APR Control in Reheat Systems

Where increased humidity control is required, the APR Control can be applied to allow use of humidistat control with significantly reduced reheat. In this case, the APR Control provides additional part-load control to eliminate potential overcooling while in the dehumidification cycle. (This is specifically for systems where application of the APR Control will not resolve (potentially) long off cycles.)

By sizing reheat (electric, gas-fired, or refrigerant hot gas) to match the range to which the APR Control can modulate, the evaporator can be kept on line in a call for dehumidification with no risk of overcooling the space. The APR Control will modulate capacity to minimize any excess cooling capacity above the capacity required for the dehumidification cycle. Space temperature remains comfortable, providing temperature neutral discharge air while allowing the system to drive down relative humidity.

Example Application

10-Ton Single-Circuit Air Conditioning System

If the entire sensible load in the conditioned space is less than the 10-ton capacity of the air conditioning system, then it is possible that using the refrigeration circuit of the system for dehumidification could overcool the space, driving space temperature down while the system is attempting to drive down the relative humidity. Normal use of the APR Control could extend the operational time of the air conditioning circuit to improve relative humidity, but capacity control of the refrigeration circuit alone might not extend the run time long enough.

The addition of the APR Control will assure that the space temperature drops less quickly on a humidistat call for dehumidification. But if the actual load in the space (at a given time) is still less than the range of the APR Control, then overcooling is possible. Rawal Devices has seen a number of systems where the addition of the APR Control has increased the runtime of the system from 5 to 15 (or more) minutes, but this may still be too short a cycle to effectively dehumidify. Experiments have shown that if a system does not operate for at least 14 to 15 minutes, none of the moisture from dehumidification hits the drain pan and thus is removed from the space. Plus, the APR Control will not turn the system on when there are long “off” times in the cycle.

A 10-ton system for dehumidification would normally require electric, natural gas-fired, or hot gas reheat of nearly 35KW to assure that the cooling will not overcool a space with no load other than the humidity (latent load).

With the application of an APR-2.5 (in an R-22 system) to provide 7.0 tons capacity control (65%-70% of system capacity), the system will modulate capacity from 10 tons down to 3 tons. By applying reheat to match approximately 50% or no less than 3 tons (36,000 btus equal to approximately 10KW of heat) of the cooling capacity, there will be no risk of overcooling the space while providing significant savings on operation of the reheat for dehumidification exclusive operation.

See diagram

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