GB2478779A

GB2478779A - Method of controlling the rotational speeds of a plurality of fans

Info

Publication number: GB2478779A
Application number: GB1004573A
Authority: GB
Inventors: Paul John Clarke
Original assignee: Ebm Papst UK Ltd
Current assignee: Ebm Papst UK Ltd
Priority date: 2010-03-19
Filing date: 2010-03-19
Publication date: 2011-09-21
Anticipated expiration: 2030-03-19
Also published as: GB2478779B; GB201004573D0

Abstract

The method controls the rotational speeds of a plurality of fans (F1-F3) sharing a common plenum wherein each fan has a respective sensor (S1-S3) supplying an input signal to a control unit. The control unit provides separate fan speed demand (FSD) outputs to control the speed of each fan and runs a pre-programmed algorithm for controlling the fans based upon the received input signals. The method comprises the steps of processing the input signals to obtain respective FSD inputs for each of the fans, producing separate FSD outputs for controlling the speed of each fan on the basis of the FSD inputs, determining a minimum FSD input for each fan for a given set of operating parameters, selecting the fan with the highest determined FSD output as a primary fan and designating the other fans as secondary fans, and limiting the maximum differences in the FSD outputs for the primary fan and each other fan, at different FSD outputs for the primary fan. An alternative method of controlling rotational speeds of a plurality of fans is also claimed.

Description

INDEPENDENT FAN SPEED CONTROL

This invention relates to a method of controlling the rotational speeds of a plurality of fans which share a common plenum. The invention also extends to a fan system including a plurality of fans drawing air through a plenum, the fans being controlled by a method of this invention.

A very common use of fans is for the cooling of equipment or rooms.

Mainly, the cooling effect is controlled by changing the speeds of the fans.

Controlling the fans to minimise their rotational speed has three major advantages: 1. Increased system reliability through components lasting longer 2. Reduced noise 3. Reduced power consumption.

Current ventilation systems consisting of two or more fans usually drive all of the fans at the same speed but this fails to balance the requirements of energy efficiency with the need to produce an efficient movement of air within the system. This can be due to a number of issues including pressure differences across the equipment or room (which can be seen as a single plenum) through which the fans draw air and differing loads on the system demand due to conflicting input requirements. These effects may be masked by the control strategy that is used to control the speeds of operation of the fans.

In the case of the cooling of equipment, a multiplicity of sensors may be used to monitor the temperature of critical components. The system then controls the fan speeds based on the highest temperature. This process achieves the temperature requirements but may over-cool other parts of the system, which results in the system being energy inefficient.

In the case of the ventilating or cooling of a room, single or multi-point temperatures may be taken and used as the average temperature for the whole room. The control process then alters the fan speeds against this one set point.

The system is inefficient at controlling the temperature because the process assumes that there are neither hot nor cold spots within the room, but usually the temperature in the room is not uniform. The system is ineffective at cooling the heat load due to the lower fan speeds obtained by driving the fans with just one set point averaged for the whole room.

As a result of the above problems, so-called independent fan speed control systems are being employed, where the speed of each fan is individually controlled. This implies that for a system with multiple sensor or input requirements, each fan speed can be controlled to give maximum cooling with minimal energy losses and hence higher efficiency. These systems still have a single plenum, which may be as large as an entire building, an office space, a data centre or the like, or may be as small as a plenum within a fan coil, an IT rack or a void within compact equipment.

These systems in general work well as long as the load on the system is balanced over all the sensor inputs and fans. The system however has a weakness in that the independent speed control is open to an unstable runaway effect that can cause the system efficiency to drop rapidly and then fail to operate correctly. This instability can be caused for example by small variations in fan performance, decision making within fan controller software or because changes in the load on the fans pushes the control process out of balance.

An example of this is where two fans are used to pressurise the area under a floor of a data centre, to achieve cooling across the room. With independent fan speed control, the process determines the air pressure under the floor and adjusts the speeds of the fans dependent on the determined pressures. Due to the system not being 100% perfect, the control process at some point will make a decision to increase the speed of one fan but then to decrease the speed of another fan. Over a period of time the process will continue to make the same decision and hence continue to increase the speed of one fan while reducing the speed of the other fan. Eventually, this runaway effect takes over and one fan will run at full speed while the other fan will run at a minimum speed or stop completely.

To prevent this runaway effect occurring, the control process needs to monitor and limit the balance across all the fans in the system, by providing independent speed control for those fans arranged so that when critical points in demand are met, efficient air movement is achieved in the system while minimising the power consumed by the system as a whole.

According to this invention, there is provided a method of controlling the rotational speeds of a plurality of fans sharing a common plenum and each having a respective sensor for supplying an input signal to a fan speed control unit arranged to provide separate fan speed demand (FSD) outputs for controlling the speed of each fan, the control unit running a pre-programmed algorithm for controlling the fans on the basis of the signals from the sensors, which method comprises the steps of: -processing the input signals of the sensors to yield respective FSD inputs for each of the associated fans; -producing separate FSD outputs for controlling the speed of each fan on the basis of the FSD inputs; -for a given set of operating parameters, determining a minimum FSD input for each fan; -selecting the fan with the highest determined required FSD output as a primary fan and designating the other fans as secondary fans; -limiting the maximum differences in the FSD outputs for the primary fan and the or each secondary fan, at different FSD outputs for the primary fan.

The control method of this invention allows for individual fan speed control regardless of the number of fans, sensors, load or control set points by means of limiting the difference in fan speeds so keeping the system within controllable limits. This ensures the system remains in balance so that a runaway effect will not occur. This leads to maximising airflow efficiency while allowing for reduced noise levels, in turn reducing power usage in the system as is a whole and reducing maintenance requirements.

According to a second, more developed control method of this invention, there is provided a method of controlling the rotational speeds of a plurality of fans sharing a common plenum and each having a respective sensor for supplying an input signal to a fan speed control unit arranged to provide separate fan speed demand (FSD) outputs for controlling the speed of each fan, the control unit running a pre-programmed algorithm for controlling the fans on the basis of the signals from the sensors, which method comprises the steps of: -for a given set of operating parameters, processing the input signals of the sensors to yield respective FSD outputs for each of the associated fans; -selecting the fan with the highest determined required FSD output as a primary fan and designating the other fans as secondary fans; -determining the maximum secondary FSD output required for each secondary fan that is required to maintain system performance when the primary fan is operating with a 100% FSD output; -programming the control unit to set a minimum primary fan FSD output for an input signal for the selected primary fan below a first pre-set value and to increase the primary fan FSD output up to 100% for corresponding increases in primary input signals above the pre-set value up to 100% FSD; -programming the control unit to set a minimum secondary FSD output for each input signal for the secondary fans below a second pre-set value and to increase the secondary fan FSD output up to the determined maximum secondary FSD output required for each secondary fan for corresponding increases in secondary input signals above the pre-set value up to 100% FSD; is and -selecting an offset between the first and second pre-set values on the basis of fan performance curves, system resistances and pressure differences whereby the secondary fans contribute to the flow through the primary fan thereby to minimise the total energy consumption of the plurality of fans.

It will be appreciated that by using the control methods of this invention, the runaway effect of known systems may be eliminated by ensuring that a secondary fan (which may be the only other fan in a two-fan installation) is controlled so as to limit its contribution to the air drawn by the primary fan. The demand on the primary fan should not be increased without a related increase in the speed of the secondary fan. The function by which the speed of the secondary fan is increased is controlled by the minimum and maximum FSD outputs for the secondary fan, in conjunction with the offset between the first and second pre-set values for the primary and secondary fans, respectively.

According to a further but closely related aspect of this invention there is provided apparatus for controlling the rotational speeds of a plurality of fans sharing a common plenum and adapted to operate on a control method also of this invention, as described above.

The plurality of sensors should each be adapted to sense a parameter associated with the required function of the plurality of fans -for example, the air temperature in a case where the fans perform a cooling function. Other possibilities would be for the sensors to sense air pressure, humidity, the presence of impurities, pollutants or contaminants.

By way of example only, one specific embodiment of a method of this invention for controlling a plurality of fans with independent fan speed control will now be described in detail, reference being made to the accompanying drawings in which:-Figure 1 illustrates a prior art fan system with a single fan in a duct; Figure 2 illustrates a prior art fan system with three fans sharing a common plenum; Figure 3 illustrates the system of Figure 2 when becoming unbalanced because of an unequal heat load; Figure 4 illustrates the system of Figure 2, showing the runaway effect; Figure 5 shows the notional fan boundary effect within the common plenum; Figure 6 illustrates the arrangement with a fan control system of this invention; and Figure 7 is a graph showing the primary and secondary intermediate and output FSD values when performing the control method of this invention.

Before describing the example of a control method of this invention, the operation of an existing multi-fan system will be explained, but first by referring to a single fan system as shown in Figure 1. Here, the efficiency of a fan to deliver a required air flow can be measured by comparing the amount of air moved through the fan with the amount of air moved over a target. In a closed system with one fan, for example mounted in a duct, the amount of air moving though the fan is equal to the amount of air moving through the duct at some distance from the fan, and so over the target. If however the duct has an opening between the fan and target, then some air leakage will be seen.

A system having two or more fans within a plenum also displays this leakage effect but in this case it is between fans. When the system is fully balanced as shown in Figure 2, all targets and fans have equal airtlows, and so the airflow through the plenum is balanced.

Taking the system of Figure 2, if the load of the targets is not balanced, a greater airflow may be required over one target than over the others. To achieve this, the speed of the fan nearest the target requiring a greater airflow may be increased and though this increases the airflow over the associated target, that will not be by the same amount. Air from the second target leaks across the plenum into the airflow through the upper fan, because the upper fan generates a larger negative pressure. This is shown in Figure 3 where the thermal load of the upper target is twice that of the other two targets.

When unbalanced as shown in Figure 3, the flow over the other two targets leaks into the flow through the upper fan and so all fans and targets in the system will be affected. This creates increased airflow on the other targets which the control method observes and so reduces the fan speeds of the fans associated with the other targets. Eventually the system reaches a point where a large amount of the airflow is through one fan and the air movement over its target is significantly less, such that the system as a whole no longer functions correctly, as shown in Figure 4. This illustrates the runaway effect with greatly reduced system efficiency.

For a fan in a duct there is a physical boundary through which air cannot move so that 100% of the air moving through the fan passes over the target.

When a fan is in a plenum with multiple fans, there is a notional boundary between the fans through which air may pass, as shown in Figure 5. If the fans run at the same speed then the pressure across this notional boundary is close to zero, and so there will be little or no airflow thereacross. The airflow through a fan is substantially equal to the air passing over the associated target. When the fan speeds differ, the airflow across the notional boundaries increases. For a system to continue to operate within its design limits, the faster fan must be able to generate an airflow equal to that required over its associated target plus any leakage from the other targets.

Figure 6 illustrates the control system of this invention. The overall system comprises a plurality of individual fans (Fl, F2, F3... Fn) each having a parameter sensor (Si, S2, S3... Sn) associated therewith. The system may be used to cool an asymmetric heat load (Hi, H2, H3... Hn), in which case each sensor senses the temperature of the air passing thereover. The sensors provide outputs to the controller which then produces for each fan a Fan Speed Demand (FSD) signal, supplied to a speed controller for the fan. Depending upon the system, the sensors may sense different parameters besides air temperature -for example, air pressure or humidity.

The method performed by the controller starts by implementing any control strategy or other process to determine the required FSD for each fan in the system, for a given set of operating parameters. The method by which this is achieved does not form a part of this invention but the result is a known required FSD for each fan. The fan with the highest required FSD will be regarded as the primary fan for the purpose of the control method and all the other fans will be regarded as secondary fans.

For any given system, there will be some airflow across the notional boundaries within the plenum. This airflow can be measured either as a pressure difference or a velocity difference but for the purposes of the following explanation, reference will be made solely to a pressure difference (Pa).

Using performance curves for the fans in the system, it is possible to calculate the allowed back pressure or system resistance in Pa on a given fan running at a given FSD; this should always fall within the working range of the fan. For example, with the primary fan operating at 90% FSD and another (secondary) fan operating at 20% FSD, the performance curves may show the secondary fan is running close to the stall condition, which is highly undesirable.

The considerations to be taken into account performing the calculation must also include any pressure drops that are present between the fans and the plenum, caused by the layout of the equipment and plenum -for example, the -10-distances between the fans, filters, coils and the size and shape of the plenum and other associated structures.

It is possible to calculate from these values the airflow through the primary fan using the effective pressures that the other fans are generating in the system and their effect on the total system resistance.

Referring now to Figure 7, there is shown an example of the control method of this invention, as implemented by the controller of a multi-fan installation as shown in Figure 6. In Figure 7: Line A -C is the minimum FSD for the fans in the system and typically is set at 16% of the maximum FSD output; This minimum FSD will be used for an input FSD of 16% or less.

Line A -B -D is the FSD profile of the primary fan.

Line A -C -E is the FSD profile of the or each secondary fan.

Point D is the maximum FSD output of the primary fan and typically is set is to 100% of the maximum fan speed.

Point E is the maximum FSD of the or each secondary fan such that the secondary fans allow the primary fan to meet the system requirements. Point E may or may not coincide with Point D dependant upon the overall system.

Point E can be calculated as the FSD at which the (or each) secondary fan must run so that the pressure developed by the secondary fan effects the total system resistance to a point where the primary fan (when running at maximum FSD) has an airflow that allows the system as a whole to operate within the required limits for the system -i.e. the fans generate enough airflow over the heat load to ensure efficient cooling of that load.

Offset F -the calculation of this offset is far more difficult for the following reason. The difference in FSD between two fans will not generate a linear pressure difference; for example, a 30% FSD difference for two fans running at low fan speeds will generate less pressure difference than a 10% FSD difference when the fans are running at higher speeds. Therefore the calculation of Offset F will have to take into account the aspects to be described below and should be attempted only after the difference between Points D and E has been calculated and all the pressure differences and system resistances have been taken into account.

For any given value of Offset F it is possible, using the fan performance curves and system resistances, to calculate the point at which the greatest pressure difference occurs between the fans. This will in general not be at the point where Offset F is used (i.e. the largest FSD difference) but at some point higher up the graph towards Points D and E. Using this pressure difference it is can then be seen if a secondary fan will be pushed out of its working range. It is also possible to reverse this and calculate the maximum allowed pressure difference between the primary and secondary fans running at differing fan speeds, and work back to find Offset F. It should be noted that the positions of the sensors in this system has not been described. These sensors should be located so that they will respond to changes in the parameters being sensed, in the air flowing thereover.

Therefore in calculating the values of Points D and E and the Offset F, the values should not create a pressure difference or airflow that causes these sensors no longer to lie in the stream of air that is to be monitored. This is an important issue in keeping the system balanced and working correctly. If the -12 -sensors are adapted to sense temperature but can no longer see the temperature of the air drawn from the heat load, the sensors cannot provide an appropriate output to the control system.

Briefly, the operation of the control system may be summarised by the following steps: * The system looks at all the sensor data and calculates the required FSD for all the fans.

* The fan with the highest required FSD is selected to be the primary fan and all the other fans are regarded as secondary fans.

* Using the primary fan required FSD, the primary fan is set to run at the output FSD indicated Figure 7, by the line A -B -D. * Using the required FSD5 calculated earlier and the Offset F, the secondary fans are allowed to run at the calculated value if that is above the value indicated by line A -C -E. If the calculated value is below the value indicated by line A -C -E, the secondary fan will be set to run at the minimum FSD dictated by the line A -C -E. * Offset F is the maximum permitted difference in speeds between the primary and secondary fans. That difference will gradually reduce as the primary fan speed approaches 100% FSD, to allow all the fans to operate at their maximum speed when required.

* The above control regime is repeated periodically, as the system parameters change.

The new control method allows for individual fan speed control regardless of the number of fans, sensors, load or control set points by means of limiting the difference in fan speeds and hence the airflows across the -13-notional boundaries within the plenum. The method keeps the system within controllable limits and hence balanced such that a runaway effect will not occur.

This leads to maximising airflow efficiency while allowing for reduced noise levels, in turn reducing power usage in the system as a whole and reducing maintenance requirements.

Claims

-14 -CLAIMS1. A method of controlling the rotational speeds of a plurality of fans sharing a common plenum and each having a respective sensor for supplying an input signal to a fan speed control unit arranged to provide separate fan speed demand (FSD) outputs for controlling the speed of each fan, the control unit running a pre-programmed algorithm for controlling the fans on the basis of the signals from the sensors, which method comprises the steps of: -processing the input signals of the sensors to yield respective FSD inputs for each of the associated fans; -producing separate FSD outputs for controlling the speed of each fan on the basis of the FSD inputs; -for a given set of operating parameters, determining a minimum FSD input for each fan; -selecting the fan with the highest determined required FSD output as a primary fan and designating the other fans as secondary fans; is -limiting the maximum differences in the FSD outputs for the primary fan and the or each secondary fan, at different FSD outputs for the primary fan.
2. A method of controlling the rotational speeds of a plurality of fans sharing a common plenum and each having a respective sensor for supplying an input signal to a fan speed control unit arranged to provide separate fan speed demand (FSD) outputs for controlling the speed of each fan, the control unit running a pre-programmed algorithm for controlling the fans on the basis of the signals from the sensors, which method comprises the steps of: -for a given set of operating parameters, processing the input signals of the sensors to yield respective FSD outputs for each of the associated fans; -15- -selecting the fan with the highest determined required FSD output as a primary fan and designating the other fans as secondary fans; -determining the maximum secondary FSD output required for each secondary fan that is required to maintain system performance when the primary fan is operating with a 100% FSD output; -programming the control unit to set a minimum primary fan FSD output for an input signal for the selected primary fan below a first pre-set value and to increase the primary fan FSD output up to 100% for corresponding increases in primary input signals above the pre-set value up to 100% FSD; -programming the control unit to set a minimum secondary FSD output for each input signal for the secondary fans below a second pre-set value and to increase the secondary fan FSD output up to the determined maximum secondary FSD output required for each secondary fan for corresponding increases in secondary input signals above the pre-set value up to 100% FSD; is and -selecting an offset between the first and second pre-set values on the basis of fan performance curves, system resistances and pressure differences whereby the secondary fans contribute to the flow through the primary fan thereby to minimise the total energy consumption of the plurality of fans.
3. A method as claimed in claim 2, wherein the pre-programmed algorithm serves to limit the maximum differences in the FSD outputs for the primary fan and the or each secondary fan, at different FSD outputs for the primary fan.
4. A method as claimed in any of the preceding claims, wherein each of the sensors is adapted to sense a parameter directly associated with the function -16-being performed by the plurality of fans and which responds to an change in the speed of the fans.
5. A method as claimed in claim 4, wherein each of the sensors is adapted to sense at least one of temperature, pressure and humidity.
6. A method as claimed in any of the preceding claims, wherein the plurality of fans is arranged to effect cooling of an asymmetric heat load disposed within the plenum.
7. A method as claimed in any of the preceding claims, wherein the method is repeated periodically and a selection of the primary fan is made for the operating parameters prevailing at the time of each iteration of the method.
8. A method as claimed in claim 1 or claim 2 and substantially as hereinbefore described, with reference to Figures 6 and 7 of the accompanying drawings.
9. Apparatus for controlling the rotational speeds of a plurality of fans sharing a common plenum, which apparatus is adapted to perform the control method as claimed in any of the preceding claims.