temperature, said temperature sensing means communicating with said potentiometer to vary the offset of the potentiometer whereby the offset is a function of the surrounding air temperature and thereby to minimize pressure differential between the enclosed air space and the surrounding air.
20. An air-conditioning system having a variable air volume system for conditioning an enclosed air space, the system including; a variable volume air supply fan, a variable volume air return fan, a controller for controlling the volume of air output of the return air fan in response to a control signal, and control signal generating means for generating a control signal which is a function of the volume of air supplied by the supply fan, said control signal generating means communicating with said controller, at least one supplementary air exhaust system for exhausting air from said enclosed air space, actuator means for selectively activating and de-activating each of said supplementary exhaust systems, and adjustments means for adjusting said control signal in response to the activating and de-activating of each supplementary air exhaust system whereby the volume of air returned by the return air fan is adjusted such that the ratio of the combined air volume plus exhausted air volume to air supply volume is maintained substantially constant for all conditions of each exhaust fan.
21. An air-conditioning system as claimed in Claim 20 wherein a plurality of supplementary air exhaust fans are provided.
22. An air-conditioning system as claimed in Claim 21 wherein said adjustment means comprises a potentiometer associated with each exhaust fan, the potentiometers being wired in series and fed from a constant current source, each potentiometer being operable by its associated exhaust fan to generate an offset signal which serves to offset the control signal to increase or decrease the air output volume of the air return fan by an amount corresponding to the air exhausted by the active supplementary air exhaust systems.
23. In an air-conditioning system wherein it is necessary to generate a control signal which is proportional to the volume of air delivered by a variable volume air fan to a conditioned space, the improved method of generating said control signal comprising the steps of;
i) generating a first signal which is proportional to the speed of the fan, ii) converting the first signal by means of a comparator which is programmed with the speed/air volume output curves of the selected variable volume air fan to produce said control signal, and generating a second signal which is proportional to the static pressure in the system and integrating the second signal and adjusting the second signal to reduce it by an amount proportional to the volume of the third signal thereby to provide a forth signal which is the second signal with a zero flow offset.
i) generating a first signal which is proportional to the speed of the fan, ii) converting the first signal by means of a comparator which is programmed with the speed/air volume output curves of the selected variable volume air fan to produce said control signal, and generating a second signal which is proportional to the static pressure in the system and integrating the second signal and adjusting the second signal to reduce it by an amount proportional to the volume of the third signal thereby to provide a forth signal which is the second signal with a zero flow offset.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000419624A CA1196710A (en) | 1983-01-17 | 1983-01-17 | Variable air volume system controls |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000419624A CA1196710A (en) | 1983-01-17 | 1983-01-17 | Variable air volume system controls |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1196710A true CA1196710A (en) | 1985-11-12 |
Family
ID=4124363
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000419624A Expired CA1196710A (en) | 1983-01-17 | 1983-01-17 | Variable air volume system controls |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1196710A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1988010403A1 (en) * | 1987-06-18 | 1988-12-29 | Halton Oy | Procedure and means for controlling volumetric flow rate in air-conditioning installations |
-
1983
- 1983-01-17 CA CA000419624A patent/CA1196710A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1988010403A1 (en) * | 1987-06-18 | 1988-12-29 | Halton Oy | Procedure and means for controlling volumetric flow rate in air-conditioning installations |
| US4934397A (en) * | 1987-06-18 | 1990-06-19 | Halton Oy | Procedure and means for controlling volumetric flow rate in air-conditioning installations |
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