evaporator ice formation

Somebody mentioned several days ago that ice formation on an a refrigeration evaporator can be caused by low freon (or perhaps air in the system, I don't recall which). I've been wondering about that for the past few days. Why would low freon or air in the system cause ice formation? I'm just curious as to what the theory behind that is.
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Vic Dura wrote:

When the refrigerant charge is low the evaporator coil gets starved for refrigerant and this results in reduced pressure at the inlet piston or expansion valve, thus allowing the refrigerant to vaporize at a lower temperature - below 32 degrees. At this point the first part of the coil will freeze. Then, since ice is a fairly good insulator the refrigerant will now travel further through the coil before encountering an exposed surface. More ice forms and the process continues. Gradually most or all of the evaporator coil will be covered with ice. This of course blocks air flow through the coil.
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Travis Jordan wrote:

What a concise, helpful answer. Thank you.
Jim
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On Wed, 29 Jun 2005 12:49:15 GMT, in alt.home.repair RE: Re:

Ok, that makes perfect sense to me. Thanks for the clear explanation.
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wrote:

I think another way to look at it is when the compressed freon enters the condenser it is a hot gas. If there is the correct amount of freon, just the right amount of heat will be removed such that it will change to a liquid but still be quite hot. The hot liquid expanded through the evaporator gets the correct amount of cooling.
If the freon is low there may be only a small amount of liquid exiting the condenser and it will be cooler than normal. When it expands in the evaporator it can produce temperatures below freezing.
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Rich256 wrote:

If you consider 80 to 110 deg "quite hot".

Not necessarily, it could be hotter than normal. It depends upon how low the charge is.

hvacrmedic
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Vic Dura writes:

It does seem paradoxical, but it is true:
http://groups-beta.google.com/group/alt.home.repair/msg/0420eda678cc180f
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Richard J Kinch wrote:

http://groups-beta.google.com/group/alt.home.repair/msg/0420eda678cc180f
But not quite for the reason stated: " With a low charge, you only get a dribble of liquid into the evap, which is at too low a temp because of the too-low suction pressure "
The evaporator temperature is a result of the vaporization of the refrigerant, not "too-low suction pressure".
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Travis Jordan wrote:

It's a result of the vaporization under too-low of a suction pressure. More to the point, it's a result of the coil's skin temperature dropping below freezing, because of the vaporization under too low of a suction pressure.
hvacrmedic
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This is Turtle.
What you spoke about here has been on my mine for YEARS and ties into the evaperator coils on hvac system up at 38 psi to 42 psi in Louisiana and other lower humidity areas at 50 psi. Do you know any places where a fellow could read up on this freezing of the coil at lower temp and pressures of the evaperator coil.
TURTLE
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Travis Jordan writes:

No. Vaporization occurs from heat transfer, not temperature. Heat and temperature are two different things, which most people confuse.
Look at the saturation pressure-vs-temperature tables.
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Richard J Kinch wrote:

You are correct - and that is what I meant to say.
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This is Turtle.
there is a point in the Back pressure and head pressure points where at this one point that the evaperator will freeze up on low freon. In the Louisiana area it is on R-22 at 38 to 42 psi back pressure and other parts of the country up as high as 50 psi back pressure. A system can freeze up on low freon but usely only at one temp it will do it at.
TURTLE
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Many refrigeration systems are designed for the evaporator to run below 32F, so they have a defrost feature.
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