APR Control – Operational and Functional Description

By: Rawal  09-12-2011
Keywords: humidity control

Rawal Devices – Humidity control – APR Control

The hot gas enters the desuperheating chamber at an angle creating a circular hot gas path, wiping the walls of the chamber and giving up excess system heat. The gas then travels up the “mixed gas lift tube,” losing energy as it returns to the system’s compressor suction inlet. The increase in suction pressure resulting from the hot gas and suction mix reduces the compression differential in the compressor, causing a decrease in required compressor horsepower.

Through the evaporator coil, the APR Control senses the enthalpy of the return air. A reduction in the enthalpy of the return air causes a drop in coil temperature that causes a drop in the suction pressure. This triggers instant response in the APR Control hot gas bypass valve and causes it to bypass some of the compressor discharge through the APR Control desuperheating chamber to the compressor suction return. This results in a decrease in hot gas to the condenser coil and ultimately less liquid refrigerant to the evaporator coil.

At the evaporator (in an R-410a system), the APR Control attempts to hold the DX coil at 105 p.s.i. by changing the rate of flow of liquid refrigerant into the DX coil and adjusting the dehumidification window of the evaporator coil. Simultaneously, the difference in system BTU capacity and the current BTU load is continually being bypassed in the form of hot gas through the APR Control’s desuperheating chamber and into the suction line at the compressor.

The overall effect is a reduction in cooling capacity of the DX coil corresponding to the reduction in the load. Most importantly, the APR Control will not decrease the maximum or designed capacity of the system when it is needed!

Keywords: humidity control

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We strongly recommend isolation valves to assist with trouble shooting the installation, as well as charging the system, and adjustment during the initial setup of the equipment. Please note that at all times, for all types of APR Controls, it is necessary to protect the valves of the APR Control from excessive heat associated with this work.


APR Control –System and User Benefits

In the design of small direct expansion systems, the APR Control gives the contractor or engineer the opportunity to select equipment based on maximum possible load and have continuous capacity modulation down to the minimum required load – all at increased system efficiencies. Drives the system into a constant temperature & stabilized humidity mode by creating long operating cycles.



In addition, they contain charts with control dimensions as well as sizes for the connections to the air conditioning system's hot gas, liquid, and suction lines. Rawal Devices’ specification sheets show the models and capacities of the APR Controls. Please contact Rawal Devices engineers if you have any questions. Rawal Devices – Humidity control – APR Control.


Selection Criteria

Most systems that employ a low minimum of outside air will benefit from about 50% modulation on a simple single-stage system, or even on the lead stage/circuit of a simple multiple-stage system. We are pleased to assist both new and long-term customers with selection of the best APR Control model (percentage of modulation) for their particular application.



By matching system capacity to continuously changing load conditions, the APR Control-enhanced system is able to maintain better comfort conditions, as well as provide a number of unique solutions that standard air conditioning systems are unable to achieve. From residences to hospitals, computer rooms to offices, pharmaceutical clean rooms to manufacturing facilities, the APR Control has always provided the solution.


Air Conditioning SystemsThe Inevitable Design Flaw

The air conditioning system is “short cycling,” meaning that for each 5 minutes the air conditioning system is actively cooling the space, there are 20 minutes when the system is merely recirculating air and not dehumidifying it. A standard thermostat, when set at 70°F, will turn the air conditioning on at 72°F and cool the space, at full capacity, until the temperature is 68°F and the thermostat is satisfied.