Service Hotline: 10/20/2008
October 20, 2008
From Roland Galatas
I have a 3-1/2-ton residential, split a/c system with R-22, and a fixed orifice metering device. I was checking the refrigerant charge the other day, when I noticed the pressure on the low side would not stay at a steady pressure. It was going between 55 to 74 psig. This pressure was not sporadic, but a gradual fall and rise. The ambient temperature outside was 90°F, inside wet bulb temperature was 62°, return air was 73°, and supply air was 63°.
What do you think the problem is?
By Emerson Climate Technologies
To properly analyze this system, you really need to know the suction line temperature at the compressor to allow you to calculate the superheat. It would also be helpful to know the head pressure that correlates to the suction pressure readings of 55 and 74 psig. Without that information, I’ll have to make some assumptions as to what is probably happening.
From your description, it sounds like the system is running low on refrigerant, resulting in the low (55-psig) suction pressure. As the suction pressure drops, the head pressure also drops, resulting in less pressure differential across the fixed orifice and a reduction in flow. As this continues, the head pressure slowly builds back up until there is sufficient refrigerant and pressure available to again feed the fixed orifice properly, allowing the suction pressure to come back up to a more normal level (74 psig). This process continues in a cyclical pattern resulting in the gradual rise and fall of suction pressure that you witnessed.
Manufacturers of split residential a/c equipment generally have very explicit instructions as to how to check their systems for proper charge, and these instructions should be followed. Typically, the manufacturer adheres a charging chart on the inside of one of the access covers. These charts aid the technician in determining if the unit is charged properly having only basic information such as outdoor temperature, suction line temperature, and suction pressure. If the measured suction line temperature is higher than what the chart shows, the system is undercharged. Conversely, if the measured suction line temperature is lower than that indicated on the chart, the system is overcharged and some of the refrigerant must be recovered. If you use the chart included in your unit, I’m quite sure you will find it indicating a low charge, and adding refrigerant to bring it within the manufacturer’s recommendation will eliminate the gradual rise and fall of suction pressure that you are experiencing.
By John West
My question involves checking the integrity of the compressor. I have been informed that if it can pull down to 15-20 inches of vacuum then the valves are good.
Is this a common practice?
From Emerson Climate Technologies
Using vacuum is not a valid method to test a compressor’s valves/efficiency. There are two reasons for this:
1. If the compressor has two pistons but one connecting rod is broken yet the valves are OK, then the compressor could still pump down.
2. If the compressor is designed for air conditioning duty, the bore and stroke as well as the discharge and suction ports would be designed for high gas pumping volume at low compression ratios but would also have a large re-expansion volume. This will result in much of the compressed gas staying in the cylinder at high compression ratios, re-expanding during the suction stroke and keeping the compressor from reaching a vacuum with even a moderate head pressure. This could result in a false condemnation of a good compressor.
An inefficient compressor could be indicated by unexpected high suction pressure and low discharge pressure. If this is the case, go online and download the curve sheet for the particular compressor model and do the following:
1. Take an accurate reading of the suction and discharge pressure as close to the compressor as possible, as well as an amperage reading on the common line to the compressor and the voltage.
2. Find the amperage corresponding to the pressure readings on the data sheet. If the measured amperage is more than 10 percent below that on the data sheet, the compressor is inefficient.
There is one more step to make this a more accurate exercise. The data sheet lists the voltage that was used to generate the information. If the unit voltage is different, then divide the spec sheet voltage by the field voltage and multiply this times the amperage on the spec sheet. Now compare the amperage.
By John West
My question involves getting oil to properly return to the compressor. We have an 11-ton and a 16-ton compressor in the outside condensing unit. We discovered that when the contractor installed the unit, the 11-ton compressor side had the 16-ton expansion valve and was flooding. The 16-ton side is running OK. We corrected the expansion valve but have about 2-1/2 gallons of oil in the system that will not return. The evaporator is about 20 feet above the condenser and the line set is run underground about 50 feet. We have pumped the compressor down to 20 inches of vacuum and it held up to 10 inches. There is an unloader on one side. We have tried covering the condenser and elevating the discharge pressure. We have inspected all three valve plates and all look OK. Any suggestions?
From Emerson Climate Technologies
This is a case where more information is needed to give an accurate answer.
What indicates the oil is not returning? Are these semi-hermetic compressors? Does the system have an oil failure control that keeps tripping? Does the sight glass empty so no oil can be seen in it?
If the sight glass always shows some oil while the compressor is running, then the oil is returning but the compressors pump out any excess oil as soon as it hits the crankcase. Many semi-hermetic compressors will only tolerate an oil level below one-half to one-third sight glass and as more oil is added to the system, the compressor’s oil pumping rate goes up while the oil level in the sight glass stays the same. If, what is described here is the case, remove oil from the compressor until the oil level will no longer come up to the normal running level.
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Publication date: 10/20/2008