Compressor question for Low temp R-507 application

[worksalot]

I made a similar post about 3 months ago that received no replies. Upon re-reading it, I realize the post was unclear and so have posted it again, this time with clarifications.

I have an application where I want to remove heat from a nominal 1 Ton load at -70C. I already have a 40 Ton R-507 compound system installed operating at -48C SST. It seems logical to find a small booster compressor to suck R-507 out of a small evaporator operating at -70C and discharge it into the much larger existing -48C compound system.

Most of the condensed R-507 from the compound system would supply the -48C evaporator and a small portion would supply the -70C evaporator. All of the gas would be compressed in a conventional 2 stage compound system and a small portion of it would be effectively compressed in a 3 stage compound system.

This would effectively make a 3 stage compound system out of the small load operating at -70C and leave the -48C system intact but with a slightly larger load.


If oil management is a problem, another solution would be to create a small R-23/R-507 cascade system. This would use some of the -48C R-507 to run a condenser on a closed loop R-23 system operating at -70 SST and -48C discharge dewpoint.

I have sent inquiries to numerous suppliers with limited success. To date, we have only one quote for a 2.5 Kw recip R-23 compressor and R-507 condenser for $140K. This is much more than I expected to pay for this unit.

Any suggestions who could supply a small R-507 compressor operating at -70C SST and discharge dewpont of -48C (about 3 psia suction and 15 psia discharge pressure) are welcome.

Failing that, suggestions for suppliers of a small R-23 compressor with R-507 inter-stage condenser are welcome.

[Peter_1]

Worksalot, see you have enough knowledge to handle this setup, therefore some thoughts/questions.

Are you not afraid that you will go below atmospheric pressure with your 3th stage in your compound?
If you have non-condensables entering your 3th stage, then they will contaminate the 2 stage compound.

Is this a DX or a pump system?

Adding a 3th stage in your compound will be the cheapest solution but I should fear for leaks at below atmospheric pressures.

I should go for the second choice: and independent R23 condensed with the R507 of -48°C.

We have some low temperature systems running on -100°C and we use for this normal Copeland piston compressors. You will have to add a pressure/expansion vessel on the R23 to avoid excessive pressure build-up.

1TR = +/- 3.2 kW I guess and if I do a simulation with Bitzer at -70°C (2 stage/R404a), the I need an S4G-12.2Y-40P ( 2,5 kW) or a S6J-16.2Y-40P (3.66 kW)

[mad fridgie]

What is your application for SST of -70C
It would seem capital cost is your problem more than technical method.
If you are for instance do storage, then a claud air cycle system would be cheap, using the -48C as the gas cooler,
or if a freeze drier or the like you could use pulsing CO2 ( dry ice powder production -78C, static is produced, use SS tubing (attracts the powder CO2) for heat exchanger ), then use a oil free scroll air compressor (atlas copco), condensing (BPHE) with the -48C R507 more than exceptable compression ratio.
Both options should be a fraction of what you have been quoted

[worksalot]

Peter_1, Thank you for your help.

I have experience with older R-22 systems operating below atmospheric pressure with a simple bubbler to remove non-condensibles though there is inevitable refrigeration loss while removing non-condensibles. If the leaks are small, the problem is small and manageable.

The -70C evaporator will be a shell and tube type exchanger. It could be a flooded, thermosiphon, or direct expansion design. The decision between these choices is largely one of oil management. DX is easiest. The other two are roughly equivalent in complexity.

You provided part numbers to a couple compressors. Whose compressors were they? I did a google search on them and it tried to take me to a website that would not launch.

Would you anticipate problems using a copeland recip compressor of suitable displacement to accomplish the 3rd stage compression of R-507?

Would you anticipate unusual oil management problems in R-507 at this temp? If so, can you offer guidance on solutions?

As you say, the R-23 solution is always available and straightforward, though I believe the current quote I have is much too high in cost.

[worksalot]

The application involves removing heat in a chemical process heat exchanger.

In the CO2 soultion you suggest, are you suggesting simple evaporation of CO2, or a CO2 refrigeration cycle?

In either case, it seems unattractive to a simple R-507 compressor. An alternative to mechanical refrigeration is LN2. At the prices I am currently quoted, it is preferable to mechanical refrigeration.

[Peter_1]

....
You provided part numbers to a couple compressors. Whose compressors were they? I did a google search on them and it tried to take me to a website that would not launch.

Would you anticipate problems using a copeland recip compressor of suitable displacement to accomplish the 3rd stage compression of R-507?

Would you anticipate unusual oil management problems in R-507 at this temp? If so, can you offer guidance on solutions?

As you say, the R-23 solution is always available and straightforward, though I believe the current quote I have is much too high in cost.

Bitzer compressors from Germany, for me the Rolls-Royce amongst the compressors.
You can download at http://www.bitzer.de/eng/productservice/software/1 their selection software.
We mostly work with their compressors and therefore I tend to take this software to do simulations.

I don't see problems using Copeland compressors. As long as you stay within the safe margin of the compressor motor.
The systems we have running has one on ethylene with a SH of at least 70°C to 90°C. Never had any serious problems with it.

You always can add some R134a in it to rinse the system with R134a which remains liquid at these temperatures.
We do take care about sloping the tubes the right way and take for suction speeds at least 10 m/s horizontal and 15 m/s vertical.

Removing non-condensables the way you describe it is no longer allowed in Europe.

[D.D.KORANNE]

R-23 & r-404a or 507 in a cascade system could provide you an answer . However , costs are higher with controls & accessories. A phe can act as cascade condenser . With indepedent system oil management can be handled better .