Handbook of air conditioning and refrigeration / Shan K

18.4 CHAPTER EIGHTEEN
cooling load intensity of 10 to 20 Btu/h� ft 2 (31.5 to 63 W/m 2 ) the Air Diffusion Performance Index
considering the turbulence effect were about 10 percent less than those that did not take the impact
of Tu into account. At higher cooling loads and airflow rates, the effect of Tu became larger.
Actually, the drop in ADPI is less than 10 percent for cooling load intensity between 10 and 20
Btu/h� ft 2 (31.5 and 63 W/m 2 ) due to the impact of Tu because the points of air velocity located on
the centerline of the test room are only a small portion of the total number of measured points.
In cooling mode operation, ADPI is an appropriate index to use to evaluate a space air diffusion
system. For heating mode operation, the temperature gradient between two points in the occupied
zone may be a better indicator of the thermal comfort of the occupants and the effectiveness of a
space air diffusion system. Usually, the temperature gradient is less than 5°F (2.8°C) within an
occupied zone.
Space Diffusion Effectiveness Factor
The effectiveness of a space air diffusion system can also be assessed by using a space diffusion effectiveness
factor for air temperature � T or for air contamination � C. Both factors are dimensionless.
Effectiveness factor � T compares temperature differentials and � C compares contamination differentials
as follows:
� T � T re � T s
T r � T s
� C � C ex � C s
where T � temperature, °F (°C)
C � concentration of air contamination, �g/m3 Cr � Cs (18.4)
In Eq. (18.4), subscript re represents the recirculating air, ex the exhaust air, r the space air or air at
the measuring point, and s the supply air. When � � 1, the space air diffusion is considered effective.
If � � 1, a portion of supply air has failed to supply the occupied zone and exhaust through
the return or exhaust inlets directly. Parameters in both the numerator and denominator must be in
the same units.
Ventilation Effectiveness and Air Exchange Effectiveness
� T ex � T s
T r � T s
ASHRAE Handbook 1997, Fundamentals, defines ventilation effectiveness as a description of an air
distribution system’s ability to remove internally generated contaminants from a zone, space, or
building. Air change effectiveness � ex is defined as air system ability to deliver ventilation air to a
zone, space, or building. The age of air � age, in minutes or hours, is the time period that outdoor
ventilation air has been in a zone, space, or building, and age can be evaluated for an existing building
using the trace gas method. Chamberlin et al. (1999) proposed the following when growth
(step-up) and decay (step-down) tracer methods are used:
� age � (� end � � initial)� 1 � C avg
C end �
� (� stop � � start)(C avg � C start)
(18.5a)
where C avg � time-averaged concentration of tracer gas measured in test zone from when tracer
gas injections began to when last sample is taken, �g/m 3 or ppm
C end, C start � concentration of tracer gas when last sample was taken at time � end and when first
sample was taken at � start, �g/m 3 or ppm