Compressor work 2

[mehaque101]

I am having trouble calculating the work done by compressor. I basically have temperature (degree Celsius) and pressure (bara) data at the suction and discharge of the compressor which is obtained by doing the experiment. Average suction temp and pressure is 40 deg Celsius and 1 bara. Average discharge temp and pressure is 60 degree Celsius and 12 bara. When I use the thermodynamics table to get the enthalpies and find the work done by compressor my result is close to one or negative. Kindly help me to figure out what I am doing wrong.

 

[EmmanuelTop]

Don't use the temperature after the discharge cooler. Use the actual discharge temperature before cooling.

Dejan IVANOVIC
Process Engineer, MSChE

 

[zdas04]

Here is an extract from my 5-day course showing an Energy Traverse for a two stage recip. If I disregard the heat lost to the atmosphere in the cooler like Dejon says to do then the thermodynamics and Arial Performance match up perfectly. Sorry about the units, I developed the slides for a U.S. audience and have lacked the energy to recreate it in SI (and if I did it would be in kPa instead of bar so it still woldn't have been in your units).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Spell out your calculation or post a sheet then we can figure it out.

remember the thing that doesn't change form one side of the compressor to the other is mass.

Hence 1 kg of something (you don't say what you are compressing) at 60C and 12 bara vs 1 kg of something at 40C and 1 bara simply must have a higher stored energy caused by the compressor.

How you account for the energy in the pressure side of it might be your issue?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

We note that compression ratio here is 12 - so for a regular compressor, that would typically be 3 stages (or 2 if discharge temp on each stage are still acceptable). Is this what you've got ?

If you've got an oil flooded compressor (with 1 or 2 stages), you have to add the oil cooler heat rejection duty to the dH gained from compressor suction to discharge to get to total compressor shaft power.


Power delivered by the motor driver would be the sum of the above plus any other driven ancillaries plus other losses.

 

[mehaque101]

Thanks every body for your great responses. I would like to add to the description of my model. I am working with a 234 liter, 2 door, no-frost refrigerator.