Author John Phillips
The condensing unit is a key component of all vapor compression air conditioning systems. We are all familiar with condensing units as the noisy exterior portions of air conditioning systems that blow hot air to the environment.
The purpose of the condensing unit is to compress refrigerant that is in a low pressure vapor state and then to remove the heat of compression from the resulting high pressure vapor so that it will condense into a high pressure liquid before it reaches the air conditioning system expansion valve and evaporator.
A simple condensing unit often found in residential air conditioning systems has a hermetically sealed compressor, a condensing coil, and a condensing unit fan. In addition to hermetically sealed compressors, these units have brazed fittings at all tubing and coil joints to form fully sealed refrigerant systems that cannot be opened without cutting.
A typical residential HVAC Unit.
Larger condensing units often have compressors and condensing coils that can be removed for service or replacement without cutting or brazing. These larger units sometimes have multiple compressors and fans that allow continued operation if some of the components are inoperable. The larger condensing units are usually repaired in place.
Failed compressors are one of the most common problems encountered with condensing units. Compressors can fail for a variety of reasons including normal mechanical wear, inadequate refrigerant or lubrication, component defects, and electrical malfunctions. Testing and inspection is usually required to determine the cause of a stalled compressor. Electrical testing can often determine the cause of a stalled compressor without removal or disassembly of the compressor. Mechanical inspections require removal or isolation of the system refrigerant from the compressor and disassembly of the compressor. Disassembly of a hermetic compressor is destructive and requires replacement of the compressor.
Lightning damage is often suspected when a compressor or a condensing fan is inoperative. Compressors have large electric motors that can provide an attractive ground for a lightning strike, particularly in buildings that do not have a good electrical ground. Often, rather than lightning damage, electrical problems such as a bad motor start relay, a damaged start or run capacitor, or the proper function of a protective safety sensor can prevent a compressor from operating.
Hail damage for units with exposed condensing coils is another common problem with condensing units. Condensing coils often have aluminum fins that bend easily. Hail can deform the aluminum fins enough to cause some reduction in heat transfer across the coils. Coil fins can normally be straightened with a fin comb if the hail impacts are minor enough and the fins are not badly corroded.
An assortment of air conditioning condensing units at an office building.
This is the second of three blogs concerning air conditioning systems. The third blog will discuss air conditioner evaporators and condensate removal systems as well as problems that can be encountered with this equipment.
John Phillips, senior consulting engineer at Warren, has more than 30 years of crane and heavy equipment experience and more than 19 years of experience in forensic engineering. A licensed professional engineer in South Carolina, North Carolina, Georgia, Louisiana and Ohio, he’s NCEES registered both as a model engineer and with The United States Council for International Engineering Practice, USCIEP. John has designed crane systems, supervised installation, tested and certified lifting equipment even serving as a project engineer for maintenance and certification of nuclear weapon lifting and handling systems. John is a certified fire and explosion investigator and fire and explosion investigator instructor by the National Association of Fire Investigators. John is a member of the American Society of Materials and American Society of Testing and Materials, as well as a voting member of ASTM Ships & Marine Forensic Sciences, Forensic Engineering, and Performance of Buildings committees.
