Handbook of air conditioning and refrigeration / Shan K

Air Conditioning Rules
Graphical Method
To determine the supply air condition, the following air conditioning rules are helpful:
1. For a given summer indoor design temperature T r, design space sensible cooling load Q rs, and
specific supply system heat gain q s,s,
● A lower air off-coil temperature Tcc and supply air temperature Ts always result in a greater
supply air temperature differential �Ts � Tr � Ts and a lower space relative humidity �s. ● On the other hand, a higher Tcc means a smaller Ts and a higher �r. 2. A greater �Ts reduces the volume flow rate and, therefore, fan size, fan energy use, and terminal
and duct sizes. These reduce both first cost and operating costs. At the same time, a greater �Ts, needs a lower Tcc to maintain an indoor design temperature Tr. Excluding the cold air distribution
in the ice storage system, a �Ts between 15 and 20°F (8.3 and 11.1°C) is most widely used.
3. Temperature of air leaving the cooling coil Tcc is closely related to the temperature of the chilled
water entering and leaving the coil, Twe and Twl, or to the evaporating temperature inside the DX
coil Tev. A lower Tcc or Twe needs a correspondingly lower Tev and, therefore, higher power input
to the refrigeration compressors.
4. For a given Tcc, Qrs, and qs,s, a supply volume flow rate that is greater than the required value
results in a lower space temperature T r, a higher space relative humidity � r, and a smaller �T s. On
the other hand, a smaller than required results in a higher T r, a lower � r, and a greater �T s.
5. When an air system is used to serve a single-zone conditioned space, T s and T cc should be properly
arranged so that the indoor design temperature and relative humidity can be maintained for
energy-efficient operation.
6. For an air system serving multizones, with many space temperatures and relative humidity requirements,
T we and T cc or T ev should satisfy the lowest T cc requirement. In actual practice, T we
and T cc, or T ev, are often determined by previous experience with similar applications.
The following graphical method may be used to determine the condition of supply air s for an air
system serving a single zone (including constant-volume single-zone systems) whose T r and � r
must be maintained.
1. According to the data given in Sec. 20.14, the relative humidity of the air leaving a cooling coil
� cc with 10 or more fins per inch can be estimated as
4-row coil: �cc � 93 percent
6-row coil: �cc � 96 percent
8-row coil: �cc � 98 percent
AIR SYSTEMS: BASICS AND CONSTANT-VOLUME SYSTEMS 20.63
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2. Draw the space conditioning line sr from the state point r, parallel to the line of known sensible
heat ratio SHR s, as shown in Fig. 20.24.
3. State point of supply air s must lie on line sr.
4. The horizontal line cc-s represents the magnitude of the supply system heat gain. As segment
cc-s is moved up and down, there exists a position where cc lies exactly on the selected � cc
curve for the coil, s lies on line sr, and T s � T cc exactly matches the temperature increase caused
by supply system heat gain.
5. In practice, this required T cc must be reconciled with the selected coil’s performance for a given
coil configuration, T we, and air and water velocities.
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