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[Art Montemayor]

George:

I have to assume something because you don’t tell us the type of compressor you’re using, the number of stages, the design suction pressure and the design discharge pressure (which should correspond to the temperature that you are condensing the NH₃ at.

I assume you have a single-stage, reciprocating, refrigeration compressor that is normally discharging at a pressure of 200 psig – which corresponds to a condensed liquid NH₃ at 101 ^oF. This is about normal, although I like to condense at a much cooler temperature (which will yield a lower discharge pressure on the compressor.

Please refer to the attached Excel schematic sketch and thermodynamic tables for NH₃. You will note that the tables give the saturated liquid pressure at the corresponding saturated temperature.

A mechanical refrigeration cycle employs a refrigerant condenser that condenses the refrigerant at its saturation point. This point is fixed by the temperature at which you are able to cool the refrigerant. If you are using cooling water at 90 oF it is possible that you are able to cool and condense the NH₃ to a temperature of ^oF. In order for you to have a credible compressor discharge pressure of only 60 psig, you would have to be cooling/condensing the NH₃ at a temperature of approximately 41 ^oF. In order to cool to that level, you would require your cooling water to be at 30-35 oF. I don’t believe that is what you have. For the lower discharge pressures you would require even colder cooling water or coolant. Even though you say you have a new condensing unit I don’t believe the figures.

I believe that your pressure gauges are broken or not reading correctly --- that is, if your system is continuing to operate and give you refrigeration.

Refer to the sketch I furnish you. Your PI 1 and PI 2 should read very close to each other. The only difference should be the condenser’s pressure drop. Your TI 1 should correspond with PI 2 in accordance with what you read from the NIST tables I furnish on the spreadsheet. The only way that the temperature and pressure do not correspond to each other in accordance with the NIST tables would be if you have impure NH₃ refrigerant or have a lot of non-condensables in your system.

I hope this helps. This is all I can really comment on because you don’t furnish enough basic data to know all that is going on.

Attached Files

•  Ammonia_Discharge_Pressure.xls   68KB   155 downloads