[QUOTE]Originally posted by stevebfl
[B]I would really like a thermodynamic explanation of that statement:
"Low refrigerant can cause the evaporator core to freeze up after a while".

Steve,

My HVAC savvy buddy confirmed my suspicion that the reason low refrigerant causes evaporator coils to freeze is simply low suction side pressure. As he showed me both on manifold gauges and a refrigerant spec sheet, anything below 30.1 psi (in the case of R-12) means evaporator coil temps fall below freezing and thusly begin to freeze the water condensation formed on the outside of the coils. This eventually causes the airflow to diminish and the output air temp to rise (since air is no longer passing over the evaporator core).

Obviously there comes a point where the amount of refrigerant is no longer sufficient to absorb enough heat to allow the coils to freeze but it is certainly possible (and even likely) for an evaporator without a low-temp shut off switch to freeze up due to a somewhat low refrigerant charge. Practical experience tells me this usually happens when a typical R-12 based car system is low by 1/2-3/4 of a pound.

I hope this answers your question.

Regards,

///MikeR
'93 190 2.6 Sportline