Sizing Compressor for Refrigeration System

First thing in the morning, call your local contractor.

It depends on lots of stuff such as refrigerant, type of condensing (air vs water cooled), and evaporating temperature (air conditioning vs cool storage vs frozen storage vs process freezing), and overall system size (bigger = more efficient). There are some rules of thumb, but they aren't very precise.

Yes please tell about this rule of thumb i need only an approximate size. anyways we always put a unit 1.5x the requirement. Nonetheless, I mean if you see a compressor lying around and you need to approximate its cooling capacity, how you will do it? What is the quick reference? Is there a data in the nameplate that you can use as reference? Correct me if i am wrong most compressor do not have a cooling capacity on the nameplate.

Say if you need a 1 ton cooling capacity compressor, what is the approximate FLA required? and how, if it is possible to use FLA of compressor as an approximate reference.

First, decide what temperature range you want to cool to.

Then choose a refrigerant, there are several some better than others for specific temperature ranges. Now, will that compressor pump that refrigerant? Well, it may; or it may be overloaded.

Generally speaking, hermetic compressors are designed for a single temperature range, semi-hermetics might let you pump in different temperature ranges, and open compressor's (those that are somehow coupled to a motor - again the size of the motor depending on the load) are most capable of working under different conditions.

From the RLA you can approximate a horsepower rating of the compressor/pump at some defined temperature.

So you could use the horsepower to calculate the BTU's/Hr, and then using the refrigerant properties information for the refrigerant you chose do the following:

Obtain the Pressure-Enthalpy Diagram and properties table (you will need them for entropy) for the refrigerant you chose. While you have the diagram and tables out, pick out a condensing and evaporation temperature.

Now, you can do the math!

With proper use of the property values at the load (in BTU's/Hr), and applying the right efficiency factors you can come reasonably close to answering the question you ask.

The problem is that we don't do this often in the trade. There are a few guys who might when looking a very specific region of the refrigeration cycle or in research, but by and large we use manufactures catalogs - after having a whole lot more information than you have provided.

Knowing how to get information from the charts and tables will not hurt you, and might keep you from making a mistake, but in actuality you can do a whole lot more and do it better by having the manufacturers' specifications for the compressor.

It is all about temperature range (which you have not given) and the load - load, the fundamental question of most every engineering problem.

As far as carrying around a little piece of paper with some constants on it, I do not. It is frustrating that so many variables change as the compressor runs and the space cools down or the load changes. I used to struggle with that in the field (but I don't anymore).

Thanks guys, in summary i really need to go to my table and depend on the manufacturer charts. I am not an expert on this, I am just trying to make a short cut if there is. More info will be appreciated.

Have you decided any thing? It seems to me as you want to use a compressor lying without use in you back yard.

First you should see in the compressor name plate its rated parameters i.e. flow rate, suction pressure & temp, discharge pressure & temp, service fluid, shaft power and motor rating. If it can handle refrigerants then choose a refrigerant which suits to its suction and discharge condition.

Now, coming to capacities, if compressor shaft power is 'P' kw then evaporator capacity should be about '4P' kw and condenser capacity shall be '5P' kw for cooling application. For freezing application it shall be '2P' and '3P' respectively.

What is the load, in BTU's/hour? What temperatures - above or below freezing?

What kind of compressors are you talking about, a hermatic or a semihermatic?

Single pass evaporator or condenser: Good luck with that.

http://www.ra.danfoss.com/TechnicalInfo/Literature/Manuals/06/key_to_compressor_type_designation_08-2010_ei050a202.pdf