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Why the Dehumidification Process in AC Systems are Ineffective

May 13, 2014

Previously, you learned how the dehumidification process typically works in a standard direct expansion air conditioning system.

Here, you’re going to learn the two problems with the current dehumidification process that we at Rawal Devices keep getting asked about, and what you can do about it.

Problem 1: Not Enough Run Time

One of the reasons the humidity in a space is high is due to a lack of run time. When the system is running, air from the conditioned space returns to the evap-air handling unit via the return vent and duct then moves across the evaporator coil so the dehumidification process can take place.

However, if there isn’t adequate runtime, not enough air will return from the conditioned space and move across the evaporator coil. When this happens, the moisture on the coil returns to the airstream and heads back into the previously conditioned space.

When there isn’t adequate runtime, humidity falls, but only temporarily.

Problem 2: Excessive Off Time

Excessive “OFF” time occurs when the sensible heat in the room isn’t sufficient to trigger the AC system “ON”.

When there is excessive “OFF” time, humidity will build up in the space to a point where thermostat runtime and the AC system’s dehumidification are not enough to overcome humidity in the space. This is especially noted when there is an extreme amount of humidity in the environment like you might see on a rainy day.

When there is excessive “OFF” time, your customers perceive the air to be stagnant and “stuffy”. To combat this, occupants will often adjust the thermostat to get the AC to start sooner and provide the illusion that the system is doing its job. As you know, that doesn’t work and while the space is cooler it is feeling less comfortable – and counter to plan, relative humidity is actually rising!

How to Remove Humidity

While these two problems appear to end at the same place, the short runtime (w/ short “OFF” time) problem can likely benefit from an APR Control alone, immediately. Installation of the APR Control, properly set-up and adjusted to the appropriate operational range (suction pressure), will help extend runtime or the “ON” part of the on/off cycle.

The APR Control modulates the refrigeration circuit reducing system capacity, keeping the thermostat from being satisfied and allowing the system the runtime period necessary for proper and thorough humidity removal from the conditioned space.

Due to the nature of the excessive “OFF” time problem, when there is an absence of sensible heat, there is no guarantee that increasing the run time will improve space comfort. The “OFF” time may be too long and the moisture built up in the conditioned space cannot be removed in the “ON” time available. A system that runs 5 minutes “ON” with 30 minutes “OFF” might not be significantly improved by merely doubling, or tripling the runtime (to as high as 15 minutes) “ON” if the 30 minute “OFF” cycle problem remains.

The space and air conditioning systems may require some sort of further enhancement. That enhancement could include the APR Control in combination with some sort of reheat to balance against the excess cooling capacity. Or the APR Control could be used with a humidistat to drive cooling operation on rising space humidity levels, and override simple thermostatic control, extending runtime for an effective solution.

We have added these types of fixes on jobs when needed over the last 20 years, eliminating the need for major equipment changes or system re-engineering. Let us know what types of application problems you are contending with, I like to think we will be able end the on/off cycle making your life easier!

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