CA2055101C - System for controlling the differential pressure of a room having laboratory fume hoods - Google Patents

Abstract

A system whereby the pressure of a room containing one or more fume hoods, such as a laboratory, is to maintained at a predetermined level relative to the pressure of a reference space in the building, which may be the pressure in an adjacent corridor or an adjacent room or the like. The system involves a room controller which is part of the heating, ventilating and air conditioning apparatus of the building. The room controller is of the type which can receive electrical signals from each fume hood controller, which signals are proportional to the volume of air that is being exhausted through each fume hood. The volume indicating signals communicated from each of the fume hood controllers to the room controller enable the system to modulate the volume of air that is being supplied to the room and thereby maintain the differential pressure at the desired level with relatively quick response times.

Description

n ~

2 PRESSURE OF A ROOM HAVING LABORATORY FUME HOODS

3 Cross Reference to Related C~nA~i ~n Applications

4 1. Title: Apparatus for Controlling the Ventilation of a Laboratory Fume Hood 6 Inventors: Osman Ahmed, Steve Bradley, Steve Fritsche and 7 Steve Jacob 8 Serial No.: 2,055,126 9 Filed: November 7, 1991 2. Title: Apparatus for Determining the Position of a 11 Moveable Structure Along A Track 12 Inventors: David Egbers and Steve Jacob 13 Serial No.: 2,055,258 14 Filed: November 12, 1991 3. Title: A Method and Apparatus for Determining the 16 Uncovered Size of an Opening Adapted to be 17 Covered by Multiple Moveable Doors 18 Inventors: Osman Ahmed, Steve Bradley and Steve Fritsche 19 Serial No.: 2,055,147 Filed: November 7, 1991 21 4. Title: Laboratory Fume Hood Control Apparatus Having 22 Im~loved Safety Considerations 23 Inventors: Osman Ahmed 24 Serial No.: 2,055,100 Filed: Nov~mher 7, 1991 26 The present invention relates generally to the 27 control of the ventilation of laboratory fume hoods, and more 28 particularly to a system for controlling the differential 29 pressure of a room having one or more laboratory fume hoods located therein.
31 Fume hoods are utilized in various laboratory 32 environments for providing a work place where potentially 33 dangerous chemicals are used, with the hoods comprising an 34 enclosure having moveable doors at the front portion thereof which can be opened in various amounts to permit a B~' .

CA 020~101 1998-12-09 1 per-qon to gain access to the interior of the enclo~ure for the 2 purpose of conducting experiments and the like. The enclo~ure 3 i~ typically connected to an exhau-at system for removing any 4 noxiou~ fumes so that the per~on will not be exposed to them while performing work in the hood. Fume hood controller~
6 which control the flow of air through the enclosure have grown 7 more -~ophisticated in recent year~, and are now able to more 8 accurately maintain the de~ired flow characteri~tics to 9 efficiently exhau-~t the fumes from the enclosure a~ a function of the de-~ired average face velocity of the opening of the 11 fume hood.
12 The average face velocity i8 generally defined as the 13 flow of air into the fume hood per square foot of open face 14 area of the fume hood, with the size of the open face area being dependent upon the position of one or more moveable 16 doors that are provided on the front of the enclosure or fume 17 hood, and in most types of enclo~ures, the amount of bypa~-~
18 opening that is provided when the sash door or doors are 19 clo~ed.
The fume hoods are typically exhau~ted by an exhaust 21 sy~tem that generally include~ a blower that i~ capable of 22 being driven at variable speed~ to increase or decrease the 23 flow of air from the fume hood to compensate for the varying 24 size of the opening or face. Alternatively, there may be a ~ingle blower connected to the exhau~t manifold that i-~ in 26 turn connected to the individual ducts of multiple fume hoods, 27 and dampers may be provided in the individual ducts to control 28 the flow from the individual ducts to thereby modulate the 29 flow to maintain the de~ired average face velocity.
The doors of ~uch fume hood~ can be opened by rai~ing 31 them vertically, often referred to as the -~ash position, or 32 some fume hoods have a number of doors that are mounted for 33 sliding movement in typically two ~et~ of tracks. There are 34 even doors that can be moved horizontally and vertically, with the tracks being mounted in a frame ass~hly that is 36 vertically moveable.
37 The volume of air that i~ drawn through each of . . . .

CA 020~101 1998-12-09 1 the fume hoodq is at lea-qt partially a function of the 2 uncovered portion of the opening thereof, and if a relatively 3 constant average face velocity i_ maintained, then a larger 4 open area of the fume hood will result in more air being drawn into the fume hood and exhausted from it. Since the total 6 number of fume hoods that are preqent in laboratory rooms can 7 be quite large in many installation_, it should be appreciated 8 that a sub-qtantial volume of air may be removed from the 9 laboratory room during operation. Also, since the HVAC sy_tem qupplie-q air to the laboratory room, there may be a 11 subqtantial change in the volume of air reguired to be 12 supplied to a room dep~n~; ng upon whether the fume hood~ are 13 frequently being opened, or other changes occur.
14 Becau-qe much of the work that i8 performed in many laboratories involve~ chemical-q which may be dangerou~, it iQ
16 often desirable to maintain the differential pre~-qure within 17 the laboratory at a lower pressure than the hallwayq out-qide 18 of the laboratory or adjacent room~. If the laboratory haq 19 qeveral fume hoods which are exhau-qting air from the room, the amount of air -qupplied to the laboratory will nece-qsarily be 21 greater than a comparably -qized room without fume hoods, and 22 there may be increa~ed difficulty in maintAin;ng the desired 23 differential pre~qure between the laboratory and a reference 24 space if the fume hood~ have their sa~h doors frequently opened.
26 Accordingly, it is one of the primary objects of the 27 present invention to provide an improved system for 28 controlling the differential pre-qsure between a laboratory 29 room which has a number of fume hood-q located within it and the reference ~pace that is preferably adjacent to the 31 laboratory.
32 Another object of the pre-qent invention i-q to provide a-qystem 33 which integrates a fume hood controller apparatu~ with the 34 heating, ventilating and air conditioning control equipment for a laboratory room in which the fume hood_ are located, and 36 which achieve_ greatly increaqed control of the ~VAC eguipment 37 qo that the CA 020~101 1998-12-09 differential pressure of the room relative to the pre~ure of 2a reference space such as a corridor or an adjacent area can 3be maintained at a desired level.
4Another one of the primary objects of the present 5invention is to provide such an imploved system for 6controlling the pressure of a laboratory room which is 7achieved by providing current and accurate information to the 8ErvAc system about the volume of air flow from each of the fume 9hoods.
10Stated in more detail, it is an object of the present 11invention to provide an improved system for providing control 12of the differential pressure of a room having a number of fume 13hoods with respect to the pre~ure in a reference space such 14as a corridor or the like, and wherein the room ha~ a room 15controller for controlling the volume of air that is supplied 16and exhausted from the room, by providing signals from each 17fume hood controller to the room controller which indicates 18the volume of air that is being exhausted by each fume hood.
19These and other objects will become apparent upon 20r~Afl; ng the following detailed description of the present 21invention, while referring to the attached drawings, in which:
22FIGIJRE 1 i~ a schematic block diagram of system of 23the present invention which includes a room controller of a 24heating, ventilating and air conditioning monitoring and 25control apparatus of a building, and several fume hood 26controllers;
27FIG. 2 is a block diagram of a fume hood controller, 28shown connected to an operator panel, the latter being shown 29in front elevation;
30FIG. 3 is a diagrammatic elevation of the front of a 31representative fume hood having vertically operable sash 32doors;
33FIG. 4 is a diagrammatic elevation of the front of a 34representative fume hood having horizontally operable sash 35doors;
36FIG. 5 is a cross section taken generally along CA 020~101 1998-12-09 1 the line 5-5 of FIG. 4;
2 FIG. 6 is a diagrammatic elevation of the front of a 3 repreqentative combination Qa-~h fume hood having horizontally 4 and vertically operable saQh door~;
FIG. 7 is an electrical schematic diagram of a 6 plurality of door saQh position indicating ~witrh;ng means;
7 FIG. 8 i~ a cro~ ~ection of the door sash position 8 switching mean~;
9 FIG. 9 is a ~chematic diagram of electrical circuitry for determining the position of sash doors of a fume hood;
11 FIG. 10 is a block diagram illuatrating the relative 12 positionQ of FIGS. lOa, lOb, lOc, lOd and lOe to one another, 13 and which together comprise a ~chematic diagram of the 14 electrical circuitry for a fume hood controller mean~;
FIGS. lOa, lOb, lOc, lOd and lOe, which if connected 16 together, compri~e the schematic diagram of the electrical 17 circuitry for the fume hood controller mean~;
18 FIG. 11 is a flow chart of the general operation of 19 the fume hood controller;
FIG. 12 i~ a flow chart of a portion of the operation 21 of the fume hood controller of the pre~ent invention, 22 particularly illustrating the operation of the feed forward 23 control scheme, which is included in one of the embodiments 24 of the fume hood controller mean~;
FIG. 13 i8 a flow chart of a portion of the operation 26 of the fume hood controller mean-q, particularly illustrating 27 the operation of the proportional gain, integral gain and 28 derivative gain control ~cheme; and, 29 FIG. 14 i~ a flow chart of a portion of the operation of the fume hood controller means, particularly illustrating 31 the operation of the calibration of the feed forward control 32 scheme.

34 Detailed De~cription It should be generally under~tood that a fume hood 36 controller controls the flow of air through the fume CA 020~101 1998-12-09 1 hood in a manner whereby the effective size of the total 2 opening to the fume hood, including the portion of the opening 3 that is not covered by one or more sash door~ will have a 4 relatively constant average face velocity of air moving into the fume hood. This means that regardless of the area of the 6 uncovered opening, an average volume of air per unit of 7 surface area of the uncovered portion will be moved into the 8 fume hood. This protects the persons in the laboratory from 9 being exposed to noxious fumes or the like because air is always flowing into the fume hood, and out of the exhaust 11 duct, and the flow i9 preferably controlled at a predetermined 12 rate of approximately 75 to 125 cubic feet per minute per 13 square feet of effective surface area of the uncovered 14 opening. In other words, if the sash door or doors are moved to the maximum open position whereby an operator has the 16 maximum access to the inside of the fume hood for co~ cting 17 experiments or the like, then the flow of air will most likely 18 have to be increased to maintain the average face velocity at 19 the predetermined desired level.
Broadly stated, the present invention relates to a 21 system whereby the pressure of a room containing one or more 22 fume hoods, for example, such as laboratory or the like is to 23 be maintained at a predetermined level relative to the 24 pressure of a reference space in the building, which may be the pressure in an adjacent corridor or an adjacent room or 26 the like. It is often highly desirable to maintain the 27 differential pressure in a laboratory room at a reduced level 28 relative to the reference space, in order to contain the 29 noxious fumes so that they will not permeate beyond the room.
The system involves a room controller which is part of the 31 heating, ventilating and air conditioning apparatus of the 32 building. The room controller is of the type which can 33 receive electrical signals from each of the fume hood 34 controllers, which signals are proportional to the volume of air that is being exhausted through each fume hood. Since 36 each fume hood can be exhausting an amount of air that can 37 vary considerably dep~n~; ng upon its initial ~_ -7-1 setting of the desired average face velocity and the degree by 2 which the sash doors are opened, it is very advantageou-~ that 3 the volume indicating signal~ be communicated from each of the 4 fume hood controllers to the room controller so that it can modulate the volume of air that i~ being supplied to the room 6 which assists it in maintAin;ng the differential pressure at 7 the de-Qired level with relatively quick response times.
8 Turning now to the drawings, and particularly 9 FIG. 1, a block diagram is shown of several fume hood controllers 20 interconnected with a room controller 22, an 11 exhaust controller 24 and a main control console 26. The fume 12 hood controllers 20 are interconnected with the room 13 controller 22 and with the exhaust controller 24 and the main 14 control console 26 in a local area network illustrated by line 28 which may be a multico~Allctor cable or the like. The room 16 controller, the exhaust controller 24 and the main control 17 console 26 are typically part of the building main HVAC system 18 in which the laboratory rooms containing the fume hoods are 19 located. The fume hood controllers 20 are provided with ~oweL
through lines 30, which is at the proper voltage via a 21 transformer 32 or the like.
22 The room controller 22 preferably is of the type 23 which i~ at least capable of providing a variable air volume 24 to the room, and may be a Landis & Gyr Powers System 600 SCU
controller. The room controller 22 is capable of 26 communicating over the LAN lines 28. When response time is 27 critical, the room controller preferably receives fume hood 28 exhaust flow information from the fume hood controller as an 29 analog signal directly through dedicated lines. This will bypass the LAN which may become busy transporting other 31 information from the fume hood controllers to the room 32 controller. The room controller preferably is a System 600 33 SCU controller and is a commercially available controller for 34 which extensive documentation exists. The ~er Reference ~AnllAl is Part No. 125-1753 for the System 600 SCU controller.

CA 020~l0l l998-l2-09 1 The room controller 22 receives signal~ via lines 81 2 from each of the fume hood controllers 20 that provides an 3 analog input-~ignal indicating the volume of air that is being 4 exhausted by each of the fume hood controllers 20 and a comparable Qignal from the exhaust flow qensor that provides 6 an indication of the volume of air that is being exhausted 7 through the main exhau-~t sy~tem apart from the fume hood 8 exhausts. These signals coupled with signal~ that are 9 ~upplied by a differential pressure senQor 29 which indicates the pressure within the room relative to the reference space 11 enable the room controller to control the supply of air that 12 i~ neces~ary to maintain the differential pressure within the 13 room at a slightly lower pressure than the reference space, 14 i.e., preferably within the range of about 0.01 to about 0.05 ;nch~s of water, which re~ults in the de-qirable lower pres~ure 16 of the room relative to the reference space. However, it i-~
17 not so low that it prevents persons inside the laboratory room 18 from opening the doors to escape in the event of an emergency, 19 particularly if the doors open outwardly from the room. Also, in the event the door~ open inwardly, the differential 21 preqqure will not be so great that it will pull the door open 22 due to exce-~sive force being applied due to such pressure.
23 The differential preQsure -~ensor 29 is preferably 24 positioned in a ~uitable hole or opening in the wall between the room and the reference space and measures the pressure on 26 one side relative to the other. Alternatively, a velocity 27 sensor may be provided which measures the velocity of air 2 8 moving through the opening which i-~ directly p o~ol~ional to 29 the pressure difference between the two spaces. Of cour~e, a lower differential pressure in the room relative to the 31 reference space would mean that air would be moving into the 32 room which is al~o capable of being detected.
33 Referring to FIG. 2, a fume hood controller 20 i-~
34 illustrated with its input and output connector port-~ being identified, and the fume hood controller 20 i~ connected to CA 020~101 1998-12-09 1 an operator panel 34. It should be understood that each fume 2 hood will have a fume hood controller 20 and that an operator 3 panel will be provided with each fume hood controller. The 4 operator panel 34 is provided for each of the fume hoods and it is intercQnn~rted with the fume hood controller 20 by a 6 line 36 which preferably comprises a multi-cQn~rtor cable 7 having eight cQn~nctors. The operator panel has a connector 8 38, such as a 6 wire RJ11 type telephone jack, for example, 9 into which a lap top personal computer or the like may be connected for the purpose of inputting information relating 11 to the configuration or operation of the fume hood during 12 initial installation, or to ch~nge certain operating 13 parameter~ if nece~sary. The operator panel 34 is preferably 14 mounted to the fume hood in a convenient location adapted to be easily observed by a person who is working with the fume 16 hood.
17 The fume hood controller operator panel 34 include~
18 a liquid crystal display 40 which when selectively activated 19 provides the visual indication of various aspects of the operation of the fume hood, including three digits 42 which 21 provide the average face velocity. The display 40 illustrates 22 other conditions such as low face velocity, high face velocity 23 and emergency condition and an indication of controller 24 failure. The operator panel may have an alarm 44, an emergency purge switch 46 which an operator can press to purge the fume 26 hood in the event of an accident. The operator panel has two 27 auxiliary switches 48 which can be used for various customer 28 needs, including day/night modes of operation. It ig 29 contemplated that night time mode of operation would have a different and preferably reduced average face velocity, 31 presumably because no one would be working in the area and 32 such a lower average face velocity would conserve energy. An 33 alarm silence switch 50 is also preferably provided to 34 extinguish the alarm.
Fume hoods come in many different styles, sizes and 36 configurations, including those which have a single sash door 37 or a number of sash doors, with the sash doors CA 020~101 1998-12-09 1 being moveable vertically, horizontally or in both direction.
2 Additionally, various fume hoods have different amount~ of 3 by-pass flow, i.e., the amount of flow permitting opening that 4 exists even when all of the sash doors are a~ completely closed as their design permits. Other design considerations 6 invol~e whether there is some kind of filtering means included 7 in the fume hood for confining fumes within the hood during 8 operation. While many of these design considerations must be 9 taken into account in providing efficient and effective control of the fume hoods, the apparatus can be configured to 11 account for virtually all of the above described design 12 variables, and effective and extremely fast control of the 13 fume hood ventilation is provided.
14 Referring to FIG. 3, there is shown a fume hood, indicated generally at 60, which has a vertically operated 16 sash door 62 which can be moved to gain access to the fume 17 hood and which can be moved to the substantially closed 18 position as shown. Fume hoods are generally designed so that 19 even when a door qash ~uch as door sash 62 is completely clo~ed, there is still some amount of opening into the fume 21 hood through which air can pass. This opening is generally 22 referred to as the bypass area and it can be determined so 23 that its effect can be taken into consideration in controlling 24 the flow of air into the fume hood. Some types of fume hoods have a bypass opening that is located above the door sash 26 while others are below the same. In some fume hoods, the first 27 amount of movement of a sash door will increase the opening 28 at the bottom of the door shown in FIG. 3 for example, but as 29 the door is raised, it will merely cut off the bypass opening so that the effective size of the total opening of the fume 31 hood is maint~;n~ relatively constant for perhaps the first 32 one-fourth amount of movement of the sash door 62 through its 33 course of travel.
34 Other types of fume hoods may include several horizontally moveable sash doors 66 such as shown in FIGS. 4 36 and 5, with the doors being movable in upper and lower CA 020~101 1998-12-09 1 pairs of adjacent track~ 68. When the doors are positioned as 2 -~hown in FIGS. 4 and 5, the fume hood opening i~ completely 3 clo~ed and an operator may move the doors in the horizontal 4 direction to gain acces-~ to the fume hood. Both of the fume~
hood~ 60 and 64 have an exhaust duct 70 which generally 6 extend~ to an exhaust ~ystem which may be that of the HVAC
7 apparatus previously described. The fume hood 64 al~o includes 8 a filtering structure -qhown diagramm~tically at 72 which 9 filtering qtructure is intended to keep noxiou~ fumes and other contaminant~ from exiting the fume hood into the exhau~t 11 ~ystem. Referring to FIG. 6, there is shown a combination fume 12 hood which has horizontally movable doors 76 which are similar 13 to the door~ 66, with the fume hood 74 having a frame 14 ~tructure 78 which carries the doors 76 in suitable tracks and the frame ~tructure 78 i8 al-~o vertically movable in the 16 opening of the fume hood.
17 The illustration of FIG. 6 ha~ portion~ removed aa 18 ~hown by the break line~ 73 which is int~nA~A to illu~trate 19 that the height of the fume hood may be greater than iQ
otherwiae -~hown so that the frame -~tructure 78 may be raised 21 ~ufficiently to permit adequate accesQ to the interior of the 22 fume hood by a per-~on. There i-~ generally a by-pa~s area which 23 is identified as the vertical area 75, and there i~ typically 24 a top lip portion 77 which may be approximately 2 ;nCh~g wide.
Thi~ dimen~ion i-~ preferably defined ~o that it-Q effect on the 26 calculation of the open face area can be taken into 27 consideration.
28 While not -~pecifically illuatrated, other 29 combination~ are al~o po~sible, including multiple set-~ of vertically moveable qa~h door~ positioned adjacent one another 31 along the width of the fume hood opening, with two or more 32 sash doorR being vertically moveable in adjacent track~, much 33 the ~ame as residential casement windows.
34 The fume hood controller 20 i~ adapted to operate the fume hoods of various size~ and configurations a~ ha-Q been 36 described, and it i9 al~o adapted to be incorporated into a 37 laboratory room where ~everal fume hood-~ may be located and 38 which may have exhaust duct~ which merge into . .

CA 020~l0l l998-l2-09 1 a common exhaust manifold which may be a part of the building 2 HVAC system. A fume hood may be a single self-contained 3 installation and may have its own separate exhaust duct. In 4 the event that a single fume hood is installed, it is typical that such an installation would have a variable speed motor 6 driven blower associated with the exhaust duct whereby the 7 speed of the motor and blower can be variably controlled to 8 thereby adjust the flow of air through the fume hood.
9 Alternatively, and most typical for multiple fume hoods in a single area, the exhaust ducts of each fume hood are merged 11 into one or more larger exhaust manifolds and a single large 12 blower may be provided in the manifold system. In such types 13 of installations, control of each fume hood is achieved by 14 means of separate dampers located in the exhaust duct of each fume hood, so that variation in the flow can be controlled by 16 a~ o~riately positioning the damper associated with each fume 17 hood.
18 The fume hood controller is adapted to control 19 virtually any of the various kinds and ~tyles of fume hoods that are commercially available, and to this end, it has a 21 number of input and output ports (lines, connectors or 22 connections, all considered to be equivalent for the purposes 23 of the present description) that can be connected to various 24 sensors that may be used with the controller. As shown in FIG.
2, it has digital output or DO ports which interface with a 26 digital signal/analog pressure transducer with an exhaust 27 damper as previously described, but it also has an analog 28 voltage output port for controlling a variable speed fan drive 29 if it is to be installed in that manner. There are five sash position sensor ports for use in sensing the position of both 31 horizontally and vertically moveable sashes and there is also 32 an analog input port provided for connection to an exhaust air 33 flow sensor. A digital input port for the emergency switch is 34 provided and digital output ports for outputting an alarm horn signal as well as an auxiliary signal is provided.
36 As has been previou~ly discu~ed and in accord-.. . _ , , . _~

CA 020~101 1998-12-09 1 ance with the present invention, an analog voltage output port 2 is also provided for providing a volume of flow signal to the 3 room controller 22. This port is connected to the room 4 controller by individual lines 81 which extend from each of the fume hood controllers 22.
6 From the foregoing discussioA, it should be 7 appreciated that if the desired average face velocity i8 8 desired to be maintained and the sash position is changed, the 9 size of the opening can be dramatically changed which may then require a dramatic change in the volume of air to maintain the 11 average face velocity. While it is known to control a 12 variable air volume blower as a function of the sash position, 13 the fume hood controller apparatus improves on that known 14 method by incorporating additional control schemes which dramatically improve the capabilities of the control system 16 in terms of maintaining relatively constant average face 17 velocity in a manner whereby reactions to perturbations in the 18 system are guickly made.
19 To determine the position of the sash doors, a sash position sensor is provided adjacent each movable sash door 21 and it is generally illustrated in FIGS. 7, 8 and 9.
22 Referring to FIG. 8, the door sash position indicator 23 comprises an elongated switch mechanism 80 of relatively 24 simple mechanical design which preferably consists of a relatively thin polyester base layer 82 upon which is printed 26 a strip of electrically resistive ink 84 of a known constant 27 resistance per unit length. Another polyester base layer 86 28 is provided and it has a strip of electrically con~l~ctive ink 29 88 printed on it. The two base layers 82 and 86 are adhesively ho~ to one another by two beads of adhesive 90 31 located on opposite sides of the strip. The base layers are 32 preferably approximately five-thou~andths of an inch thick and 33 the beads are approximately two-thousandths of an inch thick, 34 with the beads providing a spaced area between the con~llctive and resistive layers 88 and 84. The switching mechanism 80 36 is preferably applied to the fume hood by a layer of adhesive 37 92.

1 The polyester material is sufficiently flexible 2 to enable one layer to be moved toward the other in 3 response to an actuator 94 carried by the appropriate door 4 sash to which the strip is placed adjacent to so that when the door sash is moved, the actuator 94 moves along the 6 switching mechanism 80 and provides contact between the 7 resistive and conductive layers which are then sensed by 8 electrical circuitry to be described which provides a 9 voltage output that is indicative of the position of the actuator 94 along the length of the switching mechanism.
11 Stated in other words, the actuator 94 is carried by the 12 door and therefore provides an electrical voltage that is 13 indicative of the position of the door sash.
14 The actuator 94 is preferably spring biased toward the switching mechanism 80 so that as the door is 16 moved, sufficient pressure is applied to the switching 17 mer-hAnism to bring the two base layers together so that the 18 resistive and conductive layers make electrical contact 19 with one another and if this is done, the voltage level is provided. By having the switching mechanism 80 of suffi-21 cient length so that the full extent of the travel of the 22 sash door is provided as shown in FIG. 3, then an accurate 23 determination of the sash position can be made. It should 24 be understood that the illustration of the switching mec-hAnism 80 in FIGS. 3 and 5 is intended to be diagram-26 matic, in that the switching mer-hAnism is preferably 27 actually located within the sash frame itself and 28 accordingly would not be visible as shown. The width and 29 thickness dimensions of the switching mechAn;sm are so small that interference with the operation of the sash door 31 is virtually no problem. The actuator 94 can also be 32 placed in a small whole that may be drilled in the door or 33 it may be attached externally at one end thereof so that it 34 can be in position to operate the switch 80. In the vertical moveable sash doors shown in FIGS. 3 and 6, a 36 switching mer-~Anism 80 is preferably provided in one or the 37 other of the sides of the sash frame, whereas in the fume 38 hoods havinq horizontally movable doors, it is preferred 1 that the switching mechAn;sm 80 be placed in the top of the 2 tracks 68 so that the weight of the movable doors do not 3 operate the switching mechanism 80 or otherwise damage the 4 same. It is also preferred that the actuator 94 is located at one end of each of the doors for reasons that are described 6 in the aforementioned cross-referenced application entitled 7 Apparatus for determining the position of a moveable structure 8 along a track, by Egbers et al., CAnA~;An Application Serial 9 No. 2,055,258.
Turning to FIG. 9, the preferred electrical 11 circuitry which generates the position indicating voltage is 12 illustrated, and this circuitry is adapted to provide two 13 separate voltages indicating the position of two door sashes 14 in a single track. With respect to the cross-section shown in FIG. 5, there are two horizontal tracks, each of which carries 16 two door sAshes and a switching mechanism 80 is provided for 17 each of the tracks as is a circuit as shown in FIG. 9, thereby 18 providing a distinct voltage for each of the four sash doors 19 as shown.
The switch;~g mechanism is preferably applied to 21 the fume hood with a layer of adhesive 92 and the actuator 94 22 is adapted to bear upon the switching mechanism at locations 23 along the length thereof. Referring to FIG. 7, a diagrammatic 24 illustration of a pair of switch;ng mechanisms is illustrated such as may occur with respect to the two tracks shown in 26 FIG. 5. A switching mechanism 80 is provided with each track 27 and the four arrows illustrated represent the point of contact 28 created by the actuators 94 which result in a signal being 29 applied on each of the ends of each switching mechAn;sm, with the magnitude of the signal representing a voltage that is 31 proportional to the distance between the end and the nearest 32 arrow. Thus, a single switching mechanism 80 is adapted to 33 provide position indicating signals for two doors located in 34 each track. The circuitry that is used to accomplish the voltage generation is shown in FIG. 9 and includes one of 36 these circuits for each track. The resistive element is shown 37 at 84 and the con~)ctive element 88 is also B~l -1 illustrated being connected to ground with two arrows being 2 illustrated, and represented the point of contact between 3 the resistive and conductive elements caused by each of the 4 actuators 94 associated with the two separate doors. The circuitry includes an operational amplifier 100 which has 6 its ou~u~ connected to the base of a PNP transistor 102, 7 the emitter of which is connected to a source of positive 8 voltage through resistor 104 into the negative input of the 9 operational amplifier, the positive input of which is also connected to a source of positive voltage of preferably 11 approximately five volts. The collector of the transistor 12 102 is connected to one end of the resistive element 84 and 13 has an output line 106 on which the voltage is produced 14 that is indicative of the position of the door.
The circuit operates to provide a constant 16 current directed into the resistive element 84 and this 17 current results in a voltage on line 106 that is propor-18 tional to the resistance value between the collector and 19 ground which changes as the nearest point of contact along the resistance changes. The operational amplifier operates 21 to attempt to drive the negative input to equal the voltage 22 level on the positive input and this results in the current 23 applied at the output of the operational amplifier varying 24 in direct proportion to the effective length of the resistance strip 84. The lower portion of the circuitry 26 operates the same way as that which has been described and 27 it similarly produces a voltage on an output line 108 that 28 is proportional to the distance between the connected end 29 of the resistance element 84 and the point of contact that is made by the actuator 94 associated with the other sash 31 door in the track.
32 Referring to the composite electrical schematic 33 diagram of the circuitry of the fume hood controller, if 34 the separate drawings FIGS. lOa, lOb, lOc, lOd and lOe are placed adjacent one another in the manner shown in FIG. 10, 36 the total electrical schematic diagram of the fume hood 37 controller 20 is illustrated. The operation of the cir-38 cuitry of FIGS. lOa through lOe will not be described in 1 detail. The circuitry is driven by a microprocessor and 2 the important algorithms that carry out the control 3 functions of the controller will be hereinafter described.
4 Referring to FIG. 10c, the circuitry includes a Motorola MC
68HCll microprocessor 120 which is clocked at 8 MHz by a 6 crystal 122. The microprocessor 120 has a databus 124 that 7 is connected to a tri-state buffer 126 (FIG. 10d) which in 8 turn is connected to an electrically programmable read only 9 memory 128 that is also connected to the databus 124. The EPRON 128 has address lines A0 through A7 connected to the 11 tri-state buffer 126 and also has address lines A8 through 12 A14 connected to the microprocessor 120.
13 The circuitry includes a 3 to 8-bit multiplexer 14 130, a data latch 132 (see FIG. 10d), a digital-to-analog converter 134, which is adapted to provide the analog 16 outputs indicative of the volume of air being exhausted by 17 the fume hood, which information is provided to room 18 controller 22 as has been previously described with respect 19 to FIG. 2. Referring to FIG. 10b, an RS232 driver 136 is provided for transmitting and receiving information through 21 the hand held terminal. The circuitry illustrated in FIG.
22 9 is also shown in the overall schematic diagrams and is in 23 FIGS. 10a and 10b. The other components are well known and 24 therefore need not be otherwise described.
As previously mentioned, the fume hood control 26 apparatus utilizes a flow sensor preferably located in the 27 exhaust duct 70 to measure the air volume that is being 28 drawn through the fume hood. The volume flow rate may be 29 calculated by measuring the differential pressure across a multi-point pitot tube or the like. The preferred embodi-31 ment utilizes a differential pressure sensor for measuring 32 the flow through the exhaust duct and the fume hood control 33 apparatus utilizes control schemes to either maintain the 34 flow through the hood at a predetermined average face velocity, or at a minimum velocity in the event the fume 36 hood is closed or has a very small bypass area.
37 The fume hood controller can be configured for 38 almost all known types of fume hoods, including fume hoods CA 020~l0l l998-l2-09 1 having horizontally movable sash doors, vertically movable 2 sash doorq or a combination of the two. As can be seen from 3 the illustrations of FIGS. 2 and 10, the fume hood controller 4 is adapted to control an exhaust damper or a variable speed fan drive, the controller being adapted to output signals that 6 are compatible with either type of control. The controller 7 is also adapted to receive information defining the physical 8 and operating characteristics of the fume hood and other 9 initializing information. Thi~ can be input into the fume hood controller by means of the hand held terminal which is 11 preferably a lap top computer that can be connected to the 12 operator panel 34. The information that should be provided 13 to the controller include the following, and the dimensions 14 for the information are also shown. It should be appreciated that the day/night operation may be provided, but is not the 16 preferred ~mhodiment of the ~ystem; if it is provided, the 17 information relating to such day/night operation should be 18 included.
19 Operational information:
1. Time of day;
21 2. Set day and night values for the average face 22 velocity (SVEL), feet per minute or meters per 23 second;
24 3. Set day and night values for the minimum flow, (MINFLO), in cubic feet per minute;
26 4. Set day and night values for high velocity limit 27 (HVEL), F/m or M/sec;
28 5. Set day and night values for low velocity limit 29 (LVEL), F/m or M/sec;
6. Set day and night values for intermediate high 31 velocity limit (MVEL), F/m or M/sec;
32 7. Set day and night values for intermediate low 33 velocity limit (IVEL), F/m or M/sec;
34 8. Set the proportional gain factor (RP), analog output per error in percent;
36 9. Set the integral gain factor (KI), analog output 37 multiplied by time in minutes per 1 error in percent;
2 10. Set derivative gain factor (KD), analog 3 output multiplied by time in minutes per 4 error in percent;
11. Set feed forward gain factor (KF) if a 6 variable speed drive is used as the control 7 equipment instead of a damper, analog output 8 per CFM;
9 12. Set time in seconds (DELTIME) the user prefers to have the full exhaust flow in 11 case the emergency button is activated;
12 13. Set a preset percent of last exhaust flow 13 (SAFLOQ) the user wishes to have once the 14 emergency switch is activated and DELTIME is expired.
16 The above information is used to control the mode 17 of operation and to control the limits of flow during the 18 day or night modes of operation. The controller includes 19 programmed instructions to calculate the steps in para-graphs 3 through 7 in the event such information is not 21 provided by the user. To this end, once the day and night 22 values for the average face velocity are set, the con-23 troller 20 will calculate high velocity limit at 120% of 24 the average face velocity, the low velocity limit at 80%
and the intermediate limit at 90%. It should be understood 26 that these percentage values may be adjusted, as desired.
27 Other information that should be input include the follow-28 ing information which relates to the physical construction 29 of the fume hood. It should be understood that some of the information may not be required for only vertically or 31 horizontally moveable sash doors, but all of the infor-32 mation may be required for a combination of the same:
33 14. Input the number of vertical segments;
34 15. Input the height of each segment, in inches;
16. Input the width of each segment, in inches;
36 17. Input the number of tracks per segment:
37 18. Input the number of horizontal sashes per 38 track;

CA 020~l0l l998-l2-09 1 19. Input the maximum ~ash height, in inrhes;
2 20. Input the -qa-qh width, in inrh~;
3 21. Input the location of the sash sensor from left 4 edge of saqh, in ; nrhe~;
22. Input the by-pass area per -qegment, in ~quare ; nche,~;
7 23. Input the minimum face area per segment, in 8 -qquare ; nrh~;
9 24. Input the top lip height above the horizontal sash, in ; n~heg;
11 The fume hood controller 20 i-q programmed to control 12 the flow of air through the fume hood by carrying out a serie-q 13 of inqtructions, an overview of which is contained in the flow 14 chart of FIG. 11. After start-up and outputting information to the di~play and determining the time of day, the controller 16 20 reads the initial sa~h position~ of all doors (block 150), 17 and thiq information is then used to compute the open face 18 area (block 152) . If not previou~ly done, the operator can 19 set the average face velocity set point (block 154) and this information i-q then u~ed together with the open face area to 21 compute the exhau-qt flow set point (SFLOW) (block 156) that 22 iS necesqary to provide the predetermined average face 23 velocity given the open area of the fume hood that ha-q been 24 previou~ly measured and calculated. The computed fume hood exhau-qt qet point i-q then compared (block 158) with a preqet 26 or required minimum flow, and if computed -qet point i~ leqs 27 than the minimum flow, the controller ~et~ the set point flow 28 at the preset minimum flow (block 160). If it is more than 29 the minimum flow, then it i-q retained (block 162) and it iq provided to both of the control loop~.
31 If there i-q a variable speed fan drive for the fume 32 controller, i.e., ~everal fume hoods are not connected to a 33 common exhaust duct and controlled by a damper, then the 34 controller will run a feed-forward control loop (block 164) which provide~ a control ~ignal that i-q -qent to a -qumming 36 junction 166 which control signal repre~entq an open loop type 37 of control action. In thi-q control action, CA 020~l0l l998-l2-09 1 a predicted value of the speed of the blower i9 generated 2 based upon the calculated opening of the fume hood, and the 3 average face velocity set point. The predicted value of the 4 speed of the blower generated will cause the blower motor to rapidly change speed to maintain the average face velocity.
6 It qhould be understood that the feed forward aspect of the 7 control is only invoked when the sash position has been 8 changed and after it has been changed, then a second control 9 loop performs the dominant control action for maintaining the average face velocity conqtant in the event that a variable 11 speed blower is used to control the volume of air through the 12 fume hood.
13 After the sash position has been changed and the feed 14 forward loop has established the new air volume, then the control loop switches to a proportional integral derivative 16 control loop and this is accomplished by the set flow signal 17 being provided to block 168 which indicates that the 18 controller computes the error by determining the absolute 19 value of the difference between the set flow signal and the flow signal as measured by the exhaust air flow sensor in the 21 exhaust duct. Any error that is computed is applied to the 22 control loop identified as the proportional-integral-23 derivative control loop (PID) to determine an error signal 24 (block 170) and this error signal i8 compared with the prior error signal from the previous sample to determine if that 26 error is less than a ~eA~hAnA error (block 172). If it is, 27 then the prior error signal is maintained as shown by block 28 174, but if it is not, then the new error signal is provided 29 to output mode 176 and it is applied to the summing junction 166. That summed error is also compared with the la~t output 31 signal and a determination is made if this is within a 32 deadband range (block 180) which, if it is, results in the 33 laqt or previous output being retained (block 182). If it is 34 outqide of the ~eA~hAn~, then a new output signal i9 provided to the damper control or the blower (block 184).
36 In the event that the last output i~ the output as 37 -qhown in block 182, the controller then reads the CA 020~101 1998-12-09 1 mea~ured flow (MFLOW) (block 186) and the sash position~ are 2 then read (block 188) and the net open face area is recomputed 3 (block 190) and a determination made aq to whether the new 4 computed area le~s the old computed area is les-q than a ~eA~hAn~ (block 192) and if it i~, then the old area i-q 6 maintained (block 194) and the error i~ then computed again 7 (block 168). If the new area le~q the old area iq not within 8 the ~eA~hAn~, then the controller compute~ a new exhau~t flow 9 set point as shown in block 156.
One of the significant advantage~ of the fume hood 11 controller is that it is adapted to execute the control-qcheme 12 in a repetitive and extremely rapid manner. The exhau~t-qensor 13 provides flow qignal information that is inputted to the 14 microproce-qqor at a speed of approximately one qample per 100 milli-qecond-q and the control action described in connection 16 with FIG. 11 i-q completed approximately every 100 17 milli-qeconds. The -qa-qh door po~ition ~ignalq are ~ampled by 18 the microproceqqor every 200 milli~econdq. The result of such 19 rapid repetitive sampling and executing of the control actions result~ in extremely rapid operation of the controller. It has 21 been found that movement of the ~ash will reqult in adju~tment 22 of the air flow 30 that the average face velocity is achieved 23 within a time period of only approximately 3-4 second~ after 24 the -qa~h door reposition has been qtopped. Thiq representq a dramatic improvement over exiqting fume hood controller-q.
26 In the event that the feed forward control loop is 27 utilized, the ~eguence of in~tructions that are carried out 28 to accomplish rnnn;ng of this loop i~ qhown in the flow chart 29 of FIG. 12, which ha~ the controller using the exhauqt flow set point (SFLOW) to compute the control output to a fan drive 31 (block 200), which is identified aq signal AO that i-q computed 32 as an intercept point plus the set flow multiplied by a slope 33 value. The intercept i~ the value which i~ a fixed output 34 voltage to a fan drive and the ~lope in the eguation correlate-q exhau-qt flow rate and CA 020~101 1998-12-09 1 output voltage to the fan drive. The controller then reads the 2 duct velocity (DV) (block 202), takes the last duct velocity 3 ~ample (block 204) snd eguates that as the duct velocity value 4 and starts the timing of the maximum and minimum delay times (block 206) which the controller uses to insure whether the 6 duct velocity has reached steady state or not. The controller 7 determines whether the maximum delay time has expired (block 8 208), and if it has, provides the output signal at output 210.
9 If the max delay ha~ not expired, the controller determines if the absolute value of the difference between the last duct 11 velocity sample and the current duct velocity sample is less 12 than or egual to a deadband value (block 212). If it is not 13 les~ than the ~eA~h~n~ value, the controller then sets the 14 last duct value as equal to the present duct value sample (block 214) and the controller then restarts the minimum delay 16 timing function (block 216). Once this is accomplished, the 17 controller again determines whether the max delay has expired 18 (block 208). If the absolute value of the difference between 19 the last duct velocity and the present duct velocity sample is less than the deadband, the controller determines whether 21 the minimum delay time has expired which, if it has as -~hown 22 from block 218, the output is provided at 210. If it has not, 23 then it determines if the max delay has expired.
24 Turning to the proportional-integral-derivative or PID control loop, the controller runs the PID loop by carrying 26 out the instructions shown in the flow chart of FIG. 13. The 27 controller uses the error that is computed by block 168 28 (see FIG. 11) in three separate paths. With respect to the 29 upper path, the controller uses the preselected proportional gain factor (block 220) and that proportional gain factor is 31 used together with the error to calculate the proportional 32 gain (block 222) and the proportional gain is output to a 33 su~ming junction 224.
34 The controller also uses the error signal and calculates an integral term (block 226) with the integral term 36 being equal to the prior integral sum (ISUM) plus the CA 020~l0l l998-l2-09 1product of loop time and any error and this calculation is 2compared to limits to provide limits on the term. The term is 3then used together with the previously defined integral gain 4constant (block 230) and the controller than calculates the 5integral gain (block 232) which is the integral gain constant 6multiplied by the integration sum term. The output is then 7applied to the summing junction 224.
8The input error is also used by the controller to 9calculate a derivative gain factor which is done by the 10controller using the previously defined derivative gain factor 11from block 234 which is used together with the error to 12calculate the derivative gain (block 236) which is the 13reciprocal of the time in which it is re~uired to execute the 14PID loop multiplied by the derivative gain factor multiplied 15by the current sample error minus the previous sample error 16with this result being provided to the summing junction 224.
17The control action performed by the controller 20 as 18illustrated in FIG. 13 provides three separate gain factors 19which provide steady state correction of the air flow through 20the fume hood in a very fast acting manner. The formation of 21the output signal from the PID control loop takes into 22consideration not only the magnitude of the error, but as a 23result of the derivative gain segment of control, the rate of 24change of the error is considered and the change in the value 25of the gain is proportional to the rate of change. Thus, the 26derivative gain can see how fast the actual condition is 27changing and works as an "anticipator" in order to minimize 28error between the actual and desired condition. The integral 29gain develops a correction signal that is a function of the 30error integrated over a period of time, and therefore provides 31any necessary correction on a continuous basis to bring the 32actual condition to the desired condition. The proper 33combinations of proportional, integral and derivative gains 34Will make the loop faster and reach the desired conditions 35without any overshoot.
36A significant advantage of the PID control action CA 020~101 1998-12-09 1 i-q that it will compensate for perturbations that may be 2 experienced in the laboratory in which the fume hood may be 3 located in a manner in which other controllers do not. A
4 common occurrence in laboratory rooms which have a number of fume hoods that are connected to a common exhaust manifold, 6 involves the change in the pressure in a fume hood exhaust 7 duct that was caused by the sash doors being moved in another 8 of the fume hoods that is connected to the common exhaust 9 manifold. Such pressure variation~ will affect the average face velocity of those fume hoods which had no change in their 11 sash doors. However, the PID control action may adjust the 12 air flow if the exhaust duct sensor determines a change in the 13 pressure. To a lesser degree, there may be pressure 14 variations produced in the laboratory caused by opening of doors to the laboratory itself, particularly if the 16 differential pressure of the laboratory room is maint~;ne~ at 17 a lesser pressure than a reference space such as the corridor 18 outside the room, for example.
19 It i~ necessary to calibrate the feed forward control loop and to thi-~ end, the instructions illustrated in the flow 21 chart of FIG. 14 are carried out. When the initial 22 calibration is accomplished, it is preferably done through the 23 hand held terminal that may be connected to the operator panel 24 via connector 38, for example. The controller then determines if the feed forward calibration is on (block 242) and if it 26 is, then the controller sets the analog output of the fan 27 drive to a value of 20 percent of the maximum value, which is 28 identified as value AO1 (block 244). The controller then sets 29 the last sample duct velocity (LSDV) as the current duct velocity (CDV) (block 246) and starts the maximum and minimum 31 timers (block 248). The controller ensures the steady state 32 duct velocity in the following way. First by checking whether 33 the max timer has expired, and then, if the max timer has not 34 expired, the controller determines if the absolute value of the last sample duct velocity minus the current duct velocity 36 is less than or equal to a dead band (block 270), and if it 1 is, the controller determines if the min timer has expired 2 (block 272). If it has not, the controller reads the 3 current duct velocity (block 274). If the absolute value 4 of the last sample duct velocity minus the current duct velocity is not less than or equal to a dead band (block 6 270), then the last sample duct velocity is set as the 7 current duct velocity (block 276) and the mintimer is 8 restarted (block 278) and the current duct velocity is 9 again read (block 274). In case either the max timer or min timer has expired, the controller then checks the last 11 analog ouL~uL value to the fan drive (252) and inquires 12 whether the last analog output value was 70 percent of the 13 maximum output value (block 254). If it is not, then it 14 sets the analog output value to the fan drive at 70 percent of the max value AO2 (block 256) and the steady state duct 16 velocity corresponding to A01. The controller then repeats 17 the procedure of ensuring steady state duct velocity when 18 analog output is AO2 (block 258). If it is at the 70 19 percent of max value, then the duct velocity corresponds to steady state velocity of AO2 (block 258). Finally, the 21 controller (block 262) calculates the slope and intercept 22 values.
23 The result of the calibration process is to 24 determine the duct flow at 20% and at 70% of the analog output values, and the measured flow enables the slope and 26 intercept values to be determined so that the feed forward 27 control action will accurately predict the necessary fan 28 speed when sash door positions are changed.
29 From the foregoing detailed description, it should be appreciated that an improved system has been 31 described which has advantages over the prior art in terms 32 of effectively maintaining a desired differential pressure 33 in a room where a plurality of fume hoods are present.
34 While various embodiments of the present invention have been shown and described, it should be 36 understood that various alternatives, substitutions and 37 equivalents can be used, and the present invention should 38 only be limited by the claims and equivalents thereof.

1 Various features of the present invention are set 2 forth in the following claims.

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Claim 1. A system for controlling the differential pressure at a predetermined level between a room such as a laboratory or the like and a reference space such as a corridor or the like, both of which are located in a building having a building heating, ventilating and air conditioning apparatus, and in which room a plurality of fume hoods are located, the fume hoods being of the type which have at least one moveable sash door adapted to at least partially cover the opening as the fume hood sash door is moved, each of the fume hoods having an exhaust duct that is in communication with an exhaust apparatus for expelling air and fumes from the room, each of the fume hoods having a means for measuring the actual flow of air through its associated exhaust duct and generating an actual flow signal that is indicative of the actual flow of air through the exhaust duct, each fume hood being controlled by a fume hood controller means for controlling a flow modulating means associated with each fume hood and its associated exhaust duct to maintain a desired face velocity through the uncovered portion of the opening, said system comprising:
   room controlling means for controlling at least the volume of air that is supplied to the room from the heating and air conditioning apparatus of the building;
   means for interconnecting each of said fume hood controller means to said room controlling means so that the signals representing the actual flow from each of the fume hood controllers is communicated to said room controlling means, said room controlling means being adapted to receive and sum the communicated signals from each of said fume hood controller means and thereby determine the volume of air being exhausted from the room by the fume hoods, said room controlling means utilizing said adjusting means to vary the volume of air that is supplied to the room to replace the volume of air being exhausted from the room at a rate necessary to maintain the differential pressure at said predetermined level.
2. Claim 2. Apparatus as defined in claim 1 wherein said interconnecting means comprises conducting means extending from each fume hood controller means to said room controlling means.
3. Claim 3. Apparatus as defined in claim 1 wherein each of said fume hood controller means is adapted to transmit a voltage level that is proportional to the actual flow of air through the exhaust duct of the fume hood to which the fume hood controller means is connected.
4. Claim 4. Apparatus as defined in claim 1 wherein said actual flow measuring means is positioned to measure the air flow in the exhaust duct of the fume hood.
5. Claim 5. Apparatus for controlling the differential pressure at a predetermined level within a room in a building having a building heating, ventilating and air conditioning and exhaust system with reference to the pressure in another space in the building such as a corridor or the like, and in which room a plurality of fume hoods are located, each of the fume hoods being of the type which have at least one moveable sash door adapted to at least partially cover the opening as the fume hood sash door is moved, each of the fume hoods having an exhaust duct that is in communication with the exhaust system for expelling air and fumes from the fume hoods, each of the fume hoods having a means for measuring the actual flow of air through the exhaust duct and generating an actual flow signal that is indicative of the actual flow of air through the exhaust duct, each fume hood being controlled by a fume hood controller means for controlling a flow modulating means associated with each fume hood and its associated exhaust duct to maintain a desired face velocity through the uncovered portion of the opening, said apparatus comprising:
   room controlling means for controlling at least the volume of air that is supplied to the room by the building heating and air conditioning system;
   means for interconnecting each of said fume hood controller means to said room controlling means so that the actual flow signals from each of the fume hood controllers is communicated to said room controlling means, said room controlling means being adapted to receive and sum the actual flow signals from each of said fume hood controller means and thereby determine the volume of air being exhausted from the room through the fume hood exhaust ducts, said room controlling means adjusting the volume of air that is supplied to the room to replace the volume of air being exhausted from the room at a rate sufficient to maintain the differential pressure at said predetermined level.