Handbook of air conditioning and refrigeration / Shan K

where � v � standard deviation of air velocity fluctuations, fpm (m/s)
v m � mean air velocity, fpm (m/s)
Experiments showed that if the space temperature Tx � 73.4°F (23°C) and vm � 30 fpm (0.15
m/s), the percentage of dissatisfied occupants may increase from about 10 to 15 percent when Itur increases from 0.1 to 0.5. A parameter called the effective draft temperature �, in °F (°C), which
combines the effects of uneven space air temperature and air movement, is often used to assess the
deviations of local magnitudes from the mean value, and it is defined as
(18.2)
where T x, v x � space air temperature and velocity at specific location, °F (°C) and fpm (m/s)
T r � mean space air temperature or set point, °F (°C)
In Eq. (18.2), a is a conversion constant to combine the effects of space air temperature and air
movements; its value is 0.07 when T is expressed in °F and v is expressed in fpm, and 8 when T is
expressed in °C and v in m/s.
The desirable mean space air velocity v rm, in fpm (m/s), is closely related to the space air
temperature T r to be maintained, the metabolic rate, and the clothing insulation of the occupant.
According to Fig. 4.8, for a space relative humidity � � 50 percent, T rad � T r, a metabolic rate of
400 Btu/h (117.2 W), and a clothing insulation of 0.6 clo (0.093 m 2 � °C/W), their relationship may
be given as follows:
Air Diffusion Performance Index
� � T x � T r � a(v x � v rm)
T r, °F 70 72 74 76 78 80
v rm, fpm 16 20 25 32 40 55
AIR SYSTEMS: SPACE AIR DIFFUSION 18.3
To an extent, the magnitude of effective draft temperature � also reflects the degree of thermal comfort
that can be provided by comfort air conditioning systems. During cooling mode in commercial
and public buildings, if the space temperature is maintained between 75 and 78°F (23.9 and
25.6°C), T rad � T r, space air velocity v r � 55 fpm (0.28 m/s), and the space relative humidity is between
30 and 70 percent, then most sedentary occupants feel comfortable when � 3°F � � �
� 2°F (� 1.7°C � � ��1.1°C). The air diffusion performance index (ADPI), in percent, which
evaluates the performance of space air diffusion, is calculated as
ADPI �
where N� � number of points measured in occupied zone in which � 3°F � � ��2°F
(� 1.7°C � � ��1.1°C)
N � total number of points measured in occupied zone
N� � 100
N
(18.3)
The higher the ADPI, the higher the percentage of occupants who feel comfortable. Maximum
ADPI approaches 100 percent.
Because air velocity fluctuations create drafts, therefore, they also affect ADPI. Abu-Ei-Hassan
et al. (1996), based on the impact of turbulence intensity Tu recommended by Fanger et al. (1988),
conducted tests to determine the effect of turbulence intensity Tu (velocity fluctuations) on the
ADPI. Abu-Ei-Hassan et al. took the centerline velocity of the test room, which has nearly the same
flow characteristic as the centerline velocity of the air jet, as the air velocities of the local points,
and the ADPIs thus calculated as Air Diffusion Performance Index that take into consideration the
Tu impact. The results showed that for a sidewall outlet space air diffusion system with a room