Sightglass oil problem

Q   I found five out of five compressors on an R-22 low-temperature rack system with no oil in the sightglasses. The oil reservoir was filled to the top. I changed the oil filter and all the compressors, and filled it up with oil. Then I rechecked the net oil pressure, which ranged from 18 pounds of pressure net to 35 pounds of pressure net.

Should I have cleaned the pickup screens and sensors like I would have if oil controls were tripping, even though there was enough oil?

A   First let's discuss the initial problem: no oil in the compressor sightglasses. A plugged oil filter certainly would cause this and replacing it would again restore the flow of oil from the reservoir to the compressors. 

There could be another reason for low compressor oil levels with a full reservoir: insufficient pressure differential between the oil reservoir and the compressor crankcases. A brief review of the oil system will explain this potential. 

A low-pressure oil system consists of an oil separator, oil reservoir and differential check valve. As the oil/refrigerant mixture flows from the compressor discharge though the oil separator, a high percentage of the oil will be left behind. The oil level in the separator will rise, eventually causing the separator float valve to open and allowing the oil to flow from the separator into the reservoir.

After a certain amount of oil is transferred to the reservoir, the oil level in the separator will fall, causing the float valve to close. Oil will not flow from the separator to the reservoir again until the separator removes enough oil from the discharge vapor to raise the float to a level that will open the float valve.

This oil is at near discharge pressure. Because the compressor oil level control was not designed to receive oil at such a high differential pressure (oil supply pressure minus crankcase pressure), a differential check valve is used to bleed the reservoir pressure down to some minimal differential above the common suction pressure.

The oil remains in the reservoir until the compressor needs it. As the oil level in a given crankcase falls, the float valve in the compressor's oil level control will open. This results in a flow path for the oil: from the reservoir, through the filter, through the oil level control float valve and into the crankcase (see Figure 1).

It is important to use an oil filter with the ability to remove small damaging particles from the oil. A 2-micron- to 3-micron-particle size rating with an efficiency of more than 95 percent would be advisable. A filter with such a rating will remove particulate that a standard filter-drier or suction filter will not and, therefore, has the potential to plug up sooner as it's removing more. When this happens the symptom described above will occur.

A potential problem can occur if a differential check valve with a small differential rating is used. For example, if a 5 psi differential check valve is used, there could be instances where there is no net differential due to the system dynamics. Operating the rack at 38 psig and using a 5 psi differential check valve would result in a 43 psig oil pressure in the reservoir.

Now, realize that the oil separator float valve is not always open. During the period when the oil separator is not feeding oil to the reservoir, if a compressor cycles off and allows the common suction pressure to increase to 43 psig, the differential between the reservoir and the compressor crankcases will be lost. Using a differential check valve with a higher rating, say 20 psi, will eliminate this possibility from occurring.

Now to the second part of the question. You have to ask yourself what would be the causes for low oil pressure? A dirty oil pickup screen would cause this. The plugged up oil filter is evidence that there was contaminant in the system, so it is logical to assume that there might be contaminant in the crankcase too. Not all of the oil in the crankcase has had the opportunity to pass through the oil filter.

Because oil separators are less than 100 percent efficient, some oil will make it through the separator with the discharge vapor. It will eventually return to the compressor via the suction line. This oil is only filtered by the liquid filter-drier (and suction filter if in use) and, therefore, may contain particulate matter that the oil filter would have filtered out. There is no way to determine if the oil screen is dirty without inspection.

The low oil pressure reading may be the result of defective sensors (if electronic sensors are used), worn bearing journals, a worn oil pump or a defective/plugged up oil regulating valve. Unfortunately, there is no way to pinpoint any of these as the cause without inspection.

Start with the easiest possibility that can either be verified or eliminated as the cause. By a process of elimination, you can determine the reason for low oil pressure.  Remember to check with the compressor manufacturer as to what the acceptable oil pressure and oil level should be.

Question from James D. Scott, Braintree , Mass. Answer by Dave Demma, senior application engineer supermarket sales, Sporlan Valve Division of Parker Hannifin.