Handbook of air conditioning and refrigeration / Shan K

Space Cooling Load Calculation
The types of conversion of space heat gains to space cooling loads can be grouped into the following
two categories:
1. Space sensible cooling loads Q rs,t that are only a fraction of the space sensible heat gain q rs,t.
These kinds of sensible heat gains have both radiative and convective components, and it is difficult
to separate the convective component from the radiative component, such as sensible heat
gains from exterior walls and roofs, and solar heat gains through windows. Equation (6.28) will
be used to convert these types of sensible heat gains q rs,t to cooling loads Q rs,t.
2. Space heat gains q in,t are instantaneous space cooling loads Q in,t, both in Btu/h (W), or
Qin, t � qin, t
(6.30)
This category includes all latent heat gains qrl, convective sensible heat gains, infiltration sensible
heat gain qinf, s, and sensible heat gains whose convective component can be separated from
the radiative component, such as air-to-air heat gain from windows, lights, and people.
The space cooling load Q rc, t, Btu/h (W), is their sum, or
Qrc,t � Qrs,t � Qin, t � Ql,t � Qs,t � Ql,t (6.31)
where Qs,t, Ql,t � space sensible cooling load and latent load, respectively, Btu/h (W).
As mandated in ASHRAE/IESNA Standard 90.1-1999, Optimum Start Controls, pickup loads
either for cooling or heating depend on the difference between space temperature and occupied set
point and the amount of time prior to scheduled occupancy, and is often determined by computer
software.
Heat Extraction Rate and Space Air Transfer Function
Heat Loss to Surroundings
In the calculation of the conduction heat gain through the exterior wall and roofs by using the
conduction transfer function [Eq. (6.10)] and then converting to sensible cooling load from the
room transfer function, the space temperature T r is found to be a constant. Most direct-digital control
(DDC) zone control systems are now adopting proportional-integral control mode. When the
air system and the DDC system are in operation, T r is a constant once it has achieved a stable
condition.
However, for many air systems operated at nighttime shutdown mode, T r will drift away from the
set point during the shutdown period. The space air transfer function relates the heat extraction rate at
time t, denoted by Q ex, t, Btu/h (W), to the space air temperature at time t, denoted by T r, t, °F (°C ), as
� 1
pi (Qex, t – � � Qrc,t – i�) � �
i�0
2
gi (Tr,con � Tr, t – i�)
i�0
LOAD CALCULATIONS 6.25
where p i, g i � space air transfer function coefficients, refer to ASHRAE Handbook for details
Q ex,t�� � heat extraction rate at time t��, Btu/h (W)
Q rc,t �i� � calculated space cooling load at time t�i�, Btu/h (W)
T r, con � assumed constant space air temperature, °F (°C)
(6.32)
There is always a radiant heat loss from the outer surface of the building to the sky vault without
clouds because atmospheric temperature is lower at high altitudes, as described in Sec. 3.12. In
many locations in the United States, there are also radiant heat losses to the surroundings at