A Few questions about how a refrigerator works

[misko]

I am trying to understand how refrigerator works from the thermodynamic perspective. I've read many articles and watched several youtube videos but still trying to figure out following stuff.

1. What exactly is the purpose of compressor? Ok it compresses vapor and thus increases it's pressure and temperature which is then condensed in the condenser... but why is the compression needed here required step? Can't we just have some pump that keeps refrigerant substance flowing without compression? Beside that, isn't compressor adding unnecessary heat to the refrigerant fluid by doing work on it? I mean, compressor's piston is pushing the vapor which is a mechanical work which then increases internal energy of the refrigerant which then we must get rid off because the whole purpose of the refrigerator is to cool down, so adding any additional heat to the system is not a good thing. If we had only pump without compression we would still move refrigerant fluid to the condenser but without compression that would add unnecessary heat. What I am getting wrong here?

2. Once refrigerant leaves compressor it goes to the condenser where it cools down and liquefies. But how that happens? How exactly condenser works? On diagrams I don't see what exactly makes condenser a condenser, it's just pipes that go up-down (or left-right) on the back of the fridge. How that makes vapor liquefy? The volume of condenser pipes is small so vapor can't cool down due to gas expansion process. What am I missing here?

3. Then there is throttling device (eg. capillary tube) that uses Joule-Thomson effect to turn liquid back to vapor by decreasing the pressure on the other end. This is fine so only one question: is this step also necessary? Why don't we just keep using liquid if it was already cooled down in the condenser?

4. What would be the theoretically simplest refrigerator? Maybe I would understand everything better if I start from there. In general all refrigerator schemes I saw have following four elements: compressor, condenser, evaporator, throttling device. Is this minimal that is needed or can we go more basic?

 

[NFuller]

What exactly is the purpose of compressor? Ok it compresses vapor and thus increases it's pressure and temperature which is then condensed in the condenser... but why is the compression needed here required step?

A refrigerator is a heat pump. The compressor is where the work is done to pump the heat out of the system.

isn't compressor adding unnecessary heat to the refrigerant fluid by doing work on it?

No, this is were the heat exits the thermodynamic cycle.

If we had only pump without compression we would still move refrigerant fluid to the condenser but without compression that would add unnecessary heat.

Without compression, no work is done on the refrigerant gas. It would stay at constant pressure, volume, and temperature so there would be no thermodynamic cycle and no heat would move through the system.

Once refrigerant leaves compressor it goes to the condenser where it cools down and liquefies. But how that happens?

By exchanging heat with the cooler air around the condenser coil.

The volume of condenser pipes is small so vapor can't cool down due to gas expansion process.

I'm not sure what you mean here.

Then there is throttling device (eg. capillary tube) that uses Joule-Thomson effect to turn liquid back to vapor by decreasing the pressure on the other end. This is fine so only one question: is this step also necessary?

Yes, this is were the gas rapidly drops below room temperature.

What would be the theoretically simplest refrigerator?

The reverse Carnot cycle would be the simplest in theory.

If your goal is to truly understand the thermodynamics of refrigeration, you need a good math background and should get a college level textbook on the subject.

 

[esale]

You can only condense (no-pun intended) the concept of a refrigeration system so much. However, if you really need it put simply, you can think of it as a heat sink and go from there adding components, as the concept is the same (move this heat away from here, and put it over there).

I think what you start talking about and conceptualizing is akin to a coolant system such as one in a car, were only a liquid is used is conjunction with a pump and radiator.

I hope this helps.

-E

 

[jartsa]

4. What would be the theoretically simplest refrigerator? Maybe I would understand everything better if I start from there. In general all refrigerator schemes I saw have following four elements: compressor, condenser, evaporator, throttling device. Is this minimal that is needed or can we go more basic?

I think this device is quite basic:

We need a bicycle pump and a long thin tube connected to the pump, and air. Oh and a physicist working the pump.

Step 1: Physicist presses the pump handle, air in the cylinder gains thermal energy. (The pressure of the air increases too)

Step 2: The thermal energy is conducted away from the air, let's say in 10 seconds time, the tube must be so thin that not much air leaves the cylinder during that time.

Step 3: The air has the same thermal energy as initially. The air uses that thermal energy to accelerate itself to high speed inside the thin tube. Let's say the speed is 100 m/s, that means 1 kg of air has kinetic energy 5kJ, which means the air lost 5 kJ of thermal energy, which means it cooled about 5 degrees. So the thin tube gets cool.

The air cooled when it used its thermal energy to accelerate itself. That's the most important part of that story.
Here's another device that hopefully is quite basic :

First a vacuum pump creates a vacuum in a tank, then we let some liquid water into the tank.

The water in the tank changes itself to vapor using its thermal energy, 1 kg of water loses 2 MJ of thermal energy when it changes itself to vapor, which means it cools by ... well I don't know how much, maybe 1 kg of liquid water doesn't even have 2 MJ of thermal energy. The water cools a lot anyway, when it uses its thermal energy to break itself apart.

 

[sophiecentaur]

As simple as possible:
You compress the refrigerant, which increases its temperature.
You cool down the hot high pressure refrigerant to as near room temperature as possible as it passes through the hot pipes at the back..
You let the refrigerant pass through a nozzle which uses energy / does work. during the expansion This cools the refrigerant down to a low temperature and it is piped into the fridge compartment panel where it takes in some heat from the slightly warmer compartment, providing the cooling.
The not-so-cool refrigerant is then compressed by the compressor which raises its temperature and so on ad infinitum.

The clever bit is to let the compressed refrigerant expand through a nozzle which takes energy from it by the Joule Kelvin effect.

 

[rcgldr]

Note that it's possible to make a refrigerator that uses a heat source to supply the energy:

https://en.wikipedia.org/wiki/Absorption_refrigerator

One type of such a refrigerator uses a combination of ammonia, hydrogen, and water:

https://en.wikipedia.org/wiki/Absorption_refrigerator#Single_pressure_absorption_refrigeration

However, the typical refrigerators found in a household operate as described by sophiecentaur, except that these include transitions between liquid and gaseous states for better efficiency:

https://en.wikipedia.org/wiki/Refrigerator#General_technical_explanation

 

[kcaldwel]

A rubber band refrigerator is pretty simple. I used to play with this when I was akid, but someone actually built a refrigerator:

 

[misko]

Thank you all for great answers, still working my way to conceptualize this process :)

As simple as possible:
You compress the refrigerant, which increases its temperature.
You cool down the hot high pressure refrigerant to as near room temperature as possible as it passes through the hot pipes at the back..
You let the refrigerant pass through a nozzle which uses energy / does work. during the expansion This cools the refrigerant down to a low temperature and it is piped into the fridge compartment panel where it takes in some heat from the slightly warmer compartment, providing the cooling.
The not-so-cool refrigerant is then compressed by the compressor which raises its temperature and so on ad infinitum.

This is what confuses me. Can you explain a bit more why is necessary to compress the refrigerant (which increases its temperature) just in order to cool it down to the room temperature in the next step? Can't we cool it down to the room temperature without first raising its temperature by compression?

It seems to me that compression is just providing additional heat to the refrigerant that we must get rid off anyway in the next step (condensation) so why adding this heat in the first place?

The clever bit is to let the compressed refrigerant expand through a nozzle which takes energy from it by the Joule Kelvin effect.

Where is this energy going then? We are talking about internal energy right? In Joule Kelvin effect, isn't this internal energy just redistributed between potential and kinetic of the refrigerant molecules such that potential energy goes up and kinetic goes down and thus temperature goes down. This means that total energy stays the same even though temperature goes down.

 

[sophiecentaur]

In Joule Kelvin effect, isn't this internal energy just redistributed between potential and kinetic of the refrigerant molecules such that potential energy goes up and kinetic goes down and thus temperature goes down.

You are actually answering your own question here by discussing KE and PE. When you compress the refrigerant (it is not an ideal gas - that would never work), you reduce the spacing between the molecules (even more if it liquifies) which reduces the Potential Energy. That increases the Temperature (KE). The temperature then reduces in the cooling pipes at the back so the total energy is lower than when you started. Expanding the refrigerant will cool it down and, once inside the fridge, it absorbs heat from the contents.
The Potential Energy between the molecules is Negative (basically an attractive field) so pushing them further together will take it more negative - resulting in an increase in (+) Kinetic Energy.

 

[russ_watters]

1. What exactly is the purpose of compressor? Ok it compresses vapor and thus increases it's pressure and temperature which is then condensed in the condenser... but why is the compression needed here required step? Can't we just have some pump that keeps refrigerant substance flowing without compression?

If you use a pump, you'll just be pumping room-temperature refrigerant around your cycle, so there wouldn't be any refrigeration. The point of the compressor is to create a higher temperature in order to enable heat rejection.

Beside that, isn't compressor adding unnecessary heat to the refrigerant fluid by doing work on it?

Yes, but that is fairly small compared to the benefit. A typical air conditioner moves 4 kW of heat for every 1 kW of input power (heat of compression).

2. Once refrigerant leaves compressor it goes to the condenser where it cools down and liquefies. But how that happens? How exactly condenser works? On diagrams I don't see what exactly makes condenser a condenser, it's just pipes that go up-down (or left-right) on the back of the fridge. How that makes vapor liquefy? The volume of condenser pipes is small so vapor can't cool down due to gas expansion process. What am I missing here?

I'm really not sure what you are missing: it's just a heat exchanger (precisely the opposite of the evaporator). The refrigerant is a hot gas and it cools in the heat exchanger, which makes it condense. Just like water condenses on the outside of a cold drink.

3. Then there is throttling device (eg. capillary tube) that uses Joule-Thomson effect to turn liquid back to vapor by decreasing the pressure on the other end. This is fine so only one question: is this step also necessary? Why don't we just keep using liquid if it was already cooled down in the condenser?

In the condenser, it is cooled down to ambient temperature. You want it colder than that. Your refrigerator cools things down below freezing!

4. What would be the theoretically simplest refrigerator? Maybe I would understand everything better if I start from there. In general all refrigerator schemes I saw have following four elements: compressor, condenser, evaporator, throttling device. Is this minimal that is needed or can we go more basic?

The only simpler ones are open-cycle, skipping the second half by not recovering the refrigerant and re-circulating it. Your sweat/skin works that way.

 

[russ_watters]

I think this device is quite basic:

We need a bicycle pump and a long thin tube connected to the pump, and air.

Yes, but it still has four stages, similar to the normal refrigerator but without the phase change:
1. Compression
2. Heat rejection
3. Expansion
4. Heat absorption

 

[sophiecentaur]

Note that it's possible to make a refrigerator that uses a heat source to supply the energy:

That cycle is much harder to understand and they are pretty inefficient things. I don't know how they compare with peltier coolers (also not efficient).
I am very impressed with the small, low voltage compressor fridge units. They will deep freeze (mine = -18C) a small volume for useful camping / boating applications. Quiet, too!

 

[russ_watters]

This is what confuses me. Can you explain a bit more why is necessary to compress the refrigerant (which increases its temperature) just in order to cool it down to the room temperature in the next step? Can't we cool it down to the room temperature without first raising its temperature by compression?

In a normal refrigerator (for your food - not an air conditioner), it is still below ambient temperature before compression. So you can't reject heat from it unless you compress it.

It seems to me that compression is just providing additional heat to the refrigerant that we must get rid off anyway in the next step (condensation) so why adding this heat in the first place?

This is maybe where looking at the diagram will help:

Enthalpy is the horizontal scale at the bottom. The horizontal distance between 1 and 2 is much smaller than the horizontal distance between 2 and 5. So the heat of compression is only a small fraction of the total energy given away in the condenser.

Where is this energy going then? We are talking about internal energy right? In Joule Kelvin effect, isn't this internal energy just redistributed between potential and kinetic of the refrigerant molecules such that potential energy goes up and kinetic goes down and thus temperature goes down. This means that total energy stays the same even though temperature goes down.

The answer you got wasn't well put: yes, as you can see from the diagram above, the enthalpy is constant in the expansion. Just the temperature changes.

 

[jerromyjon]

It might be worth starting at the cooling portion, which can be easily seen with a spray can, of brake cleaner for example. When you press the plunger on top of the can the room temperature fluid cools drastically while exiting the nozzle and cools even more as the fluid evaporates off of the surface you spray it onto. The rest of the cycle is re-condensing the gas back into a fluid, pressurizing (which makes heat), and cooling back (as close as possible) to room temperature.