Handbook of air conditioning and refrigeration / Shan K

6.24 CHAPTER SIX
Infiltration
Infiltration is the uncontrolled inward flow of outdoor air through cracks and openings in the building
envelope due to the pressure difference across the envelope. The pressure difference may be
caused by any of the following:
1. Wind pressure
2. Stack effect due to the outdoor and indoor temperature difference
3. Mechanical ventilation
In summer, for low-rise commercial buildings that have their exterior windows well sealed, and if a
positive pressure is maintained in the conditioned space when the air system is operating, normally
the infiltration can be considered zero.
For high-rise buildings, infiltration should be considered and calculated in both summer and
winter. Infiltration is discussed again in Sec. 20.4.
As soon as the volume flow rate of infiltrated air , cfm (m3 V˙ inf /min), is determined, the space
sensible heat gain from infiltration qinf, Btu/h (W), can be calculated as
q s, inf � 60V˙ inf � oc pa (T o � T r)
(6.26)
where � o � density of outdoor air, lb/ft 3 (kg/m 3 ). The space latent heat gain from infiltration q l, inf,
Btu/h (W), can be calculated as
q l,inf � 60V˙ inf � o (w o � w r)h fg, 32
where w o, w r � humidity ratio of outdoor and space air, respectively, lb/lb (kg/kg)
h fg,32 � latent heat of vaporization at 32°F, Btu/lb (J/kg)
Cooling Load Conversion Using Room Transfer Function
(6.27)
The conversion of space sensible heat gains q rs,t, Btu/h (W), having radiative only or radiative and
convective components to space sensible cooling loads Q rs,t, Btu/h (W), using the transfer function
method and room transfer function coefficients can be expressed as follows:
Qrs,t � � (v0 qs,t � v1qs,t – � � v2qs,t – 2� � ���) � (w1 Qrs,t – � � w2 Qrs,t – 2� � ���)
i�1
where i � number of heat gain components in same group
��time interval
t�n� �time at t�n�
(6.28)
Here v 0, v 1, v 2,...,w 1, w 2,...are the coefficients of the room transfer function; refer to the
ASHRAE Handbook for details. Their relationship can be expressed from Eqs. (6.8) and (6.9) as
K(z) � v 0 � v 1z �1 � v 2z �2 � ���
1 � w 1z �1 � w 2z �2 � ���
(6.29)
The magnitude of the coefficients depends on the duration of the time interval, fraction of
the radiative component, and heat storage capacity because of the 14 influential parameters
of zone characteristics, such as zone geometry, height, exterior wall construction, interior shade,
furniture, zone location, glass percentage, and type of partition, midfloor, slab, ceiling, roof,
and floor.