Handbook of air conditioning and refrigeration / Shan K

where m˙ ex,r, m˙ eu � mass flow rate of air exhausted, including air exhausted from the restroom and
from the AHU, lb/min (kg/min). In most conditioned space, a space positive pressure � pim is thus
formed because of a surplus of outdoor air � m˙ inf entering the system, which is then exfiltrated
through the gaps and openings on the building shell to outdoors and can be calculated as
where � inf � volume flow rate of exfiltration, cfm (m3 /min)
Ae,l � effective leakage area on building shell, ft2 (m2 )
�r � space air density, lb/ft3 (kg/m 3 V˙
)
�pro � space and outdoor pressure difference, in. WC (Pa)
(23.4)
● As discussed in Sec. 20.3, the space and outdoor pressure difference often has a value between
� 0.005 and 0.03 in. WC (1.25 and 7.5 Pa). If a pressure sensor in a space pressurization control
system needs to sense a 10 percent change in p im, that is, typically � 0.0005 in. WG (0.13 Pa), it
would be difficult and expensive to specify such a level of accuracy for most comfort systems.
● For an average leaky one-story office building with an exterior wall area of 10,000 ft 2 (929 m 2 )
and a floor area of 20,000 ft 2 (1858 m 2 ), as discussed in Sec. 23.2, from Eq. (20.2)
From Eq. (23.4), for this average leaky office building, the effective leakage area is
A e,l �
�V˙ inf
4005√�p ro
Also from Eq. (23.4) the outdoor air intake required to maintain various space positive pressures
and to exfiltrate them in this average leaky office building with 10,000 ft 2 (929 m 2 ) of exterior wall
is as follows:
VAV Systems Return/Relief Fan Volume Flow Control
AIR SYSTEMS: MINIMUM VENTILATION AND VAV SYSTEM CONTROLS 23.17
�m˙ inf ��V˙ inf� r
�V˙ inf � 4005A e,l √�p ro
V˙ leak � C flow A w(�p) 0.65 � 1.0(10,000)(0.3) 0.65 � 4572 cfm (2157 L / s)
� 4572
4005√0.3
� 2.08 ft 2 (0.193 m 2 )
Required outdoor air intake
�pro, in. WC (Pa) Exterior wall area, cfm/ft 2 (L/s�m2 ) Floor area, cfm/ft 2 (L/s�m2 )
� 0.005 0.059 (0.30) 0.0295 (0.15)
� 0.01 0.083 (0.42) 0.0417 (0.21)
� 0.02 0.118 (0.60) 0.059 (0.30)
� 0.03 0.144 (0.73) 0.072 (0.36)
� 0.05 0.186 (0.95) 0.093 (0.47)
� 0.1 0.263 (1.33) 0.132 (0.67)
For a VAV system with either a supply and relief fan combination as shown in Fig. 22.3, or a supply
and return fan combination as shown in Figs. 22.1 and 22.4, if the volume flow rate of supply air reduces
during part-load, the amount of return air extracted along points ru should be reduced accordingly
in order to provide the required outdoor ventilation air and to maintain a desirable space pressure.
Modulating the speed of the return/relief fan with adjustable-frequency drives, and thus its
volume flow, to maintain space pressure within predetermined limits is a simple, direct, and effective
means of return/relief fan volume control.
Place a pressure sensor in a representative floorspace. The location of the indoor pressure sensor
must be away from the door and openings to the ouside, as well as away from the elevator lobbies.
The outdoor reference pressure should be typically 10 to 15 ft (3 to 4.5 m) above the building and