Handbook of air conditioning and refrigeration / Shan K

6.38 CHAPTER SIX
where hci � convective heat-transfer coefficient of surface i, Btu/h�ft2�°F (W/m2�°C) �a � density of space air, lb/ft 3 (kg/m 3 )
hmi � convective mass-transfer coefficient of surface i, ft/s (m/s)
wt is � humidity ratio corresponding to surface i at time t, lb/lb (kg/kg)
h fg � latent heat of vaporization at surface temperature, Btu/lb (J/kg)
F1:i, Fp:i, Fm:i � shape factor between surface of lights, occupants, appliances and surface i
L r,Or, Mr � radiative portion of heat energy from lights, occupants, and appliances and machines,
Btu/ h (W)
Solving for gives
T i t��t
In Eq. (6.58) Bi is the Biot number and can be expressed as
The stability limit of the surface nodes requires that
(6.58)
(6.59)
(6.60)
Similarly, according to the principle of conservation of mass, the moisture content at surface node i
is given as
(6.61)
The temperature and moisture content at other surface nodes such as the floor, ceiling, exterior
walls, glass, and Plexiglas of the lighting fixture can be found in the same manner.
According to ASHRAE Handbook 1993, Fundamentals, the radiative and convective portions of
the heat gains are as follows:
Space Air Temperature and Cooling Loads
t��t Ti � 2Fo�T t i�1 � Bi�T r t � � t t t
a hmi hfgX i(w r � wis)
hci � F 1:i L r
h ci A i
X i t��t � (1 � 2Fomass)X i t � 2Fomass X t i�1 � 2� ah mi �t X i t (w t r � w t is)
Radiative, percent Convective, percent
Fluorescent lights 50 50
People 33 67
External walls and roofs 60 40
Appliance and machines 20–80 80–20
If the infiltrated air is ignored, the heat balance on the space air or the plenum air can be described
by the following relationship:
Internal energy of supply air � convective heat transfer from building structures
� convective heat transfer from internal loads � internal energy of space air
t��t 60V˙ s�scpaT s � � n
t��t t��t t��t
hck Ak(T i � T r ) � Lc � Oc � Mc � 60V˙ s�sc T pa r
k�1
� F p:i O r
h ci A i
� F m:i M r
h ci A i �� � [1 � 2Fo(1 � Bi)]T i t
Bi � h c �x
k
Fo(1 � Bi) � 1/2
� b �x