Efficiency of worn screw compressors

[gepman]

Does anyone have any published information or studies on the efficiency of heavily used (or worn out) refrigeration screw compressors? I would like to know some typical values. I need to calculate the efficiency on some existing old screw compressors. The only way that I can think of measuring the efficiency is to 1) measure the input power to the motor; 2) measure the suction temperature, suction pressure, discharge temperature, and discharge pressure; 3) measure the refrigerant mass flow (most likely on the discharge pipe); 4) calculate the useful work in the refrigeration cycle from items 2 and 3 and divide it by the input power in item 1.

I have two 350hp ammonia screw compressors in an engine room with about another 2000hp of compressors which I cannot shut off.

 

[chilled]

I'm not sure that I've ever heard of studies being done on 'worn out' equipment.

Sounds like the compressors need an overhaul, stop trying to be a hero.

 

[imok2]

Knowing the refrigerants isentropic index, suction pressure, discharge pressure and swept volume one can use this tool to experiment by changing these variables and watching the effects.
Go hers:

http://fridgetech.com/calculators/

to refrigeration vapor compression

 

[gepman]

I don't make postings to waste people's time. I am interested in calculating the efficiency so that a new or rebuilt compressor can qualify for a $0.14/kWh saved on an annual basis incentive from the local power utility. If the efficiency improves 10% on a fully loaded 350hp motor operating 8760 hours/year the incentive could be over $38,000.

 

[gepman]

imok2

Thanks for the new calculation tool, which I have not seen before. It looks very useful but it does not help me calculate the efficiency of this particular compressor since it is the isentropic efficiency (which is assumed to be 100% in this calculator) that changes with the worn compressor. The reason that I am interested in a study or a typical value of inefficiency for worn screw compressors is that I want to determine if the potential utility incentive is worth the time to measure the refrigerant flow and perform the calculations.

PS My previous reply was in response to chilled

 

[MintJulep]

gepman:

I think your proposed method would return the total system efficiency, not the compressor efficiency.

With the data you would collect I think you need to compare the actual change in refrigerant state through the compressor to a theoretical ideal compressor.

 

[siboatyard]

We are $0.17/kWh and have a similar issue in getting rent credits for more efficient equipment. Our screw air compressor runs at 90 amp 208 volts and idles at 50 amp. In addition to the efficiency of the compressor, would it be productive to research buying a more efficient motor?

 

[sterl]

1) No slide valve screw compressor actually "idles". Pressed a little bit, the manufacturers will tell you that they turn down to maybe 12%, some models more like 18%.

2) Motor efficiency at part load a considerable topic...But that motor pulling 50-amps is either way outsized to the compressor or the compressor isn't running at all well...

3) If your input energy is high but your moved energy is low, the excess power went into one of 3-fluids: The refrigerant; the oil; or the air. Depending on how the machine is (oil) cooled, one of the 3 should be showing some pretty high temperatures and the one that would change substantially in a machine thats slipping and recompressing badly is the discharge temp.

4) Measuring refrigerant mass flow is not an easy undertaking and usually involves keeping everything else about the plant as constant as possible...Which usually means shutting down all of the other machines. The only other "reasonable" way is to create a singular path from high side through outlet pressure regulator through desuperheater to fixed pressure reduction device to compressor suction, and measuring the mass flow from desuperheater to suction....That's how the compressor manufacturers measure mass flow on the big machines, originally. For a 350-Hp Machine: Sounds like about a 3" High Pressure Source would be required....

5) How old are the machines? If they are 15 to 20 years old, the rotor profiles are considerably different from a more modern high efficiency machine, as are the operating controls. If rotor improvements plus upgraded controls can convert you from a Fixed-Vi machine to a variable Vi arrangement, it won't take long to pay for a 24-7 machine at a 6 or 7% gain in efficiency....

 

[gepman]

siboatyard
My experience with replacing older motors for more efficient motors is that it only pays back for equipment that is used for a large number of hours per year (at least over 50%). There are two good motor payback analysis tools that I use, one is Baldors BEST 2.0 available at

http://www.baldor.com/support/software_download.asp?type=BE$T Energy Savings Tool

and the other is MotorMaster 4+ available at

http://www1.eere.energy.gov/industry/bestpractices/software.html#mm

Efficiency of motors at part load isn't as bad as most people think. You cant look at amps because the power factor is much worse at part load and most people can't measure power only amps. See

http://www1.eere.energy.gov/industry/bestpractices/pdfs/10097517.pdf

If you are really interested in compressed air efficiency see AirMaster at the same link that MotorMaster is at. There is also the Compressed Air Challenge Sourcebook at

http://www.compressedairchallenge.org/content/library/pdfs/compressed_air_sourcebook.pdf

sterl

1)No arguement here. I will just add that the efficiences of the compressor at slide valve positions less than 50% is much less than near 100%. Also air compressors (as mentioned by siboatyard usually operate somewhat differently than refrigeration compressors since air compressors usually are controlled on discharge pressure and refrigeration compressors are usually controlled on suction pressure. The capacity control mechanisms are usually different.

2) See what I said in response to siboatyard.

3) That is a good idea although on a refrigeration compressor I doubt that the (ambient) air will get heated much more than normally happens.

4) I think you combined your air compressors and refrigeration compressors in this one. I have never seen a discharge pressure regulator on a refrigeration system since it would only waste energy and most systems don't have desuperheaters although I have seen them. I was just going to put a flowmeter (probably a differential pressure type like an Annubar) in the discharge line of the compressor and calculate mass flow by measuring velocity and compensating for temperature and pressure since these are measured by the compressor control system. I don't see any reason that won't work although I might not get 10 pipe diameters of straight pipe. Usually there is about 5 - 10' of pipe between the discharge flange of the compressor and the discharge header where the other compressors are connected into the system.

5) The machines are over 15 years old. I did get a response finally from a major (refrigeration) screw compressor manufacturer and they told me that screw compressors loose very little efficiency since the rotors never touch the housing and are sealed by the oil. They said that even if debris or liquid slugging damages the rotors they loose only 5 - 10 % efficiency. So I think that I won't bother trying to measure the refrigerant flow and give up on trying to get a utility rebate on the refrigeration compressor replacement (unless of course the new compressor is just more efficient than the old compressor was in new condition).