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22 TRG-TRC005-EN
notes
period three
Evaporators
The face-split coil configuration, also called horizontal-split or parallel-flow, is
split into parallel sections. A portion of the air passes through the top section,
the remainder passes through the bottom section, and the two airstreams mix
downstream of the coil.
At lower loads, only one section of the face-split coil is active. A portion of the
air passes through the active lower section and is cooled, while the rest of the
air passes through the inactive top section and remains unconditioned. The two
airstreams mix downstream of the coil, producing average temperature and
humidity conditions. At higher loads, both sections of the coil are activated,
providing a more uniform leaving-air temperature.
In a VAV application, where the leaving air is controlled to a constant
temperature, the active section of coil must supply colder air than the desired
average temperature at part load. Consequently, the refrigerant must get colder
at part load, eventually reaching a condition where the coil surface is cold
enough that the water condensing from the air will create frost on the coil.
Resetting the supply air temperature upward can help to avoid this problem,
but this may result in space humidity problems.
In constant-volume applications, where the leaving-air temperature varies to
respond to changing loads, this is not as much of a concern since the average
mixed temperature rises at part-load conditions. Therefore, face-split coils are
well suited for constant-volume applications. They provide better part-load
humidity control than could be obtained from a large coil controlled by a single
expansion valve. In a constant-volume application, the lower section of a face-
split coil should be activated first and deactivated last. This sequence prevents
moisture that has condensed from the air flowing through the active coil
section from flowing over the fins of the inactive coil section, where it could
carry over into the supply airstream.
Face-Split Arrangement
80ºF
80ºF
[27ºC]
[27ºC]
30ºF
30ºF
[-1.1ºC]
[-1.1ºC]
55ºF
55ºF
[13ºC]
[13ºC]
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80ºF
80ºF
[27ºC]
[27ºC]
Figure 33