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It is possible to use one of several methods to modulate or to control the level of the moisture removal capacity of the dehumidifier. The specific application usually dictates the appropriate method. However, there are some situations in which the best approach may involve some combination or variation of the five methods. These methods are described below and are presented in increasing order of their effectiveness or their ability to maintain a constant output.

METHOD ONE:
On/Off control of the entire dehumidifier including both process and reactivation air flows using a humidistat or dewpoint sensor to monitor the return air to the dehumidifier. This control procedure typically is used with the system whenever the dehumidifier is installed as an independent unit and is not tied into a make-up or air circulation system. (plus or minus 5% RH)

METHOD TWO:
On/Off control of the reactivation heaters and blower. It is an inexpensive method of control to install, however, the effluent process air will tend to have greater variation in moisture content or dewpoint than will be the case with units controlled by other means, as outlined below. (plus or minus 4% RH)

METHOD THREE:
Modulation of reactivation heat using step or proportional controllers, SCR, etc. to vary the supply of energy to the heaters or to adjust the number of heating elements being energized in some way. This modulation of the reactivation temperature input to the dehumidifier offers the best opportunity to maintain a very close span or uniform effluent process condition. The consumption of energy also is minimized by the system’s inherent effort to supply only the minimum reactivation energy required to satisfy the controlled space humidistat or dewpoint sensor. (plus or minus 2% RH)

METHOD FOUR:
Controlling the rotor/bed rotational speed is the most effective and responsive system for capacity control. However, the controller must also be of rapid response time. (plus or minus 1% RH)

METHOD FIVE:
Bypass process air around the desiccant rotor with a modulating damper. This also will give (plus or minus 1% RH).