Handbook of air conditioning and refrigeration / Shan K

6.6 CHAPTER SIX
Influence of Stored Heat
If refrigeration is used for cooling during the cool-down period, then it usually lasts less than
1 h, depending mainly on the tightness of the building and the differences of the space temperatures
and humidity ratios between the shutdown and cool-down periods. If the cool-down of the space air
and the building envelope is by means of the free cooling of outdoor air, the cool-down period may
last for several hours.
During winter, the supply of warm air during the warm-up period raises the space temperature
and lowers the space relative humidity. Because of the higher space temperature, heat transfer from
the space air to the colder building envelope and furnishings, and the heat energy required to raise
the temperature of the building envelope, forms the warm-up heating load.
Conditioning Period. This period commences when the space air temperature has fallen or risen
to a value within the predetermined limits. It ends when the air system is shut down. In summer,
cold and dehumidified air is usually supplied to the space to offset the space cooling load and to
maintain a required temperature and relative humidity. During winter, warm air is supplied to
compensate for heat losses from the conditioned space. Space temperature is often controlled and
maintained within predetermined limits by control systems.
In commercial buildings located in areas with a cold winter, the air system may operate in dutycycling
mode during nighttime, unoccupied periods in winter, or intermediate seasons to maintain a
night setback temperature. In duty-cycling mode, the fan and heater turn on and off to maintain a
desired temperature. Space temperature is set back at night, e.g., to 55 or 60°F (12.8 or 15.6°C), to
prevent freezing of water pipes and to produce a comparatively smaller temperature lift during a
warm-up period. The operation of the air system in a 24-h diel cycle in winter is then divided into
night setback period, warm-up period, and conditioning period. The required load on the heating
coil during a morning warm-up period at winter design conditions is usually the winter design heating
load.
The curve of solar heat gain from a west window is shown in Fig. 6.2, as well as the cooling load
curve due to this solar heat gain for a conditioned space operated at night shutdown mode in summer.
The difference between the maximum solar heat gain q hgm and the maximum cooling load Q clm
during the conditioning period indicates the amount of heat stored inside the building structures.
This difference significantly affects the size of air conditioning equipment required. The amount
of heat stored depends mainly on the mass of the building envelope (whether it is heavy, medium,
or light), the duration of the operating period of the air system within a 24-h cycle, and the characteristics
of heat gain, whether radiant heat or convective heat predominates. For solar radiation
transmitted through a west window, Q clm may have a magnitude of only 40 to 60 percent of q hgm.
ASHRAE Handbook divides the mass of the building construction into the following three
groups:
Heavy construction: approximately 130 lb/ft 2 (634 kg/m 2 ) floor area
Medium construction: approximately 70 lb/ft 2 (342 kg/m 2 ) floor area
Light construction: approximately 30 lb/ft 2 (146 kg/m 2 ) floor area
6.2 COOLING LOAD AND COIL LOAD CALCULATIONS
Components of Cooling Load
Cooling load calculations for air conditioning system design are mainly used to determine the volume
flow rate of the air system as well as the coil and refrigeration load of the equipment—to size