When liquid refrigerant enters the evaporator coil temperatures may be as low as 10 degFat that point - that is at the top of the coil at the point of refrigerant entry. In normal operation of a refrigeration system, air movement across the evaporator coil provides enough warmth that frost or ice do not form on the coil.
In fact, as one sees in a refrigeration class, releasing liquid refrigerant into a coil over which air is not being blown will quickly result in frost formation on the coil surfaces, beginning at the point of entry of refrigerant into the coil.
At the point on the cooling coil (with no air blowing across it) where no more frost forms on the coil, we know that there is no more liquid refrigerant in the coil. That is, at this point in its travel through the cooling coil all of the liquid refrigerant that has been introduced has boiled (evaporated) to a gas. Now as all vapor, the refrigerant begins to absorb sensible heat and its temperature will increase. There are pressure increases at this point in the coil too, but they are insignificant.
In normal operation an Others air conditioning system is cooling air by moving it across a refrigerant-cooled "evaporator coil" or "cooling coil" in the air handler.
Cooling air passing over the coil also removes moisture from that air - a key factor in making indoor air comfortable in hot weather.
Normally the moisture that's removed from building air forms condensate on the surfaces of the cooling coil, runs down that surface to a collector pan, and is drained away.
The cause of this problem could be as simple as a dirty air filter or it could be crimped, disconnected ductwork or even improperly-sized ductwork.
A clogged capillary tube or a frozen, dirty, stuck thermostatic expansion valve can cause this trouble.
Any or all of those conditions cause the level of refrigerant in the cooling coil to be too low; if there is some refrigerant but not enough the coil may become abnormally cold, freezing the condensate that forms on the cooling coil surface as moisture condenses out of air moving across the coil. This freezing condensate liquid can form frost and may build up into a coil icing problem or frost may appear on the cooling coil's refrigerant suction line.
When the surface of a cooling coil or suction line drops below 32 degF (say from too little refrigerant in the system or too little flow of warmer air across the cooling coil) frost formation is likely on that surface. Conversely, when the air conditioning system is working properly the surface temperatures on the cooling coil and on the refrigerant lines stay above 32 degF.
In some installations the evaporator coil tend want to drop below 32 F even in normal operation, but air movement across the coil keeps its temperature higher, and thus avoids freezing. On some commercial refrigeration or air conditioning systems where lower temperatures are common, a defrost cycle is designed into the equipment.
If an icing problem is occurring on commercial cooling systems, in addition to checking the refrigerant charge and air flow, the service technician will also check out the defrost cycle timer.
in the duct system can interfere with proper airflow through the duct system, reducing air delivery into the building, and can be mistaken for a blocked or dirty cooling coil.