Direct Expansion Coils

Most evaporator coils are designed with either 1/2” or 3/8” O.D. tubes. However, there are certain applications where it is desirable to use 5/8” tube coils. Perhaps the most critical factor in designing a refrigerant evaporator coil is the need to keep the velocity of the refrigerant flow up high enough that the oil that is present remains entrained within the refrigerant. This is one of the primary reasons that so many DX coils are manufactured with 3/8” tubes. The smaller tube diameter facilitates the mixing of the oil within the refrigerant. However, on coils with longer tube lengths, the smaller diameter can lead to excessive refrigerant pressure drops so larger tubes may be used.

The most commonly used tube material is copper, and there are a variety of tube wall thickness options to handle different applications. Aluminum is the most frequently used material for the fins on a coil, because of good heat transfer characteristics and low cost. There are many different fin designs that will either enhance heat transfer or reduce air pressure drop, depending on the requirement. Coil frames (or casings, flanges, whatever you might call them) are normally galvanized, but stainless steel is a good consideration to increase the longevity of the coil. Coil connections are almost universally copper.

Keep in mind that although there are a huge variety of material options available, unless there is a design reason to specify something special, most coils will be purchased based on the cost alone. So using the “standard” coil material is generally the approach to take. We are strong believers in stainless steel frames for cooling coils, assuming that there is condensation resulting from the cooling process (which is not always the case in dry climates).

Heat pump coils in their most basic form are simply evaporator coils that become condenser coils when the system is operating in the heating mode (also known as the reverse cycle).


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