About the capacities

[david2008]

Hello,

I am confusing about different capacities in refrigeration system. There are evaporator capacity, condensing unit capacity, compressor capacity and so on. What is the relation between them? How to match them? Thank you.

Regards,
David

[GXMPLX]

If you have to hook speakers to an amplifier, you first check they match the power output of the amplifier. If they are too big, the energy of the emplifier will hardly move them and if too small they will brake. In either extremes they won't sound good.

There are four main components that should match in a refrigeration system.

The compressor capacity is the main one and you try to match other componets to it.

Condensing unit capacity is synonymous, only that includes two main components already matched, condenser and compressor.

The other pair is the evaporator and expansion valve. Paired because if you have more than one evaporator, the expansion valve should be matched with it and the sum of all evaporators should match the compressor.

How to match them is explained in many books. You have to check variables like ambient conditions and cool room conditions you want and an idea of the pressure drop you'll be having in the pipes, the approximate size your evaporator should be to cool the room and the load you want. With this info and the refrigerant, you estimate the high and low pressure the compressor should work with, then select the compressor to give you it's real capacity (closet to what you want) at these conditions, then estimate how much power it consumes to give you the size of the condenser.

If you have experience you don't need to follow this order and depending on the type of system you have to check at other conditions.

Seldom things match completely in the first selection, for you have discrete component sizes. So you have to re check everything.

Some crazy people, like the one who writes this, sometimes draw graphs with the locus of equilibrium points between components vs ambient and cool room conditions to check if the system will match the demand under several external conditions.

Others save themselves the trouble and work on only one operation point and hope the real system will work at least around this conditions. They are more down to earth but may sometimes have problems at different conditions.

[david2008]

GXMPLX, thanks for your long reply. I understand better now. Usually, which unit will be used in capacities, hp or BTU/h? For example, we call a system is 2 hp system, it is refer compressor? But the capacities of compressor will change with the ambient temp condition.


One more question, when I read a book, it states that the capacity of evaporator will increase when the evaporator temperature decrease. I don’t understand this sentence. From my understanding, the lower of the evap tem, the more difficult to get a certain amount cooling energy. Am I right?

David

[GXMPLX]

GXMPLX, thanks for your long reply.

That was a very short version. No need to thank.


I understand better now. Usually, which unit will be used in capacities, hp or BTU/h?

These are both units of the same physical quantity: power= (Energy/time) but in refrigeration we use it for different purposes.



For example, we call a system is 2 hp system, it is refer compressor? But the capacities of compressor will change with the ambient temp condition.


Yes it refers to the power consumed by the compressor, generally nominal power at some blurry specified condition.

In the old days there were very few motor-compressor combinations so people took the custom of naming them by nominal power to simplify.
This came to byte them in the ass years later when for the same nominal power motor you have at least five or six different design compressors (but of the same type) giving completely different capacities.

Now a days only in Air conditioning there is a bad but still a relation between nominal power of the motor in HP and capacity in BTU/h of the compressor.

This is generally not so in refrigeration, though you may find some lousy relation if you stick to one manufacturer one type of compressor.



One more question, when I read a book, it states that the capacity of evaporator will increase when the evaporator temperature decrease. I don’t understand this sentence. From my understanding, the lower of the evap tem, the more difficult to get a certain amount cooling energy. Am I right?

David

You won’t understand it if taken out of context.

If you keep room temperature constant and lower the temperature of the evaporator, the temperature difference is higher and the evaporator will absorb more energy.

But you are correct in general heat transfer coefficients make energy flow more difficult at lower temperatures and capacity will reduce.

The laws governing this process depend on many things and you must try to keep something constant in order to understand how it works.

You cannot strip conclusions from its hypothesis.

[david2008]

I got it. Thank you GXMPLX.

[Peter_1]

Some crazy people, like the one who writes this, sometimes draw graphs with the locus of equilibrium points between components vs ambient and cool room conditions to check if the system will match the demand under several external conditions.

This is indeed a lot of work but it gives you for sure a better understanding how everything changes when certain parameters varies, like outside temperature, room temperature, ....