by Jacques Gordon
I just received my June 2014 magazine and read your article about A/C Temp Testing right off the bat. I’ve taken numerous A/C classes and read many books and magazine articles, and recently I’ve started watching instructional videos about Central Air Conditioning in buildings. I’ve noticed some differences in the way Mobile A/C is taught versus the way fixed A/C is taught. With fixed A/C they make a much greater emphasis on Super Heat and Sub Cooling when adjusting the refrigerant charge than is made in Mobile A/C. They also use two terms I never hear in regards to A/C in cars; Saturation and Sensible Heat. In stationary A/C, Saturation is the (temperature/pressure) point at which the refrigerant is changing states in either direction: it can’t be either 100% liquid or 100% vapor at that point. When it is in transition from one state to another, it is in Saturation and any heat absorbed or lost changes the state of the refrigerant, but NOT its temperature. Above or below Saturation, if it absorbs or loses heat, it is called sensible heat and is said to be Super Heated or Sub Cooled. This was never explained to me while studying Mobile A/C.
I love your method of observing temperature at the evaporator, condenser and metering device for measuring refrigerant charge. Of those three, which is the most important to get correct within the spec? I noticed during the tests you talked about some readings being inside the window, but others were on the edge or slightly off. Do you concentrate on getting one of these values within spec more than the other two, even if one of the others might be on the edge of the temp value?
Thanks for writing Allan.
I’ll address your last question first. Temperature testing as described in the article is not a method of measuring or adjusting the refrigerant charge, it’s a diagnostic tool. When you know system pressures and vent temperature, those other temperature readings can help you determine if the system is undercharged or overcharged. That’s one difference between stationary and mobile A/C: the stationary techs adjust the refrigerant charge to achieve the desired temperatures and pressures, but the correct refrigerant charge in a mobile system is a quantity specified by the manufacturer.
I have never studied stationary A/C, but I know it’s very different from mobile systems. In a stationary system, the refrigerant flow rate and the air flow rate through the condenser are always the same. In a mobile system, air flow through the condenser varies with vehicle speed, and refrigerant flow varies with engine/compressor rpm. While the basic principals are the same, design and function are very different.
As you point out, the term ‘saturation’ defines the temperature and pressure at which the refrigerant will change phase from liquid to vapor or vice-versa, and the refrigerant’s temperature does not change during the phase change. Well, the vehicle in the magazine article has an orifice tube A/C system that has a flooded evaporator. That means the evaporator is always filled with liquid refrigerant (when the refrigerant charge is correct). The phase change takes place after the refrigerant leaves the evaporator. So the question is: if the evaporator is always filled with liquid refrigerant below its saturation point, why does its temperature not increase as it absorbs heat from the cabin?
First we have to understand ‘convection’ and ‘heat transfer coefficient’ as defined in Newton’s law of cooling. Convection is defined as energy flowing through a solid into a fluid that is moving. The fluid can either be forced to move or free to rise naturally, but either way, the heat moves through the solid because the fluid on the other side moves and carries it away. Heat transfer coefficient is the proportion of heat energy that can flow through this solid/fluid system. It depends less on the properties of the materials involved and more on design parameters like flow patterns, whetted area and geometry.
So, heat flows from the air in the cabin through the (solid) evaporator fins and tubes to the moving (liquid) refrigerant via convection. If the liquid refrigerant was not being forced through the evaporator, its temperature would increase as it absorbs heat. But the refrigerant is flowing, and the heat transfer coefficient of a fluid in motion is very high. So the inlet and outlet temperature of liquid refrigerant flowing through the evaporator will be the same because the system is designed to be that way.
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