cfm Tech Tips – Compressor Failure at Startup
When a condensing unit or heat pump is assembled at the factory, it is charged with enough refrigerant to operate with a matching indoor coil and a prescribed amount of refrigerant piping (15 feet for York). The compressor contains the correct charge of oil in its crankcase. One of the basic characteristics of a refrigerant and oil mixture in a sealed system is the fact that refrigerant is chemically attracted by oil and will vaporize and migrate through the system to the compressor crankcase even though no pressure difference exists to cause the movement. Upon reaching the crankcase the refrigerant may condense to a liquid. This migration continues until the oil is saturated with liquid refrigerant. The specific gravity of the liquid refrigerant is greater than the specific gravity of the oil allowing liquid refrigerant to displace oil at the bottom of the compressor crankcase. The bottom of the crankcase is where the oil pump picks up oil when the compressor is running.
Most compressor failures that occur at start-up are not the fault of the compressor, but can be traced to the following chain of events. Liquid refrigerant is a very good solvent and will wash the oil off the surfaces that it comes in contact with. With refrigerant displacing oil in the sump, at start up refrigerant is drawn into the oil pump instead of oil. It washes the oil out of the bearings and allows metal-to-metal contact of the bearing surfaces. Immediately the heat of friction galls the bearings and some permanent damage occurs. If the “oil wash out” time is short enough that the compressor survives, it now faces another problem. Upon start up, the crankcase pressure suddenly drops. This change in pressure causes the liquid refrigerant immediately to boil off into vapor. Since it is entrained in the oil, it causes a violent foaming action of the oil to occur. The suction port inside the compressor shell picks up the foam, and much of the oil gets pumped out into the condenser coil. The oil must travel through the condenser coil and evaporator coil before returning to the compressor. Some run time is required before enough oil returns to the compressor for proper lubrication. Meanwhile, any oil remaining in the compressor is suspended as foam and is unavailable to the oil pump. The first two minutes are when most failures occur. Even if the compressor lives through the first few minutes, some bearing damage most likely will occur that will shorten the life of the compressor.
Because refrigerant is chemically attracted by oil, this phenomenon will occur even at high ambient temperatures. As stated in the installation instructions, a condensing unit or heat pump must be open to the system a minimum of 8 hours, with crankcase heat applied when applicable, before enough separation of refrigerant and oil occurs to minimize bearing damage at start up. If it becomes necessary to start a new unit before waiting the prescribed 8 hours, it is imperative that you coax the refrigerant out of the oil. (Please refer to the back of this page for the correct procedure.) Using common sense and caution during the startup process of a new unit will eliminate most compressor failures.
Apply heat to the compressor crankcase with a heat lamp, heat gun, heater blanket or some other safe method. Continue applying heat until the crankcase is 25° warmer than ambient temperature.
Energize the compressor for a maximum of 2 seconds then let it set for ½ minute. Repeat the process three times adding 1 second to each successive run cycle, for a total of four operations. Now energize the compressor for 15 seconds and leave it off ½ minute.
On 2 Seconds, off 30 Seconds
On 3 Seconds, off 30 Seconds
On 4 Seconds, off 30 Seconds
On 5 Seconds, off 30 Seconds
On 5 Seconds, off 30 Seconds
The short run time between each cycle minimizes the pressure change in the compressor shell and greatly reduces oil foaming. The ½ minute off time allows the oil to settle back into the sump. It is now safe to operate the compressor without loss of lubrication. This entire process takes less than 5 minutes.
- cfm Tech Tips – Compressor Failure at Startup – March 12, 2018
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