Bitzer screw - not pumping down

[charkkol]

Hello everybody,
I am new to the forums and this is my first thread post. I am a Ref engineer/technician and face a problem described below and I want your help,
I have a rack system consisting of 2 Bitzer open screws model OSK 7471-N for negative temperature, with economizer/subcoolers for each compressor, and Evaporative condenser, controlled by an ELIWELL EWCM 900/S Compressors and Fans controller. The system serves 9 freezer rooms of different capacities. Since last week the compr. No.2 fails to pump down the system but instead stops at 0.35 bar and would not go any lower whatever the time. Compr. No.1 pumps down the system easily and quickly when it is its turn to operate as last compressor but No.2 not. So I put compr. No.1 to always be the last to stop (through the EWCM 900 Programing) until I fix this problem. One more detail is that when de-energizing the solenoid valve of the economizer/subcooler (No.2) then the compressor No.2 will pump down the system without problem. Does this look like a compressor internal problem (i.e. pressure relieve valve blocked open) or what?
Thank you very much for your attention - waiting for your comments
charkkol

[sedgy]

hi charkkol,
I would have thought ´35 bar would be ok < no liquid< what gas are you on? it must be the L-P switch stoping the comp? at what psi dose N o2 switch off ?

[Sandro Baptista]

Is solenoid valve of the ECO nº2 is off when the pump down starts?

[charkkol]

hi Sedgy
- I am on R404a
- 0,35 would be Ok if I didn't have 2 of the rooms at -35 C temp. and long pipe distances with signifigant pressure drop, so I have to go close to 0 pressure to sutisfy these condition. But anyway the problem is that the compressor (2) cannot perform the pump down to 0,05 bar pressure that the other compressor (1) can perform, so something is not wright with this apecific compressor. The compresors are controlled electronically by the ELIWELL controller i.e no extra pressure switch (A pressure transducer gives pressure signal to ELIWELL), so the ELIWELL stops the last compressor at 0.05 bar.

[charkkol]

Hi Sandro Baptista,

No, the solenoid valve is ON all the way until the compressor stops. Now as I mentioned in my original post, if I manually de-energize the solenoid valve then the compressor No.2 will also pump down the system to 0.05 bar without problem.

[charkkol]

Clarification: when I said in my original post "stops at 0.35 bar" I mean that the compressor is still working but without lowering the suction pressure any lower,i.e. by "stops" I meant stops lowering the pressure any more. If I leave the compressor operating and assume all the evaporator solenoid valves are off, the compressor suction will stay at .35 bar and then I will have compressor and oil overheating.

[Sandro Baptista]

Maybe the relieve valve is damage. But by the way...why don't you close automatically the feeding of the ECO when start pumping down? it would down the pressure faster as the main liquid been not cooling would generate vapor at the intermediate suction pressure.

[NoNickName]

You already diagnosed the problem. You can't pump down with the eco port open. It doesn't make sense to economise a compressor that's going to be switched off.

[Sandro Baptista]

You already diagnosed the problem. You can't pump down with the eco port open. It doesn't make sense to economise a compressor that's going to be switched off.

Plenty agree with NoNickName.

[charkkol]

Hi NoNickName,
I don't agree because the eco port is not big enough to make it hard for the compressor to pump down, also compressor No.1 is pumping down easily with the eco port open, so the problem is only with compressor No.2. Also it is not possible to adjust the eco solenoid valve to close below a certain pressure because there are conditions that require operating pressures of as low as 0,3 bar when for example only one evaporator solenoid valve is open, so we cannot inform the system when to close the eco solenoid valve

[NoNickName]

Bad design. Intermediate pressure is higher than evaporating pressure, therefore at low suction pressure, most of the refrigerant comes from the ECO instead than the suction.
If C1 pumps down with ECO valve open, it means that the valve is closed nonetheless i.e. coil burnt.
All on all, no reason to pump down a screw compressor, because when it counterrotates, it fills the vacuum, annihilating the pump down.
Disable the pump down altogether.

[Magoo]

Try closing eco port valve, if still not pumping down I would suggest rotor clearance wear, is it noisy?. May also be why oil is getting hot with the gas slippage fro discharge back to suction. Does it have unloaders? If it does check internals for wear in the block ports.
Being an N is it running ammonia.