two stage verses economized single stage

[Clarkson]

I am presently installing a economized single stage system for R717 with a -28 deg. F suction. I am calculating about a 4% electrical operating penalty verses two stage, with lower initial capital as the offset. What do others think about this approach.

[RANGER1]

Frigoscandia ( They now have a new name ) do it all the time and they are considered a leader in spiral freezing food products .
Depends probably on how big plant is for this "4%" to be feasable .

If its only 1 compresor and maybe a small holding one as well it would be feasable .
Of course only screw comps could be used for the duty you propose .

[Clarkson]

Yes I am using screw compressors and I am also running them at > 90% of there capacity on a steady state load.

[Magoo]

It comes down to capital outlay versus savings, what is design capacity and operating design conditions.

[Clarkson]

My design conditions are for 87 deg. F condensing, -28 deg. F suction at a 400 Ton load using two fully loaded compressors. My load is full or nothing, which is why I leaned towards the lower capital cost of installation. This seems like a perfect application.

[Sridhar1312]

Earlier with R12 we have done comfortable single stage installations without any issues. Now with what refrigerant it is planned. According to me on certain low temperature applications it is the functional need which supercedes the cost or power.

[Segei]

Yes I am using screw compressors and I am also running them at > 90% of there capacity on a steady state load.
Make sure that this is real 90% capacity. Sometimes slide valve indicator shows 90% but real capacity is 50-60%. Efficiency of single stage compressor with economizer will drop to efficiency of single stage compressor at 70-80% of compressor capacity(real).

[US Iceman]

Segei brings up a good point here. Slide valve position provided by the indication (display or dial gauge) is not equal to the same percentage of capacity.

Depending on the compressor the 90% indicated could be just enough to cause the compressor to be operating where the side port is uncovered. This can affect the economizer (will need back-pressure regulators to maintain the economizer temperature/pressure).

The slide valve position can drastically affect the part load performance also. It may not be just 4% penalty. It could be more as it depends on the actual capacity developed by the compressor at the indicated slide position and the resulting power input.

[Clarkson]

Iceman,

I like the back pressure valve idea.

Thanks

[US Iceman]

Iceman,

I like the back pressure valve idea.

Thanks

Well... don't laugh but I've seen people use economized screws with a flash tank and no BPR to control the flash tank pressure. When the screw unloads the liquid feed pressure goes to suction pressure. :rolleyes:

Glad to help...

[JERRYCOOL]

Well... don't laugh but I've seen people use economized screws with a flash tank and no BPR to control the flash tank pressure. When the screw unloads the liquid feed pressure goes to suction pressure. :rolleyes:

Glad to help...

Yes. It calls 'floated condition'. And it looks like this method could give better performance from York's software.