Direct expansion (DX) heating, cooling, and cooling coils are heat exchangers that condition air through heat transfer between a fluid circulating in copper or steel tubes and aluminum, copper, or stainless steel fins. Air is forced through the finned surface by a fan or duct, and reciprocal heat exchange occurs with the water, steam, or refrigerant contained in the tubes, allowing the desired temperature and humidity conditions to be achieved indoors.

A heating coil distributes heat into the air using hot water or steam, while a cooling coil extracts heat from the air using cold water and removes the condensed moisture through a collection tray. In VRF/VRV systems, a direct expansion (DX) coil allows the refrigerant to expand and condense directly within the coil, with the circuit and distribution system designed specifically for the chiller.

Standard aluminum fins are distinguished by their high thermal conductivity and cost-effectiveness. Epoxy-coated aluminum (blue/gold) offers corrosion resistance in marine, chemical, or outdoor environments. Copper or stainless steel fins are used in specific applications. The tubes are generally made of copper; stainless or alloy steel is considered depending on corrosion, temperature, and pressure requirements.

The selection file must include supply and exhaust air flow rates, inlet and outlet air temperatures, target dew point or relative humidity, water inlet/outlet temperatures and flow rate or capacity, allowable pressure drop, electrical and noise limits, installation orientation, and dimensional constraints. For direct expansion (DX) systems, additional information is required regarding the refrigerant type, saturation temperatures and pressures, evaporation and condensation conditions, number of circuits, and expansion valve element.

In cooling coils, a properly sloped and insulated condensate collection tray, a double-walled drain pipe, and an appropriate air velocity prevent water backflow and runoff. Outdoors, the coil body is insulated to prevent thermal bridging. In areas prone to freezing, glycol is used, a minimum flow rate is maintained, an antifreeze thermostat is used, and bypass/valve systems are implemented.

The fins must be regularly dusted and cleaned with compressed air or low-pressure water without damaging them. The condensate collection tray and siphon should be checked periodically. In hydronic systems, filters and sludge separators are kept in place, and water quality is monitored according to the manufacturer's recommendations. Regular maintenance reduces fan capacity loss and energy consumption, thus significantly extending battery life.