- #1
fourthindiana
- 162
- 25
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:
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:
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?
______________________________________________________________________________________
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.
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?