How does a metering device on an A/C lower the pressure?

In summary, the conversation discusses the role of the metering device in an air-conditioning system and how it affects the temperature of the refrigerant. The student has some questions about how the metering device works and what causes the temperature of the refrigerant to decrease. They also mention a classmate's explanation that the refrigerant's temperature decreases due to the metering device changing the refrigerant from liquid to a mixture of liquid and vapor. The student is unsure about this explanation and seeks clarification from their instructor.
  • #1
fourthindiana
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Preface: Some people call the metering device on an air-conditioner the "expansion device". When I refer to the metering device in this post, metering device is synonymous with expansion device.
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I'm a student attending a HVAC program at a trade school. My instructor has drawn pictorial diagrams of air-conditioning systems that cool houses before. In a typical pictorial diagram that my instructor has drawn of a residential air-conditioning system, the refrigerant will typically be labeled as 100 degrees Fahrenheit immediately after the refrigerant leaves the condenser coil. When the refrigerant is in the middle of the evaporator coil, my instructor will typically label the refrigerant as 40 degrees Fahrenheit.

Here are some facts I know already about air-conditioners:
  • I know that the metering device changes the pressure of the refrigerant from high pressure to low pressure.
  • I know that when the refrigerant enters the metering device, the refrigerant is 100% liquid.
  • I know that when refrigerant goes through the metering device, the metering device changes the refrigerant from 100% liquid to a flashgas mixture of 65% liquid/35% vapor (or 75% liquid/25% vapor).
  • I know all about how it takes a lot of heat just to change the state of a liquid to a vapor without increasing the temperature.
I have a fairly knowledgeable classmate that sits next to me who I frequently ask HVAC related questions to if my instructor is too busy or otherwise unavailable to help me. I will call this knowledgeable classmate of mine "Classmate." Classmate claims that the metering device changing 25-35% of the liquid refrigerant into flashgas causes the temperature of the refrigerant to decrease because it takes a lot of heat to turn the liquid refrigerant into vapor. Although I agree with Classmate that it takes a lot of heat to turn the liquid refrigerant into vapor, I don't see how this would lower the temperature of the refrigerant.

To my knowledge, my instructor has never make an incorrect statement about HVAC service procedures. However, my instructor is not a scientist or an engineer, and he has made incorrect statements about science before. I asked my instructor how the metering device indirectly causes the refrigerant to become cooler. My instructor told me that the metering device indirectly causes the temperature of the refrigerant to drop because the metering device causes the pressure of the refrigerant to decrease. Now there are two ways to this could be taken to answer the question of how the metering device causes the temperature of the refrigerant to get cooler. I will color way #1 orange:
the metering device causes the refrigerant to get cooler by lowering the pressure. By Gay-Lussac's Gas Law, when the pressure decreases, the temperature decreases.​
By way #1, the refrigerant boiling is totally irrelevant to this question and need not be mentioned. End of story. I will color way #2 green: The metering device causes the refrigerant to get cooler by lowering the pressure of the refrigerant. When the pressure of the refrigerant decreases, the boiling point of the refrigerant decreases. Then the refrigerant's lower boiling point allows the refrigerant to start boiling. So the refrigerant starts boiling. The boiling takes an enormous amount of heat. This lowers the temperature of the refrigerant. My instructor said that way #2 is the way that the metering device's lowering the pressure of the refrigerant indirectly causes the temperature of the refrigerant to decrease.

To me, way #2 sounds it is saying that the sensible heat of refrigerant is decreased because the refrigerant is converting part of its sensible heat to latent heat. I have never heard of this before.

Is what I described as way #2 how a metering device indirectly causes refrigerant to get cooler? In other words, does the fact that the metering device lowers the pressure of the refrigerant to the point that the refrigerant starts boiling cause the temperature of the refrigerant to decrease?

Do you people agree with Classmate's explanation of this?

Am I missing something?
 
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  • #2
I know that the metering device on a residential air-conditioning system changes the pressure of the refrigerant from high to low. However, I don't know how a metering device changes the pressure of the refrigerant from high to low.

I know that the metering device on a residential air-conditioning system changes the refrigerant from 100% liquid to a 65% liquid/35% vapor or 75% liquid/25% vapor flashgas mixture.

How does a metering device change the pressure of the refrigerant from high to low?

How does a metering device turn 100% liquid refrigerant into a 65% liquid/35% vapor flashgas mixture?
 
  • #3
fourthindiana said:
Classmate claims that the metering device changing 25-35% of the liquid refrigerant into flashgas causes the temperature of the refrigerant to decrease because it takes a lot of heat to turn the liquid refrigerant into vapor...

Do you people agree with Classmate's explanation of this?
Yes, classmate is correct.
I will color way #1 orange:
the metering device causes the refrigerant to get cooler by lowering the pressure. By Gay-Lussac's Gas Law, when the pressure decreases, the temperature decreases.
As you correctly noted in the description of the process, the refrigerant is not a gas when it enters the metering device, it is a liquid. So gas expansion can't be the answer.
The metering device causes the refrigerant to get cooler by lowering the pressure of the refrigerant. When the pressure of the refrigerant decreases, the boiling point of the refrigerant decreases. Then the refrigerant's lower boiling point allows the refrigerant to start boiling. So the refrigerant starts boiling. The boiling takes an enormous amount of heat. This lowers the temperature of the refrigerant.
My instructor said that way #2 is the way that the metering device's lowering the pressure of the refrigerant indirectly causes the temperature of the refrigerant to decrease.
Yes, this is correct.
To me, way #2 sounds it is saying that the sensible heat of refrigerant is decreased because the refrigerant is converting part of its sensible heat to latent heat. I have never heard of this before.
Sure you have. Every morning when you get out of the shower and feel cold. Every time you sweat.
fourthindiana said:
How does a metering device change the pressure of the refrigerant from high to low?
By being an obstruction. Any obstruction causes a loss of pressure and therefore energy in the flow going through it.

[note: identical topic threads merged]
 
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  • #4
Thank you for clearing this up for me, Russ Waters.
 
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1. How does a metering device on an A/C lower the pressure?

A metering device on an A/C is responsible for regulating the flow of refrigerant into the evaporator coil. It does this by creating a restriction in the refrigerant line, causing a drop in pressure and temperature. This drop in pressure allows the refrigerant to absorb heat from the surrounding air, resulting in cool air being circulated into the room.

2. What is the purpose of a metering device on an A/C?

The main purpose of a metering device on an A/C is to control the flow of refrigerant into the evaporator coil. This ensures that the refrigerant is at the correct pressure and temperature for efficient heat transfer, resulting in cool air being produced by the A/C.

3. How does a metering device affect the efficiency of an A/C?

A metering device plays a crucial role in the efficiency of an A/C. By regulating the flow of refrigerant, it ensures that the refrigerant is at the correct pressure and temperature for efficient heat transfer. This results in the A/C using less energy to produce cool air, making it more efficient.

4. Are there different types of metering devices used in A/C systems?

Yes, there are different types of metering devices used in A/C systems such as thermostatic expansion valves (TXVs), capillary tubes, and electronic expansion valves (EEVs). Each type has its own unique way of regulating the flow of refrigerant and is chosen based on the specific needs of the A/C system.

5. What happens if the metering device on an A/C is not functioning properly?

If the metering device on an A/C is not functioning properly, it can lead to issues such as improper cooling, reduced efficiency, and even damage to the A/C system. It is important to regularly maintain and replace the metering device if needed to ensure the A/C is functioning at its best.

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