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COIL SELECTION CRITERIA


Coil selection criteria favored by design engineers have not changed much in the past 30 years. Typically, a 10-degree (45-degree entering water temperature, 55-degree leaving water temperature) with a pressure drop not to exceed 10 feet (approximately 4.3 PSI) is used for chilled water coils; a 20-degree (180-degreee entering water temperature, 160-degree leaving water temperature) is used for hot water coil selections. Then there are the ubiquitous safety actors used by engineers when calculating loads. It all adds up to a recipe for systems that are oversized, have insufficient part load control capability and are inherently inefficient.

For example, any manufacturer will provide ARI certified coil selection software that emulates coil performance, and all of them have very similar characteristics. They will all produce approximately 80% of design capacity at 50% flow. A typical selection is shown below. Note that the selection with a 20% safety factor requires 46 GPM for 100% capacity, while a coil selected without a safety factor requires only 16 GPM to produce 80% of the "actual" design load.

Coils (and heat transfer devices in general) must maintain minimum fluid velocities to maintain a sufficiently high Reynolds Number (http://www.engineeringtoolbox.com/reynolds-number-d_237.html) to ensure turbulence and thereby heat transfer. At low flow rates coils will go laminar and will lose heat transfer capability.

Contributing to this phenomenon is the original low pressure drop requirement, done to attempt to minimize the system's pumping head. This low pressure drop means low tube velocities, so as the load decreases the coil has inadequate "turn-down" capability and will alternate between turbulent and laminar conditions. Obviously, most systems will operate at or below 80% design capacity for the vast majority of operating hours, so the implications are obvious: a system will be over-pumped and thereby experience lower than design DT. Coil selection is a critical component to chilled water/hot water HVAC system efficiency, as are the control valves that are intended to control flow rates delivered to the coils and thereby control supply air temperatures supplied to occupied spaces.

 

      Copyright Energy Environment Economics - 2003