Handbook of air conditioning and refrigeration / Shan K

LOAD CALCULATIONS 6.31
Internal load density (ILD), W/ft 2 (W/m 2 ), indicates the total internal heat gains of people, lights,
and equipment, and it can be calculated as
where SHGp, LHGp � sensible and latent heat gains for occupants, respectively, Btu/h (W)
Afl � floor area, ft2 (m2 )
WA,l, WA,e � lighting and equipment power density, respectively, W/ft 2 (W/m2 ILD �
)
SHGp � LHGp � WA,l � WA,e 3.413 Afl (6.41)
Both infiltration sensible heat gain q inf, s and infiltration latent heat gain q inf, l, in Btu/h (W), are instantaneous
space cooling loads; they also can be expressed as
Q inf, s � q inf, s Q inf, l � q inf, l (6.42)
where Q inf, s, Q inf, l � infiltration sensible and latent cooling loads, respectively, Btu/h (W).
Example 6.1. A return air plenum in a typical floor of a multistory building has the following
construction and operating characteristics:
Wattage of electric lights
Return air volume flow rate
Density of return air � r
Fraction of heat to plenum F lp
U values: exterior wall of plenum
suspended ceiling
floor
Area of ceiling and floor
Area of exterior wall of plenum
CLF of electric lights
CLTD of exterior wall of plenum
Space temperature
1.5 W/ft 2 (16.1 W/m 2 )
11,800 cfm (334 m 3 /min)
0.073 lb/ft 3 (1.168 kg/m 3 )
0.5
0.2 Btu/h�ft 2 �°F (1.136 W/m 2 �°C)
0.32 Btu/h�ft 2 �°F (1.817 W/m 2 �°C)
0.21 Btu/h�ft 2 �°F (1.192 W/m 2 �°C)
11,900 ft 2 (1106 m 2 )
1920 ft 2 (178 m 2 )
1.0
24°F (13.3°C)
75°F (23.9°C)
Determine the return air plenum temperature and the space cooling load from the electric lights
when they are recess-mounted on the ceiling and the ceiling plenum is used as a return plenum.
Solution. From Eq. (6.17),
q lp � F lpq s, l � 0.5 � 1.5 � 11,900 � 3.413 � 30,461 Btu/h (W)
Since 60V˙ r �r cpa � 60 � 11,800 � 0.073 � 0.243 � 12,559 Btu / h��F,
and given Eq. (6.22), the
temperature of return air in the return plenum is
� 75 �
T p � T r �
� 75 � 39,933
18,866
q lp � U wp A wpCLTD wp
60V˙ r � r c pa � U cl A cl � U flA fl
30,717 � 0.2 � 1920 � 24
12,559 � 0.32 � 11,900 � 0.21 � 11,900
� 77.12�F (25.07�C)