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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.
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