Handbook of air conditioning and refrigeration / Shan K

Operating Characteristics
Because the volume flow rates of the supply air and recirculating air are different, the flow resistance
of the supply system and return system as well as the flow resistance of the fixed part and
variable part of the air system pressure loss should be calculated separately.
Consider a VAV rooftop packaged system with a supply fan and an exhaust fan serving a typical
floor in a commercial building, as shown in Fig. 22.2. This air system has the following operating
characteristics at design conditions:
Supply volume flow rate
Total pressure losses:
20,000 cfm (9438 L/s)
Across recirculating damper at design flow 0.4 in. WC (100 Pa)
Filters and coils 2.5 in. WC (625 Pa)
Supply main duct, �pd, var
0.85 in. WC (212 Pa)
Recirculating system 0.15 in. WC (37 Pa)
VAV box, branch duct, and diffuser 0.75 in. WC (187 Pa)
Effective leakage area on building shell 0.5 ft2 (0.46 m2 )
Volume flow of exhaust fan 3000 cfm (1416 L/s)
Minimum outdoor ventilation air required 3350 cfm (1581 L/s)
Space pressure � 0.03 in. WG (7.5 Pag)
For simplicity, the difference in air densities between supply, return, and exhaust air is ignored.
In minimum outdoor air recirculating mode, the pressure losses of the outdoor louver can be neglected.
A barometric relief damper is mounted in the conditioned space and is opened when the
space positive pressure is greater than 0.2 in. WG (50 Pag). The supply fan has the same �pt�V˙ characteristics as in Example 20.3.
The following control systems are installed to maintain the required operating parameters:
● Zone temperature control
● Discharge temperature control
● Duct static pressure control
● Minimum outdoor ventilation air control
● High pressure limit control
All these control systems are described in Chap. 23.
In summer, this air system can be operated in cooling mode, recirculating mode, air economizer
mode, or warm-up or cool-down mode.
Recirculation Mode and Design Volume Flow
AIR SYSTEMS: FAN COMBINATION AND SMOKE CONTROL 22.9
Minimum outdoor air is extracted through the outdoor intake and is mixed with the recirculating air
in the mixing box in a rooftop packaged unit. The mixture is conditioned in the packaged unit. After
conditioning, the supply air flows through the supply fan and the supply duct and is discharged to
the conditioned space. In the conditioned space, a certain volume flow rate is exhausted through an
exhaust fan, and another small portion is exfiltrated through the gaps and openings on the building
shell if the space is maintained at a positive pressure between � 0.005 and � 0.03 in. WG (1.25
and 7.5 Pag). The major portion of the space air is returned to the rooftop unit, where it is mixed
with the outdoor air for recirculation.