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 DEBRIS AND FILTRATION—AND OIL BALANCING   By Paul Weissler  - Back to Index Page

When debris is an issue, you need filtration. Sure, there are accessible screens that can be replaced or cleaned (orifice tube, perhaps expansion valve and/or receiver-dryer).

Inspecting them usually indicates how much debris got through the condenser.

Suction side “thimble” screens don’t look very big, but their shape gives them a surprising amount of surface area and they save a lot of compressors from debris that had blown back into the suction side.

A nasty problem with debris: it often doesn’t stay put unless it’s trapped in a filter or screen. So a technician changes some parts, but not the condenser, runs the system and soon encounters a failure from an apparent restriction.

This time he replaces the condenser, but debris already came out and plugged an orifice tube or expansion valve, which previously was replaced. So the parts all may be new, but because of the sequence of replacement, there’s still debris in the system. Moral: the more complete the repair the first time around after catastrophic failure, the better, and if a suction screen and liquid line filter go with it, you’ve got some backup protection.

Flushing (if done correctly for the full system) should remove all the oil, so you have to make a suitable adjustment when you replace the compressor, condenser, etc. There isn’t a “standard” procedure. A replacement compressor may come with no oil, a full charge or some amount in between. Because one compressor goes into systems with different oil capacities, the best bet:

Rremove as much oil as possible from the new compressor (it takes time. Turn the shaft slowly until you are at the point of very diminished returns).

Then add oil to each component according to the manufacturer’s recommendations for new parts. There’s surely some oil left in the compressor and what you add may result in an ounce or two extra in the system, but it beats an ounce or two short.

A far greater problem is from just bolting on a compressor with a full oil charge when there may be more than a full charge remaining in the system from the OE charge and any additions during previous service. 

If it’s the first compressor failure, no indication of much debris, and the technician decides not to flush components, he should remember to adjust the oil level in the new compressor, referring to what he could drain from the failed one. However, if that compressor has lasted a long time, he should remember that one of the reasons that compressors repeat-fail is because the system is loaded with oil. Perhaps previous shops kept topping up a leaking system and added a few ounces of oil each time.

Then the technician installs a new compressor with a full charge of oil and combined with debris left in the system, encounters a repeat failure. He replaces that compressor, maybe changes the accumulator or receiver-dryer and a couple of hoses too, but the new compressor contains still another full charge of oil. Yes, even if the compressor doesn’t hydro-lock, there’s so much oil in the system that cooling is bad. Perhaps the technician tries adding more refrigerant to improve cooling, and the head pressures go up. Well, you can see where this is leading.

But before putting back all those trim pieces that he had to take off to gain access, the technician also should leak check with a premium electronic detector. Loss of refrigerant causes a performance drop, and if a technician has been focused on the compressor and sees pressures that are in the low-normal range, he concludes the compressor failed again. As we’ve pointed out in previous issues, loss of refrigerant that leads to poor cooling may not produce big changes in system pressures. But it reduces oil circulation (the refrigerant carries the oil, of course). So when the compressor fails once more –although perhaps

not until next season – the technician is complaining to his parts supplier about compressor quality.



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