Experimental Investigation to Determine Accumulation of Lubrication Oil in a Single Tank Evaporator with Tank at the Top at Different Compressor Operating Speeds

A number of experiments were conducted to determine oil accumulation rates for a single tank laminate evaporator with the tank at the top. The tank at the top results in a U-shaped evaporator plate design at the bottom where oil can collect under extreme operating conditions. A typical 4 pass laminate evaporator was used for testing. R-134a with an oil circulation ratio of 3% was used for this study. The AC system was run for extended periods (2∼4 hours) at different compressor speeds to simulate the variation of load on the evaporator. Evaporator samples were taken off from the test stand and weighed after recovering refrigerant. The evaporators were designed such that it could be isolated by shutting off valves at the inlet and the outlet connections.

The experimental data shows some accumulation of the oil (a max of 18.6 gm) in an evaporator when the air conditioning system is operated at the worst case scenario, i.e., at lower compressor speeds with tank at the top for extended periods of time (4 hours). However, the accumulation was not significant to cause the compressor to fail. It seems that the maximum accumulation of the oil in the evaporator levels off after 30∼90 minutes of operation. At higher compressor speeds, the oil that had accumulated in the U shaped section of the evaporator at low compressor speeds is carried out of the evaporator by the high refrigerant velocities.

Tests were also conducted to see the amount of oil that is necessary to coat the inside surface area of an evaporator for a virgin core. For the evaporator core under investigation, 3.0∼3.6gm of oil was required to coat the inside heat transfer area. This translates to a thickness of the oil layer of approximately 5∼6 microns.