Air Conditioning SystemsThe Inevitable Design Flaw

By: Rawal  09-12-2011
Keywords: Air Conditioning, Air Conditioning Systems, Humidity

Rawal Devices – Humidity control – APR Control

Air conditioning systems are primarily on/off devices of a fixed capacity, controlled by the limits of the thermostat. A standard thermostat, when set at 70°F (with +/- 2°F range), 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. This is repeated continuously.

Standards for air conditioning design require that the system has sufficient capacity to meet several design criteria (i.e. outdoor temperature, sunshine, internal occupancy, and ventilation requirements). As most of these criteria are maximums for the summer peak, the system will be designed larger than necessary most of the time. When the system is oversized, the temperature may remain cool, but the unit does not operate long enough to sufficiently dehumidify the space. 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. During the off cycle, the humidity level rises and the combination of low temperature and high relative humidity creates an uncomfortable space.

Keywords: Air Conditioning, Air Conditioning System, Air Conditioning Systems, Conditioning Systems, Humidity, humidity control

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APR Control – Operational and Functional Description

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 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.



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.