Building Design and Construction Handbook - Merritt - Ventech!

HEATING, VENTILATION, AND AIR CONDITIONING 13.93
actuation may be economical, particularly for larger valves and dampers. When a
pneumatic output signal is required from the controller, a simple digital-topneumatic
relay converts the electronic signal to a variable pressure signal. Pneumatic
actuators provide the additional advantages of excellent reliability and low
maintenance cost.
13.37 INDUSTRIAL AIR CONDITIONING
Certain manufacturing processes call for close control of temperature and relative
humidity. A typical control system is shown in Fig. 13.45. The humidistat may be
of the single-pole, double-throw type.
On a rise in room humidity, the refrigeration compressor cuts in for the purpose
of dehumidification. Since the sensible capacity of the cooling coil usually is much
higher than the sensible load, whereas the latent capacity does not differ too much
from the latent load, undercooling results. The room thermostat will then send
enough steam or hot water to the reheat coil to maintain proper room temperature.
A small drop in room relative humidity will cut off the compressor. A further
drop in room relative humidity will energize the water or steam solenoid valve to
add moisture to the air. The thermostat is also arranged to cut off all steam from
the reheat coil when the room temperature reaches the thermostat setting. However,
there may be periods when the humidistat will not call for the operation of the
refrigeration compressor and the room temperature will climb above the thermostat
setting. An arrangement is provided for the thermostat to cut in the compressor to
counteract the increase in room temperature.
When calculating the load for an industrial system, designers should allow for
the thermal capacity and moisture content of the manufactured product entering
and leaving the room. Often, this part of the load is a considerable percentage of
the total.
FIGURE 13.45 Circuit for close control of temperature and humidity.