testing compressor pumping performance?

[coldkeeper1]

I have had a 10-year-old USA-manufactured 4 compressor r-22 medium temperature produce storage rack under service contract for about a year. The four semihermetic units are all Copeland: 15 HP, 15 HP, 10 HP and 7.5 HP. Just recently, I´ve noticed that in order to keep adequate temperature levels in all rooms, the system had to keep up to three of the compressors running instead of the regular two (usually one of the 15 HP units with either the 10 HP or 7.5 HP unit). Cooling loads remain pretty much the same, and I guess weather is not an issue down here since we have mostly the same conditions all year around. Because of this situation (three compressors running) , suction manifold pressures are abnormally low: in the 20´s. I was asking myself if there is the possibility that one of the running compressors has experienced a drop in pumping capacity, forcing the operation of an additional compressor, therefore lowering the common suction pressure. Is there any way I could evaluate the pumping performance of a compressor before deciding to pull it off the line for an internal check: valves, rings, etc.? Any suggestions welcomed.

[brianubaldo]

hi coldkeeper,

if your usual discharge pressure gets low and your usual suction pressure gets high it means you have problems in seals.. (piston ring, Valve plate, Fluffer Valve)

if your current gets high than the usual current in the nameplate.. [check outdoor first] if its ok then maybe you have problems in.. [bearings, Drive shaft etc..]

before you open your system for overhauling try to get a long screw driver.. put the tip in the body of your compressor then the handle in your ears.. try to listen whats going on inside the compressor..

[nike123]

Please describe capacity control logic for that system!

[tony--1]

If the suction pressure is lower now then before . has anyone ajusted the pack pressure . if not i would say pack controls are faulty . we need to no what is controling the pack more info plz ? .

[750 Valve]

mate like nike123 says - whats the go with the control logic?!?!
is your plant staging comps on room temp? have you got control of the suction pressure - this is the preferred way to stage compressors on and off

[Porkington]

Whats with all the 13 and 14 year old Refrigeration Engineers these days?

[NH3LVR]

Fluffer Valve

What is a Fluffer Valve?

[coldkeeper1]

Thank you very much for your responses.

The rack pressure is controlled by a CPC REFLECS controller. Seems to be an old version, maybe as old as the rack itself: about 10 years, maybe older. The control strategy is pressure based: there is a pressure sensor in the common suction manifold, and the compressors are staged according to the suction pressure setpoint. When I got here, this setpoint was 35 psi (sorry for the English units, but this rack is not SI configured) and I haven´t changed it at all since all rooms were operating nicely with this setting. I haven´t re-adjusted any of the EPRs either.

About my age, I misstyped my date of birth when registered in the forum. It isn´t 13 but rather 23. I was born in 1985.

[SteinarN]

If your suction set point is 35 psi but the suction is actually running at some 20 psi, then your pressure transducer or your control regulator probably is faulty.

[coldkeeper1]

Good evening SteinarN, and thank you for your comments.
The digital controller does allow the suction pressure to fluctuate around 35 psig a bit. I guess the software which evaluates actual pressure vs setpoint presure vs power of operating compressors decides how much allowance. The fact is that the suction manifold pressure sometimes goes down to 25-27 psig before the compressors begin come off line.

[nike123]

Good evening SteinarN, and thank you for your comments.
The digital controller does allow the suction pressure to fluctuate around 35 psig a bit. I guess the software which evaluates actual pressure vs setpoint presure vs power of operating compressors decides how much allowance. The fact is that the suction manifold pressure sometimes goes down to 25-27 psig before the compressors begin come off line.
You need to know in detail how control logic should behave and what inputs could lead to behavior you experiencing, and than check if that input gives false or right data.

[nike123]

I was asking myself if there is the possibility that one of the running compressors has experienced a drop in pumping capacity, forcing the operation of an additional compressor, therefore lowering the common suction pressure. Is there any way I could evaluate the pumping performance of a compressor before deciding to pull it off the line for an internal check: valves, rings, etc.? Any suggestions welcomed.

This is from Thermagroup manual (http://www.thermagroup.com/documents/FieldCompressorGuide_000.pdf):
Close in the compressor suction valve tight, & link out LP control switch.
Attach a compound gauge with a coarse Vacuum scale and operate compressor. Allow machine to pump down to below -0.8 bar (-12 psig).
If at least -0,6 bar (-9 psig) is not achieved, suspect valves are faulty. Do not allow compressor to run in Vacuum for more than 1 minute.
If -0.8 bar (-12 psig) achieved, stop compressor & monitor vacuum rise rate. Normal criterion for a new/re-manufactured compressor is not more 0,16 bar (2.5 psi) rise / minute. A reasonable field machine should not rise more than 0,3 bar (5 psi) minute.

[coldkeeper1]

To Brianubaldo:

Thank you for your suggestion. I tried to detect any strange noise with the long screwdriver applied to the compressor. I couldn´t hear anything strange, but maybe it´s because I´m still not very good detecting internal failures by sound differences.

To Nike123:

I´ll find out more details about the pressure control strategy. The problem is that there is no rack manual at the site. There is no controller manual either.
The procedure that you suggested seems to be a quick way to gain insight on the compressor pumping perormance. I´ll try it out. Thank you.

[SteinarN]

Another way to do a quick assessment of the compressors performance is to measure the discharge gas temperature. Leaky valve plates will lead to a significant increase in the discharge gas temperature. Measure the temperature at the discharge line close to the compressor. The compressor must be running for at least 5 minutes for the temperature to reach its ultimate level. Compare the temperatures for all 3 compressors. If the temperature is within say 5K of each other then there probably is not any fault with either one of the compressors.

By the way, I suppose it is enough refrigerant in the system and that the condensing temperature hasnt increased compared to what it was earlier when only 2 compressors was operating simultaneously.

Regarding the suction pressure, is it constantly below 30 psi, or is it averaging close to setpoint of 35 psi?

[Peter_1]

Steinarn good point but you could plot also the cycle on a log/P and see how high the discharge may go theoretically.You will not see big differences between the theoretical and the practical value.
Aren' the scution line filters clogged where your measurement point is before the filters?
In the compressors are also mesh filters which sometimes clogs.
Measure once on suction pressure on the compressor head, the service valve before and after the main suction filter and see if you have big differences.
I've seen many times clogged mesh filters inside the compressors (water, mixed with oil and other brown substances.. :) )

[750 Valve]

Aren' the scution line filters clogged where your measurement point is before the filters?
In the compressors are also mesh filters which sometimes clogs.
Measure once on suction pressure on the compressor head, the service valve before and after the main suction filter and see if you have big differences.
I've seen many times clogged mesh filters inside the compressors (water, mixed with oil and other brown substances.. :) )


I'd start here, its a very good idea - seen it a lot of times too.