How does metering device in an A/C cause flash gas?

[sevensages]

TL;DR Summary

When the refrigerant in an air-conditioner goes through the metering device, the metering device lower the pressure of the refrigerant a great deal and the refrigerant turns into a mixture of 75% liquid, 25% flash gas. How does lowering the pressure of the refrigerant cause 25% of the refrigerant to turn into flash gas?

When refrigerant enters the metering device of an air-conditioner, the refrigerant enters the metering device as 100% liquid. When the refrigerant in a straight air-conditioner goes through the metering device, the metering device lowers the pressure of the refrigerant a great deal. The refrigerant leaves the metering device as 75% liquid and 25% flash gas. Since the only thing that the metering device directly does to the refrigerant is lower the refrigerant's pressure, I think that the metering device's lowering the pressure of the refrigerant must be what causes 25% of the refrigerant to turn into flash gas immediately after the refrigerant leaves the metering device.

But outside of metering devices in air-conditioners, I have never heard of lowering the pressure on a liquid causing part of the liquid to change phase from liquid to gas. I don't think that the ideal gas laws would explain why lowering the pressure on a liquid would cause part of the liquid to turn into a gas because the ideal gas laws don't apply to liquid, as far as I know.

In an air-conditioner, why does the metering device's lowering the pressure of the liquid refrigerant cause 25% of the liquid refrigerant to change phase into gas?

If there is a "Law" of physics with a name that explains this, what is the name of this law of physics?

 

[Bystander]

See "Latent Heat."

 

[russ_watters]

How does lowering the pressure of the refrigerant cause 25% of the refrigerant to turn into flash gas?

Lowering the pressure lowers the boiling point too. If you lower the boiling point below the current temperature of the liquid, it starts boiling.

If you do that quickly, it happens in a flash.

Look up phase diagrams.

But outside of metering devices in air-conditioners, I have never heard of lowering the pressure on a liquid causing part of the liquid to change phase from liquid to gas.

Same effect, different context:
-Using liquid propane for fuel.
-Cooking at lower temperatures at higher altitudes.
-Humidity vs temperature.

[Edit: fixed bad typo]

 

[russ_watters]

See "Latent Heat."

That's the type of energy you get, but it doesn't tell you where it came from or why.

Sensible heat is converted to latent heat. But why? The boiling point provides a boundary of sorts between sensible and latent heat. Changing the pressure moves the boundary, forcing some to be converted.

 

[sevensages]

Lowering the pressure lowers the boiling point too. If you lower the boiling point below the current temperature of the liquid, it starts boiling.

If you do that quickly, it happens in a flash.

Look up phase diagrams.

Same effect, different context:
-Using liquid propane for fuel.
-Cooking at lower temperatures at higher altitudes.
-Humidity vs temperature.

[Edit: fixed bad typo]

I know that the refrigerant in an air-conditioner gets colder right when it goes through the metering device. I know of Gay-Lussac's rule that when the pressure goes down, temperature goes down too, but I think that Gay-Lussac's rule only applies to gases.

Is the phase change of 25% of the refrigerant from liquid to flash gas the only reason that the refrigerant gets colder when the refrigerant goes through the metering device? If there is another reason that the refrigerant gets colder when it goes through the metering device, what is the other reason?

 

[hutchphd]

The amount that actually changes phase is determined by how much heat is available. Most of the heat comes from the original temperature of the liquid being somewhat above the new low pressure boiling point temperature (and its heat capacity). Nothing magic about 25%.

 

[jrmichler]

Spend some time studying pressure enthalpy diagrams (search the term). This one is from https://en.wikipedia.org/wiki/File:R22_ph.gif.

For example: Start with saturated liquid R22 at 250 kJ/kg. According to the chart, the pressure is about 16 bar. If you reduce the pressure without adding or removing heat, you follow a path straight down on the chart. Decrease the pressure to 0.1 bar (bottom of the chart) at 250 kJ/kg. The quality is 0.54, meaning that 54% of the liquid R22 has flashed to vapor.

If you add or remove heat at constant pressure, say in an evaporator or condenser, then that process follows a horizontal path on the diagram. Liquid or vapor can change temperature, or the fraction of liquid to vapor will change.

 

[russ_watters]

I know that the refrigerant in an air-conditioner gets colder right when it goes through the metering device. I know of Gay-Lussac's rule that when the pressure goes down, temperature goes down too, but I think that Gay-Lussac's rule only applies to gases.

Yeah, that's not what is happening here.

Is the phase change of 25% of the refrigerant from liquid to flash gas the only reason that the refrigerant gets colder when the refrigerant goes through the metering device?

Yes. When the boiling point is lowered, the temperature of the mixture has to drop. The liquid can't exist at a temperature above its boiling point. When it starts boiling, the some of the sensible energy of the liquid is changed into latent energy as the liquid boil. This conversion reduces the temperature of the mixture.

 

[Lnewqban]

Please, see:
https://www.spiraxsarco.com/learn-about-steam/condensate-recovery/flash-steam

Welcome!

 

[Tom.G]

I see you have gotten lots of theoretical and implementation data. Now let's try a more materialistic approach.

As you probably know, 'temperature' is the molecules (or atoms) of a substance moving around in random paths, bumping into each other and exchanging energy. At absolute zero the random motion no longer exists.

When a fluid evaporates some of those molecules at the surface have enough energy (and direction of motion) to leave the fluid into the surrounding space.

The number of molecules leaving the fluid also depends on the pressure of the gas at the fluid surface. At high gas pressures (gas molecules packed closely together) those fluid molecules trying to evaporate are more likely to immediately hit a gas molecule. The high pressure gas acts as a barrier.

When the fluid loses its higher energy molecules it has a higher proportion of lower energy molecules, which by definition is a lower temperature.

Hope this helps!

Cheers,
Tom

 

[sophiecentaur]

I see you have gotten lots of theoretical and implementation data. Now let's try a more materialistic approach.

As you probably know, 'temperature' is the molecules (or atoms) of a substance moving around in random paths, bumping into each other and exchanging energy. At absolute zero the random motion no longer exists.

When a fluid evaporates some of those molecules at the surface have enough energy (and direction of motion) to leave the fluid into the surrounding space.

The number of molecules leaving the fluid also depends on the pressure of the gas at the fluid surface. At high gas pressures (gas molecules packed closely together) those fluid molecules trying to evaporate are more likely to immediately hit a gas molecule. The high pressure gas acts as a barrier.

When the fluid loses its higher energy molecules it has a higher proportion of lower energy molecules, which by definition is a lower temperature.

Hope this helps!

Cheers,
Tom

IMO, this is the explanation that the OP needs. Energy is taken away by the lost, fast molecules on the liquid surface. The liquid loses energy and gets colder. The same energy is returned when the gas is re-compressed and it gets hotter in the ' other' heat exchanger .