Note: Descriptions are shown in the official language in which they were submitted.
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1 LABORATORY FUME HOOD CONTROL APPARATUS
2 HAVING IMPROVED SA~FETY CONSIDERATIONS
3 Cross Reference to Related Applications
4 1. Title: Apparatus for Determining the Position of a
S Moveable Structure Along a Track
6 Inventors: David Egbers and Steve Jacob
7 Serial No.: 52496
8 2. Title: A System ~'or Controlling the Di~ferential
9 Pressure o~ a Room Having Laboratory Fume
Hoods
11 Inventors: Osman ~hmed, Steve Bradley
12 Serial No.: 52497
13 3. Title: A Method and Apparatus for Determining the
14 Uncovered Siæe o~ an Opening Adapted to be
Covered by Multiple Moveable Doors
16 Inventors: Osman ~hmed, Steve Bradley and Steve
17 Fritsche
18 Serial No.: 52498
19 4. Title: Apparatus for Controlling the V'entilation of
Laborato~y Fume Hoods
21 Inventors: Osman Ahmed, Steve Bradley, Steve Fritsche
22 and Steve Jacob
23 Serial No.: ~2370
24 ~he present invention relates generally to the
control of the ventilation of laboratory fume hoods, and
26 more particularly to an improved method and apparatus for
27 controlling the ventilation o~ fumes from one or more fume
28 hoods that are typically located in a laboratory
29 ~nvironment.
Fume hoods are utilized in various laboratory
31 environments ~or providing a work place where potentially
3~ ~.anqerolls chemicals are used, w.i.th the hoods comprisin~ an
2~
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1 enclosure having moveabla doors at the front portion
2 thereof which can be openPd in various amounts to permit a
3 person to gain access to the interior o~ the enclosure for
the purpose o~ con~ucting exper.iments and the like. The
enclosure is typically connected to an exhaust sy~tem for
6 removing any noxious fumes so that the person will not be
7 exposed to them ~hile performing work in the hood.
8 Fume hood controllers which control the ~low of
9 air through the enclosure have become more sophisticated in
recent years, and are now able to more accurately maintain
11 the desired flow characteristics to efficiently exhaust the
12 fumes from the enclosure as a function o~ the desired
13 average face velocity of the opening of t~e fume hood
14 required to effectively exhaust the fume hoodO The average
face velocity i5 generally defined as the flow of aix into
16 the fume hood per square foot o~ open face area of the fume
17 hood, with the size of the open face area being dependent
18 upon the position of one or more moveable doors that are
19 provided on the front of the enclosure or fume hood, and in
most types o~ enclosures, the amount of bypass opening that
21 is provided when the door or doors are closed.
~2 ~he fume hoods are exhausted by an exhaust system
23 that includes one or more blowers that are capable of being
24 driven at variable speeds to increase or decrease the flow
of air from the ~ume hood to compensate for the varying
26 size of the opening or face. Alternatively, there may be
27 a single blower connected to the exhaust manifold that is
28 in turn connected to the individual ducts of multiple ~ume
29 hoods, and dampers may be provided in the individual ducts
to control the flow from the individual ducts to thereby
31 modulate the flow to maintain the desired a~erage face
32 velocity. There may also be a combination o~ both of the
33 above described ~ystems.
34 The doors of such $ume hoods can be opened by
raising them vertically, o~ten referred ~o as the sash
36 position, or some fume hoods have a number of doors that
37 are mounted for sliding movement in typically two sets of
38 tracks. There are even doors that can be moved
1 horizontally and vertically, wi~h the tracks being mounted
2 in a frame asse~bly that is vertically moveable.
3 Prior art fume hood controllers have included
4 sensing means for measuring the position o~ the doors and
then using a signal proportional to the sensed position to
6 thereby vary the speed of the blowers or the p~sition of
7 the dampers. While uch contro~ has represented an
8 improvement in the control of fume hood~, there are
9 circumstances that arise that require further adjustment of
the exhausting of such hoods that such a con~roller cannot
11 perform. Significant improvement~ are disclo~ed in the
12 above referenced cross related applications, and
13 particularly Apparatus for Controlling the Ventilation o~
14 Laboratory Fume Hoods by Ahmed et al., Serial NoO 52370.
It is desirable for some fume hoods to have a
16 filtering means typically located in the upper portion o~
17 the fume hood enclosure between the working area and the
18 exhaust duct for the purpose of retaining noxious ~umes and
19 effluents. The filter medium for such filtering means
often can become loaded with residue or the like which over
21 time will tend to restrict the flow of air through the
22 filter medium. The resistance to flow throu~h the medium
23 and out of the exhaust duct will result in inefficiency of
24 operation of the fume hood, and can also create a
potentially hazardous condition. The inability of a fume
26 hood to efficiently expel air will also increase the energy
27 requirements during operation of the fume hood.
28 Accordingly, it i5 a primary object o~ the
29 present invention to provide an improved apparatus ~or
controlling the ventilation of laboratory fume hoods which
31 apparatus has desirable safety features as well as
32 maintaining good energy efficiency.
33 Another object of the present invention is to
34 provide such an improved apparatus which is adapted to
determine if a ~ilter medium is loaded beyond a predeter~
36 mined amount and provide a signal that is indicative of
37 such a condition.
38 Still another object of the present invention is
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1 to provide such an improved apparatus which provides a
2 visual or audible indication in response to the loading
3 signal being generated~
4 Yet another object of the present invention is to
provide such an improved apparatus which ha~ emergancy
6 switches near each fume hood, with the switch controlling
7 the fume hood when actuated so that the fum~ hood can
8 operate in an emergency mode, but al~o provide an
9 indication to a central building console of a building
supervisory and control system for heating ventilating and
11 air conditioning apparatus.
12 Another object of the present invention is to
13 provide an improved apparatus which has additional
14 desirable safety features, including the feature of
controlling the differential pressure within the room to a
16 level that is slightly less than the pressure within a
17 reference space such as a corridor, adjacent room or the
18 like, in the ev~nt of an emergency chemical spill or the
19 like within a fume hood which results in the fume hood
increasing its exhaust flow to an emergency level. ~hi~ is
21 highly desirable so that any person~ within the room can
22 open an outwardly opening external door to the room to
23 escape from the room. Also, the slight dif~erence in the
24 differential pressure will not normally result in an
inwardly opening door being forced open.
26 These and other objects will become apparent upon
27 reading the following detailed description of the present
28 invention, while referring to the attached drawings, in
29 which:
FIGU~E 1 i~ a ~chematic block diagram of
31 apparatus of the present invention shown integrated with a
32 room controller of a heating, ventilating and air
33 conditioning monitoring and control system of a building;
34 FIG. 2 is a ~lock diagram of a fume hood
controller, shown connected to an operator panel, the
36 latter being shown in front elevation;
37 FIG. 3 is a diagra~matic elevation of the front
38 of a representative fume hood having vertically opera~le
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1 sash doors;
2 FIG. 4 is a diagrammatic elevation of the front
3 of a reprasentative ~ume hood having horizontally operable
4 sash doors;
FIGo 5 is a cross s~ction taken generally along
6 the line 5-5 of FIG. 4;
7 FIG. 6 is a diagrammatic elevation o~ the ~ront
8 of a representative combination sash fume hood having
9 horizontally and vertically operable sash doors;
lo FIG. 7 is an electrical schematic diagram of a
11 plurality of door sash position indicating switching means;
12 FIG. 8 is a cross section of the door sa~h
13 position switching means;
14 FIG. 9 is a schematic diagram of electrical
circuitry for determining the position of sash doors of a
16 fume hood;
17 FIG. lO is a block diagram illu~trating the
18 relative positions of FIGS. lOa, lOb, lOc, lOd and lOe to
l9 one another, and which together comprise a schematic
diagram of the electrical circuitry ~or the fume hood
21 controller means embodying the present invention;
2~ FIGS. lOa, lOb, lOc, lOd and lOe, which if
23 connected together, comprise the schematic diagram of the
24 electrical circuitry for the fume hood controller means
embodying the present invention;
26 FIG. 11 is a flow chart of the general operation
27 of the fume hood controller of the present invention;
28 FIG. 12 is a flow chart of ~ portion of the
29 operation of the fume hood controller of the present
invention, particularly illustrating th2 operation of the
31 feed orward control scheme, which is included in one of
32 the preferred embodiments of the present invention;
33 FIG. 13 is a flow chart of a portion of the
34 operation of the fume hood controller of the present
invention, particularly illustrating the operation of the
36 proportional gain, integral gain and derivative gain
37 control scheme, which embodies the present invention; and,
38 FIG. 14 .is a flow chart of a portion of the
ao
1 operation of the fume hood controller of the present
2 invention, paxticularly illustrating the operation of the
3 calibration of the feed forward control scheme.
4 ~etailed Descrip~ion
It should be generally under~tood that a fume
6 hood controller controls the rlow of air through the fume
7 hood in a manner whereby the effective ~ize o~ the to~al
8 opening to the fume hood, including the portion of the
9 opening that is not covered by one or more sash doors will
have a relatively constant average face velocity of air
11 moving into the fume hood. This means that regardless of
12 the area of the uncovered opening, an average volume of air
13 per unit of surface area of the uncovered portion will be
14 moved into the fume hood. This protects the persons in the
laboratory from being exposed to noxious fumes or the like
16 because air i~ always flowing into the fume hood, and out
17 of the exhaust duct, and the flow is preferably controlled
18 at a predetermined rate of approximately 75 to 125 cubic
19 ~eet per minute per square feet o~ effective surface area
of the uncovered opening. In other words, if the sash door
21 or doors are moved to the maximum open position whereby an
22 operator has the maximum ac~-ess to the inside of the ~ume
23 hood for conducting experiments or the like, then the flow
24 of air will most likely have to be increased to maintain
the average face velocity at the predetermined desired
26 level.
27 Since the total num~er of fume hoods that are
28 present in laboratory rooms can be quite large in many
29 installations, it ~hould be appreciated that a substantial
volume of air may be removed from the laboratory room
31 during operation. Also, since the HVAC system supplies air
32 to the laboratory roomr there may be a substantial change
33 in the volume of air required to ~e supplied to a room
34 depending upon whether the fume hoods are frequently being
opened, or other changes occur.
3~ Because much of the work that is performed in
37 many laboratories involves chemicals which may be
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1 dangerous, it i~ often desirable to maintain the
2 differential pressure within the laboratory at a lower
3 pressure than the hallways outside o~ the laboratory or
4 adjacent rooms. I~ the laboratory has several fume hoods
which are exhausting air from the room, the amount of air
6 supplied to the laboratory will necessarily be greater than
7 a comparably sized room without fume hoods, and there may
8 be increased diffioulty in maintaining the desired
9 differential pressure within the laboratory if the fume
hoods have their sash doors freguently opened.
11 If the differential pressure in a laboratory
12 room is maintained at a reduced level relative to the
13 reference space, noxious fumes which may escape from a fume
14 hood due to an accident or other cause will not permeate
beyond the room. The system involves ~ room controller and
16 an exhaust controller which are part of the heating,
17 ventilating and air conditioning apparatus of the building.
18 The room controller is of the type which can receive
19 electrical signals from the ~ume hood controllers, which
signals are proportional to the volume of air that is being
21 exhausted through the fume hoods. Sinre each fume hood can
22 be exhausting an amount of air that can v~ry considerably
23 depending upon its initial setting of the desired average
24 face velocity and the amount by which the sash doors are
opened, it is very advantageous that th~ volume indicating
26 signals be communicated from each of the ~ume hood
27 controllers to the room controller so that it can modulate
28 the volume of air that is being supplied to the room which
29 assists it in maintaining the differential pressure at the
desired level with relatively quick response times.
31 Broadly stated, the present invention is directed
32 to an improved fume hood controlling apparatus that is
33 adapted to provide desirable operational safety features
34 for persons who use the fume hoods to perform experiments
or other work, and also for the operator of the ~acility in
36 which the fume hoods are located~ More particularly~ the
37 apparatus of the present invention, in one of its preferred
38 embodiments is ~or use with fume hoods of the type which
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1 include a filtering means located between the fume hood
2 enclosure and the exhaust duct and the apparatus i~ adapted
3 to determine if a filter medium is loaded beyond a
4 predetermined amount and provide a loading signal that is
indicative of such a condition. The apparatus also
6 provides a visual or audible indication in response to the
7 loading signal being g~nerated. ~he apparatus also has
8 emergency switches near each Pume hood, with the switch
9 controlling the fume hood when actuated so that the fume
hood can operate in an emergency mode, and also provides an
11 indication to a central building console of a building
12 supervisory and control system for heating ventilating and
13 air conditioning apparatus.
14 In another embodiment, the system has additional
desirable safety features, including the feature of
16 controlling the differential pressure within the room to a
17 level that is sliyhtly less than the pressure within a
18 reference space such as a corridor, adjacent room or the
19 like, in the event of an emergency chemical spill or the
like within a ~ume hood which results in the fume hood
21 increasing its exhaust flow to an emergency level. This is
22 highly desirable so that any persons within the room can
23 open an outwardly opening external door to the room to
24 escape from the room. Also, the ælight dif~erence in the
differential pressure will not normally result in an
26 inwardly opening door being forced open. To this end, the
27 system utilizes emergency switches adjacent each fume hood
28 and also an emergency ~witch that i8 preferably located
29 outside of the room containing the ~ume hoods.
Turning now to the drawings, and particularly
31 FIG. 1, a block diagram is shown of several fume hood
32 controllers 20 embodying the present invention intercon-
33 nected with a room controller 22, an exhaust controller 24
34 and a main control console 26. The ~ume hood controllers
20 are interconnected wikh the room controller 22 and with
36 the exhaust controller 24 and the main control console 26
37 in a local area network illustrated by line 28 which may be
38 a multiconductor cable or the like. The room controller,
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1 the exhaust controller 24 and the main control console 26
2 are typically part of the building main HVAC system in
3 which the laboratory rooms containing the fume hoods are
4 located. The fume hood controllers ~0 are provided with
power through line 30, which is at the proper voltage via
6 a transformer 32 or the like.
7 The room controller ~2 pre~erably is of the type
8 which is at least capable o~ providing variable air volume
9 of air that is supplied to tha room, and may be a Landis &
Gyr Powers System 600 SCU controller. The room controller
11 22 is capable of communicating over the LAN lines 28 and is
12 interconnected with the exhaust controller which is
13 preferably part of the same hardware as the room
14 controller, i.e., it is part of the System 600 SCU
controller. The System 600 SCU controller is a
16 commer~ially available controller for which extensive
17 documentation exists. The User Reference Manual, Part ~o.
1~ 125-1753 for the System 600 SCU controller is specifically
19 incorporated by reference herein.
The room controller 20 receives signals via lines
21 81 from each of the fume hood controllers 20 that provides
22 an analog input signal indicating the volume of air that is
23 being exhausted by each of the fume hood controllers 20 and
24 a comparable signal from the exhaust controller 24 that
provides an indication of the volume of air that is being
26 exhausted through the main exhaust system apart from the
27 fume hoo~ exhausts. These signals coupled with ~ignal~
28 that are supplied by a differential pressure sensor 29
29 which indicates the pressure within the room relative to
the reference space enable the room controller to control
31 the supply of air that is necessary to maintain the
32 differential pressure within the room at a slightly lower
33 pressure than the reference space, i.e., pre~erably within
34 the range of about 0.05 to about 0.1 inches of water, which
results in the desirable lower pressure o~ the room
36 relative to the reference space. However, it is not so low
37 that it prevents persons inside the laboratory room from
38 o~enin~ the doors to escape in the event o an emergency,
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1 particularly i~ the doors open outwardly from the xoom.
2 Also, in the event the doors open inwardly, the differ-
3 ential pressure will not be so great that it will pull the
4 door open due to excessive ~orce being applied due to such
pressure.
6 ~he sensor 2~ i5 preferably positioned in a
7 suitable hole or opening in the wall between the room and
8 the reference space and measures the pressure on one side
9 relative to the other. Alternatively, a v~locity sensor
may be provided which measures the velocity o~ air moving
11 through the opening which is directly proportional to the
12 pressure difference between the two spaces. of course, a
13 lower differential pressure in the room relative to tha
14 reference space would mean that air would be moving into
the room which is also capable of being detected.
16 Referring to FIG. 2, a fume hood controller 20 is
17 illustrated with its input and output connector ports being
18 identified, and the fume hood controller 20 is connected to
19 an operator panel 34. It should be understood that each
fume hood will have a fume hood controller 20 and that an
21 operator panel will be provided with each fume hood
22 controller. The operator panel 34 is provided for each of
23 the fume hoods and it is interconnected with the fume hood
24 controller 20 by a line 36 which preferably comprises a
multi-conductor cable having six conductors. The operator
26 panel has a connector 38, such as a ~ wixe ~J111 type
27 telephone jack for example, into which a lap top personal
28 computer or the like may be connected for the purpose of
29 inputting information relating to the configuration or
operation of the ~ume hood during initial installation, or
31 to change certain operating parameters i~ necessary. The
32 operator panel 34 is preferably mounted to the fume hood in
33 a convenient location adapted to be easily observed by a
34 person who is working with the ~ume hood.
The fume hood controller operator panel 34
36 includes a liquid crystal display 40 which when s~lectively
37 activated provides the visual indication of various aspects
38 of the operation of the fume hood, including three digits
1 42 which provide the average face velocity. The display ~o
2 illustratQs other conditions ~uch as low face velocity,
3 high face velocity and amergency condition and an indi-
4 cation of controller failure.
The operator panel may have an alarm 44, and an
6 emergency purge switch 46 which an operator can press to
7 purge the fume hood in the event of an accident. In this
8 regard, the fume hood controller is programmed to prefer-
9 ably open the exhaust damper or control the blower so that
it will exhaust the maximum amount of air that is possible
11 in the even the purge switch 46 is activated. Alterna-
12 tively, the amount of air can be preset to another value,
13 if desired, such as 75% of maximum.
14 The operator panel has two auxiliary switches 48
which can be used for various customer needs, including
16 day/night modes of operation. I~ is contemplated that
17 night time mode of operation would have a different and
18 preferably reduced averaye face velocity, presumably
19 because no one would be worXing in the area and such a
lower average face velocity would conserve energy. An
~1 alarm silence switch 50 is also preferably provided to
22 extinguish the alarm.
23 Fume hoods come in many different styles, sizes
24 and configurations, including those which have a single
sash door or a number of sash doors, with the sash doors
26 being moveable vertically, horizontally or in both direc-
27 tions. Additionally, various fume hoods have different
28 amounts of by-pass flow, i.e., the amount of flow per-
29 mitting opening that exists even when all of the sash doors
are as completely closed as their design permits. Other
31 design considerations involve whether there is some kind of
32 filtering means included in the fume hood for confining
33 fumes within the hood during operation. While many of
34 these design consid~rations must be taken into account in
providing efficient and effective control of the fume
36 hoods, the apparatus o~ the present invention ~an be
37 configured to account for virtually all of the above
3~ describ~d ~es;gn variabl~s, and e~fective and extremel~
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1 fast control of the fume hood ventilation i5 provided.
2 Re~erring to FIG. 3, there is shown a fume hood,
3 indicated generally at 60, which has a vertically operated
4 sash door 62 which can be moved to gain access to the fume
hood and ~hich can be moved to the substantially closed
6 position as shown. Fume hoods are generally designed ~o
7 that even when a door sash ~uch as door sash 62 is com-
8 pletely closed, there is ~till some amount of opening into
9 the fume hood through which air can pass. This opening is
generally referred to as the bypass area and it can be
11 determined so that its effect can be taXen into considera-
12 tion in controlling the flow of air into the ~ume hood.
13 Some types of fume hoods have a bypass opening tha~ is
14 located above the door sash while others are below the
same. In some fume hoods, the first amount of movement of
16 a sash door will increase the opening at the bottom of the
17 door shown in FIG. 3, for example, but as the door is
18 raised, it will merely cut of~ the bypass opening so that
19 the effective ~ize of the total opening of the fume hood is
maintained relatively constant for perhaps the first one-
21 fourth amount of movement o~ the sash door 62 through its
22 course of travel.
23 Other types of Pume hoods may include several
24 horizontally moveable sash doors 66 such as -hown in FIGS.
4 and 5, with the doors being movable in upper and lower
26 pairs of adjacent tracks 68. When the doors are positioned
27 as shown in FIGS. 4 and 5, the Eume hood opening is
28 completely closed and an operator may move the doors in the
29 horizontal direction to gain access to the fume hood. Both
of the fumes hoods 60 and 64 have an exhaust duct 70 which
31 generally extends to an exhaust system which may be that of
32 the HVAC apparatus previously described.
33 The fume hood 64 is of the type which includes a
34 filtering structure shown diagra~matically at 72 which
filtering structure is intended to keep noxious fumes and
36 other contaminants from exiting the fume hood into the
37 exhaust system. The filtering ~tructure includes a filter
38 medium wh;~h is adapted to entrap fumes and effluents and
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1 keep them from bein~ exhausted, and the ~ilter mfPdium may
2 become loaded over time as a result of residue accumulation
3 on the medium. When the r~sidue builds up, a greater
4 resistance to air flow through the medium is experienced,
which is potentially dangerous if air cannot be e~hausted
6 from the fume hood. Also, more energy is required to
7 remove the air from the fume hood due to the increased
8 xesistancfe to flow.
9 In accordance with an important aspe¢t of the
present invention, a di~ferential pressure sensor,
11 generally indicated at 55, is provided and measures the
12 differential pressure of one side of the filtering
13 structure relative to the other. The sensor is adapted to
14 provifde an analog input voltage to the fume hood controller
20 that is proportional to the degree of loading of the
16 filter medium. When the signal reaches a predetermined
17 level, the fume hood controller 20 detects the same and
18 provides a warning indication on the operator panel 34,
19 which alerts anyone using the fume hood of such condition.
Alternatively, the predetermined signal level may be
21 detected by the controller and it can be adapted to sound
22 the alarm 44.
23 Referring to FIG. 6, there i5 shown a combination
24 fume hood which has horizontally movable doors 76 which are
similar to the doors 66, with the fume hood 74 having a
26 frame structure 7~ which carries the doors 76 in suitable
27 tracks and the frame structure 78 is also vertically
28 movable in the opening o~ the fume hood.
29 Other types of fume hoods may include several
horizontally moveable sash doors 66 such as shown in FIGS.
31 4 and 5, with the doors being movable in upper anfd lower
32 pairs of adjacent tracks 68. When the doors are positioned
33 as shown in FIGS. 4 and 5, the fume hood opening is
34 completely closed and an operator may move the doors in the
horizontal direction to gain access to the fume hood. Both
36 of the fumes hoods ~0 and 64 have an exhaust duc~ 70 which
37 generally extends to an exhaust system which may be that of
~ the H~T~ f~naratus previously described1 The fume hood 64
1 also includes a filtering structure shown diagrammatically
2 at 72 which filtering structure is intended to keep noxious
3 fumes and other contaminants frsm exiting the ~ume hood
4 into the exhaust system. ~eferring to ~IG. 6, there is
shown a Gombination fume hood which has horizontally
6 movable doors 76 which are similar to the doors 66, with
7 the fume hood 74 having a frame struc~ure 78 which carries
8 the doors 76 in suitable tracks and th~ frame ~tructure 78
9 is also vertically movable in the opening of the fume hood.
The illustration of FIG. 6 has portions removed
11 as shown by the break lines 73 which is intended to
12 illustrate that the height of the fume hood may be greater
13 than is otherwise shown so that the frame structure 78 may
14 be raised sufficiently to permit adequate access to the
interior of the fume hood by a person. There is generally
16 a by-pass area which is identified as the verti~al area 75,
17 and there is typically a top lip portion 77 which may be
18 approximately 2 inches wide. This dimension is preferably
19 de~ined so that its ef~ect on the calculatîon of the open
face area can be taken into consideration. Similarly, the
21 dimension of the lower sash portion 79 of the frame is
22 similarly defined for the same reason.
23 While not specifically illustrated, other
24 combinations are also possible, including multiple sets of
vertically moveable sash doors positioned adjacent one
26 another along the width of the fume hood opening, with two
27 or more sash doors being vertically moveable in adjacent
28 tracks, much the same as residential casement windows.
29 The fume hood controller 20 is adapted to operate
the fume hoods of various sizes and con~igurations as has
31 been described, and it is also adapted to be incorporated
32 into a laboratory room where several fume hood~ may be
33 located and which may hav2 exhaust ducts which merge into
34 a common exhaust manifold which may be a part of the
building HVAC system. A fume hood may be a single self-
36 contained installation and may have its own separate
37 exhaust duct. In the event that a single fume hood is
~R installe~, it is typical that such an installation would
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1 have a variable speed motor driven blower associated with
2 the exhaust duct whereby the speed of the motor and blower
3 can be variably controlled to thereby adjust the flow of
4 air through the fume hood.
Alternatively, and most typically ~or multiple
6 fume hoods in a single area, th~ exhaust ducts of each Pume
7 hood are merged into one or more larger exhaust mani~olds
8 and a single large blower may be provided in the manifold
9 system. In such types of installations, control o~ each
fume hood is achieved by means of separate dampers located
11 in the exhaust duct of each ~ume hood, so that variation in
12 the flow can be controlled by appropriately positioning the
13 damper associated with each fume hood.
14 The fume hood controller is adapted to control
virtually any of the various kinds and styles o~ ~ume hoods
16 that are commercially available, and to thi~ ~nd, it `has a
17 number of input and output ports (lines, connectors or
18 connections, all considered to be equivalsnt for the
19 pul~oses of describing the present invention) that can be
connected to various sensors that may be used with the
21 controller. As shown in FIG. 2, it has digital output or
22 DO ports which interface with a digital signal/analog
23 pressure transducer with an exhaust damper as previously
24 described, but it also has an analog voltage output port
for controlling a variable speed fan drive i~ it is to be
26 installed in that manner. There are five sash position
27 sensor ports for use in sensing the position of both
28 horizontally and vertically moveable sashes and there is
29 also an analog input port provided for connection to an
exhaust air flow sensor 49.
31 A digital input port for a second emergency
32 switch 51 is provided ~nd digital output ports for
33 outputting an alarm horn signal as well as an auxiliary
34 signal is provided. An analog output port is also provided
for providing a volume of flow signal to the room
36 controller 22. This port is connected to the room
37 controller by the individual lines 81 which extend from
38 each of the fume hood control]ers 20~
2~
1 From the foregoing discussion, it should be
2 appreciated that if the average ~ace velocity is desired to
3 be maintained and the sash position is changed, the size of
4 the opening can be dramatically changed which may then
require a dramatic change in the volume of air to mai~tain
6 the average face velocity. While it i5 known to control a
7 variable air volume blower as a function of the sash
8 position, the fume hood controller apparatus o~ the present
9 invention improves on that known method by incorporating
additional control schemes which dramatically improve the
11 capabilities of the control system in terms of maintaining
12 relatively con~tant average face velocity in a manner
13 whereby reactions to perturbations in the ~ystem are
14 quickly made. Such impxovements are illustrated, dPscribed
and claimed in the above referenced cross related
16 applications.
17 To determine the position of the sash doors, a
18 sash position sensor is provided adjacent each movable sash
19 door and it is generally illustrated in FIGS. 7, 8 and 9.
Referring to FIG. 8, the door sash position indicator com-
21 prises an elongated switching mechanism 80 of relatively
22 simple mechanical design which preferably consists of a
23 relatively thin polyester base layer 82 upon which is
24 printed a strip of electrically resistive ink 84 of a known
constant resistance per unit length. Another polyester
26 base layer 86 is provided and it has a strip of elec-
27 trically conductive ink 38 printed on it. The two base
28 layers 82 and 86 are adhesively bonded to one another by
29 two beads of adhesive 90 located on opposite ~ides oP the
strip. The base la~ers are preferably approximately five-
31 thousandths of an inch thick and the beads are approxi-
32 mately two-thousandths of an inch thick, with the beads
33 providing a spaced area between the conductive and resis-
34 tive layers 88 and 84. The switching mechanism ~0 is
preferably applied to the ~ume hood by a layer o~ adheeive
36 92.
37 The polyester material is su~ficiently flexible
38 to enable one layer to be mov~d to~-ard th0 other so that
2(~ Q
-17-
1 contact is made in response to a preferably spring biased
2 actuator 94 carried by the appropriate sash door to which
3 the strip is placed adjacent to so that when the sash door
4 is moved, the actuator 94 moves along the switching
mechanism 80 and provid~s contact between the resistive and
6 conductive layers which are then ~ensed by electrical
7 circuitry to be described which provides a voltage output
8 that is indicative of the position of the actuator 94 along
9 the length of the switching mechanism. Stated in other
words, the actuator 94 i5 carried by the door and therefore
ll provides an electrical voltage that is indicative of the
12 position of the sash door.
13 The actuator 94 is preferably spring biased
14 toward the switching mechanism ~o so that as the door is
moved, sufficient pre~sure is applied to the switching
16 mechanism to bring the two base layers together so that the
17 resistive and conductive layers make electrical contact
18 with one another and if this is done, the voltage level is
19 provided. By having the switching mechanism 80 of
sufficient length so that the full extent of the travel of
21 the sash door is provided as shown in FIG. 3, then an
22 accurate determination of the sash position can be made.
23 It should be understood that the illustration of
24 the switching mechanism 80 in FIGS. 3 and 5 is intended to
be diagrammatic, in that the switching mechanism is
26 preferably actually located within the sash frame itself
27 and accordingly would not be visible as shown. The width
28 and thickness dimensions of the switching mechanism 80 are
29 so small that interference with the operation o~ the sash
door is virtually no problem. The actuator 94 can also be
31 placed in a small hole that may be drilled in the sash door
32 or it may be attached externally at one end thereof so that
33 it can be in position to operate the ~witching mechanism
34 80. In the vertical moveable saeh doors shown in FIGS. 3
and 6 t a switching mechanism 80 is pref~rably provided in
36 one or the other of the sides of the sash frame, whereas in
37 the fume hoods having horizontally movable doors, it is
38 preferred that the ~witching mechanism 80 be placed in the
~ Q ~
-18-
1 top of the tracks 6~ so that the weight of ths movable
2 doors do not operate ~he switching mechanism 80 or other-
3 wise damage the same~ It is also pre~erred that the
4 actuator 94 be located at one end of 0ach of the doors ~or
reasons that are described in th~ cross-xeferenced appli-
6 cation entitled "~ method and apparatus ~or determining the
7 uncovered si~e of an opening adapted to be covered by
8 multiple moveable doors" by Ahmed et al., Serial No. 52498.
9 Turning to FI~. 9, the pre~erred electrical
circuitry which generates the position indicating voltage
ll is illustrated, and this circuitry is adapted to provide
12 two separate voltages indicating the position o~ two sash
13 doors in a single track. With respect to the cross-section
14 shown in FIG. 5, there are two horizontal tracks, each of
which carries two sash doors and a switching mechanism 80
16 is provided for each of the tracks as is a circuit as shown
17 in FIG. 9, thereby providing a distinct voltage for each of
18 the four sash doors as shown.
19 The switching mechanism 80 is preferably applied
to the fume hood with a layer of adhesive 92 and the
21 actuator 94 is adapted to bear upon the switching mech~nism
22 80 at locations along the length thereo~. Referring to
23 FIG. 7, a diagrammatic illustration of a pair of switching
24 mechanism 80 is illustrated such as may occur with respect
to the two tracks shown in FIG. 5. A switching mechanism
26 80 is provided with each track and the ~our arrows
27 illustrated represent the point of contact created by the
28 actuators 94 which result in a signal being applied on each
29 of the ends of each switching mechanism, with the magnitude
of the signal representing a voltage that is proportional
31 to the distance between the end and the nearest arrowO
32 Thus, a single switching mechanism 80 is adapted to provide
33 position indicating signals for two doors located in each
34 track. The circuitry that is used to accomplish the
voltage generation is shown in FIG. 9 and in ludes one of
36 these circuits for each track. The resistive element is
37 shown at 84 and the conductive element 88 is also illus-
38 trated being connected to ground with two arrows being
~C ~30
1 illustrated, and represented the point of contact between
2 the resistive and conductive elements caused by each of the
3 actuators 94 associated with the two separate doors. The
4 circuitry includes an operational amplifier lU0 which has
its output connected to the base of a PNP tran~istor 102,
6 the emitter vf which is connected to a source of positive
7 voltage through resistor 104 into the negative input of the
8 operational amplifier, the po~itive input of which is also
9 connected to a source of po~itive voltage of preferably
approximately five volts. The collector of the transistor
11 102 is connected to one end of the res.istive element 84 and
12 has an output line 106 on which the voltage is produced
13 that is indicative of the position of the door.
14 The circuit operates to provide a constant cur-
rent directed into the resistive element 84 and this cur-
16 rent results in a voltage on line 106 that is proportional
17 to the resistance value between the collector and ground
18 which changes as the nearest point of contact along the
19 resistance changes. The operational amplifier operates to
attempt to drive the negative input to equal the voltage
21 level on the positiY2 input and this results in the current
22 applied at the output of the operational amplifier varying
23 in direct proportion to the effective length of the
24 resistance strip 84. The lower portion of the circuitry
operates the same way as that which has been described and
26 it similarly produces a voltage on an output line 108 that
27 is proportional to the distance ~etween the connected end
28 of the resistance element 84 and the point of contact that
29 is made by the actuator 94 associated with the other sash
door in the track.
31 Referring to the composite electrical schematic
32 diagram of the circuitry of the fume hood controller, if
33 the separate drawing~ FIGS. lOa, lOb, lOc, lOd and lOe are
34 placed adjacent one another in the manner shown in FI~. 10,
the total electrical schematic diagram of the fume hood
36 controller 20 is illustratedO The operation of the
37 circuitry of FIGS. lOa through lOe will not be described in
38 detail. The circuitry is driven by a microprocessor and
--20--
1 the important algorithms that carry out the control
2 functions of the controller will be hereinafter described.
3 Referring to FIG. lOc, the circuitry includ~s a Motorola MC
4 68HCll microproGesscr 120 which is clocked at 8 MHz by a
5 crystal 122. The microprocessor 120 has a data~us 124 that
6 is connected to a tri-state bu~fer 126 (FIG. lOd) which in
7 turn is connected to an electrically programmable read only
8 memory 128 that is also connected to the databus 124. The
9 EPROM 128 has address lines AO through A7 connected to the
tri-state buffer 126 and also has address lines A8 through
11 A14 connected to the microprocessor 120.
12 The circuitry includes a 3 to 8-bit multiplexer
13 130, a data latch 132 (see FIG. lOd), a diyital-to-analog
14 converter 134, which is adapted to provide the analog
outputs indicative of the volume of air being exhausted by
16 the fume hood, which information is provided to room
17 controller 22 as has been previously described with respect
18 to FIG. 2. Referring to FIG. lOb, an ~S232 driver 136 is
19 provided for transmitting and receiving information through
the hand held terminal. The circuitry illustrated in FIG.
21 9 is also shown in the overall schematic diagrams and is in
22 FIGS. lOa and lOb. The other components are well known and
23 therefore need not be othexwise described.
24 As previously mentioned, the apparatus utilizes
the flow sensor 49 preferably located in the exhaust duct
26 70 to measure the air volume that is being drawn through
27 the fume hood. The volume flow rate may be calculated by
28 measuring the differential pressure across a multi-point
2~ pitot tube or the like. hood. The volume may be measured
with an air valve, flow meter or by measuring the
31 differential pressure across an ori~ice plate or the like.
32 The preferred embodiment utilizes a differential pressure
33 sensor for measuring the flow through the ~xhaust duct and
34 the apparatus of the present invention utilizes control
schemes ~o either maintain the ~low through the hood at a
36 predetermined average face velocity, or at a minimum
37 velocity in the event the fume hood is closed or has a very
38 small bypass area.
-21-
1 The fume hood con~roller can be con~igured ~or
2 almost all known types of ~ume hoods, including fume hoods
3 having horizontally movable sash doors, vertically movable
4 sash doors or a combination of the two. As can be seen
5 from the illustrations o~ FIGS. 2 and 10, the fume hood
6 controller is adapted to control an exhaust damper or a
7 variable speed fan drive, the controller beîng adapted to
8 output signals that are compatible with either type of
9 control. The controller is also adapted to receive
information defining the physical and operating
11 characteristics of the fume hood and other initializing
12 information. This can be input into the fume hood
13 controller by means of the hand held terminal which is
14 pre~erably a lap top computer that can be connected to the
operator panel 34. The information that should ~e prvvided
16 to the controller include the following, and the dimensions
17 for the information are also shown:
18 Operational information:
l9 l. Time of day;
2. Sat day and night values ~or the average
21 face velocity (SVEL), feet per minute or
22 meters psr second;
23 3. Set day and night values for the minimum
24 flow, (MINFLO), in cubic .~eet per minute;
4. Set day and night values for high velocity
26 limit (HVEL), F/m or M/sec;
27 5. Set day and night value~ for low velocity
28 limit (LVEL), F/m or M/sec;
29 6. Set day and night values for intermediate
high velocity limit (MVEL), F/m or M/sec;
31 7. Set day and night values for intermediate
32 low velocity limit (IVEL), F/m or M~sec;
33 8. Set the proportional gain factor (KP),
34 analog output per error in percent;
9. Set the integral gain factor (KI)I analog
36 output multiplied by time in minutes per
37 error in percenk:
38 10. Set derivative gain ~actor (KD), analog
~5~0
22-
1 output multiplied by time in minutes p~r
2 error in percent;
3 11. S~t feed forward gain ~actor (KF) if a
4 variabl~ seed drive is used as the control
equipment in~tead of a ~ E, analog output
6 per CFM;
7 The above information is used to control the mode
8 of operation and to con~rol t~e limits of flow during the
9 day or night modes of operation. The controller includes
programmed instructions to calculate the steps in
11 paragraphs 3 through 7 in the event such infoxmation is not
12 provided by the user. To this end, once the da~ and night
13 values for the average face velocity are set, the
14 controller 20 will calculate high velocity limit at 120% of
the avcrage face velocityf the low velocity limit at 80%
16 and the intermediate limit at 90%. It should be understood
17 that these percentage values may be adjusted, as desired~
18 Other information that should be input include th~
19 following information which relates to the physical
construction of the ~ume hood. It should be understood
21 that some o~ th inPormation may not be required for only
22 vertically or horizontally moveable sash doors, but all of
23 ~he information may be required for a combination of the
24 same:
12. Input the number of vertical segments;
26 13. Input the height of each segment, in inches:
27 14. Input the width of each segment, in inches;
28 15~ Input the number of tracks per segment;
29 16. Input the number of horizontal sashes per
track;
31 17. Input the maximum sash height, in inches:
32 18. Input the ~ash width, in inches;
33 lg. Input the location of the sash sensor from
3~ left edge of sash, in inches;
20. Input the by-pass area per segment, in
36 square inches;
37 21. Input the minimum face area per segment, in
38 square in¢hes~
z~
23~
1 22. Input the top lip height above the
2 hori~ontal sash, in inches;
3 23. Input the bottom lip height below Aorizontal
4 sash, in inrhes.
The ~ume hood controller 20 is programmed to
6 control the flow o~ air through the ~ume hood by carrying
7 out a series of instructions, an overview of which is
8 contained in the flow chart of FIG. 11. ~fter ~tart~up and
9 outputting in~ormation to the display and determining the
time o~ day, the controller 20 reads the initial sash
11 positions of all doors (block 150), and this information is
12 then used to compute the open face area (block 152). If
13 not previously done, the operator can set the average ~ace
14 velocity set point (block 154) and this information is then
used together with the open face area to çompute the
16 exhaust flow set point (SFLOW) (block 156) that is
17 necessary to provide the predetermined average face
18 velocity given the open area of the fume hood that has been
19 previously measured and calculated. The computed fume hood
exhaust set point is then compared (block 158~ with a
21 preset or required min.imum flow, and if computed set point
22 is less than the minimum flow, the controller sets the set
23 point flow at the preset minimum flow (block 160). If it
24 is more than the minimum ~low, then it is retained (block
162) and it is provided to both of the control loops.
26 If there is a variable speed fan drive fox the
27 fume controller, i.e., several fume hoods are not connected
28 to a common exhaust duct and controlled by a damper, then
29 the controller will run a feed-forward control loop (block
164) which provides a control signal that is sent to a
31 summing junction 166 which control signal represents an
32 open loop type o~ control action. In this control action,
33 a predicted value of the speed of the blower is generated
34 based upon the calculated opening of the fume hood, and the
average face velocity set point. The predicted value o~
36 the speed of the blower generated will cause the blower
37 motor to rapidly change speed to maintain the average ~ace
38 velocity. It should be understood that the feed ~orward
-24-
1 aspect of the control is only invoked when the sash
2 position has been changed and a~ter it has been changed,
3 then a second control loop performs the dominant control
4 action for maintaining the average face velocity constant
in the event that a variable speed blower is used to
6 control the volume of air through the fume hood.
7 After the sash position has been changed and the
8 feed forward loop has established the new air volume, then
9 the control loop switches to a proportional integral
derivative control loop and this is accomplished by the set
11 flow signal being provided to block 16~ which indicates
12 that the controller computes the error by determining the
13 absolute value of the difference between the set flow
14 signal and the flow signal as measured by the exhaust air
flow sensor in the exhaust duct. Any error that i5
16 computed is applied to the control loop ide~tified as the
17 proportional-integral-derivative control loop ~PID) to
18 determine an error signal (block 170) and this error signal
19 is compared with the prior error signal from the previous
sample to determine if that error is less than a deadband
21 error (block 172). If it is, then the prior error signal
22 is maintained as shown ky block 174, but if it is not, then
23 the new error signal is provided to output mode 176 and it
24 is applied to the summing junction 166. That summed error
is also compared with the last output signal and a
26 determination is made if this is within a deadband range
27 (block 180) which, if it is, results in the last or
28 previous output being retained (block 182). If it is
29 outside of the deadband, then a new output signal i~
provided to the damper control or the blower (block 184).
31 After the sash position has been changed and the
32 feed forward loop has established the new air volume, then
33 the control loop switches to a proportional integral
34 derivative control loop and this is accompli~hed by the set
flow signal being provided to block 168 which indicates
36 that the controller computes t~e error by determining the
37 absolute value of the difference between the set ~low
38 signal and the flow signal as measured by the exhaust air
-25-
1 flow sensor in the exhaust duct. Any error that is
2 computed is applied to the control loop identi~ied as the
3 proportional-integral-derivative control loop (PID) to
4 determine an error signal (~lock 170) and this error signal
is compared with ~he prior error signal ~rom the previous
6 sample to determine if that error i5 less than a deadband
7 error (block 172). If it is, then the prior error signal
8 is maintainad as shown by block 174, but i~ it is not, then
9 the new error signal is provided to output mode 176 and it
is applied to the summing junction 166. That summed error
11 is also compared with the last output signal and a
12 determination is made if this is within a deadband range
13 (block 180) which, if it is, result6 in the last or
14 previous output being retained (blocX 182). If it is
outside of the deadband, then a new output signal is
;6 provided to the damper control or the blower (block 184).
17 In the event that the last output is he output
18 as shown in block 182, the controller then reads the
19 measured flow (MFLOW) ~block 186) and the sash positions
are then read (block 188) and the net open ~ace area is
21 recomputed (block 190) and a determination made as to
22 whether the new computed area less the old computed area is
23 less than a deadband (block 192) and if it is, then the old
24 area is maintained (block 194) and the error is then
computed again (block 168). If the new area less the old
26 area is not within the deadband, then the controllex
27 computes a new exhaust flow set point as shown in block
28 156.
29 One of the significant advantages of the fume
hood controller is that it is adapted to execute tha
31 control scheme in a repetitive and extremely rapid mannex.
32 The exhaust sensor provides flow signal infsrmation that is
33 inputted to the microprocessor at a speed of approximately
34 one sample per 100 milliseconds and the control action
described in connection with FIG. 11 i~ completed
36 approximately every 100 milliseconds. The sash door
37 position signals are sampled by the microprocessor every
38 200 milliseconds. The result of such rapid repetitive
-~6-
1 sampling and executing of the control actions results in
2 extremely rapid operation of the controller. Ik has been
3 found that movement of the sash will result in adjustment
4 of the air flow so that the av~rage face velocity is
achieved within a time period of only approximately 3-4
6 seconds after the sash door reposi~ion has been stopped.
7 This represents a dramatic improvement over existing fume
8 hood controllers.
9 In the event that the feed forward control loop
is utilized, the sequence of instructions that are carried
11 out to accomplish running of this loop is shown in the flow
12 chart of FIG. 12, which has the controller using the
1~ exhaust flow set point (SFLOW) to compute the control
14 output to a fan drive (block 200), which is identified as
signal AO that is computed as an intercept point plu5 the
16 set flow multiplied by a slope value. The intercept is the
17 value which is a fixed output voltage to a fan drive and
18 the slope in the equation correlates exhaust flow rate and
19 output voltage to the fan drive. The controller then reads
the duct velocity (DV) (block 202), takes the last duct
21 velocity sample (block 204) and equates that as the duct
22 velocity value and starts the timing of the maximum and
23 minimum delay times (block 206~ which the controller uses
24 to insure whether the duct velocity has reached steady
state or not. The controller determines whether the
26 maximum delay time has expired ~block 208), and if it has,
27 provides the output signal at output 210. If the max delay
28 has not expired, the controller determines if the absolute
29 value of the difference between the last duct velocity
sample and the current duct velocity sample is less than or
31 equal to a dead band value (block 212). If it is not less
32 than the dead band value, th~ controller then sets the last
33 duct value as equal to the present duct value ~ample (block
34 214) and the controller then restarts the minimum delay
timing function (block 216). Once this is accompliæhed,
36 the controller again determinPs wh~ther the max delay has
37 expired (block 208). If the absolute value of the
38 difference between the last duct velocity and the present
;~5~:1~0
-27-
1 duct velocity sample is less than the dead band, the
2 controll2r determines whether the minimum delay time has
3 expired which, if it has as shuwn from block 21~, the
4 output is provided at 210. If it has not, then it
determines if the max delay has expired.
6 Turning to the proportional-integral-der.lvative
7 or PID control loop, the controller runs the PID loop by
8 carrying out the instructions shown in the flow chart of
9 FIG. 13. The controller uses the error that is computed by
block 168 (see FIG. 1~) in three separate paths. With
11 respect to the upper path, the controller uses the
12 preselected proportional gain factor (block 220) and that
13 proportional gain factor is used together with the error to
14 calculate the proportional gain (block 222) and the
proportional gain is output to a summing junction 224.
16 Th~ controller also uses the error signal and
17 calculates an integral term (block 226) with the integral
18 term being equal to the prior integral sum (ISUM) plus the
19 product of loop time and any error and this calculation is
compared to limits to provide limits on the term. The term
21 is then used together with the previou~ly defined integral
22 gain constant (block 230) and the controller than
23 calculates the integral gain (block 232) which is the
24 integral gain constant multiplied by the integration sum
term. The output is then applied to thP summing junction
26 2~4.
27 The input errox is also used by the controller to
28 calculate a derivative gain factor which is done by the
29 controller using the previously defined derivative gain
factor from block 234 which is used together with the error
31 to calculate the derivative gain (block 236) which is the
32 reciprocal of the time in which it is required to execute
33 the PID loop ~ultiplied by the derivative gain *actor
34 multiplied by the current sample ~rror minus the previous
sample error with this result being provided to the summing
36 junction 224.
37 The control action performed by the controller 20
38 as illustrated in ~IG. 13 provides three separate gain
-28-
1 factors which provide steady state correction o~ the air
2 ~low through the fume hood in a very ~ast acting manner.
3 The formation of the output signal from the PID control
4 loop takes into consideration not only the magnitude of the
error, but as a result of the derivative gain segment of
6 control, the rate o~ change of the error is considered and
7 the change in the value of the gain is proportional to the
8 rate of change. Thus, ~he ~eriva~ive gain can ~ee how ~ast
g the actual condition is changing and works as an
"anticipator" in order to minimize error between the actual
11 and desired condition. The integral gain develops a
12 correction signal that is a function of the error
13 integrated over a period of time, and therefore provides
14 any necessary correction on a continuous ba~is to bring the
actual condition to the desired condition. The proper
16 combinations of proportional, integral and derivative gains
17 will make the loop faster and reach the desired conditions
18 without any overshoot.
19 A significant advantage of the PID control action
is that it will compensate for perturbations that may be
21 experienced in the laboratory in which the ~ume hood may be
22 located in a manner in which other controllers do not. A
23 common occurrence in laboratory rooms which have a number
24 of fume hoods that are connected to a common exhaust
manifold, involves the change in the pressure in a fume
26 hood exhaust duct that was caused by the sash doors being
27 moved in another of the fume hoods that is connected to the
28 common exhaust manifold. Such pressure variations will
29 affect the average face velocity o~ those fume hoods which
had no change in their sash doors~ However, the PID
31 control action may adjust the air flow if the exhaust duct
32 sensor determines a change in the pressure. To a lesser
33 degree, there may be pressure variations produced in the
34 laboratory caused by opening of doors to the laboratory
itself, particularly if the differenkial pressure of the
36 laboratory room is maintained at a lesser pxessure than a
37 referenco space such as the corridor outside the room, for
38 example.
-29-
1 It is necessary to calibrate the feed forward
2 control loop and to this end, the in~truc~ions illustrated
3 in the flow chart of FIG. 14 are carried ou~. When the
4 initial calibration is accomplished, it is preferably done
through the hand held tsrminal that may be connected to the
6 operator panel via connector 3~, for example. ~he
7 controller then determines if the feed forward calibration
8 is on (block 242) and i~ it is, then ~he controller sets
9 the analog output of the ~an drive to a value of 20 percent
of the maximum value, which is identified as value A01
11 (block 244). The controller then sets the last sample duct
12 velocity (LSDV) as the current duct velocity ~CDV) (block
13 246) and starts the maximum and minimum timers (block 248)~
14 The controller ensures the steady state duct velocity in
the following way. First by checking whether the max timer
16 has expired, and then, if the max timer has not expired,
17 the controller determines if the absolute value of the last
18 sample duct velocity minus the current duct velocity is
19 less than or equal to a dead band (block 270), and if it
is, the controller determines if the min timer has expired
21 (block 272). If it has not, the controller reads the
22 current duct velocity (block 274). If the absolute value
23 of the last sample duct velocity minus the current duct
24 velocity is not less than or equal to a dead band (block
270), then the last sample duct velocity is set as the
26 current duct velocity (block 276) and the mintimer is
27 restarted (block 278) and the current duct velocity is
28 again read (block 274). In case either the max timer or
29 min timer has expired, the controller then checks the last
analog output value to the fan drive (252) and inquires
31 whether the last analog output value was 70 percent of the
32 maximum output value ~block 254~. If it is not, then it
33 sets the analog output value to the fan drive at 70 percent
34 of the max value A02 (block 256) and the steady state duct
velocity corresponding to A01. The controller then repeats
36 the procedure of ensuring steady state duct velocity when
37 analog output is A02 (block 258). I~ it is at the 70
38 percent of max value, then the duct velocity corresponds to
~ J~
-30-
1 steady state velooity of A0~ (block 258). Finally, the
2 con~roller (~lock 2S2) calculates the slope and intercept
3 values.
4 The resul~ of the calibration process i5 to
determine the duct flow at 20% and at 70% of the analog
6 output values, and the measured flsw enables the slope and
7 intercept values to be dete~mined so that the feed forward
8 control action will accurately pr~dict the necessary fan
g speed wh~n sash door positions ar~ changed.
From the foregoing detailed description, it
11 should be appreciated that an improved system and apparatus
12 for controlling fume hoods and the room in which they are
13 contained has been shown and described. The many desirable
14 safety features insure increased safety for those present
in a room containing fume hoods. The apparatus detects
16 excessive loading of a ~ilter medium and provides an audio
17 and/or video indication of that condition. ~he system
18 controls the air ~upply into the room, taking into
19 consideration the volume o~ air that is being exhausted by
the fume hoods within it and ths amount of air being
21 exhausted by the HVAC equipment for the room, and controls
22 the differential pressure of the room so that in the event
23 of an emergency, an unusual emergency fume hood ex~aust
24 mode of operation can be instituted without trapping an
2~ individual in the room or causing external doors fxom
26 opening into the room, which may rapidly dissipate the
27 desired lower differential pressure within the room.
28 While various embodiments of the present
29 invention have been shown and described, it should be
understood that various alternatives, substitutions and
31 equivalents can be used, and th~ present invention should
32 only be limited by the claim~ and equivalents thereof.
33 Various features of the present invention are set
34 forth in the ~ollowing claims.
