Evaporator coils come in a variety of shapes and sizes, depending on the type of installation, the amount of cooling capacity needed, and the manufacturer. It is the source of cooling as air passes through the furnace or air handler. They are constructed of aluminum finned copper tubing. The copper tubing runs perpendicular to the aluminum fins, making U-turns back and forth until the desired coil size is achieved. Added cooling capacity without an increase in length and width is accomplished by adding more rows of copper tubing.

Slant coils and horizontal coils have a slab appearance, similar to the radiator in an automobile. They can be installed in ductwork running horizontally or in an air handler. An A-coil is shaped like a capital A without the crossbar. It can be installed on top of a fuel burning furnace heat exchanger or in an air handler. The newest design is the multi-flex coil which is a series of A-coils linked together at the base. The multi-flex coil can be installed in any position when encased in a special cabinet. All evaporator coils must have a drain pan to collect the water that condenses as the air flowing across the coil cools. The water can drain away by gravity or be pumped away.

The cooling effect that takes place inside the coil requires a pressure drop in the refrigerant. This drop can be accomplished in a number of ways: capillary tube, piston or orifice, or thermostatic expansion valve.

A capillary tube is a thin copper tube of predetermined length into which the compressed liquid refrigerant is pumped. The length of the tubing causes the pressure drop and subsequent cooling effect of the refrigerant.

A piston or orifice blocks the flow of refrigerant and forces it through a tiny hole, creating the needed pressure drop.

A thermostatic expansion valve meters the flow of refrigerant to meet the cooling demand of the coil. It determines this demand by way of a sensing bulb attached to the outlet tube on the coil. Because it can meter the flow to meet demand, the expansion valve can keep the coil at optimum cooling potential.