62-11640
4–8
4.8
AUTOFRESH AIR EXCHANGE
AutoFresh Air Exchange is a factory installed option
that allows control of the atmospheric conditions within
the refrigerated compartment. When air is being
exchanged, two air ports are opened. The upper port
allows fresh ambient air to enter while stale air exits
through the lower port. Air Exchange is allowed with
setpoints above 28°F (-2.2°C). It is always closed when
in defrost or in a Start-Stop OFF cycle regardless of
Configuration or Functional Parameter settings.
Operation is dependent on two Configurations and one
or two Functional Parameters. When AutoFresh Air is
installed at the factory, the AUTO FRESH AIR Configu-
ration is set to INSTALLED. The control point at which
it becomes active is set in the control system Configu-
rations. The AUTO FRESH AIR DELAY Configuration
may be set to SETPOINT or a time value of 0 to 48
hours. If set to SETPOINT, control will begin when the
control temperature is 1.5°F (0.8°C) away from set-
point. If a time value is entered, it will become active
after the time has expired and the system is no longer
in Pulldown or Pull-up even if the control temperature
has not reached 1.5°F (0.8°C) away from setpoint.
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 that overrides 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
If DTT sensor alarm is active, the RAT is less than
45.0°F (7.2°C).
OR
If the alarms for both DTT and RAT alarms are active,
then defrost can be initiated when the SAT 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).
OR
The control system has calculated there is a loss of
heat transfer due to ice on the coil. This method of
defrost initiation is called a saturation based demand
defrost. This method is allowed when the unit has been
in low speed or electric mode cool for five minutes, the
evaporator fans have been in operation for seven min-
utes, and the evaporator expansion valve is operating
at less than 90%.
OR
When the RAT - 1DTT temperature is greater than
36°F (20°C) for 15 continuous minutes. This method of
defrost initiation will only be used for 3 consecutive
defrosts. After three consecutive defrost initiations by
this method the counter will be reset to allow this
defrost initiation again only after a defrost cycle is initi-
ated by another method (timer, air switch or manual).
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.
Summary of Contents for Vector 8500
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