Older Residential SystemsCOMMENT:
From Dave Anderlik
Retired from Carrier Corp.
I have some concern with Dan Kramer’s response to a Hotline question concerning the charging of older residential units. It is often suggested from those in the field that charging can be done by pressure and feel.
That was maybe accepted 40 years ago, but not today. Manufacturers are very sensitive to the amount of charge required in a system. By using the feel method, you will usually overcharge.
If the manufacturer gives the refrigerant weight in the system — whether it is a split and especially if it is a package system — you weigh in the charge. Then, if the readings (especially superheat and subcooling) using an electronic digital thermometer are not correct, you have a system problem and it is not the refrigerant charge.
The other approved method of charging a system with a fixed metering device is by the superheat method. If it is a TXV system, you charge by subcooling.
We know there are always exceptions, but let’s recommend to the beginners and everyone else the proper method.
From Dan Kramer
Patent Attorney and Specialist Grade Member of RSES
The basic premise of my comment was that the system charge was unknown. This could arise from a missing label or one that did not contain charge information, or from a field-installed system where the lengths of the lines are not standard. Therefore, charging by weight was not an option.
The second premise was that there are some techniques that a refrigeration tech should know or should learn. Among these is the technique of charging by feel and pressure gauge, especially when some of the more modern instruments are not available to the technician.
I disagree that the proper method of charging a fixed restrictor system is by superheat alone. Charging such a system by superheat alone means the tech is ignoring the head pressure and the subcooling at the restrictor. That suggests an overcharge if the restrictor is “restricted.” Monitoring the head pressure and subcooling, even by touch, will prevent overcharge arising from a choked up cap.
Yes, charging a TXV system by subcooling at the TXV inlet is reasonably sound. However, what does the tech do if he or she has charged to a normal subcooling, but the superheat at the evaporator outlet is still 35 or 40 degrees F? To suggest that the tech should go to the trouble of pulling the charge and replacing the valve without giving the TXV adjustment a turn or two is somewhat unreasonable.
So, I think that each of us has learned how to do some things in ways we were taught were right.
Plugged Cap TubesCOMMENT:
From Duane Frost
Complete Temperature Systems
I want to report on problems we had with plugged cap tubes on one- and two-year old equipment. This was with under-the-counter, self-contained equipment, specifically two-door R-404A freezers and two- or three-door R-134a medium-temperature units.
We originally cured two of the two-year-old units — one with R-134a and one with R-404A — by replacing the cap tube with original size tubing only to have them plug again after 60 days.
Our second repair used large capillary tubing and an adjustment of the charge. That was 14 months ago.
Then when another five R-134a units failed in less than one year, the manufacturer was asked to correct the situation. We received a complete refrigeration system to drop into the reach-in. We were told at that time that the replacement systems were modified and contained a larger capilliary tube.
The manufacturer told us the units must be kept super clean and out of hot environments. My question to them: “Aren’t kitchens usually hot?”
Now, our customer’s units were kept clean because of their daily cleanup and our monthly service contract. Meanwhile, the local manufacturer’s rep and the manufacturer got involved in trying to compensate our customer for the replacement cost incurred.
One final thought: The labor involved in changing just the capilliary tube is approximately eight hours if you bring the unit to the shop, or six hours if done in the field. I would recommend a complete refrigeration system replacement that would take about three hours.
I do hope something is being done about correcting the synthetic oils the manufacturers are using.
From Dan Kramer
Patent Attorney and Specialist Grade Member of RSES
POE lubricants are uniformly demanded by refrigerant and lubricant manufacturers for use with HFC refrigerants. But POEs have an increased tendency to form sludges, especially at higher discharge temperatures. This is because they appear to be more reactive than mineral oil and also because they exhibit much greater solubility and attraction for water which contributes to their reactiveness.
At the same time, there are many situations where mineral oil and HFC refrigerants work fine together. Carrier and others have reported on tests that show satisfactory oil return, even with heat pumps at low suction temperatures.
There are perfectly valid reasons for mineral oil to work satisfactorily with these new refrigerants. I have reported on most of them in a paper that was published in the November 1999 issue of ASHRAE Journal. I have this paper saved in a tif format and would be happy to e-mail it to anyone who e-mails a request to me at DEK50@columbia.edu. This paper has a list of references, some of which address the matter of sludge formation.
From Hessdl (screen name)
I was reminded of the problems we had with R-22 some 35 or 40 years ago. It was caused by oil breakdown due to excessive discharge temperatures. Back then, it came from undersized condensers (30 degree TDs) and the greater heat of compression of R-22 compared to R-12. This went away in low temp with the advent of R-502 and in a/c with increased condenser capacity that reduced compression ratios.
To the question of how clean is clean, it depends on the ambient at the condenser and the compression ratio. Measure the temperature at the discharge valve to find out. If it is high enough to cause oil breakdown, then it is not clean enough or the condenser fan is not moving enough air. We found this most often in equipment from factories located where the summers were mild. They seemed to forget that things warm up in the Southwest.
By Somhvac (screen name)
We are having problems on a few a/c systems with oil leaving the compressor and not coming back. We raised the head pressure and the oil seemed to come back for a short time. Also our compressors seem cooler than normal.
Could it be that our condenser cooling water is too cold? Could it have too much of a charge?
From Sheri Wilkerson
Manager, Service Training
There is insufficient information of system design to identify all the possibilities of oil logging.
The low head pressure could cause insufficient refrigerant flow through the evaporator causing the oil to log. The head pressure could be raised during cold ambient conditions by various methods. Assuming this is a water-cooled unit, the head pressure could be raised by any of the following means: fan cycling on the cooling towers, bypass the water around the cooling tower, or even sump heaters in the tower.
Considering your question on the system charge, the overcharge would cause excessive head pressure on a expansion valve system and could cause a liquid floodback condition on a fixed orifice system.
An undercharged system would be more of a cause for oil logging than one that is overcharged. The oil is carried out of the evaporator by mass flow. A system that operates at low charge or at extended low load conditions may allow for oil to log out in the evaporator.
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Publication date: 02/11/2002