Direct Expansion Coils

DX Coils

DX coils are commonly used in air conditioning refrigeration applications and many types of commercial refrigeration systems. Air to be cooled or dehumidified is circulated through the finned surface. Multiple rows and various tube patterns are used to achieve the desired heat transfer from the air to the circulating refrigerant. Copper tubes with copper or aluminum fins are most commonly used for efficient heat transfer.

Air conditioning systems need to operate efficiently at maximum design loads as well as partial load conditions. Many types of system capacity controls are used to match the load requirement with system capacity. With refrigerant evaporator coils, this generally takes place in the form of surface reduction and/or one of the following:

Single Circuit
Face Split Circuits
Row Split Circuits
Intertwined Circuits

VAV (Variable Air Volume) systems use buildup coil banks or large air handling units with one refrigeration system. To balance system capacity to load requirements, some form of face or row control is required. During the time of reduced capacity, it is imperative to maintain a fully active dehumidification process at the heat transfer coil. This is best accomplished by row control on standard DX coils or by use of "intertwined" refrigerant circuits. Row control is used for partial load surface balance. Whichever circuit is deactivated first results in full face area operation of the remaining circuit, keeping the full volume of air in contact with active coil surfaces.