Issue with 2 stage semi hermetic compressor for blast freezer application

[Rakesh]

Dear RE friends,

I'am Rakesh from India, we are facing some trouble with one of the two stage semi hermetic Bitzer compressor installed for blast freezing purpose for a sausage plant.
The plant was working well until recently we noticed that the one of the Units showing some trouble with its performance. The compressor is running but with less discharge pressure. The overall performance was found poor. The sound is having slight difference.

The units are 2 No's of 25 HP Bitzer make units, connected to a single evaporator blast coil Guentner make.

The reading are as shown below.

at -15 deg C, suction pressure is 8 psi
and discharge pressure is 340 psi for normal ODU.

at -15 deg C, suction pressure is 10 psi
and discharge pressure is 250 psi. for faulty ODU.

the unit which is running ok is found having sufficient inlet and outlet pressure.and the discharge pressure is ok with high temperature, and sub cooler is also working properly.

please help me to solve this issue.

[chemi-cool]

A broken suction valve?

[cadwaladr]

I agree as my old retired friend used to say quote off with its head!

[Rakesh]

I agree as my old retired friend used to say quote off with its head!

Dear Cadwaladr.

Please make it further, as the issue has not yet solved.

Thanks for your support.

[Glenn Moore]

Hi Rakesh
You need to remove the cylinder heads from the compressor and check the valve plate for valve breakages. This must be done urgently as if a broken valve reed falls into the cylinder bore, the cylinder and piston could also be damaged beyond repair . The longer you run it the more damage will be done to the compressor motion work

[Rakesh]

Thanks for your reply, Mr.Chemi cool, but we have checked it for any damages. it is ok

[Rakesh]

HI Mr.Glenn Moore,
Thanks for your message.
As you have mentioned above, we checked with each and every part of compressor for any malfunction. but the pressure difference is there between suction and discharge.
I'am in doubt whether there might be some trouble with the sub-cooler portion of the compressor.
Please update me if there is anything I need to check in detail about that.

[RANGER1]

Rakesh,
Can you tell us a bit more information.

Refrigerant type
ambient temperature
condenser type & are both compressors discharge into common discharge line or each have it's own condenser.
amps on both compressors
check if unloaders are working & amps change when loaded/unloaded
discharge temperatures
check discharge valve leak back test, as maybe some kind of internal relief valve or leakage.
check discharge pressure gauges correct

Discharge pressure seems high on good unit, but realize your ambient could be 50deg C with roads melting bitumen, as seen on news.

[Rakesh]

Refrigerant type = R404A ( The units are designed to work at -40 deg room temp)
ambient temperature = 35 deg C
condenser type & are both compressors discharge into common discharge line or each have it's own condenser = both are having separate lines ,the Guntner IDU's units are having dual circuits.

amps on both compressors =
faulty ODU - 17 Amp x 1.8 = 30.6 Amps ( Part winding) for ( -5 deg C room temp)
normal ODU - 20 Amp x 1.8 = 36 Amps ( part winding ) for (-5 deg C room temp)

check if unloaders are working & amps change when loaded/unloaded
( that option is not provided in the machinery)

discharge temperatures = 75 deg C for faulty, 85 deg C for normal compressor.

check discharge valve leak back test, as maybe some kind of internal relief valve or leakage.
check discharge pressure gauges correct = no troubles like that.

One more thing we found is that, the output temp of the sub cooler is same as inlet. so we guess some trouble is there with subcooler side. what shall we do to assess the same.

your suggestions are invited.

[RANGER1]

Refrigerant type = R404A ( The units are designed to work at -40 deg room temp)
ambient temperature = 35 deg C
condenser type & are both compressors discharge into common discharge line or each have it's own condenser = both are having separate lines ,the Guntner IDU's units are having dual circuits.

amps on both compressors =
faulty ODU - 17 Amp x 1.8 = 30.6 Amps ( Part winding) for ( -5 deg C room temp)
normal ODU - 20 Amp x 1.8 = 36 Amps ( part winding ) for (-5 deg C room temp)

check if unloaders are working & amps change when loaded/unloaded
( that option is not provided in the machinery)

discharge temperatures = 75 deg C for faulty, 85 deg C for normal compressor.

check discharge valve leak back test, as maybe some kind of internal relief valve or leakage.
check discharge pressure gauges correct = no troubles like that.

One more thing we found is that, the output temp of the sub cooler is same as inlet. so we guess some trouble is there with subcooler side. what shall we do to assess the same.

your suggestions are invited.

On this type of system subcooler is critical for performance.
Can you describe sub cooler, is it a plate heat exchanger with TX valve?
Tx valve should control superheat compared to it's suction pressure which would assume is entering between stages or between discharge of first stage & suction of second stage.
More than likely a damaged TX valve.

[Rakesh]

Yeah, as you have mentioned, it is a plate heat exchanger with TX valve.
But some other technician had replaced it earlier, because of this trouble in sub-cooling finally the result was the same. They couldn't rectify the trouble.

What if the refrigerant charge is low, will this make a difference in this sub-cooling effect? while the unit was in pumb down stage. the liquid quantity was less, it was around 60 %.

Or if any faulty gas ( mix of other gases other than R404a or with different gas ratios ) has been charged in initial stages, will it affect the performance. If so, what will be the trouble.

Please advice.

[Frigorista]

Hello Rakesh. Your problem sounds like you have a leak in the compressor valves and there is a by pass between the HP sector and the LP one. Maybe you have the problem in the cylinder segments instead of the valves. Regarding the subcooler, if there isn't any subcooling in the liquid line, it could be due to two things. Or the expansión valve is not working, or you have vapour refrigerant still condensing inside the subcooler and cause of that you have a constant temp in the exchanger. Hace you checked de subcooling just at the outlet of the condenser? Probably you are getting vapour in the subcooler instead of liquid.

[Rakesh]

Dear All,

At last the trouble has been rectified . We pressure tested the whole system with Nitrogen, and vaccumised, Fresh Refrigerant was filled , resulting in good super cooling , with good efficiency.

We think the earlier troubling shootings and the maintenance work done by other teams without proper refrigerant filling and correct practise in checking and identifying the troubles has made this machinery in such a condition. Polluted refrigerant etc. may be

Its working now , with a good performance.

Thank you all for your kind support.