Basic Documentation

Chapter 8–Laboratory Temperature and Humidity Control Systems
Humidification Systems
Since laboratory ventilation rates are considerably higher than for most other
applications, it is very difficult to inject moisture in the room or into the supply air near
the room and achieve uniform moisture dispersion and adsorption. Therefore,
humidification is best provided as the primary HVAC air handling unit rather than at
the individual rooms.
Steam grid humidifiers are widely used for humidification in high ventilation rate
applications because they can inject high rates of moisture at a temperature that
enables ready absorption and uniform distribution. Since laboratory and health
facilities generally need steam for sterilization and many other purposes, a steam
grid humidifier is a very practical way to also provide the moisture. When using
steam for humidification, note that it must be “clean steam” generated from potable
water and not contain any chemicals as are typically used in boiler water for steam
generation.
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Clean steam can be generated from boiler generated steam by utilizing
the boiler steam as a heat source for intermediate heat exchangers which then
generate steam from potable water. The deposits that accumulate in such heat
exchanger applications will need to be periodically cleaned. It may be advisable then
to utilize multiple heat exchanger units thus enabling each to be periodically shut
down for cleaning.
Other types of humidification equipment may be suitable for laboratory applications.
However, care must be utilized in selection to ensure the units have sufficient
capacity and are responsive to the airflow variations that can occur particularly in
VAV laboratory ventilation systems. Also, be sure that the operation of the
humidification units will not pose the potential for moisture to settle out and
accumulate in the duct system resulting in fungi and bacterial growth. The current
focus on indoor air quality issues in particular requires increased vigilance on the part
of a HVAC designer to ensure against creating such potential problems.
Humidity Control for General Applications
Humidity control at the primary air handling unit is applicable to laboratory rooms or
facilities where the humidity can vary from 40 to 60% throughout the year, and where
no room will have an unusually high humidity gain. Figure 17 shows a general
humidity control arrangement where one typical laboratory provides feedback for
controlling a humidifier in the primary air handling system that serves several room.
The humidity sensor should be located in a laboratory which is typical or
representative of the other laboratories that are served by the same primary HVAC
system. Note that if tighter individual room humidity control is required or if there will
be some rooms with high levels of moisture gain, this control arrangement will not be
adequate.
The control arrangement shown in Figure 17, using only one humidity sensor in one
laboratory room, can be improved if necessary by incorporating a humidity sensor in
several (or all) rooms and then averaging the inputs in the room controller. This
would add to the cost but will provide better average control since more laboratories
would have a say in the process. This would be an appropriate arrangement if none
of the laboratory rooms would be very acceptable as the typical laboratory room.
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Clean steam is differentiated from pure steam which requires distilled water for generation. Pure steam is not normally
required for laboratory humidification systems.
80 Siemens Building Technologies, Inc.