Low back pressure compressor application

[Abe]

Compressors usually are classed for either low back or high back pressure. Is it okay to use a low back pressure compressor for a high back pressure application, ie: using R404 for instance for a chiller room. I know it is not possible to use a high back pressure compressor for a freezer application

[Dan]

Hi, Aiyub. I am not sure I understand your question. But I assume you are talking about motor-compressors where the motors are internal to the device.

I will take a stab anyway. With Copeland semi-hermetic compressors, for example, you will notice that the same compressor can be used for medium temperature r12 or 134a and R502 or 404a duty. But you cannot use that compressor for r404a medium temperature duty.

The motor will be too small for the large displacement of the higher suction pressures. Often you will see OEM people such as self-contained reach-in manufacturers utilize a compressor that should not be used this way, but they get approval from Copeland as long as they install a suction pressure limiting device for the normally unapproved duty. It is mostly done to save money on the compressor.

Carlyle, on the other hand, has broader applications available because they do not size their motors as tightly for a specific application. So it gets into which manufacturer, which model, etc when you consider changing duty on a compressor.

The most important consideration is the motor. But compressor design is important too.

Dan

Dan

[Dan]

Good Lord. I did it again. My nick name is Dan Dan because I sign my name twice. At least now I know why I do it. Thoughtfully responding, then urgent phone call, sign off, then have one more hurried thought and sign off again.

Anyway, I missed a point. The heat of the motor in a suction cooled motor compressor requires that the gas flow is predictable over it. A medium temperature compressor might have a motor that was sized for a predictable power factor, gas flow, and amperage duty that will not portray itself in a low temperature application with the same refrigerant. I think the danger with using a high pressure compressor for low temperature application would be motor overheating as a result of reduced cooling effect from the refrigerant flow.

Would love to hear from the others as to whether I am on or off track here.

This is a very good question, in my opinion.
One Dan.

[terrygoodrich]

Low temp compressors definitely cannot be used with medium or high temperature evaporators because they will overload. They have a larger displacement to compensate for the lower suction pressure. That is why many low temp systems include a crankcase pressure regulator for warm box pulldowns or when recovering from defrosts. On the other hand, a compressor designed for high back pressure will be underloaded when used on low temp systems. This can result in motor overheating on a hermetic, or refrigerant cooled semi-hermetic compressor. The superheat must be kept to a minimum in the suction gas. I have had very good luck with a few Copelaweld CR model compressors used on freezers. Although, they are not recommended for such applications, my experience has been very good. Another consideration is that the compressor will not have the capacity when underloaded. For instance, a three HP air-conditioning compressor will not pump the same volume as a three HP freezer compressor.

[Abe]

Let me rephrase my question. Supposing I was putting up a chiller room to operate at +1 Celcius. I would in this case specify a compressor which was rated as "high back pressure" by the manufacturer. No way will this compressor be suitable for operating or use for a freezer application, ie: for a room operating at say minus 18 Celcius.

Now if I had a compressor which was rated by the manufacturer as a "low back pressure" then it is designed for gases like R502, 404, R69, etc. If I were to use this compressor for a chiller room +1 C, and put into it R134A, then in my opinion it will work, and work well.

Am I right or correct in assuming this??

Terra

[terrygoodrich]

My previous comments didn't account for using different refrigerants. You are correct that a compressor designed for R-502 or an equivalent refrigerant at low temp. will be approximately the correct volumetric capacity for R-12 or an equivalent refrigerant at medium temp. As you can see there are many factors to consider. The main concern is to stay within the operating envelope of the compressor and prevent overloading or overheating.