Building Design and Construction Handbook - Merritt - Ventech!

HEATING, VENTILATION, AND AIR CONDITIONING 13.75
where q s � internal sensible load
�T � temperature difference between room and air leaving coil (usually
18�F)
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 q s, 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.
where Q � air flow, ft 3 /min
A c � coil face area, ft 2
V � air velocity, ft/min
FIGURE 13.33 Multizone air-handling unit.
Q
V � (13.36)
A c
The number of rows of coils can be determined by getting the manufacturer’s
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.