For an economical installation, the air-handling units should be chosen to handle the least amount of air without danger of sweating ducts and grilles. In most comfort cooling, an 18F discharge temperature below room temperature will be satisfactory.
If a fresh-air duct is used, the required fresh air is mixed with the return air, and the mix is sent through the cooling coil; i.e., the fresh-air load is taken care of in the coil, and discharge air must take care of the internal load only. Then, from Eq. (13.30), the amount of air to be handled is
If a fresh-air duct is not installed, and the outside air is allowed to infiltrate into the premises, we use Eq. (13.30) with the sensible part of the load of the infiltration air added to qs , because the outside air infiltrating becomes part of the internal load.
Once the amount of air to be handled is determined, choose a coil of face area such that the coil-face velocity V would be not much more than 500 ft /min. Some coil manufacturers recommend cooling-coil-face velocities as high as 700 ft /min.
But there is danger of moisture from the cooling coil being carried along the air stream at such high velocities.
The number of rows of coils can be determined by getting the manufacturers capacity ratings of the coils for three, four, five, six, etc., rows deep and choosing a coil that can handle not less than the sensible and latent load at the working suction temperature.
Multizone air-handling units (Fig. 13.33) are used to control the temperature of more than one space without use of a separate air-handling unit for each zone.
When a zone thermostat calls for cooling, the damper motor for that zone opens the cold deck dampers and closes or throttles the warm deck dampers. Thus, the same unit can provide cooling for one zone while it can provide heating for another zone at the same time.
This style of unit is not typically used in new installations, but may be encountered in existing facilities. Various controllers are available to raise these units closer to efficiencies expected in current designs.