Selection Criteria

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

Rawal Devices – Humidity control – APR Control

Applying the APR Control to new and existing systems requires proper evaluation and selection.

These criteria include:

  • System capacity – both total and staging
  • System configuration – single or multiple circuits
  • System type – package or split system
  • Refrigeration type – R-22, R-410a, R-407c and others for which we can provide capacity control and modulation
  • Specific to split systems – suction line sizing and routing with specific attention to suction gas velocity for oil entrainment back to the compressor at low load conditions

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. Most systems that employ a low minimum of outside air (ventilation 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.

When system application deviates from the base standard, which is why we often get called upon, Rawal Devices engineers look at the whole application, including outside air introduction, space usage, internal base load of the space, and (on existing systems) what is the condition of the space and how is the unit currently operating (cycle timing/runtimes). In addition, we also review how the controls impact system runtime and performance, the availability of upgrading controls, or adding some small amount of reheat where it may be necessary.

Please note that when using these sheets, you must take staging and circuiting into account. This means that a 10-ton system is often the result of two 5-ton stages, and a single APR Control installed on the lead circuit/stage might be sufficient for the needs of the system. So selection needs to be based on the 5-ton stage and all the factors that might impact the operation of that stage.

Keywords: humidity control

Other products and services from Rawal


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.



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.