Value for heat of compresion

[Tip-speed]

Fellow service techs:

When doing a heat balance formula for an open drive chiller, what value is added to the condenser for the heat of compresion o the refrigerant? hermetics have to figure kw, but we must still have a penalty for compression even without the motor heat in an open drive. thank you.


ti speed

[US Iceman]

You should add the motor power raiting (mecanical power), you can obtaine the information on motor nameplate.

Agreed. One thing to watch for though is the discharge temperature you calculate in the heat balance will be greater than the actual value found by a thermometer.

It's similar to the same thing on a Mollier diagram. The isentropic lines will state one temperature and the actual temperature will be greater.

Just use the calculated number with some careful consideration.

[Tip-speed]

thanks for the replies, still a little confused, is the power of the motor such that an assumption can be made that the transmission of power into the condenser from the driven compressor will be a value that can be calculated even iif it is open-drive, thus no motor heat?

[mad fridgie]

The rated motor power is shaft power, not input power.

[US Iceman]

To clarify a point: motor heat is the heat that an open drive motor rejects to the ambient air. It's basically the difference between 100% efficiency and the rated motor efficiency, which is fairly small.

The shaft input power is the total energy input into the gas for all practical purposes although some is lost to overcoming friction and windage of the rotating parts of the compressor.

If the compressor is driven by V-belts, these absorb some of the input power also. An average number for this is 3-5%.

You can calculate numbers and arrive at an answer, but... what you are trying to do has to be taken with a grain of salt. Calculated discharge temperatures or gas enthalpy have a way of being a difficult number to ascertain and verify with actual operating data.