Handbook of air conditioning and refrigeration / Shan K

22.20 CHAPTER TWENTY-TWO
Air Economizer Mode
system �p ret increased from 0.15 to 0.7 in. WC (37 to 74 Pa). The fan total pressure of the supply
fan is then
0.2 � 0.4 � 2.5 � 0.85 � 0.75 � 0.03 � 4.73 in. WC (1176 Pa)
During the minimum outdoor air recirculating mode, the recirculating air turns 90° and flows
through the recirculating damper and has a greater total pressure loss than the 3000 cfm (1416 L/s)
exhaust air exhausted through exhaust damper and louver. Assume that the dynamic loss of the 90°
turn of the recirculating air is 0.2 in. WC (50 Pa). Then the total pressure loss of the recirculating
damper is 0.4 in. WC (100 Pa) and the fan total pressure of the return fan is
0.7 � 0.2 � 0.4 � 0.03 � 1.27 in. WC (316 Pa)
The pressure-volume characteristics at the key points are shown below:
�pret, in. WC Point o Point m Point r Point ru
0.15
0.15
p, in. WC
V
0 ��0.52 � 0.03 ��0.12
· , cfm 3,350 20,000 20,000 16,650
0.70
0.70
p, in. WG
V
0 � 0.6 � 0.03 1.0
· , cfm 3,350 20,000 20,000 19,650
At 50 percent of the design flow rate, if the return fan is turned off and the exhaust damper after
point ru is closed, then the fan total pressure of the supply fan will be higher than 1.72 in. WC (428
Pa), as in the supply fan and exhaust fan combination. The space pressure p r drops to � 0.03 in.
WG (� 7.5 Pag). Total pressure at point ru may be lower than � 0.07 in. WG (� 17 Pag) and at
point m lower than � 0.26 in. WG (� 65 Pa) because of the pressure loss of air flowing through the
turned-off return fan.
If the total pressure loss of the return system is at 0.7 in. WC (174 Pa) and 3000 cfm (1416 L/s)
of air is exhausted through the return fan instead of the exhaust fan, and also if the return fan is operating
at 50 percent of design flow rate, then the fan total pressure of the return fan is about
(10,350/20,000) 2 (0.67 � 0.2 � 0.4) � 0.34 in. WC (85 Pa) and the total pressure at ru is about
0.16 in. WG (40 Pag).
During air economizer mode operation at the design volume flow rate, the supply and exhaust volume
flow rate and the pressure losses of various sections of the supply air system are the same as in
minimum outdoor air recirculating mode. The fan total pressure of the supply fan is 4.73 in. WC
(1176 Pa).
Because the total pressure at point ru is raised to 1.0 in. WG (249 Pag) to overcome the pressure
loss of the exhaust flow passage and the discharge velocity pressure at the outlet of 0.40 in. WC
(100 Pa), the return fan total pressure is then
�p ret.f � 0.15 � 0.03 � 0.60 � 0.40 � 1.12 in. WC (278 Pa)
If the total pressure loss of the return system is increased to 0.70 in. WC (174 Pa) and the return air
is exhausted through the return fan and exhaust damper, during air economizer mode operation at
design flow rate, then the return fan total pressure is the same as in the recirculating mode
�p ret.f � (0.70 � 0.03) � 0.60 � 0.40 � 1.67 in. WC (415 Pa)
Pressure-volume characteristics at the key points in air economizer mode at design volume flow,
when the total pressure loss of the return system is 0.15 in. WC (37 Pa) and space air is exhausted
through the exhaust fan, are as follows: