Handbook of air conditioning and refrigeration / Shan K

22.18 CHAPTER TWENTY-TWO
air is extracted by the supply fan to mix with the 3350 cfm (1581 L/s) of outdoor air in the mixed
plenum at design volume flow. The excessive amount (350 cfm or 165 L/s) of supply air raises
the space pressure until the 20,000 cfm (9438 L/s) supply air is balanced with the sum of 16,650
cfm (7857 L/s) recirculating air, 3000 cfm (1416 L/s) exhaust air, and 350 cfm (165 L/s) exfiltrated
air at � 0.03 in. WG (7.5 Pag) space positive pressure.
● The relief fan is often controlled by a DDC unit controller according to the input from the space
pressure sensor to maintain a space positive pressure within predetermined limits.
● The main drawback of the supply and relief fan combination is the increase in pressure at the
mixed plenum p m when the supply volume flow rate of a VAV system is reduced at part-load operation
during minimum outdoor air recirculating mode. The higher the pressure loss of the return
system, the greater the increase in p m. Higher pressure in the mixed box and, therefore, a lower
�p om ( p o � p m) cause a deficiency in outdoor air intake. Various outdoor ventilation controls have
been developed recently and are discussed in Chap. 23. The operation of a relief fan reduces both
mixing box pressure and space pressure.
22.6 SUPPLY FAN AND RETURN FAN COMBINATION
Recirculation Mode
Supply fan and return fan combinations are widely used, especially in air systems that serve large
conditioned areas. A return fan is located upstream from the junction of the recirculating flow passage
and the exhaust air passage, point ru, as shown in Fig. 22.4a. Consider an AHU or PU with a
supply fan and return fan combination and an exhaust system. Its operating characteristics are the
same as those in Sec. 22.4 except for the following:
● A return fan is installed with the following pressure-volume characteristics:
V · , cfm 10,000 15,000 20,000
�p t, in. WC 1.47 1.23 1.10
● The pressure loss of the exhaust damper after point ru in the exhaust passage is 0.4 in. WC (100
Pa) and is 0.2 in. WC (50 Pa) for the air louver at the maximum exhaust volume flow rate. At
minimum outdoor airflow, louver pressure losses can be ignored. As in the supply and relief fan
combination, a minimum outdoor air damper and an economizer damper are installed, and both
have a total pressure loss of 0.4 in. WC (100 Pa) at design airflow.
During minimum outdoor air recirculation mode, outdoor air is drawn through the outdoor air louver,
duct, and damper and is mixed with recirculating air. The mixture then flows through the filters
and coils and is supplied to the conditioned space. At design volume flow, a small part is exhausted,
another part is exfiltrated, and the remaining portion is extracted by the return fan flowing through
the return system. At point ru, almost all the return air is recirculated through the recirculating
damper.
During minimum outdoor air recirculating mode, at design volume flow rate:
● If the return fan is not operating and the exhaust damper is closed, then the fan total pressure of
the supply fan is higher than 4.65 in. WC (1156 Pa) because of the air flowing through the turnedoff
return fan.
● If the return fan is operating, 3000 cfm (1416 L/s) of air is exhausted through the exhaust passage
and exhaust damper instead of from the separate exhaust fan. The total pressure loss of the return