Handbook of air conditioning and refrigeration / Shan K

Steam Humidifier Control
Dew Point Control
AIR SYSTEMS: MINIMUM VENTILATION AND VAV SYSTEM CONTROLS 23.27
spaces that have been unoccupied less than or greater than 24 h and provide an optimum start or
stop time of the air system.
A simple and effective way that is worthwhile to try is as follows: Record the outdoor air temperature,
space temperature, relative humidity, internal load, and equipment and damper operating
conditions at suitable time intervals, say, 10 min to 1 h, during the warm-up or cool-down period;
analyze and determine the optimum start time once sufficient data have been accumulated. Modify
the results until they are verified by experience.
Some manufacturers also provide cycling capacity during morning warm-up to maintain the
space temperature reached before the occupied period.
If both cool-down and purge mode or warm-up and purge mode operations are required, and the
outdoor air is significantly hot and humid, or colder than the space air during the occupied period,
then the purge operation must be performed first, and the cool-down or warm-up operation follows
until the space temperature is maintained within predetermined limits. During purge mode, the minimum
outdoor air, economizer, and exhaust dampers are opened 100 percent, and the recirculating
damper is closed. During warm-up or cool-down mode, the minimum outdoor air, economizer, and
exhaust dampers are closed, and the recirculating damper is opened 100 percent.
Only when the outdoor air can be used for free cooling, and to squeeze out the contaminated
indoor air at the same time, are the cool-down and purge operations combined into a single
operation.
A steam humidifier is the most widely used humidifier when a warm air supply is required in winter in
an AHU or PU. A humidity sensor is often located in the supply duct just after the supply fan discharge,
to sense the relative humidity of the discharge air. The DDC controller receives the sensed signal and
positions the two-way valve to modulate the steam flow rate entering the steam humidifier, as shown in
Figs. 15.42 and 15.43, until the relative humidity of the discharge air meets the required set point.
When the fan is turned on, the humidifier control is energized. The steam valve closes when the
fan is turned off.
If a steam humidifier is installed, it is critical that the humidifying capacity not be exceeded, so
that a wet surface does not occur in the AHU as well as in the supply duct in order to maintain an
acceptable indoor air quality.
For air systems serving conditioned space that needed both close control of the space temperature
and relative humidity with a sensible ratio of the space conditioning line SHR s above 0.9, a
dew point control is often used. Dew point control involves using a water cooling coil, DX coil,
or air washer to cause a specific amount of conditioned air to cool and dehumidify or occasionally
cool and humidify to point cc, such as that on Fig. 23.10. Because of the high relative
humidity of the departing air, the temperature T cc is approximately equal to the dew point on the
saturation curve.
When the indoor space temperature is to be maintained at 75°F (23.9°C), with a relative humidity
of 50 percent, T cc � 55°F (12.8°C), � cc � 97 percent, and SHR s � 0.95, the state point of the
conditioned space r must move approximately along the horizontal dew point line. Either the sensible
cooling load is absorbed by the supply air or reheating is provided to the supply air by a reheating
coil to compensate the sensible load at part load, if the space temperature is maintained at 75°F
(23.9°C), the relative humidity must be at 50 percent. A temperature sensor senses the space temperature
T r, and a DDC controller modulates the capacity of the reheating coil according to the
sensed space temperature to maintain T r � 75°F (23.9°C) with � cc � 50 percent.
Dew point control is simple and effective. Its main drawback is its need for reheating which
wastes energy. Dew point control is mainly used for industrial applications such as clean rooms.