What really happens is when the cabinet temperature goes below –10 degrees, the back pressure goes into vacuum, and the system cycles on the low-pressure switch.

Since I could not find a pressure-temperature chart for 408A, I could not check the superheat of the system. I was wondering if 408A is the right refrigerant for this kind of application. If yes, then I could not understand why the back pressure is so low. I checked the piping for blocking. It seems to be all right. Also, there is no moisture problem.

ANSWER:
From Gus Rolotti
Technical Manager
Atofina Chemicals Inc.
Philadelphia, PA

I am assuming that the system was originally an R-502 system that was properly retrofitted to R-408A, but had stopped working prior to your servicing it.

First of all, you asked if you are using the right refrigerant. If the system was originally designed to operate with R-502, then R-408A is the right refrigerant for the application. If the system was not originally designed for use with R-502, then you need to ensure that the proper conversion was made. If not, you can either convert properly or switch to the correct refrigerant. You should be able to determine all this based on the nameplate on the compressor and/or system and, hopefully, on the retrofit information labels. Incidentally, I cannot emphasize enough how important these informational labels are for the future service of the system, either by you or any other technician.

If your cabinet temperature was –10 degrees, then your coil should be at least –10 degrees or colder. From the pressure-temperature saturation charts, you can see that the corresponding pressure is approximately 20 psig and not the 10 psig you measured. The corresponding temperature for the 10 psig you measured is about –25 degrees.

Since this is not what you have, you probably have either an obstruction in one of your lines or filters (debris, etc.), your TXV could be undersized, or you are low on charge.

An important tool in troubleshooting your system would be to measure its superheat. You said you did not have a pressure-temperature chart. You can obtain one through your refrigerant supplier, or you can download one from our website at www.forane.com.

No Data Plate

I want to offer some additional comments to the questions dealing with charging systems with no data plate that appeared in the Jan. 8 and May 14 issues of The News.

Just because a system has a sight glass, it should not be a direct indication of charge until the glass is full. Using 10 degrees to 15 degrees suction superheat is a good general rule as measured at the compressor service valve. Low-temp systems can run suction superheats as high as 50 degrees.

In addition, liquid subcooling is just as important as superheat when “blindly” charging a system.

Superheat is important in maintaining compressor longevity and proper motor cooling. However, to focus only on superheat when charging a system that you have no idea what the weighed charge should be, could be asking for trouble.

Most important is to determine whether or not the system requires a critical charge. Systems that utilize a TXV metering device and a liquid receiver can usually charge to a full glass and 99% of the time will be trouble free as long as no other system problems are present.

This will not always hold true for the replacement refrigerants that we are seeing these days and on some special applications. But let’s assume a standard R-22 air conditioning application. Critical-charge systems require a little more investigating on the tech’s part as to what method he should use to charge the system. The tech should determine the refrigerant and the type of metering device if not known.

For example, a system that uses a fixed orifice to meter the refrigerant flow to the evaporator would require a superheat-type charging method to obtain a correct charge. Any fixed metering device is relying solely on the pressure differential across it to determine its flow rate. For this reason, evaporator load, suction superheat, and ambient temps become critical in determining the proper charge.

However, the same system that has a TXV should be charged using a subcooling method to obtain both the proper charge and the maximum evaporator efficiency. Of course, this does not mean you should ignore the suction superheat when charging a TXV system. This metering device has the ability to throttle the refrigerant flow in an attempt to maintain its set superheat value. It requires only quality liquid from the condenser and it takes care of the rest, based on the evaporator load.

It is quite common to see a system that is fully charged, but at the same time has poor evaporator performance and poor superheat due to a faulty metering device. Attempting to charge this system based only on a superheat reading would never work. If you were able to get the superheat anywhere near 10 degrees, the system would more than likely be way overcharged.

Obtaining the saturated condensing temperature and the amount of liquid subcooling when charging confirms that you have quality liquid refrigerant, as well as giving the technicians a baseline to work from.

To attempt to achieve a 100% efficient evaporator with a lower superheat adjustment is silly. First off, there is no such thing as a 100% efficient evaporator because of the flash gas that occurs during initial expansion at the evaporator inlet. Lowering the superheat setting on the TXV will not reduce the amount of flash gas. Subcooling the liquid, however, will reduce flash gas at the TXV, and allow the efficiency of the evaporator to rise without lowering the superheat. Given the same load on two evap coils — with 0 degrees subcooling and 10 degrees suction superheat — it will be less efficient than that having 10 degrees of subcooling with the same superheat.

Also, “X” degrees of superheat is X degrees no matter how long the line is. Yes, long line runs tend to pick up some extra superheat. But I would be more inclined to pay attention to line sizing and piping practices to maintain compressor life in long suction lines.

There is no question that superheat is important when charging a system. But subcooling is something that also should always be checked when charging and/or determining the performance of any system — data plate or not.

Brazing Help

I want to offer some additional assistance to the writer of a question on brazing that appeared in the June 4, 2001 issue of The News. I would suggest checking out the website www.lokringusa.com for more assistance in this area.

With the new energy restrictions, technicians will see more and more of the aluminum/copper joints.

Publication date: 01/01/2002