Trane HUV Installation Operation & Maintenance page 47

Direct Expansion Refrigerant
Cooling Coil Selection
Trane unit ventilators are available
with direct expansion refrigerant
cooling coils. These coils come
complete with a thermal expan-
sion valve. Trane unit ventilators
with refrigerant coils will operate
as a system with most properly
sized condensing units.
The proper selection of a DX split
system unit ventilator for a class-
room application versus an office
application requires different con-
siderations. This is due to the dif-
ference in the ventilation as well as
the cooling requirements for the
two applications.
• The office application is more
like a DX split system that is
applied in a residence than the
typical classroom. The ventila-
tion air introduced into the
unit ventilator is small and the
number of occupants per
square foot of office space is
less than the classroom. In
most cases this application is
relatively trouble free if the
unit ventilator and the con-
densing unit are sized prop-
erly.
UV-PRC001-EN
Selection Procedure
Direct Expansion Coils
• The classroom application
requires special consider-
ations in order to prevent
objectionable discharge air
temperatures and nuisance
trips. Because of the high
number of occupants in a
classroom, the space will
require cooling even when the
outside air temperature is very
mild. With the mild ambient
conditions, the system can
create colder discharge air
temperatures than what is
desired by the occupants and
can even trip the frost stat on
the unit. A common way to
prevent this is to order the
condensing unit with a head
pressure control device. Other
preventative measures
include field installation of
either hot gas bypass or an
evaporator minimum pressure
regulator.
Another problem with split sys-
tems is OVERSIZING. If the unit
ventilator is oversized for the loads
in the space, the compressor will
have exceedingly short run times.
Since unit ventilator provides con-
tinuous ventilation in the occupied
mode, the space will be heated by
the ventilation air and then the
compressor will be enabled for a
short time to cool down the space.
This short cycling is detrimental to
the compressor in the condensing
unit. Additionally, in humid cli-
mates the ventilation air will be full
of moisture and the short run
times for the compressor will keep
the unit from extracting the hu-
midity. This can result in class-
room humidities above 60
percent.
If the condensing unit is oversized,
the suction temperature of the re-
frigerant will be excessively low
and will cause nuisance trips on
the frost stat. The best solution for
DX unit sizing, is 400 cfm per ton
of cooling capacity. If 400 cfm per
ton is followed, then most prob-
lems can be avoided. It is also wise
to not only look at the design se-
lection for the system, but also
look at a typical low ambient con-
dition to see if the suction temper-
atures are below an acceptable
level.
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