2 replies to this topic

[choychoy]

Hi,

My plant has a centrifugal compressor discharged to a condenser. Based on my observations, when the condenser cooling water inlet temperature drops, the condensing temperature and hence pressure drops. This will closely correlate to the compressor amps. In this case, the amps will drops in accordance.

From a thermodynamic point of view, judging from the pressure vs specific enthaply curve. The lift required to "lift" the gas from lower pressure to higher pressure is reduced since the discharge pressure is reduced, assuming suction pressure is constant. Also, assuming mass of gas compressed is constant, amps of the compressor will drop.

However, I chanced upon the work shaft equation of the compressor which is directly proportional to the mass of gas compressed and compression ratio. Also, the polytropic head developed by the compressor is roughly
discharge pressure - suction pressure = density of gas at suction X polytropic head

With a lower discharge pressure, delta pressure drops and thus polytropic head required should drop if the density of gas compressed is still the same. This will lead to higher inlet volumetric flowrate of the gas compressed(since the compressor performance curve slops downwards to the right).

My question is if the inlet volumetric flowrate increases, but the suction density of gas remains constant, thus based on ideal gas law, the number of moles(mass) should increase. Also, if I am not wrong, compressor amps is a stronger function of mass than compression ratio, thus shouldn't the amps then increase?

I know the second part of the explanation does not coincide with reality and the "lift" explanation, but I can't reason it out.

Pls explain if possible.

[Art Montemayor]

Choychoy:

On Jan 20th you started a thread in the Student Forum asking for help on what may be the same chiller as the one you mention in this thread. We worked and wrote you lengthy responses by Jan 22 with specific answers and comments - as well as detailed drawings to help you identify what you were trying to explain. We never heard from you again on the assistance we gave you then and on the specific questions we asked you then.

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[choychoy]

Hi,

I believed you misunderstood me. Upon your replies to my question on the chiller, I am still reading up myself on basic chiller operations as I felt I did not have sufficient knowledge on that topic. If I just reply for the sake of replying, then I think I am doing you guys a disservice.

I am basically reading up old textbook and searching for more information on the internet and that it how I ended up with another question.

Thus, I did not reply to that thread as I felt that I am not ready yet.

Frankly speaking, this thread is somehow related to that thread but with a different question.

Thus, if anyone can help with my question.