The cooling coil or evaporator coil is where building indoor air cooling actually takes place. The liquid air conditioning refrigerant entering the cooling coil through the metering device (a capillary tube or THERMOSTATIC EXPANSION VALVE) is increasingly changed to gas form as it "boils" or evaporates as the liquid refrigerant flows through the cooling or "evaporator" coil, so that at the end of the cooling coil the refrigerant is totally in gaseous form.
This state change (liquid to gas refrigerant) absorbs energy, cooling the tubing and fins of the cooling coil and thus indirectly, cooling and dehumidifying indoor air that is blown across the coil.
A cooling coil which is blocked by debris or ice and frost, or which is damaged can obstruct air flow and reduce air conditioning system output. The air conditioning system evaporator coil and problems include ice and frost build-up, dirt or debris blocking air flow through the coil, and damaged or leaky cooling coils.