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One of
the widest uses of heat exchangers is for air
conditioning of buildings and vehicles. This class
of heat exchangers is commonly called air coils, or
just coils due to their often-serpentine internal
tubing. Liquid-to-air, or air-to-liquid HVAC coils
are typically of modified crossflow arrangement. In
vehicles, heat coils are often called heater cores.
On the liquid side of these heat exchangers, the
common fluids are water, a water-glycol solution,
steam, or a refrigerant. For heating coils, hot
water and steam are the most common, and this heated
fluid is supplied by boilers, for example. For
cooling coils, chilled water and refrigerant are
most common. Chilled water is supplied from a
chiller that is potentially located very far away,
but refrigerant must come from a nearby condensing
unit. When a refrigerant is used, the cooling coil
is the evaporator in the vapor-compression
refrigeration cycle. HVAC coils that use this
direct-expansion of refrigerants are commonly called
DX coils.
On the air side of HVAC coils a significant
difference exists between those used for heating,
and those for cooling. Due to psychrometrics, air
that is cooled often has moisture condensing out of
it, except with extremely dry air flows. Heating
some air increases that airflow's capacity to hold
water. So heating coils need not consider moisture
condensation on their air-side, but cooling coils
must be adequately designed and selected to handle
their particular latent (moisture) as well as the
sensible (cooling) loads. The water that is removed
is called condensate.
For many climates, water or steam HVAC coils can be
exposed to freezing conditions. Because water
expands upon freezing, these somewhat expensive and
difficult to replace thin-walled heat exchangers can
easily be damaged or destroyed by just one freeze.
As such, freeze protection of coils is a major
concern of HVAC designers, installers, and
operators.
The introduction of indentations (1/08/1934) placed
within the heat exchange fins controlled
condensation, allowing water molecules to remain in
the cooled air. This invention allowed for
refrigeration without icing of the cooling
mechanism. Inventor John C. Raisley Patent number
2,046,968 issued July 7th 1936
The heat exchangers in direct-combustion furnaces,
typical in many residences, are not 'coils'. They
are, instead, gas-to-air heat exchangers that are
typically made of stamped steel sheet metal. The
combustion products pass on one side of these heat
exchangers, and air to be conditioned on the other.
A cracked heat exchanger is therefore a dangerous
situation requiring immediate attention because
combustion products are then likely to enter the
building. |
