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Laboratory design parameters: laboratory layout as shown below, arranged 4 sets of 1524mm bench-mounted fume hoods, laboratory specific parameters are as follows:
Laboratory size = 18’ x 30’ x 10’ (5,400 ft3)=5.486m*9.144m*3.048m=152.9m3
1524mm bench-mounted fumehoods(4 set)=300CMHmin* 1500 CFM max*
offset = 360–1200 CMH*
Ventilation flow= 1530CMH* (ACPH = 10t/h)
Cooling flow = 2040CMH*
Design pressure= -0.001 “ H2O*=-0.25Pa
Design temperature= 72F=22.2℃
Room Pressure Control System:
(1) Model 8682 Adaptive Offset Control System mounted in the laboratory.
(2) A through-the-wall pressure sensor mounted between the corridor (referenced space) and laboratory (controlled space).
(3)Damper, pressure dependent VAV box or valve with actuator assembly mounted in supply air duct(s).
(4) Damper, pressure dependent VAV box or valve with actuator assembly mounted in exhaust air duct.
(5)Flow station mounted in supply air duct. (Required for non-venturi valve applications only.)
(6) Flow station mounted in general exhaust air duct. (Required for nonventuri valve applications only.)
(7) Flow stations mounted in fume hood exhaust duct. (Required for nonventuri valve applications only.)
Temperature Control System:
(1)Temperature Sensor (1000Platinum RTD) mounted in the laboratory.
(2) Reheat coil mounted in supply air duct(s).
Fume Hood Control System:
(1)Independent VAV Face Velocity Control system.
0.0725m2/HD1 DCT AREA 0.78 ft2 (12 in. round) Fume hood 1 duct area;
0.0725 m2/HD2 DCT AREA 0.78 ft2 (12 in. round) Fume hood 2 duct area;
0.0725 m2/HD3 DCT AREA 0.78 ft2 (12 in. round) Fume hood 3 duct area;
0.0725 m2/HD4 DCT AREA 0.78 ft2 (12 in. round) Fume hood 4 duct area;
0.0929 m2/EX1 DCT AREA 1.0 ft2 (12” *12”) General exhaust duct area;
0.3094m2/SP1 DCT AREA 3.33 ft2 (24”*20”) Supply duct area
MIN OFFSET 600CMH Minimum offset.
MAX OFFSET 1200CMH Maximum offset.
VENT MIN SET 1530CMH 10 air changes per hour;
COOLING FLOW 2040CMH Required flow to cool laboratory;
TEMP SETP 72 F/22.2℃ Temperature sensor switches from VENT MIN SET to COOLING FLOW.
-0.001 “ H2O*=-0.25Pa/ SET POINT –0.001 in. H2O Pressure differential set point.
Sequence Of Operation
Fume hood sashes are down, total hood exhaust is 1200CMH;
Supply air is 1530CMH(maintain ventilation);
General exhaust 690CMH(calculated from below).
Fume hoods + General exhaust - Offset = Supply air
1200CMH +? – 360CMH= 1530CMH
Fume hoods + General exhaust - Offset = Supply air
3600CMH + 0 – 360CMH =3240CMH
The supply air volume changes to 3240CMH (3600CMH hood exhaust –360CMH offset). The general exhaust is closed since no additional exhaust is needed for temperature or ventilation.
However, the Digital Interface Module indicates the laboratory is now -0.05Pa (not negative enough). The AOC algorithm slowly changes the offset until pressure control is maintained. In this case, the offset changes to 660CMH, which decreases the supply volume by 300CMH. The additional offset maintains the pressure differential at -0.25Pa (set point).
Fume hoods + General exhaust - Offset = Supply air
3600CMH + 0 – 660CMH =2940CMH
The hoods are shut after the experiments are loaded so the initial conditions prevail.
Fume hoods + General exhaust - Offset = Supply air
1200CMH +? – 360CMH= 1530CMH
An oven is turned on and the laboratory is getting warm. The temperature sensor sends the AOC a signal to switch to temperature minimum ventilation (COOLING FLOW). This increases the supply air to 2040CMH. The general exhaust air must also increase (damper opens) to maintain flow balance.
Fume hoods + General exhaust - Offset = Supply air
1200CMH + 1200CMH – 360CMH= 2040CMH
Three fume hoods are opened so the fume hood flow is now 4800 CMH. In order to
maintain air balance the general exhaust and supply air change to:
Fume hoods + General exhaust - Offset = Supply air
4800CMH + 0 CMH– 360CMH= 4440CMH
The control loop continuously keeps the room balance, room pressure, and temperature control satisfied.
