# How does the capillary tube in refrigerators work?

• Karagoz

#### Karagoz

How does the capillary tube decrease the pressure on the liquid when its diameter is smaller?

How small diameter of capillary tube is helpful in causing more pressure drop?

And how, when pressure drops, the liquid instantly cools down?

Do you mean capillary tube mat for cooling?

https://en.m.wikipedia.org/wiki/Capillary_tube_mat said:
A room whose temperature is controlled by the use of capillary tube mats as a surface heat-exchanger requires flow temperatures for heating or cooling which need only be a few degrees away from the desired room temperature. At the same time, due to the large number of parallel capillary tubes, the drive energy required to maintain the flow is comparatively low in relation to “single tube systems”, which are laid in the form of a meander (typical installation of underfloor heating pipes). Due to the small distance of the capillary tubes to the surface of the room area, the system reacts very quickly. The temperature of the heating or cooling medium is given off very evenly and quickly to the environment due to the large number of capillary tubes. These two properties – very good heat transmission and low pressure loss – provide an advantage in terms of energy saving. In the broad definition of the terms of the technology, the capillary tube mat is a surface heat exchanger, and can be used for the transfer of heat between two media.

How does the capillary tube decrease the pressure on the liquid when its diameter is smaller?
It's an obstruction, like a valve, so there is an energy loss associated with it. It is basically friction/drag.
How small diameter of capillary tube is helpful in causing more pressure drop?
Depends on the specifics of the scenario but in general the smaller the tube the larger the pressure drop.
And how, when pressure drops, the liquid instantly cools down?
https://en.m.wikipedia.org/wiki/Joule–Thomson_effect

How does the capillary tube decrease the pressure on the liquid when its diameter is smaller?

How small diameter of capillary tube is helpful in causing more pressure drop?

And how, when pressure drops, the liquid instantly cools down?

We want the pressure inside the evaporator to be quite low. To achieve that we do two things:
1: We make a small hole through which the (liquid) refrigerant leaks into the evaporator.
2: We attach a quite effective pump to the other end of the evaporator. The pump sucks (gaseous) refrigerant out of the evaporator.

Doesn't it make sense that those two things are the requirements for low pressure?

(The pump is the compressor)

So smaller diameter --> liquid touches more the tube --> higher friction --> more energy loss --> and more energy loss causes drop in pressure?

I disagree. The cooling effect does not occur in the capillary tube. It occurs there were the phase change occurs. The phase change occurs in the evaporator.

If the refrigerant cooled while going through the capillary tube, it lost heat energy:
mass * specific heat capacity * temperature change
That is a quite small number, not as large as heat of phase changes.

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I disagree. The cooling effect does not occur in the capillary tube. It occurs there were the phase change occurs. The phase change occurs in the evaporator.
Do you have access to a split system AC unit where you can identify the expansion valve? Touch the refrigerant line.

Evaporation(boiling) occurs at constant temperature.

Or maybe more to the point, in the evaporator you are cooling air by passing it over cold coils to transfer heat to the refrigerant. It has to already be cold to do that and it can't get colder by heating it!
If the refrigerant cooled while going through the capillary tube, it lost heat energy:
mass * specific heat capacity * temperature change
That is a quite small number, not as large as heat of phase changes.
I think you are missing the function of the capillary tube: It's to make the refrigerant lose pressure.

Do you have access to a split system AC unit where you can identify the expansion valve? Touch the refrigerant line.

Evaporation(boiling) occurs at constant temperature.

Or maybe more to the point, in the evaporator you are cooling air by passing it over cold coils to transfer heat to the refrigerant. It has to already be cold to do that and it can't get colder by heating it!

There are two fridge parts where refrigerant temperature drops: the expansion valve and those warm tubes at the back of the fridge.

The liquid that comes out of the expansion valve is cold because a small fraction of it has evaporated.

The liquid that is in the evaporator is cold, despite of it being warmed, because it is evaporating.

If 10% of evaporation occurs in the expansion valve and 90% of evaporation occurs in the evaporator, then the evaporator is the main part where cooling occurs. 'Cooling' does not mean drop of temperature here, it means staying cool, despite of being warmed.

Sorry, @Karagoz, we made a bit of a mess of this and it started with me. Let me try to clean it up...
I disagree.
Well, at least in part your disagreement is correct. I had the Joule-Thomson effect in my head due to a recent thread on air-only refrigeration and failed to switch gears. Here's the wiki on vapor compression refrigeration:
https://en.wikipedia.org/wiki/Vapor-compression_refrigeration#Thermodynamic_analysis_of_the_system

The OP is primarily about step 4-5, which is described: as:
"Between points 4 and 5, the saturated liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure. That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes). The adiabatic flash evaporation process is isenthalpic (occurs at constant enthalpy)."

To put it more simply, by reducing the pressure you reduce the boiling point to below its current temperature, which causes a temperature drop until the saturation temperature is reached, where liquid and gas can coexist.

However, I think you are still muddying the waters with confusing usage of the word "cooling" or "cooling effect":
The cooling effect does not occur in the capillary tube. It occurs there were the phase change occurs. [snip]

If the refrigerant cooled while going through the capillary tube, it lost heat energy:
mass * specific heat capacity * temperature change
That is a quite small number, not as large as heat of phase changes.
What does "cooling effect" mean? To me, the verb "cool" can mean both "to lose thermal energy" and "to make colder". Since those definitions do not overlap completely, some clarification is in order. So here is what happens in the capillary tube:
It gets colder (temperature drops) without losing thermal energy, but while doing a little bit of work. The vast majority of the temperature change though is kind of as you say, due to the phase change reducing sensible heat and increasing latent heat (boiling). So while one definition (getting colder) definitely applies, the other (releasing thermal energy) mostly doesn't. However:
The cooling effect does not occur in the capillary tube. It occurs there were the phase change occurs.[snip] The phase change occurs in the evaporator.
Since the refrigerant is neither losing thermal energy nor getting colder in the evaporator, I can't see how you can say it is "cooling" in the evaporator. Indeed, the only one of the two definitions that applies is applied backwards: the refrigerant is gaining thermal energy in the evaporator, which means it is being heated. Just like a boiling pot on a stove is at constant temperature while still being heated.
There are two fridge parts where refrigerant temperature drops: the expansion valve and those warm tubes at the back of the fridge.

The liquid that comes out of the expansion valve is cold because a small fraction of it has evaporated.

The liquid that is in the evaporator is cold, despite of it being warmed, because it is evaporating.
Yes. And just to be clear, "the warm tubes at the back of the fridge" are the condenser coils.
If 10% of evaporation occurs in the expansion valve and 90% of evaporation occurs in the evaporator, then the evaporator is the main part where cooling occurs. 'Cooling' does not mean drop of temperature here, it means staying cool, despite of being warmed.
So it is being heated but it is cooling? I think you're just mixing up the to and from of the heat transfer: the evaporator coil is cooling the air while heating the refrigerant. Again: you would never use the word "cooling" to describe what is happening to a boiling pot of water on a stove.

 Er, caveat due to a cumbersome system definition. You could say the water itself on a boiling pot on the stove is losing thermal energy to evaporation, which is "cooling" while simultaneously being heated by the stove, but that system definition isn't allowed in the refrigeration cycle because you aren't discarding the gaseous refrigerant as it boils.
@Karagoz: Here's a good explanation of how refrigerators work:
http://mocomi.com/how-does-a-refrigerator-work/

Everybody: Anything wrong with that explanation?
Just a minor quibble: It oddly splits the condenser into two separate steps/devices. It's only one step/device (the coil).

The English language is full of problems. The word "Heating" can be taken to mean 'increasing temperature' or 'supplying heat to' and a good explanation of refrigeration methods should make clear which meaning is involved each time the word 'heating' is used. I went to a wedding, many years ago, in a massive granite church on the Yorkshire Moors. It was the coldest place I have ever had to stand in. The caretaker insisted he had "had the heating on all morning". It had not heated it up (raised the temperature) at all as the place was full of holes!
We spend (waste) a lot of thread pages on PF arguing at cross purposes due to the different meanings of words. If you want to resolve a problem like this one then try to put things another way, avoiding the offending word. People will thank you.