2 stage comp for multitemp operation

[Kevin Yeo]

Hi everyone,

I have a question and would like u guys to give some opinion. Existing plant is using 2 stage compressor, 3 units to cater for 2 rooms. Each room has 2 unit coolers, total 4 units. The rooms are designed for freezer application, room temp -25 deg C initially. However, the user intends to convert these 2 rooms into multi temp, room temp from O deg C to -25 deg C. The condenser has sufficient heat rejection capacity to cater to chiller application, but the problem is with the 2 stage comp which has the highest possible SST of -20 deg C, based on its operating temperature envelope.

I'm thinking to either:-
1. Replace the 3 compressors to single stage, with CIC fitted for low temp operation or
2. Install evaporator pressure regulating valves for each and every unit coolers.
will these methods work

Thanks in advance

Rgds
Kevin

[750 Valve]

what refrigerant? R22?

If so convert to R404a/R507 this will reduce the massive discharge temps associated with LT R22 and will allow a single stage comp to be selected to do the job. No need for CIC (are you using Bitzer comps?)

Fitting an EPR to each will work, you may fing you need to do a dual temp piping arrangement with a N/O suction solenoid around the EPR for LT operation - solenoid shuts and diverts gas through EPR for MT application. The only thing is it will be a massive waste of energy running at -30°C SST to achieve only -5°C in the evaps on MT operation. You would also need to ensure pump down operation to maintain a low SST.

[Kevin Yeo]

Hi 750,

Thx for your help. Yes existing plant is working on R22 and its bitzer 2 stage compressor.

On the same job site, there is another problem which is imbalance oil return. It doesn't have a pressure equilizer line and all 3 compressors have electronic oil regulator fitted.

At the same time, the pressure differential vent valve from oil reservoir is connected to the motor (intermediate pressure?) of compressor No 3.
I have noticed comp no 3 oil sight glass is always full, comp no 2 and 1 always have low oil level. Perhaps the correct method is to equilize the crankcase pressure of 3 and the residual pressure from oil reservoir can be connected to the pressure equilizing line.

Or is it better to connect the vent line to suction line?

thx and have a nice day

rgds,
Kevin

[750 Valve]

you need that reservoir to be above the crankcase pressure - so it should vent to intermediate pressure. you say all 3 comps have electronic oil level controls, are they traxoils?

Is it possible that you have eiher of the following

1. comp 3 has a valveplate issue keeping its crankcase pressure below the level of comps 1 and 2 - thereby keeping the oil reservoir below the pressure needed to flow oil into the 2 other comps

2. you have different compressor models producing slightly different intermediate pressures

try measuring the interstage pressure of all 3 comps, this should highlight what the issue is. My bet is that comp3 has a lower interstage pressure than the other two.

[Kevin Yeo]

Hi 750,

Its Emerson electronic oil regulator, the OMB series. Thinking to measure the intermediate pressure difference before adding pressure equilizer

rgds,
Kevin

[Kevin Yeo]

Hi all,

Just a thought...For multiple 2 stage compressor rack system, say 3 compressors, is it better to use a common intercooler for 3 compressor. In this way, the interstage pressure is the same across 3 compressors and oil return to each compressor is more even (assuming oil regulator is in normal condition). However, if one of the LP valve plate has problem, it could be more difficult to sense it. Also if the PHE intercooler breaks, the entire system will fail.
For now in our situation, individual intercooler is used for each compressor with pressure equilizing line connecting all compressors. However, sometimes we notice the motor temperature of the middle compressor can be slightly higher. May I know what is the reason? Is it because the amount of cooling gas from intercooler is less in the middle compressor which is caused by liquid refrigerant distribution from liquid receiver?

rgds,
Kevin

Thanks,
Kevin

[750 Valve]

I would say it would be better to have individual heat exchangers, this way the heat exchangers are sized for the mass flow rate of each compressor, as cone comp sysles off so does the HX, if you had one HX it would be hard to control it - you may need a few stages of capacity.

Can you provide a phot of the setup you have, is the "intercooler" just subcooling the main liquid line and the suction vapour dumps into the discharge of the first stage. One of the CIC probes may be out of calibration causing it to cycle less.