Consider this: the compressor moves close to 1,000 pounds of refrigerant an hour when operating at a 50ºF evaporator saturation temperature. At a 45º saturation evaporator, it is near 800 pounds of refrigerant. The metering device moves the same amount of refrigerant in a liquid form that the compressor moves in a superheated form. Think of the work that takes place to move that much liquid and vapor gas. The compressor must receive a low pressure cold superheated gas at around 60º and move each molecule to a high pressure superheated gas to near 190º. Therefore, keeping the oil clean and void of moisture is a must. Allowing the compressor to operate in as little time as possible in the in-rush amperage mode is very important.
I recommend a start kit to accomplish a short start-up period. All of the high efficiency compressors that have expansion valve metering devices use start kits. The compressor must overcome the pressure difference at start up when the expansion valve is a positive shut-off valve. In my opinion, the start kit is an aftermarket item that should be offered to every client. The life of the compressor depends on it. I recommend a three-wire hard start kit the HVAC technician can carry, since he typically services most makes of air conditioning compressors.
And when using a start kit, the run capacitor needs to be addressed as well. If the run capacitor is weak, the start kit will start the compressor but will let it operate with higher amperage than designed. This will shorten the compressor’s life.
Another item in the circuit is the contactor. In an older system, the contactor could have pitted contacts. The pits could cause great harm to the compressor motor during the inrush period as well. The inrush amperage is near seven times the running amperage, placing it at times over 100 amps. The high amperage will cause a serious voltage drop that can only be found with an oscilloscope. Technicians do not carry one on the truck. When the pitting gets bad enough to cause a continuous voltage drop, permanent damage has already occurred. Just a visual check will show the pitting, and the tech should recommend a contactor change-out to prevent a continuous problem to the compressor windings.
Again, a start kit helps significantly cut the start-up period. A three-wire hard start kit in many cases will reduce the inrush period by 50 percent.
During this business cycle it is imperative that each technician keep a close eye on the company revenues. Profits are earned by professional technicians, and the more the technician understands about the science of the refrigeration system, the easier it is for the company to charge professional wages.
Keep all of your charges on a legitimate level and back them up with the science — not just an “I think you need to change this out” attitude.
Publication date: 04/09/2012