MadSci Network: Engineering

Re: Why does an A/C unit freeze

Date: Sun Nov 21 00:57:26 1999
Posted By: Tye Morancy, MadSci Admin
Area of science: Engineering
ID: 943050510.Eg

Hi Brandon,

That's a good question...most of us who have ever dealt with Air Conditioning, have had to deal with the AC freeze which can sometimes occur. This freezing of the AC was correctly diagnosed as possibly resulting from one of two things (or both). For one, the AC can freeze if there is improper air flow in the unit and the other is that the freon level may be low. To understand why this happens, we first need a little background on the basics of an AC unit.

Air conditioning units consist basically of a coil system of tubing which contains a "freon" fluid, a compressor, an expansion valve, and one or two fans. An air conditioner is basically a refrigerator without the insulated box. The tubing forms a closed loop which is coiled on two sides (one faces inside and the other faces outside). The tubing goes from the coil on the inside through a compressor, to the coil facing the outside, through an expansion valve, and then back to the coil inside. The compressor compresses cool freon gas, causing it to become hot, high-pressure freon gas . This hot gas runs through a set of coils so it can dissipate its heat, and it condenses into a liquid in the process. This dissipation happens as the hot freon passes through a maze of coils that typically have a fan blowing the heat from the coils to the outside of the building. The cooled freon liquid runs through an expansion value, it expands, and in the process it evaporates to become cold, low-pressure freon gas. This cold gas runs through a set of coils that allow the gas to absorb heat and cool down the air inside the building. There is typically a fan here as well blowing the cooled air around the coils into the room.

Now, back to our two problems...The first was the low air flow. If we have a low air flow as produced by the fan blowing the air cooled air inside, then the coils will have a tendency to freeze. The explanation is that the air flow, as explained above, warms the freon up as it cools the air in the process. If the air flow is low, the freon does not heat up as much, so that when the freon loops back to pass through the expansion valve, it is starting off cooler than before and thus becomes colder with each pass, until the coils freeze up on the inside and the AC fails to work properly.

The second issue of the freon being low in the unit is based on the above prinicple as well. Remember that when the freon passes through the expansion valve it expands and thus cools. Well, if there is less freon in the coils as it passes through the expansion valve, then the freon is allowed to expand even more than before because it has more room. Since it can now expand even more, it will thus cool that much more. As the freon level drops, this expansion increases more and more, until this chilling process begins to freeze up the coils on the inside and the unit fails.

I hope this answered your question...drop us a line if you need any clarification. Thank you for the question...have a good day.

Tye Morancy

"Mad Scientist"

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