Technical Section
55
Temperature Control System:
(1)
Temperature Sensor (1000
Ω
Platinum RTD) mounted in the laboratory.
(2)
Reheat coil mounted in supply air duct(s).
Fume Hood Control System:
(1) Independent
S
URE
F
LOW
VAV Face Velocity Control system.
Based on the preceding information, and knowing duct sizes, the following required menu items
can be programmed:
MENU ITEM
ITEM VALUE
DESCRIPTION
HD1 DCT AREA
0.78 ft
2
(12 in. round)
Fume hood 1 duct area
HD2 DCT AREA
0.78 ft
2
(12 in. round)
Fume hood 2 duct area
HD3 DCT AREA
0.78 ft
2
(12 in. round)
Fume hood 3 duct area
HD4 DCT AREA
0.78 ft
2
(12 in. round)
Fume hood 4 duct area
EX1 DCT AREA
1.0 ft
2
(12”
×
12”)
General exhaust duct area
SP1 DCT AREA
3.33 ft
2
(24”
×
20”)
Supply duct area
MIN OFFSET
300 CFM
Minimum offset.
MAX OFFSET
1,000 CFM
Maximum offset.
EXH CONFIG UNGANGED (Default value)
Additional menu items to program for temperature and pressure control
VENT MIN SET
900 CFM
10 air changes per hour
COOLING FLOW
1,200 CFM
Required flow to cool laboratory.
TEMP SETP
72
°
F
Temperature sensor switches from
VENT MIN SET
to
COOLING FLOW.
SET POINT
–0.001 in. H
2
O
Pressure differential set point.
Sequence Of Operation
Beginning scenario:
Laboratory is maintaining pressure control; -0.001 “ H
2
O.
Temperature requirement is satisfied.
Fume hood sashes are down, total hood exhaust is 1,000 CFM.
Supply air is 900 CFM (maintain ventilation).
General exhaust 200 CFM (calculated from below).
Fume hoods + General exhaust - Offset = Supply air
1,000 +
?
– 300 = 900
Two fume hoods are opened so that the chemists can load experiments into the hood. The face
velocity (100 ft/min) is maintained by modulating the fume hood dampers. The total fume hood
flow is now 2,500 CFM (1,000 + 1,000 + 250 + 250).
Fume hoods + General exhaust - Offset = Supply air
2,500 + 0
– 300 = 2,200
The supply air volume changes to 2,200 CFM (2,500 CFM hood exhaust –300 CFM offset). The
general exhaust is closed since no additional exhaust is needed for temperature or ventilation.
