The previous section on coil construction necessarily explained some of the terminology associated with air-cooling coils. Several other terms that also appear frequently are:
– Circuit: the route of the refrigerant from the time it enters a tube until it leaves.
– Coil depth: defines the number of rows of tubes the air passes from the entrance to the exit of the coil.
– Face area: the cross-sectional area through which the air passes as it enters the coil, or in other words the finned length of the coil multiplied by the finned width.
– Face velocity: the volume rate of air flow divided by the face area.
– Header: A common pipe, as shown in the coils of Fig. 6.15, from which all circuits are supplied refrigerant (liquid or supply header) or a common pipe which gathers refrigerant leaving all the tube circuits (return or suction header).
– Pass: The flow of refrigerant through one straight section of the circuit.
– Prime surface: the air-side area of tubes that is in contact with air.
– Return bends: or U-bends are short sections of curved pipe to direct the refrigerant leaving one pass to the entrance of the next pass.
– Secondary surface: the surface area of both sides of the fins in contact with air.
– Temperature difference: in the jargon of industrial refrigeration two temperature differences may be encountered—the TD, which is the temperature difference between incoming air and the refrigerant and the DeltaT, which is the change in air temperature through the coil.