Rotary Screw Oil-Free Compressor

[EmmanuelTop]

Good evening to forum members and happy New Year's Eve,


I am looking to find some more information related to possible causes of excessive discharge temperature for the air compressor. The compressor is two-stage rotary screw machine, oil-free, with an interstage and final discharge air cooling system.

There's nothing specific with regard to this unit; two compressors operate in parallel (lead/lag), with exactly the same process parameters (1st stage suction pressure and temperature, and 2nd stage suction pressure and temperature are equal for both machines). Compressor A exhibits high discharge temperatures (231C), while compressor B operates with normal/design discharge temperature (200C). Compressor A, apart from discharge temperature issues, also develops slightly lower air flow. At the time being, only compressor B is running which is sufficient to maintain air demands.

Both machines are packaged and enclosed within the metal box, and it's quite difficult to perform any troubleshooting without disassembling the unit. Since both compressor systems have identical performance up to the 2nd stage, my guess was that it could be related to mechanical integrity problems of the 2nd stage machine. I was hoping to get some guidance regarding this issue.

Best wishes in 2009,

 

[zdas04]

It is almost certainly plugging between the first and second stage. This lowers second stage suction pressure and increases compression ratios across the stage, resulting in the elevated temperature and lowered capacity.

I'd look for any filters that could be clogged, then the dP across the interstage cooler, then the piping.

I don't know of any IM problems that could result in elevated temperatures unless you have a leak in the piping between the stages, but a leak that big would be pretty obnoxious.

David

 

[Compositepro]

There is almost certainly an air to air heat exchanger to cool the compressed air before it leaves the compressor cabinet. The heat exchanger or filter needs to be cleaned (regularly).

 

[EmmanuelTop]

OK, we cleaned the interstage cooler and the temperature has dropped from 231 to 225C, but it's still quite high and far above the design value. Air filters appear to be clean.

Apart from what you have mentioned in the previous posts, what are other (even theoretical) reasons/causes for high discharge temperature of rotary screw oil-free compressor?

Thanks again,

 

[zdas04]

If the interstage air filters are clean, then the problem is in the piping upstream or downstream of them. I'd look for plugging in the interstage cooler since it is much easier to accumulate junk in the small ports within the cooler than in the piping.

You might want to check the pressure at the first stage discharge and the 2nd stage suction to see what the dP really is. My guess is that it is considerably more in the one compressor than the other. If you have installed gauges at these points and they don't show a difference then I'd swap the guages on the two machines and see what they say then (gauges and transducers are not nearly as trouble-free as we would hope).

David

 

[EmmanuelTop]

David,

I guess you're right; I couldn't come up with any other conclusion than what you were saying from the beginning.
Actually what I did was to gather shift operator log sheets and look at trends of both stage suction and discharge temperatures. It was obvious that the increase of 2nd stage discharge temperature was gradual, it was continuously progressing during the last three months. If you put P2/P1 versus T2/T1 on the graph, I believe you can observe if there is any mechanical issue (loss of efficiency would change the polytropic or adiabatic exponent). However, this was not the case for the air compressor. That made me think of partially plugged air cooler and/or air filter in the compressor A, which happens quite frequenly with air compressors. Another fact supporting this theory is that aftercooler of compressor A has sufficiently lower outlet temperature than the aftercooler of compressor B, in spite of tremendously higher inlet temperature. This can happen in the case of reduced airflow through the compressor A which, again, points out at partial pluggage within the compressor A assembly.

I will get back on this topic again, once when I have the machine shutdown and inspected.

 

[EmmanuelTop]

I hope to hear some feedback from you again.

After the equipment cleaning has been completed (coolers, filters, moisture separators), I've gathered process trends for temperature and pressure across both compressors (operator log sheets, not linked to DCS) for the period between 19-Jan to 07-Feb 2009. The averaged data have been plugged into the very simple HYSYS model of compressor unit in order to estimate performance parameters, such are compressor efficiency, useful work, and other parameters. All the data are attached below. Compressor B is LEAD machine, Compressor A is in LAG mode. Both compressors are loaded.

Now, the questions which have arrived:

1. Why two identical compressors have different interstage pressures? What could be the cause of this?
2. Is it possible for machine A to have some internal leakage which lowers the effective 1st stage discharge pressure, showing average efficiency to be higher than it actualy is? Somehow I don't believe in this difference in efficiencies in "A" machine, and it makes me think something else is going on there.

Any hints or advices on pathforward would be appreciated.
Thanks in advance,

http://my.engineering.com/action/file/download?file_guid=176753

 

[sidassy]

dear emanueltop ,

looks that the second stage of compressor A has a reduced performance. Most likely that the coating ( typical for oFA screw) has detoriated and as such theis stage will have higher internal leakage. The more the hot compressed air can go from discharge to suction side ( high to low pressure ) the higher it's temperature will get ! We see this on the majority of our OFA screw compressors after a couple of thousand run hours... exchanging the unperforming stage is almost your only option