Coil Selection: Chilled Water, Hot Water, DX, Evaporator, Steam, Etc.

Whether designing a new HVAC or process cooling system, performing a one-to-one coil replacement, or upgrading a coil to improve equipment performance, there are a number of details to consider when selecting a coil. In addition to choosing from the various coil types available – chilled water, hot water, DX evaporator, standard steam, steam distributing, etc. – there are various materials, dimensions, water, and air velocities, and configurations that must factor into your selection. Replacing an existing coil with an identical one is not necessarily the most appropriate choice, as there may be a number of ways to improve performance or lengthen the lifespan of the coil and related equipment.

The first step in selecting a coil is to determine its configuration, which includes coil face area, number of rows of tubes, tube diameter, number of fins and fin surface design, coil circuiting, and whether turbulators are required. Construction options that must be determined are tube material and wall thickness, fin material and thickness, casing material, header type and material, and optional coatings. Operating conditions and environment are factored into the various calculations to arrive at the optimal coil for the application. Thicker, heavier tubes translate into longer coil life, and stainless steel casings can be used where corrosion is a concern. Materials used for tubes and fins (copper, cupro-nickel, aluminum, stainless steel, etc.) should be selected according to the environment to prolong life, and surface coatings can be added to prevent corrosion.

Air velocity, water velocity, and moisture carryover are important considerations when choosing a coil. Proper coil circuiting is necessary to ensure the correct fluid velocity to prevent fouling and poor heat transfer in the event of velocities that are too low, or tube erosion due to velocities that are too high. Low air velocity can result in hot and cold spots in leaving-air temperatures, and too-high air velocity leads to excessive noise, high pressure drop, and increased risk of moisture carryover. Carryover, which occurs when water droplets are blown off the fins into the unit, is also influenced by fin spacing, materials, and configuration as well as tube spacing and coil face height.

For more information on coil selection and installation, please contact MSC at (973) 884-5000.