Note: Descriptions are shown in the official language in which they were submitted.
5 1 2 ~
1 APPARATUS FOR CONTROLLING THE
2 VENTILATION OF LABORATORY FUME HOODS
3 Cross Reference To Related Applications
4 1. Title: Apparatus for Determining the Position of a
- 5 Moveable Structure Along a Track
6 Inventors: David Egbers and Steve Jacob
7 Serial No.: 2~055~258
8 2. Title: A System for Controlling the Differential
9 Pressure of a Room Having Laboratory Fume
Hoods
11 Inventors: Osman Ahmed and Steve Bradley
12 Serial No.: 2,055,101
13 3. Title: A Method and Apparatus for Determining the
14 Uncovered Size of an Opening Adapted to be
Covered by Multiple Moveable Doors
16 Inventors: Osman Ahmed, Steve Bradley and Steve
17 Fritsche
18 Serial No.: 2,055,147
19 4. Title: Laboratory Fume Hood Control Apparatus
Having Improved Safety Considerations
21 Inventors: Osman Ahmed
22 Serial No.: 2,~55,100
23 The present invention relates generally to the
24 control of the ventilation of laboratory fume hoods, and
more particularly to an improved method and apparatus for
26 controlling the ventilation of fumes from one or more fume
27 hoods that are typically located in a laboratory
28 environment.
~'
A
2 ~ 2 b~
1 Fume hoods are utilized in various laboratory
2 environments for providing a work place where potentially
3 dangerous chemicals are used, with the hoods comprising an
4 enclosure having moveable doors at the front portion
thereof which can be opened in various amounts to permit a
6 person to gain access to the interior of the enclosure for
7 the purpose of conducting experiments and the like. The
8 enclosure is typically connected to an exhaust system for
9 removing any noxious fumes so that the person will not be
exposed to them while performing work in the hood.
11 Fume hood controllers which control the flow of
12 air through the enclosure have experience increased
13 sophistication in recent years, and are now able to more
14 accurately maintain the desired flow characteristics to
efficiently exhaust the fumes from the enclosure as a
16 function of the desired average face velocity of the
17 opening of the fume hood.
18 The average face velocity is generally defined as
19 the flow of air into the fume hood per square foot of open
face area of the fume hood, with the size of the open face
21 area being dependent upon the position of one or more
22 moveable doors that are provided on the front of the
23 enclosure or fume hood, and in most types of enclosures,
24 the amount of bypass opening that is provided when the door
or doors are closed.
26 The fume hoods are exhausted by an exhaust system
27 that includes one or more blowers that are capable of being
28 driven at variable speeds to increase or decrease the flow
29 of air from the fume hood to compensate for the varying
size of the opening or face. Alternatively, there may be
31 a single blower connected to the exhaust manifold that is
32 in turn connected to the individual ducts of multiple fume
33 hoods, and dampers may be provided in the individual ducts
34 to control the flow from the individual ducts to thereby
modulate the flow to maintain the desired average face
36 velocity.
37 The doors of such fume hoods can be opened by
38 ra~sing them vertically, often referre~ to as the sash
i 2 ~
--3--
1 position, or some fume hoods have a number of doors that
2 are mounted for sliding movement in typically two sets of
3 tracks. There are even doors that can be moved
4 horizontally and ve.rtically, with the tracks being mounted
in a frame assembly that is vertically moveable.
6 Prior art fume hood controllers have included
7 sensing means for measuring the position of the doors and
8 then using a signal proportional to the sensed position to
9 thereby vary the speed of the blowers or dampers. While
such control has represented an improvement in the control
11 of fume hoods, there are circumstances that arise that
12 require further adjustment of the exhausting of such hoods
13 that such a controller cannot perform. This is in part due
14 to the fact that such position sensing of the doors results
in essentially a prediction of the required blower speed or
16 damper position which should result in the required flow to
17 maintain a desired face velocity over the entire open area
18 of the hood for a given position of the doors without
19 actually measuring the resultant flow. This type of
control is known as an open loop type of control scheme.
21 Accordingly, it is one of the primary objects of
22 the present invention to provide an improved fume hood
23 controller apparatus, as well as an improved method, which
24 overcomes many of the disadvantages of existing
controllers.
26 Another one of the primary objects of the present
27 invention is to provide such an improved apparatus and
28 method which provides for very accurate control to thereby
29 maintain the desired average face velocity of air across
the open areas of the fume hood, which by virtue of
31 apparatus' design, also exhibits a very fast response time
32 in maintaining such control when the door positions are
33 changed or other balance upsetting circumstances change.
34 Still another object of the present invention is
to provide such an improved method and apparatus which
36 utilizes a closed loop type of control, which during its
37 operation, employs three distinct types of gain to measure
?~ 2n-ol in the d~sired ~ac~ veloci~y, with the co~trol sche~e
1 operating to reduce the error to zero in a rapid manner.
2 A more detailed object of the present invention
3 is to provide an improved method and apparatus, which in
4 all of its embodiments, employs proportional gain, integral
gain and derivative gain in achieving the desired average
6 face velocity of air inwardly through the open areas to the
7 fume hood, i.e., the areas not covered by one or more
8 doors.
9 Yet another object of the present invention is to
provide an improved method and apparatus, which in certain
11 of its embodiments, employs the above mentioned
12 proportional gain, integral gain and derivative gain, but
13 in addition, employs a feed-forward type of control which
14 occurs when one or more of the doors is moved to alter the
open area of the hood.
16 Still another object of the present invention is
17 to provide such an improved fume hood controller apparatus
18 and method which is susceptible of being integrated with
19 the heating, ventilating and air conditioning control
system of a laboratory room in which the fume hoods are
21 located.
22 Yet another object of the present invention is to
23 provide such an improved fume hood controller apparatus and
24 method, which apparatus includes a small, convenient and
effective operator panel that is located on the fume hood
26 itself at a location easily observed and convenient for
27 manipulation by a person, which operator panel is adapted
28 to monitor and control the fume hood controller at any
29 time.
A closely related object lies in the provision of
31 providing a connector on said operator panels which is
32 adapted to receive a hand held terminal that can be used to
33 change various critical parameters relating to the
34 operation of the fume hood controllers.
These and other objects will become apparent upon
36 reading the following detailed description of the present
37 invention, while referring to the attached drawings, in
38 which:
2 ~ 6
1 FIGURE 1 is a schematic block diagram of
2 apparatus of the present invention shown integrated with a
3 room controller of a heating, ventilating and air
4 conditioning monitoring and control system of a building;
FIG. 2 is a block diagram of a fume hood
6 controller, shown connected to an operator panel, the
7 latter being shown in front elevation;
8 FIG. 3 is a diagrammatic elevation of the front
9 of a representative fume hood having vertically operable
sash doors;
11 FIG. 4 is a diagrammatic elevation of the front
12 of a representative fume hood having horizontally operable
13 sash doors;
14 FIG. 5 is a cross section taken generally along
the line 5-5 of FIG. 4;
16 FIG. 6 is a diagrammatic elevation of the front
17 of a representative combination sash fume hood having
18 horizontally and vertically operable sash doors;
19 FIG. 7 is an electrical schematic diagram of a
plurality of door sash position indicating switching means;
21 FIG. 8 is a cross section of the door sash
22 position switching means;
23 FIG. 9 is a schematic diagram of electrical
24 circuitry for determining the position of sash doors of a
fume hood;
26 FIG. 10 is a block diagram illustrating the
27 relative positions of FIGS. lOa, lOb, lOc, lOd and lOe to
28 one another, and which together comprise a schematic
29 diagram of the electrical circuitry for the fume hood
controller means embodying the present invention;
31 FIGS. lOa, lOb, lOc, lOd and lOe, which if
32 connected together, comprise the schematic diagram of the
33 electrical circuitry for the fume hood controller means
34 embodying the present invention;
FIG. 11 is a flow chart of the general operation
36 of the fume hood controller of the present invention;
37 FIG. 12 is a flow chart of a portion of the
3~ o~er~tion of the fume hood controller of the present
~ û S ~
1 invention, particularly illustrating the operation of the
2 feed forward control scheme, which is included in one of
3 the preferred embodiments of the present invention;
4 FIG. 13 is a flow chart of a portion of the
operation of the fume hood controller of the present
6 invention, particularly illustrating the operation of the
7 proportional gain, -integral gain and derivative gain
8 control scheme, which embodies the present invention; and,
9 FIG. 14 is a flow chart of a portion of the
operation of the fume hood controller of the present
11 invention, particularly illustrating the operation of the
12 calibration of the feed forward control scheme;
13 FIG. 15 is a flow chart of a portion of the
14 operation of the fume hood controller embodying the present
invention, particularly illustrating the operation of the
16 calculation of the uncovered opening for a number of
17 horizontally moveable sash doors; and,
18 FIG. 16 is a flow chart of a portion of the
19 operation of the fume hood controller embodying the present
invention, particularly illustrating the operation of the
21 calculation of the uncovered opening for a number of
22 horizontally and vertically moveable sash doors.
23 Detailed Description
24 It should be generally understood that a fume
hood controller controls the flow of air through the fume
26 hood in a manner whereby the effective size of the total
27 opening to the fume hood, including the portion of the
28 opening that is not covered by one or more sash doors will
29 have a relatively constant average face velocity of air
moving into the fume hood. This means that regardless of
31 the area of the uncovered opening, an average volume of air
32 per unit of surface area of the uncovered portion will be
33 moved into the fume hood. This protects the persons in the
34 laboratory from being exposed to noxious fumes or the like
because air is always flowing into the fume hood, and out
36 of the exhaust duct, and the flow is preferably controlled
37 at a predetermined rate of approximately 75 to 125 cubic
38 feet per minute per square feet of effective surface area
1 2 6
1 of the uncovered opening. In other words, if the sash door
2 or doors are moved to the maximum open position whereby an
3 operator has the maximum access to the inside of the fume
4 hood for conducting experiments or the like, then the flow
of air will most likely have to be increased to maintain
6 the average face velocity at the predetermined desired
7 level. The capabilities and effectiveness of various
8 controllers of the prior art varies considerably.
9 Broadly stated, the present invention is directed
to an improved fume hood controlling apparatus that is
11 adapted to provide many desirable operational advantages
12 for person which use the fume hoods to perform experiments
13 or other work, and also for the operator of the facility in
14 which the fume hoods are located. The apparatus embodying
the present invention provides extremely rapid and
16 effective control of the average face velocity of the fume
17 hood, and achieves and maintains the desired average face
18 velocity within a few seconds after one or more doors which
19 cover the front opening of the fume hood have been moved.
Also, if other perturbations occur within the laboratory
21 environment where one or more fume hoods are located, the
22 control apparatus of the present invention is adapted to
23 quickly react and stabilize to maintain the desired flow
24 conditions.
Turning now to the drawings, and particularly
26 FIG. 1, a block diagram is shown of several fume hood
27 controllers 20 embodying the present invention
28 interconnected with a room controller 22, an exhaust
29 controller 24 and a main control console 26. The fume hood
controllers 20 are interconnected with the room controller
31 22 and with the exhaust controller 24 and the main control
32 console 26 in a local area network illustrated by line 28
33 which may be a multiconductor cable or the like. The room
34 controller, the exhaust controller 24 and the main control
console 26 are typically part of the building main HVAC
36 system in which the laboratory rooms containing the fume
37 hoods are located.
38 The room controller 22 preferably is of ~he type
-8- 2~126
1 which is at least capable of providing a variable air
2 volume to the room, and may be a Landis & Gyr Powers System
3 600 SCU controller. The room controller 22 is capable of
4 communicatin~ over the LAN lines 28. The room controller
preferably is a System 600 SCU controller and is a
6 commercially available controller for which extensive
7 documentation exists.
n The room controller 22 may also receive signals
~ via lines 81 from each of the fume hood controllers 20 that
provides an analog input signal indicating the volume of
11 air that is being exhausted by each of the fume hood
~2 controllers 20 and a comparable signal from the exhaust
13 flow sensor that provides an indication of the volume of
1~ air that is being exhausted through the main exhaust system
apart from the fume hood exhausts. These signals coupled
16 with signals that are supplied by a differential pressure
17 sensor 29 which indicates the pressure within the room
1~ relative to the reference space enable the room controller
1~ to control the supply of air that is necessary to maintain
the differential pressure within the room at a slightly
21 lower pressure than the reference space. Such a system is
~2 disclosed in cross referenced application, assigned to the
same assignee as the present application, entitled A system
~4 for controlling the differential pressure of a room having
laboratory fume hoods, by Ahmed, et al., Serial No.:
2~, 2,055,101.
27 The fume hood controllers 20 are provided with
2~ power through line 30, which is at the proper voltage via
29 a transformer 32 or the like.
Referring to FIG. 2, a fume hood controller 20 is
3~ illustrated with its input and output connector ports being
32 identified, and the fume hood controller 20 is connected to
33 an operator panel 34. It should be understood that each
34 fume hood will have a fume hood controller 20 and that an
operator panel will be provided with each fume hood
36 controller. The operator panel 34 is provided for each of
1 2 6
1 the fume hoods and it is interconnected with the fume hood
2 controller 20 by a line 36 which preferably comprises a
3 multi-conductor cable having eight conductors. The
4 operator panel has a connector 38, such as a 6 wire RJll
type telephone jack, for example, into which a lap top
6 personal computer or the like may be connected for the
7 purpose of inputting information relating to the
8 configuration or operation of the fume hood during initial
9 installation, or to change certain operating parameters if
necessary. The operator panel 34 is preferably mounted to
11 the fume hood in a convenient location adapted to be easily
12 observed by a person who is working with the fume hood.
13 The fume hood controller operator panel 34
14 includes a liquid crystal display 40 which when selectively
activated provides the visual indication of various aspects
16 of the operation of the fume hood, including three digits
17 42 which provide the average face velocity. The display 40
18 illustrates other conditions such as low face velocity,
19 high face velocity and emergency condition and an
indication of controller failure. The operator panel may
21 have an alarm 44, an emergency purge pushbutton 46 which an
22 operator can press to purge the fume hood in the event of
23 an accident. The operator panel has two auxiliary switches
24 48 which can be used for various customer needs, including
day/night modes of operation. It is contemplated that
26 night time mode of operation would have a different and
27 preferably reduced average face velocity, presumably
28 because no one would be working in the area and such a
29 lower average face velocity would conserve energy. An
alarm silence switch 50 is also preferably provided.
31 Fume hoods come in many different styles, sizes
32 and configurations, including those which have a single
33 sash door or a number of sash doors, with the sash doors
34 being moveable vertically, horizontally or in both
direction. Additionally, various fume hoods have different
36 amounts of by-pass flow, i.e., the amount of flow
37 permitting opening that exists even when all of the sash
38 doors are as completely closed as their design permits.
-lo- 2d~ 512~
1 Other design considerations involve whether there is some
2 kind of filtering means included in the fume hood for
3 confining fumes within the hood during operation. While
4 many of these design considerations must be taken into
account in providing efficient and effective control of the
6 fume hoods, the apparatus of the present invention can be
7 configured to account for virtually all of the above
8 described design variables, and effective and extremely
9 fast control of the fume hood ventilation is provided.
Referring to FIG. 3, there is shown a fume hood,
11 indicated generally at 60, which has a vertically operated
12 sash door 62 which can be moved to gain access to the fume
13 hood and which can be moved to the substantially closed
14 position as shown. Fume hoods are generally designed so
that even when a door sash such as door sash 62 is
16 completely closed, there is still some amount of opening
17 into the fume hood through which air can pass. This
18 opening is generally referred to as the bypass area and it
19 can be determined so that its effect can be taken into
consideration in controlling the flow of air into the fume
21 hood. Some types of fume hoods have a bypass opening that
22 is located above the door sash while others are below the
23 same. In some fume hoods, the first amount of movement of
24 a sash door will increase the opening at the bottom of the
door shown in FIG. 3 for example, but as the door is
26 raised, it will merely cut off the bypass opening so that
27 the effective size of the total opening of the fume hood is
28 maintained relatively constant for perhaps the first one-
29 fourth amount of movement of the sash door 62 through its
course of travel.
31 Other types of fume hoods may include several
32 horizontally moveable sash doors 66 such as shown in FIGS.
33 4 and 5, with the doors being movable in upper and lower
34 pairs of adjacent tracks 68. When the doors are positioned
as shown in FIGS. 4 and 5, the fume hood opening is com-
36 pletely closed and an operator may move the doors in the
37 horizontal direction to gain access to the fume hood. Both
38 of the fumes hoods 60 and 64 have an exhaus~ duct 70 which
1 2 ~
1 generally extends to an exhaust system which may be that of
2 the HVAC apparatus previously described. The fume hood 64
3 also includes a filtering structure shown diagrammatically
4 at 72 which filtering structure is intended to keep noxious
S fumes and other contaminants from exiting the fume hood
6 into the exhaust system. Referring to FIG. 6, there is
7 shown a combination fume hood which has horizontally
8 movable doors 76 which are similar to the doors 66, with
9 the fume hood 74 having a frame structure 78 which carries
the doors 76 in suitable tracks and the frame structure 78
11 is also vertically movable in the opening of the fume hood.
12 The illustration of FIG. 6 has portions removed
13 as shown by the break lines 73 which is intended to
14 illustrate that the height of the fume hood may be greater
than is otherwise shown so that the frame structure 78 may
16 be raised sufficiently to permit adequate access to the
17 interior of the fume hood by a person. There is generally
18 a by-pass area which is identified as the vertical area 75,
19 and there is typically a top lip portion 77 which may be
approximately 2 inches wide. This dimension is preferably
21 defined so that its effect on the calculation of the open
22 face area can be taken into consideration.
23 While not specifically illustrated, other com-
24 binations 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 In accordance with an important aspect of the
present invention, the fume hood controller 20 is adapted
31 to operate the fume hoods of various sizes and configura-
32 tions as has been described, and it is also adapted to be
33 incorporated into a laboratory room where several fume
34 hoods may be located and which may have exhaust ducts which
merge into a common exhaust manifold which may be a part of
36 the building HVAC system. A fume hood may be a single
37 self-contained installation and may have its own separate
38 exhaust duct. In the event that a single fume hood is
-12- ~ 1 2 ~
1 installed, it is typical that such an installation would
2 have a variable speed motor driven blower associated with
3 the exhaust duct whereby the speed of the motor and blower
4 can be variably controlled to thereby adjust the flow of
S air through the fume hood. Alternatively, and most typical
6 for multiple fume hoods in a single area, the exhaust ducts
7 of each fume hood are merged into one or more larger
8 exhaust manifolds and a single large blower may be provided
9 in the manifold system. In such types of installations,
control of each fume hood is achieved by means of separate
11 dampers located in the exhaust duct of each fume hood, so
12 that variation in the flow can be controlled by
13 appropriately positioning the damper associated with each
14 fume hood.
The fume hood controller is adapted to control
16 virtually any of the various kinds and styles of fume hoods
17 that are commercially available, and to this end, it has a
18 number of input and output ports (lines, connectors or
19 connections, all considered to be equivalent for the
purposes of describing the present invention) that can be
21 connected to various sensors that may be used with the
22 controller. As shown in FIG. 2, it has digital output or
23 DO ports which interface with a digital signal/analog
24 pressure transducer with an exhaust damper as previously
described, but it also has an analog voltage output port
26 for controlling a variable speed fan drive if it is to be
27 installed in that manner. There are five sash position
28 sensor ports for use in sensing the position of both
29 horizontally and vertically moveable sashes and there is
also an analog input port provided for connection to an
31 exhaust air flow sensor 49. A digital input port for the
32 emergency switch is provided and digital output ports for
33 outputting an alarm horn signal as well as an auxiliary
34 signal is provided. An analog voltage output port is also
provided for providing a volume of flow signal to the room
36 controller 22. In certain applications where the exhaust
37 air flow sensor is not provided, a wall velocity sensor
J~ indic~ve ~f fac~ ~/eloc~ty r~y he utili~d and ~ ~np~t
-13- ~a-J512 6
1 port for such a signal is provided, but the use of such
2 sensors is generally considered to be less accurate and is
3 not the preferred embodiment. With these various input and
4 output ports, virtually any type of fume hood can be
controlled in an effective and efficient manner.
6 From the foregoing discussion, it should be
7 appreciated that if the desired average face velocity is to
8 be maintained and the sash position is changed, the size of
9 the opening can be dramatically changed which may then
require a dramatic change in the volume of air to maintain
11 the average face velocity. While it is known to control a
12 variable air volume blower as a function of the sash
13 position, the fume hood controller apparatus of the present
14 invention improves on that known method by incorporating
additional control schemes which dramatically improve the
16 capabilities of the control system in terms of maintaining
17 relatively constant average face velocity in a manner
18 whereby reactions to perturbations in the system are
19 quickly made.
To determine the position of the sash doors, a
21 sash position sensor is provided adjacent each movable sash
22 door and it is generally illustrated in FIGS. 7, 8 and 9.
23 Referring to FIG. 8, the door sash position indicator
24 comprises an elongated switch mechanism 80 of relatively
simple mechanical design which preferably consists of a
26 relatively thin polyester base layer 82 upon which is
27 printed a strip of electrically resistive ink 84 of a known
28 constant resistance per unit length. Another polyester
29 base layer 86 is provided and it has a strip of
electrically conductive ink 88 printed on it. The two base
31 layers 82 and 86 are adhesively bonded to one another by
32 two beads of adhesive 90 located on opposite sides of the
33 strip. The base layers are preferably approximately five-
34 thousandths of an inch thick and the beads are
approximately two-thousandths of an inch thick, with the
36 beads providing a spaced area between the conductive and
37 resistive layers 88 and 84. The switching mechanism 80 is
38 preferably applied to the fume hood by a layer of adhesive
-14- XO.~512~
1 92.
2 The polyester material is sufficiently flexible
3 to enable one layer to be moved toward the other in
4 response to an actuator 94 carried by the appropriate door
sash to which the strip is placed adjacent to so that when
6 the door sash is moved, the actuator 94 moves along the
7 switching mechanism 80 and provides contact between the
8 resistive and conductive layers which are then sensed by
9 electrical circuitry to be described which provides a
voltage output that is indicative of the absolute position
11 of the actuator 94 along the length of the switching means.
12 Stated in other words, the actuator 94 is carried by the
13 door and therefore provides an electrical voltage that is
14 indicative of the absolute position of the door sash.
The actuator 94 is preferably spring biased
16 toward the switching mechanism 80 so that as the door is
17 moved, sufficient pressure is applied to the switching
18 means to bring the two base layers together so that the
19 resistive and conductive layers make electrical contact
with one another and if this is done, the voltage level is
21 provided. By having the switching means 80 of sufficient
22 length so that the full extent of the travel of the sash
23 door is provided as shown in FIG. 3, then an accurate
24 determination of the sash position can be made. It should
be understood that the illustration of the switching
26 mechanism 80 in FIGS. 3 and 5 is intended to be
27 diagrammatic, in that the switching mechanism is preferably
28 actually located within the sash frame itself and
29 accordingly would not be visible as shown. The width and
thickness dimensions of the switching mechanism are so
31 small that interference with the operation of the sash door
32 is virtually no problem. The actuator 94 can also be
33 placed in a small whole that may be drilled in the door or
34 it may be attached externally at one end thereof so that it
can be in position to operate the switch 80. In the
36 vertical moveable sash doors shown in FIGS. 3 and 6, a
37 switching mechanism 80 is preferably provided in one or the
38 other of the sides of the sash frame, whereas in the fume
~0~5126
1 hoods having horizontally movable doors, it is preferred
2 that the switching mechanism 80 be placed in the top of the
3 tracks 68 so that the weight of the movable doors do not
4 operate the switching mechanism 80 or otherwise damage the
same. It is also preferred that the actuator 94 is located
6 at one end of each of the doors for reasons that are
7 described in the cross-referenced application entitled
8 Apparatus for determining the position of a moveable
9 structure along a track, by Egbers et al., Serial No.:
2,055,258-
11 Turning to FIG. 9, the preferred electrical12 circuitry which generates the position indicating voltage
13 is illustrated, and this circuitry is adapted to provide
14 two separate voltages indicating the absolute position of
two door sashes in a single track. With respect to the
16 cross-section shown in FIG. 5, there are two horizontal
17 tracks, each of which carries two door sashes and a
18 switching mechanism 80 is provided for each of the tracks
l9 as is a circuit as shown in FIG. 9, thereby providing a
distinct voltage for each of the four sash doors as shown.
21 The switching means is preferably applied to the
22 fume hood with a layer of adhesive 92 and the actuator 94
23 is adapted to bear upon the switching means at locations
24 along the length thereof. Referring to FIG. 7, a
diagrammatic illustration of a pair of switching means is
26 illustrated such as may occur with respect to the two
27 tracks shown in FIG. 5. A switching mechanism 80 is
28 provided with each track and the four arrows illustrated
29 represent the point of contact created by the actuators 94
which result in a signal being applied on each of the ends
31 of each switching means, with the magnitude of the signal
32 representing a voltage that is proportional to the distance
33 between the end and the nearest arrow. Thus, a single
34 switching mechanism 80 is adapted to provide absolute
position indicating signals for two doors located in each
36 track. The circuitry that is used to accomplish the
37 voltage generation is shown in FIG. 9 and includes one of
38 th~se circuits for each track. The resistive element is
A
-16- ~ 0~
1 shown at 84 and the conductive element 88 is also
2 illustrated being connected to ground with two arrows being
3 illustrated, and represented the point of contact between
4 the resistive and conductive elements caused by each of the
actuators 94 associated with the two separate doors. The
6 circuitry includes an operational amplifier 100 which has
7 its output connected to the base of a PNP transistor 102,
8 the emitter of which is connected to a source of positive
9 voltage through resistor 104 into the negative input of the
operational amplifier, the positive input of which is also
11 connected to a source of positive voltage of preferably
12 approximately five volts. The collector of the transistor
13 102 is connected to one end of the resistive element 84 and
14 has an output line 106 on which the voltage is produced
that is indicative of the absolute position of the door.
16 The circuit operates to provide a constant
17 current directed into the resistive element 84 and this
18 current results in a voltage on line 106 that is propor-
19 tional to the resistance value between the collector and
ground which changes as the nearest point of contact along
21 the resistance changes. The operational amplifier operates
22 to attempt to drive the negative input to equal the voltage
23 level on the positive input and this results in the current
24 applied at the output of the operational amplifier varying
in direct proportion to the effective length of the
26 resistance strip 84. The lower portion of the circuitry
27 operates the same way as that which has been described and
28 it similarly produces a voltage on an output line 108 that
29 is proportional to the distance between the connected end
of the resistance element 84 and the point of contact that
31 is made by the actuator 94 associated with the other sash
32 door in the track.
33 Referring to the composite electrical schematic
34 diagram of the circuitry of the fume hood controller, if
the separate drawings FIGS. lOa, lOb, lOc, lOd and lOe are
36 placed adjacent one another in the manner shown in FIG. 10,
37 the total electrical schematic diagram of the fume hood
38 controller 20 is illustrated. The operation of the
-17- ~ 512~
1 circuitry of FIGS. 10a through 10e will not be described in
2 detail. The circuitry is driven by a microprocessor and
3 the important algorithms that carry out the control
4 functions of the controller will be hereinafter described.
Referring to FIG. 10c, the circuitry includes a Motorola MC
6 68HCll microprocessor 120 which is clocked at 8 MHz by a
7 crystal 122. The microprocessor 120 has a databus 124 that
8 is connected to a tri-state buffer 126 (FIG. 10d) which in
9 turn is connected to an electrically programmable read only
memory 128 that is also connected to the databus 124. The
11 EPROM 128 has address lines A0 through A7 connected to the
12 tri-state buffer 126 and also has address lines A8 through
13 A14 connected to the microprocessor 120.
14 The circuitry includes a 3 to 8-bit multiplexer
130, a data latch 132 (see FIG. 10d), a digital-to-analog
16 converter 134, which is adapted to provide the analog
17 outputs indicative of the volume of air being exhausted by
18 the fume hood, which information is provided to room
19 controller 22 as has been previously described with respect
to FIG. 2. Referring to FIG. 10b, an RS232 driver 136 is
21 provided for transmitting and receiving information through
22 the hand held terminal. The circuitry illustrated in FIG.
23 9 is also shown in the overall schematic diagrams and is in
24 FIGS. 10a and 10b. The other components are well known and
therefore need not be otherwise described.
26 As previously mentioned, the apparatus of the
27 present invention utilizes a flow sensor preferably located
28 in the exhaust duct 70 to measure the air volume that is
29 being drawn through the fume hood. The volume flow rate
may be calculated by measuring the differential pressure
31 across a multi-point pitot tube or the like. The preferred
32 embodiment utilizes a differential pressure sensor for
33 measuring the flow through the exhaust duct and the
34 apparatus of the present invention utilizes control schemes
to either maintain the flow through the hood at a predeter-
36 mined average face velocity, or at a minimum velocity in
37 the event the fume hood is closed or has a very small
38 byp3~s area.
Xi~512~
-18-
1 The fume hood controller of the present invention
2 can be configured for almost all known types of fume hoods,
3 including fume hoods having horizontally movable sash
4 doors, vertically movable sash doors or a combination of
the two. As can be seen from the illustrations of FIGS. 2
6 and 10, the fume hood controller is adapted to control an
7 exhaust damper or a variable speed fan drive, the
8 controller being adapted to output signals that are
9 compatible with either type of control. The controller is
also adapted to receive information defining the physical
11 and operating characteristics of the fume hood and other
12 initializing information. This can be input into the fume
13 hood controller by means of the hand held terminal which is
14 preferably a lap top computer that can be connected to the
operator panel 34. The information that should be provided
16 to the controller include the following, and the dimensions
17 for the information are also shown. It should be appre-
18 ciated that the day/night operation may be provided, but is
19 not the preferred embodiment of the system; if it is pro-
vided, the information relating to such day/night operation
21 should be included.
22 Operational information:
23 5. Time of day;
24 6. Set day and night values for the average
face velocity (SVEL), feet per minute or
26 meters per second;
27 7. Set day and night values for the minimum
28 flow, (MINFLO), in cubic feet per minute;
29 8. Set day and night values for high velocity
limit (HVEL), F/m or M/sec;
31 9. Set day and night values for low velocity
32 limit (LVEL), F/m or M/sec;
33 10. Set day and night values for intermediate
34 high velocity limit (MVEL), F/m or M/sec;
11. Set day and night values for intermediate
36 low velocity limit (IVEL), F/m or M/sec;
37 12. Set the proportional gain factor (KP),
38 analog output per error in percent;
2~.5~12~
--19--
1 13. Set the integral gain factor (KI), analog
2 output multiplied by time in minutes per
3 error in percent;
4 14. Set derivative gain factor (KD), analog
output multiplied by time in minutes per
6 error in percent;
7 15. Set feed forward gain factor (KF) if a
8 variable speed drive is used as the control
9 equipment instead of a damper, analog output
per CFM;
11 16. Set time in seconds (DELTIME) the user
12 prefers to have the full exhaust flow in
13 case the emergency button is activated;
14 17. Set a preset percent of last exhaust flow
(SAFLOQ) the user wishes to have once the
16 emergency switch is activated and DELTIME is
17 expired.
18 The above information is used to control the mode
19 of operation and to control the limits of flow during the
day or night modes of operation. The controller includes
21 programmed instructions to calculate the steps in para-
22 graphs 3 through 7 in the event such information is not
23 provided by the user. To this end, once the day and night
24 values for the average face velocity are set, the con-
troller 20 will calculate high velocity limit at 120% of
26 the average face velocity, the low velocity limit at 80%
27 and the intermediate limit at 90%. It should be understood
28 that these percentage values may be adjusted, as desired.
29 Other information that should be input include the
following information which relates to the physical con-
31 struction of the fume hood. It should be understood that
32 some of the information may not be required for only
33 vertically or horizontally moveable sash doors, but all of
34 the information may be required for a combination of the
same. The information required includes vertical segments,
36 which is defined to be a height and width dimension that
37 may be covered by one or more sash doors. If more than one
~^~ s~sh d~or is provi~ed for ea~h se~mer~ ~h~ r~
5 12 6
-20-
1 intended to be vertically moveable sash doors, analogous to
2 a double sash residential window. The information to be
3 provided includes the following:
4 18. Input the number of vertical segments;
S 19. Input the height of each segment, in inches;
6 20. Input the width of each segment, in inches;
7 21. Input the number of tracks per segment;
8 22. Input the number of horizontal sashes per
9 track;
23. Input the maximum sash height, in inches;
11 24. Input the sash width, in inches;
12 25. Input the location of the sash sensor from
13 left edge of sash, in inches;
14 26. Input the by-pass area per segment, in
square inches;
16 27. Input the minimum face area per segment, in
17 square inches;
18 28. Input the top lip height above the
19 horizontal sash, in inches;
The fume hood controller 20 is programmed to
21 control the flow of air through the fume hood by carrying
22 out a series of instructions, an overview of which is
23 contained in the flow chart of FIG. 11. After start-up and
24 outputting information to the display and determining the
time of day, the controller 20 reads the initial sash
26 positions of all doors (block 150), and this information is
27 then used to compute the open face area (block 152). If
28 not previously done, the operator can set the average face
29 velocity set point (block 154) and this information is then
used together with the open face area to compute the
31 exhaust flow set point (SFLOW) (block 156) that is
32 necessary to provide the predetermined average face
33 velocity given the open area of the fume hood that has been
34 previously measured and calculated. The computed fume hood
exhaust set point is then compared (block 158) with a
36 preset or required minimum flow, and if computed set point
37 is less than the minimum flow, the controller sets the set
~8 point flow at the preset minimum flow ~block 160). Tf it
-21- ~ 51~
1 is more than the minimum flow, then it is retained (block
2 162) and it is provided to both of the control loops.
3 If there is a variable speed fan drive for the
4 fume controller, i.e., several fume hoods are not connected
to a common exhaust duct and controlled by a damper, then
6 the controller will run a feed-forward control loop (block
7 164) which provides a control signal that is sent to a
8 summing junction 166 which control signal represents an
9 open loop type of control action. In this control action,
a predicted value of the speed of the blower is generated
11 based upon the calculated opening of the fume hood, and the
12 average face velocity set point. The predicted value of
13 the speed of the blower generated will cause the blower
14 motor to rapidly change speed to maintain the average face
velocity. It should be understood that the feed forward
16 aspect of the control is only invoked when the sash
17 position has been changed and after it has been changed,
18 then a second control loop performs the dominant control
19 action for maintaining the average face velocity constant
in the event that a variable speed blower is used to
21 control the volume of air through the fume hood.
22 After the sash position has been changed and the
23 feed forward loop has established the new air volume, then
24 the control loop switches to a proportional integral
derivative control loop and this is accomplished by the set
26 flow signal being provided to block 168 which indicates
27 that the controller computes the error by determining the
28 absolute value of the difference between the set flow
29 signal and the flow signal as measured by the exhaust air
flow sensor in the exhaust duct. Any error that is
31 computed is applied to the control loop identified as the
32 proportional-integral-derivative control loop (PID) to
33 determine an error signal (block 170) and this error signal
34 is compared with the prior error signal from the previous
sample to determine if that error is less than a deadband
36 error (block 172). If it is, then the prior error signal
37 is maintained as shown by block 174, but if it is not, then
3~ the new errcr signal is provided to outpvt mode 176 and it
5 1 2 ~
-22-
1 is applied to the summing junction 166. That summed error
2 is also compared with the last output signal and a
3 determination is made if this is within a deadband range
4 (block 180) which, if it is, results in the last or
previous output being retained (block 182). If it is
6 outside of the deadband, then a new output signal is
7 provided to the damper control or the blower (block 184).
8 In the event that the last output is the output
9 as shown in block 182, the controller then reads the
measured flow (MFLOW) (block 186) and the sash positions
11 are then read (block 188) and the net open face area is
12 recomputed (block 190) and a determination made as to
13 whether the new computed area less the old computed area is
14 less than a deadband (block 192) and if it is, then the old
area is maintained (block 194) and the error is then
16 computed again (block 168). If the new area less the old
17 area is not within the deadband, then the controller
18 computes a new exhaust flow set point as shown in block
19 156.
One of the significant advantages of the present
21 invention is that the controller is adapted to execute the
22 control scheme in a repetitive and extremely rapid manner.
23 The exhaust sensor provides flow signal information that is
24 inputted to the microprocessor at a speed of approximately
one sample per 100 milliseconds and the control action
26 described in connection with FIG. 11 is completed appro-
27 ximately every 100 milliseconds. The sash door position
28 signals are sampled by the microprocessor every 200
29 milliseconds. The result of such rapid repetitive sampling
and executing of the control actions results in extremely
31 rapid operation of the controller. It has been found that
32 movement of the sash will result in adjustment of the air
33 flow so that the average face velocity is achieved within
34 a time period of only approximately 3-4 seconds after the
sash door reposition has been stopped. This represents a
36 dramatic improvement over existing fume hood controllers.
37 In the event that the feed forward control loop
?Q is ut~ d! the se~e~e of instr~ctions that are ca-r;ed
~5512~
-23-
1 out to accomplish running of this loop is shown in the flow
2 chart of FIG. 12, which has the controller using the
3 exhaust flow set point (SFLOW) to compute the control
4 output to a fan drive (block 200), which is identified as
signal AO that is computed as an intercept point plus the
6 set flow multiplied by a slope value. The intercept is the
7 value which is a fixed output voltage to a fan drive and
8 the slope in the equation correlates exhaust flow rate and
9 output voltage to the fan drive. The controller then reads
the duct velocity (DV) (block 202), takes the last duct
11 velocity sample (block 204) and equates that as the duct
12 velocity value and starts the timing of the maximum and
13 minimum delay times (block 206) which the controller uses
14 to insure whether the duct velocity has reached steady
state or not. The controller determines whether the
16 maximum delay time has expired (block 208), and if it has,
17 provides the output signal at output 210. If the max delay
18 has not expired, the controller determines if the absolute
19 value of the difference between the last duct velocity
sample and the current duct velocity sample is less than or
21 equal to a dead band value (block 212). If it is not less
22 than the dead band value, the controller then sets the last
23 duct value as equal to the present duct value sample (block
24 214) and the controller then restarts the minimum delay
timing function (block 216). Once this is accomplished,
26 the controller again determines whether the max delay has
27 expired (block 208). If the absolute value of the
28 difference between the last duct velocity and the present
29 duct velocity sample is less than the dead band, the
controller determines whether the minimum delay time has
31 expired. If it has as shown from block 218, the output is
32 provided at 210. If it has not, then it determines if the
33 max delay has expired.
34 Turning to the proportional-integral-derivative
or PID control loop, the controller runs the PID loop by
36 carrying out the instructions shown in the flow chart of
37 FIG. 13. The controller uses the error that is computed by
38 block 168 (see FIG. 11) in three separate paths. wit~A
'~0~12~
-24-
1 respect to the upper path, the controller uses the
2 preselected proportional gain factor (block 220) and that
3 proportional gain factor is used together with the error to
4 calculate the proportional gain (block 222) and the
proportional gain is output to a summing junction 224.
6 The controller also uses the error signal and
7 calculates an integral term (block 226) with the integral
8 term being equal to the prior integral sum (ISUM) plus the
9 product of loop time and any error and this calculation is
compared to limits to provide limits on the term. The term
11 is then used together with the previously defined integral
12 gain constant (block 230) and the controller then
13 calculates the integral gain (block 232) which is the
14 integral gain constant multiplied by the integration sum
term. The output is then applied to the summing junction
16 224.
17 The input error is also used by the controller to
18 calculate a derivative gain factor which is done by the
19 controller using the previously defined derivative gain
factor from block 234 which is used together with the error
21 to calculate the derivative gain (block 236) which is the
22 reciprocal of the time in which it is required to execute
23 the PID loop multiplied by the derivative gain factor
24 multiplied by the current sample error minus the previous
sample error with this result being provided to the summing
26 junction 224.
27 The control action performed by the controller 20
28 as illustrated in FIG. 13 provides three separate gain
29 factors which provide steady state correction of the air
flow through the fume hood in a very fast acting manner.
31 The formation of the output signal from the PID control
32 loop takes into consideration not only the magnitude of the
33 error, but as a result of the derivative gain segment of
34 control, the rate of change of the error is considered and
the change in the value of the gain is proportional to the
36 rate of change. Thus, the derivative gain can see how fast
37 the actual condition is changing and works as an
38 "anticipator" in order to minimize error between the ac~aJ
~0~512~
1 and desired condition. The integral gain develops a cor-
2 rection signal that is a function of the error integrated
3 over a period of time, and therefore provides any necessary
4 correction on a continuous basis to bring the actual
condition to the desired condition. The proper combina-
6 tions of proportional, integral and derivative gains will
7 make the loop faster and reach the desired conditions
8 without any overshoot.
9 A significant advantage of the PID control action
is that it will compensate for perturbations that may be
11 experienced in the laboratory in which the fume hood may be
12 located in a manner in which other controllers do not. A
13 common occurrence in laboratory rooms which have a number
14 of fume hoods that are connected to a common exhaust
manifold, involves the change in the pressure in a fume
16 hood exhaust duct that was caused by the sash doors being
17 moved in another of the fume hoods that is connected to the
18 common exhaust manifold. Such pressure variations will
19 affect the average face velocity of those fume hoods which
had no change in their sash doors. However, the PID
21 control action may adjust the air flow if the exhaust duct
22 sensor determines a change in the pressure. To a lesser
23 degree, there may be pressure variations produced in the
24 laboratory caused by opening of doors to the laboratory
itself, particularly if the differential pressure of the
26 laboratory room is maintained at a lesser pressure than a
27 reference space such as the corridor outside the room, for
28 example.
29 It is necessary to calibrate the feed forward
control loop and to this end, the instructions illustrated
31 in the flow chart of FIG. 14 are carried out. When the
32 initial calibration is accomplished, it is preferably done
33 through the hand held terminal that may be connected to the
34 operator panel via connector 38, for example. The
controller then determines if the feed forward calibration
36 is on (block 242) and if it is, then the controller sets
37 the analog output of the fan drive to a value of 20 percent
38 of the maximum value, which is identified as value AOl
-26- ~J~512~
1 (block 244). The controller then sets the last sample duct
2 velocity (LSDV) as the current duct velocity (CDV) (block
3 246) and starts the maximum and minimum timers (block 248).
4 The controller ensures the steady state duct velocity in
the following way. First by checking whether the max timer
6 has expired, and then, if the max timer has not expired,
7 the controller determines if the absolute value of the last
8 sample duct velocity minus the current duct velocity is
9 less than or equal to a dead band (block 270), and if it
is, the controller determines if the min timer has expired
11 (block 272). If it has not, the controller reads the
12 current duct velocity (block 274). If the absolute value
13 of the last sample duct velocity minus the current duct
14 velocity is not less than or equal to a dead band (block
270), then the last sample duct velocity is set as the
16 current duct velocity (block 276) and the mintimer is
17 restarted (block 278) and the current duct velocity is
18 again read (block 274). In case either the max timer or
19 min timer has expired, the controller then checks the last
analog output value to the fan drive (252) and inquires
21 whether the last analog output value was 70 percent of the
22 maximum output value (block 254). If it is not, then it
23 sets the analog output value to the fan drive at 70 percent
24 of the max value A02 (block 256) and the steady state duct
velocity corresponding to A01. The controller then repeats
26 the procedure of ensuring steady state duct velocity when
27 analog output is A02 (block 258). If it is at the 70
28 percent of max value, then the duct velocity corresponds to
29 steady state velocity of A02 (block 258). Finally, the
controller (block 262) calculates the slope and intercept
31 values.
32 The result of the calibration process is to
33 determine the duct flow at 20% and at 70% of the analog
34 output values, and the measured flow enables the slope and
intercept values to be determined so that the feed forward
36 control action will accurately predict the necessary fan
37 speed when sash door positions are changed.
38 The fume hood controller is adapted to rapidly
.5 1 ~ ~
-27-
1 calculate on a periodic basis several times per second, the
2 uncovered or open area of a fume hood access opening that
3 is capable of being covered by one or more sash doors as
4 previously described. As is shown in FIG. 6, the actuator
94 is preferably located at the righthand end of each of
6 the horizontally movable doors of which there are four in
7 number as illustrated. The position indicating capability
8 of the switching mechanism 80 provides a signal having a
9 voltage level for each of the four doors which is
indicative of the position of the particular sash door
11 along its associated track. While the actuators 94 are
12 shown at the righthand portion of the sash doors, it should
13 be understood that they may be alternatively located on the
14 lefthand portion, or they could be located at virtually any
location on each door, provided that the relationship
16 between the width of the door and the position of the
17 actuator is determined and is input into the fume hood
18 controller. It should be appreciated that having the
19 location of the actuators 94 at a common position, such as
the right end, simplifies the calculation of the uncovered
21 opening.
22 While the fume hood shown in FIG. 6 is of the
23 type which has four horizontally movable doors 76 that are
24 housed within a frame structure 78 that itself is
vertically movable, the fume hood controller apparatus of
26 the present invention is adapted to be used with up to four
27 movable sash doors in a single direction, i.e.,
28 horizontally, and a perpendicularly movable sash door
29 frame. However, there are five analog input ports in the
controller for inputting position information regardless of
31 whether it is horizontal or vertical and the controller can
32 be configured to accommodate any combination of
33 horizontally and vertically movable doors up to a total of
34 five. To this end, it should be appreciated that there are
vertically movable double sash doors in certain
36 commercially available fume hoods, which configuration is
37 not specifically shown in the drawings, with the double
38 sash confi~uration being housed in a single frame structure
Z~5126
-28-
1 that itself may be horizontally movable. The fume hood
2 controller of the present invention may treat the double
3 sash door configuration in the vertical direction much the
4 same as it operates with the horizontally movable sash
doors that operate in two tracks as shown in FIG. 6.
6 Turning now to FIG. 15, the flow chart for the
7 fume hood controller operation as it calculates the
8 uncovered portion of the opening of the fume hood as
9 illustrated for the embodiment of FIG. 6 with respect to
the four horizontally movable doors. The flow chart
11 operation would also be applicable for determining the
12 uncovered area for the embodiment of FIG. 4 as well. The
13 initial step is to read each sash door position (block 300)
14 and this is done by sorting the sash doors to determine the
sash door positions relative to the left edge of the
16 opening (block 302). It should be understood that the
17 determination could be made from the right edge just as
18 easily, but the left edge has conveniently been chosen.
19 The apparatus then initializes the open area 304 as being
equal to zero and then the apparatus computes the distance
21 between the right edge of the sash door nearest the left
22 edge of the opening and the right edge of the next sash
23 door that is adjacent to it (block 306).
24 If the difference between the edges, as
determined by the actuator location, is greater than the
26 width of the sash (block 308), then the net open area is
27 set to be equal to the net open area plus the difference
28 minus the sash door width (block 310) and this value is
29 stored in memory. If the difference is less than the sash
door width, then the program proceeds to repeat for the
31 next two pair of sash doors (block 312) as shown. In
32 either event, then the program similarly repeats for the
33 next two pair of sash doors. After the controller performs
34 its repetitions to calculate any open area between all of
the sash doors, then the controller checks the distance
36 between the right edge of the nearest sash door and the
37 left track edge which is comparable to the left opening
3~ (block 314) and if the left difference is less than the
~Q~512~
-29-
1 sash door width (block 316) the controller then checks the
2 distances between the left edge of the furthest sash door
3 and the right edge of the track, i.e., the right opening
4 318. If the left difference is not less than the sash door
width, then the net open area is determined to be equal to
6 the net open area plus any left difference (block 320).
7 The controller then determines if the right
8 difference is less than the sash width (block 322) which,
9 if it is, results in the net face area being equal to the
net open area plus the fixed area (block 324) with the
11 fixed area being the preprogrammed bypass area, if any. If
12 the right difference is not less than the sash width, then
13 the controller determines that the net open area equals the
14 net open area plus the right difference (block 326). In
this way, the net open area is determined to be the
16 addition between any open areas between sash doors and
17 between the rightward sash door and the right edge of the
18 opening and the difference between the left edge of the
19 leftmost sash door and the left edge of the opening.
Turning now to FIG. 16, a flow chart of operation
21 of the apparatus for determining the uncovered area of the
22 opening for a fume hood which has multiple vertically
23 moveable sash doors is shown. The controller, when
24 initially configured, requires the input of the width of
each segment, the number of such segments, the minimum face
26 area, i.e., the bypass area, plus any other residual open
27 area with the sash doors closed, and the number of sash
28 doors per segment (block 330). The controller then sets
29 the area equal to zero (block 332) and begins the
calculation for the first segment (block 334) and sets the
31 old height equal to zero (block 336). It then begins with
32 the first sash door (block 338) and reads the sash position
33 (block 340), inputs the slope and intercept (block 342)
34 from the prior calibration routine, and calculates the
height for that sash door and segment (block 344). The
36 apparatus then determines if it is sash door number 1,
37 which if it is, forwards the height of the segment (block
3~ ob~ ~ins t~e w~th of the segment ~b~eck 3sn! <~
~f~5126
-30-
1 calculates the area by multiplying the height times the
2 width (block 358). If the sash door was not the number 1
3 sash, then the controller determines if the height of the
4 segment and sash was less than the old height, which if it
is, then the height of the segment is set as the height
6 (block 352) and the next sash door is made the subject of
7 inquiry (block 354) and the old height is retrieved (block
8 356) and the controller returns to block 338 to repeat the
9 calculations for the other segments and sash doors. After
the sash doors for a segment have been considered, and the
11 area of the segment determined (block 358), the controller
12 determines if the area for the segment is less than the
13 minimum flow area, and if it is, then the area is set to
14 the minimum flow area (block 362). If it is greater than
the minimum flow area, then the area for the segment is
16 determined to be equal to the bypass area plus the
17 calculated area for the segment (block 364). The area is
18 then calculated as the prior calculated area plus the area
19 of the segment under consideration (block 366), and the
controller then proceeds to consider the next segment
21 (block 368). After all segments have been considered, the
22 total area is obtained (block 370).
23 In accordance with an important aspect of the
24 present invention, the apparatus is also adapted to
determine the uncovered area of a combination of vertically
26 and horizontally moveable sash doors, such as the fume hood
27 illustrated in FIG. 6, which has four horizontally moveable
28 sash doors that are contained in two sets of tracks, with
29 the sets of tracks being contained in a frame structure
which is itself vertically moveable. As previously
31 mentioned, there is an upper lip 77 having a front
32 thickness of about 2 inches, the exact dimension of which
33 can vary with the manufacturer's design, a lower portion 79
34 of the frame 78, and a bypass area 75. As may be
appreciated, when the frame 78 is in its lowermost
36 position, the entire bypass area is "open" and air may be
37 moved through it. As the frame is raised, the portion of
38 the sash doors 76 which cover the opening will increasingly
Z~ j~l26
1 cover the bypass area as shown. In the particular
2 illustration of FIG. 6, the horizontally moveable doors
3 overlap and are completely closed, but the frame is shown
4 being slightly raised.
To determine the uncovered area of the
6 combination sash door fume hood, the following specific
7 steps are performed. The net open area, i.e., the
8 uncovered area, is the sum of the vertical (hereinafter "V"
9 in the equations) area and the horizontal (hereinafter "H")
area:
11 Net Open Area = V area + H area
12 with the horizontal area being determined as follows:
13 H area = H width * minimum of {panel Ht; Max of
14 (panel Ht + top lip Ht + min. face Ht -
sash Ht; 0))
16 with the H width comprising the previously described
17 operation being performed with respect to the horizontally
18 moveable sash doors. The vertical area (V area) is
19 determined by the following equation:
V area = Max of (Sash Ht * V width; minimum face
21 area)
22 To complete the determination, the Net Face Area
23 is then equal to the sum of the Net Open Area and the Fixed
24 or bypass Area:
Net Face Area = Net Open Area + Fixed Area
26 From the foregoing detailed description, it
27 should be appreciated that an extremely effective and
28 efficient fume hood controller has been shown and described
29 that offers significant advantages over the current prior
art controllers. The fume hood controller has a design
31 that permits it to be configured to control virtually any
32 type of fume hood that is now available, and it has the
33 capability to very rapidly adjust the flow of air through
34 the fume hood to maintain the average face velocity at the
desired value even when sash positions are changed or other
36 perturbations occur, and such adjustment results in steady
~0~5126
1 state operation within approximately 3 to 4 seconds.
2 While various embodiments of the present
3 invention have been shown and described, it should be
4 understood that various alternatives, substitutions and
equivalents can be used, and the present invention should
6 only be limited by the claims and equivalents thereof.
7 Various features of the present invention are set forth in
8 the following claims.
