Selection compressor

[tralex]

Hello experts
Let me ask you some questions.

I need choose compressor set for 3 low temperature island showcase (evaporation temperature is -35 C).
I have evaporator cooling capasity 2500W C for each island showcase.
Is it right, if I make next:
2500+2500+2500=7500W (total cooling capasity)
7500+15%= 8625W ( cooling capasity reserve)
Then I choose compressor with cooling capasity 8625W
(evaporation temperature is -35 C, condensing temperature is +45 C)?
What should be:
useful superheat ?
liquid subcooling ?
suction gas temperature ?

Single or multihle compressor set will better for my purpose?

Thanks for any advice

[frank]

What should be:
useful superheat ?
liquid subcooling ?
suction gas temperature ?

Hi tralex

Useful superheat is 3C, at the compressor inlet, anything more is non-useful as it increases system energy useage.

Liquid sub-cooling should be 6C and no more to ensure 100% liquid with no flash gas

Suction gas temperature depends on the refrigerant used and the design evaporator temperature.

[Peter_1]

Make that you have at least 20 K compressor superheat.
Liquid subcooling 5 K measured before your TEV
Why add a reserve capacity?
Not all the showcases will always run at the same time.
Are you sure about evaporating at -35°C?

Install at least 2 compressors. If you install one machine, how will you maintain a stable suction pressure?
And what if this machine fails?
With 2 machines you will also need an unloader.

Why not 3 independent compressors? It's much safer but of course also more expensive. You have to consider if you will offer a low price or a high safety. The decision is yours but I shouldn't hesitate to offer 3 machines.

[Kevin Yeo]

Tralex,

From wat you have mentioned about the capacity of 2.5 kW, is it a 2.5 meter showcase? Is it single well or double well island freezer. I think the selection of compressor depends on your design on defrost group.

If it is a double well island, you may separate the defrost into 2, left row and right row assuming these 3 island is lined up in a row. I agree with Peter, you do not need reserve capacity. However, make sure the store condition is not too hazardous (26 deg C, 70% RH) as recommended by most showcase manufacturer. Furthermore, with evaporating of -35 deg C, you already have a surplus of reserve capacity only on the compressor side.

Bear in mind with -35 deg C as compressor selection parameter, power input of compressor is lower and thus giving you lower required condensing capacity. My experience with these problem is to obtain exact range of showcase return air temperature (ice cream or packed frozen food) and size according to actual requied running condition.

Guys, don't we normally use 0 k subcooling when we select these components?

In summary, assuming its double well island, i suggest to use 2 compressors in a rack (with pump down option) with common suction & discharge header, condenser with 2 fan model. This rack itself would serve as unloader and you may save some money on controls and unloaders.

Gentlemen, please do not hesitate to comment on my suggestion..

[Kevin S]

Hello Tralex, I have made such compressor selections many times. Obviously more compressors are always safer and the suction will be more stable. Usually (in South Africa) because of the higher ambient temperature, we add 15% to the cabinet manufactures rated capacity ONLY when the supermarket is NOT airconditioned. The capacity you mentioned is typical for an open freezer display (frozen veg or meat) - you have to know if these three cabinets are connected together - in other words they have a common cold air system - if this is the case then you must defrost them together at the same time. In this case you will only have one solenoid valve - and because the cabinets are close together they should have similar loads and therefore one compressor will be OK. Hope this helps you.

By the way I usually stick to the "standard" ratings on most compressor selection software (20'C return gas temp (note: not superheat!) and 0K subcooling - this is a fairly conservative selection (I think these are the Asercom ratings?).