I like
@Mooseman 's Perfect Description here... and if I may... I'll add three pictures to show just how completely frozen over the evaporator under the dashboard inside the cab can really become... and elaborate on the technical reasons for why this can occur:
Instead of a Fixed Orifice...THIS last image shows an Evaporator Unit which employs a TXV (
Thermostatic e
Xpansion
Valve) that tends to be more forgiving of variable amounts of charges of refrigerant. Note the FILTHY and FROZEN AIR FILTER ON THE RIGHT SIDE!
Now even though these images are of Residential HVAC "A" Frame- like Evaporator Coils that have frozen over... their value for demonstration purposes is clear... If you observe the lower section of the first image...you can see where the Copper "L" Liquid Line ends at the Brass Coupling... and what should be a group of very fine capillary copper tubes branching out to the left and right sides that feed larger copper tubes in the angled finned areas. But as you can observe...The entire thing from the point of that Brass fitting onward... is obscured and has become just a solid block of Ice.
Just inside of that Brass Fitting is a Fixed Sized Orifice that ordinarily allows the hot, liquid refrigerant to branch out uniformly at a prescribed rate...and when the liquid reaches the larger tubes...it expands gradually as a mixture of gas and liquid at the entry point... and by the time it reaches the top of each side.,..it has fully expanded into refrigerant vapour. The energy required to make this evaporation "Change of State" to take place is "stolen" right out of the air that the AC Blower in the cab is pushing through and over the coils...and the consequence of that is that the air temperature drops and any water vapour being carried inside the air stream...collects on the Evaporator fins (often referred to as being "Beer Can Cold") and drops down into a Drain Tube to gravity feed outside under the vehicle and pool on the ground, After that...things get nice and cool and de-humidified inside the cab...as long as everything is working properly.
Ideally... Air Conditioners work best when the temperature of the evaporator coils does not drop below 38-40 degrees Fahrenheit . But if the Refrigerant Charge is Too Low... what will happen is that the Hot Liquid inside the Liquid Line will be so scarce, that as the liquid approaches the evaporator... it will prematurely "FLASH OVER" inside the entry port of the Evaporator... drawing the ambient heat from the air stream so rapidly at sub-freezing temperature... and any moisture left in the air stream will instantly freeze solid on the internal coils and fins of the Evaporator. When enough of the moisture collects in between... the Ice will simply block off the flow of the air entirely... and so what looks like it should make the air inside the cab REAL COLD... will become nothing more than a FROZEN PLUG. And now... the only way out of this Freezing Purgatory... is to isolate the leak(s) and repair them... and then charge the system gradually... after the Ice Block melts completely and the Evaporator returns to normal.
If it comes down to cases and the Compressor requires a change out... just a word of practicality. There are only Two Kinds of Compressors to consider using:
----== The Ones that WORK...and the Ones that DON'T==----
The Best:
(1) AC-Delco
(2) Delphi
The Worst:
(3) All the Rest of the Cheap, Inexpensive New and Rebuilt Ones that will FAIL in 6 Weeks!