Understanding the Efficiency and Heat Exchange in Refrigeration Systems

[Green Zach]

Hello everyone, I am a first year engineering student and I am really interested in learning about refrigeration (this isn't homework its my own personal interest). I have done a bunch of research but I am a little confused on a couple of points and was hoping that you guys could help me understand them.

So my first question: If I cool down the high pressure components of a refrigeration system, by say putting a block of ice on them, I know this will increase the efficiency of the system but I'm not sure about why. My initial thoughts were that there is probably a constant temperature difference between the high and low pressure sides of a refrigerator so by cooling down the high pressure parts the low pressure parts also get colder (in order to maintain the temp difference) which allows them to suck in heat faster. The things cooling down the high pressure components are doing so because they are drawing heat away from them. So by cooling down the high pressure components you are both drawing heat into and out of the refrigeration system faster. Is my understanding correct?

My second question: I'm not sure about the relationship between a refrigerator's exhaust heat and the energy used to run the refrigerator's compressor. My initial thoughts were that the heat energy drawn in by a refrigerator's low pressure components should be equal to the heat energy given off by a refrigerator's high pressure components which should be equal to the energy required to run the refrigerator's compressor. This is the sort of relationship I would imagine there would be as long as the it is just as easy for the low pressure components to draw heat in as it is for the high pressure components to give off heat.

Please let me know if my understanding is correct. My understanding of these two concepts may be WAY off because I don't know much about thermodynamics and I am only a first year engineering student.

 

[Mech_Engineer]

So my first question: If I cool down the high pressure components of a refrigeration system, by say putting a block of ice on them, I know this will increase the efficiency of the system but I'm not sure about why.

My initial thoughts were that there is probably a constant temperature difference between the high and low pressure sides of a refrigerator so by cooling down the high pressure parts the low pressure parts also get colder (in order to maintain the temp difference) which allows them to suck in heat faster. The things cooling down the high pressure components are doing so because they are drawing heat away from them. So by cooling down the high pressure components you are both drawing heat into and out of the refrigeration system faster. Is my understanding correct?

It's best to refer to the refrigeration system's heat trasnfer through its two constituent heat exchangers- a condenser (high pressure hot side of the system) and an evaporator (low pressure cold side of the system). Calling it the "high pressure side" and "low pressure side" is a little misleading, since the compressor and expansion valves are included in those statements.

That being said, if you cool condenser using colder air (or in your example ice), it will cool the compressed gas farther into the liquid regime. This in turn allows the liquid to absorb more energy as it is "heated" back into a gas buy the evaporator. The phase change of the refrigerant working fluid is a large constant-temperature energy sink. For the most efficient process it is desirable to utilize a large portion of the phase change from liquid to gas.

Notice step 5-1 in the following image. To move state 5 closer to the saturated liquid line on the left, it is necessary to cool the liquid beyond the saturated liquid line in step 2-4.

If you're very interested in refrigeration systems, make sure and take one or two Thermodynamics classes at your school, which covers refrigeration and a lot more.

My second question: I'm not sure about the relationship between a refrigerator's exhaust heat and the energy used to run the refrigerator's compressor.

It will depend on the working fluid being used and the prssure it is brought to in the compressor. Again the refrigeration cycle takes advantage of phase change in the working fluid (which utilizes a large amount of energy with a relatively small temperature change, step 2-4 in the diagram above), so the exhaust temperature is going to be a moving target based on a number of physical propeties and system dynamics.

 

[Green Zach]

Thank you very much for the help :). I do agree that my use of high and low pressure sides may have been a bit of an oversimplification. Sorry about that.

 

[sophiecentaur]

An answer to the question in the OP about where the energy comes from and where it goes. The total energy out, in the long run, will be the same as the total energy put in by the electrical supply, if the device is sitting in an enclosed room. i.e. the unit will just be maintaining a temperature difference within a closed system.
Most of that "energy out" will leave from the heat exchanger and some of it will leave via the motor, compressor etc., which will be warmer than ambient.

 

[Green Zach]

Thank you SO much sophiecentaur! Do you know of any sources where I can read up on this?