Handbook of air conditioning and refrigeration / Shan K

Air Economizer Cycle
If the difference in air density is ignored, the volume flow balance between the incoming infiltrated
air and outward exhaust air in this air system becomes
The volume flow of incoming air and exhaust air entering and leaving the system is nearly balanced.
From the above analysis, the system operating point at 50 percent design volume flow rate, point
Q, has a supply volume flow of 10,000 cfm (4719 L/s) and a fan total pressure
�p t50 � 0.75 � 0.84 � 0.13 � 1.72 in. WC (428 Pa)
During minimum outdoor air recirculating mode at 50 percent design volume flow, the volume flow
rate of outdoor air extracted through the outdoor intake passage decreases to 1870 cfm (883 L/s).
Space is maintained at a negative pressure of � 0.03 in. WG (� 7.5 Pag). An infiltration rate of 347
cfm (164 L/s) enters the space through openings and cracks in the building shell.
The pt�V˙ characteristics at key points during 50 percent design volume flow are as follows:
When an air economizer cycle is operated at a design flow rate of 100 percent of outdoor air in this
air system with a single supply fan and an exhaust fan combination, the recirculating damper is
closed, and both the minimum outdoor damper and the other, large outdoor damper for the air economizer
cycle are fully opened. Outdoor air is extracted to the mixed plenum, through the filters and
coils, and is supplied to the conditioned space. Because only a small portion of supply air is exhausted
and exfiltrated, the space positive pressure increases to about 0.2 in. WG (50 Pag). The
barometric relief damper then opens. Most of the supply air is discharged through the relief damper
outdoors.
When the supply and exhaust fan combination is operated at 50 percent design volume flow in
the air economizer cycle, space pressure is still limited to 0.2 in. WG (50 Pa), and most of the supply
air is discharged through the relief damper outdoors, as in the design volume flow.
Warm-Up and Cool-Down Mode
During the warm-up and cool-down mode, the outdoor damper is closed and the recirculating
damper is fully open, while the exhaust fan is turned off. Recirculated air from the space is conditioned
in the packaged unit and is then supplied to the space. Because of the negative pressure in
the mixing box, outdoor air is leaked into the box through the closed outdoor dampers and is exfiltrated
from the space because of the higher positive space pressure.
Pressure Variation at the Mixing Box
AIR SYSTEMS: FAN COMBINATION AND SMOKE CONTROL 22.13
V˙ o � V˙ inf � V˙ ex � 1870 � 347 � (3000 � 810) ��27 cfm (13 L / s)
Point o Point m Point r Point ru
p, in. WG
V
0 � 0.162 � 0.03 � 0.065
· , cfm 1870 10,000 10,000 8157
During the recirculating mode, when the supply volume flow rate is reduced from design flow to 50
percent of design flow, the decrease in the recirculating volume flow rate causes a corresponding
drop in the pressure loss of the recirculation system. Therefore, the pressure at the mixed plenum,
p m increases from � 0.52 to � 0.16 in. WG (� 129 to � 40 Pag). The greater the pressure loss of
the return system at the design flow rate, the higher the fluctuation of �p om. Because of the reduced
volume flow rate at a higher fan total pressure of the exhaust fan at 50 percent design flow, in order