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The first parameter is the AUTOFRESH AIR Functional
Parameter. It may be set to CLOSED, OPEN or CFM
CONTROL. When set to CLOSED, AutoFresh Air is not
operational. When set to OPEN, the ports will always
be open with setpoints above 28°F (-2.2°C).
When set to CFM CONTROL the AUTOFRESH AIR
CONTROL Functional Parameter becomes available.
The Parameter may be set form 5 to 50 CFM in 5 CFM
increments. Under CFM CONTROL, AutoFresh Air will
operate on a 20 minute cycle. During the cycle the ports
will open until the required CFM has been exchanged
and then close for the remainder of the cycle.
4.9
DEFROST
Defrost is an independent cycle overriding cooling and
heating functions in order to de-ice the evaporator.
When the unit enters defrost the MessageCenter will
display DEFROST CYCLE STARTED for the first 10
seconds and then display the default message for the
remainder of the cycle. DEFROST is displayed in the
Operator Message Panel, along with the setpoint while
the unit is in the cycle. Defrost is allowed when the DTT
is less than 40.0°F (4.4°C) or the SAT is less than
45.0°F (7.2°C), unless the alarms for both DTT and
SAT are activated, then defrost can be initiated when
the RAT is less than 45.0°F (7.2°C).
4.9.1
Defrost Initiation And Start
Defrost can be initiated by pressing the MANUAL
DEFROST key or automatically by the control system.
Control system initiation is based on coil condition or
expiration of the defrost timer.
a. Defrost based on coil condition
Defrost based on coil condition will be initiated when
blockage is sufficient to cause an air pressure differen-
tial across the coil great enough to close the contacts
of the defrost air switch (DAS).
TIP
Ice is not the only thing that will cause the air
differential to increase across the evaporator
coil. Shrink wrap, paper, plastic bags, and
other such items when caught in the return
air stream and pulled up against the evapo-
rator coil or the return air grille can also
cause the DAS contacts to close.
b. Defrost based on time
Time interval between defrosts is selected in the Func-
tional Parameters. The parameter value may be set to
1.5, 3, 6, or 12 hours (factory default setting is 6 hours.)
NOTE
The defrost timer will not count when the unit
is in defrost, the unit is in a Start/Stop off cycle
or the DTT is greater than 40.0°F (4.4°C).
NOTE
The unit will operate in high speed in the
Defrost Mode except during pump down in
high ambient defrost.
4.9.2
Normal Defrost Operation (Ambient Air
Temperature less than 80°F (26.7°C)
When the ambient air temperature is greater than 35°F
(1.7°C) the compressor discharge pressure will be
monitored during the defrost cycle.
NOTES
1. When in defrost, control of SV2 is the same as
in Heat Mode. Refer to
2. When the Ambient Air Temperature is greater
than 35°F (1.7°C) at the beginning of the defrost
cycle
and
the discharge pressure rises to 340
psig (23.1 bar), UL1 will unload for the remain-
der of the cycle.
3. If discharge pressure rises to 350psig (23.8
bar), the SV1 valve will open for 1 second and
continue to cycle as often as every 30 seconds
to reduce the pressure to less than 350psig
(23.8 bar).
4.9.3
Normal Defrost Termination
The Defrost Mode terminates when the defrost termi-
nation temperature (DTT) and supply air temperature
(SAT)
both
rise higher than 55°F (12.8°C).
4.9.4
Normal Defrost Termination Sequence
The following sequence will be used to perform a Nor-
mal Defrost Termination:
a. The control system will place the engine in low
speed and open SV1 & SV2.
b. The control system will return the CSMV to the
same % it was prior to the initiation of defrost.
c. After a 3 second delay, SV4 will be closed.
d. After a few seconds the Clutch will be engaged.
There will be a short delay before the engine
can return to High Speed.
e. At this point, the control system will return to
normal temperature control and the data
recorder will record a Defrost Termination
Event.
SV1
SV2
SV4
Engine
Speed
Note 3
Note 1
Open
High
Clutch
Output
UL1 & UL2
CSMV
Engage
Note 2
Load
100% Open
Summary of Contents for Transicold X4 7300
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