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SECTION 3
3-15 T-268-07
modulation valve (SMV) and suction solenoid valve
(SSV) with the compressor energized.
When pulling down from a control temperature that is
more than 5_C(9_F) above set point, both valves will
be open t o reduce the pulldown time unless suction
solenoid override or current limiting is activated. See
section 2.10 for explanation of suction solenoid
override. The current limit function will restrict the
valves if the current is above the selected value. When
the controlling probe temperature reaches set point, the
suction solenoid valve will close.
When the controlling probe temperature enters the
in-range temperature tolerance as selected at function
code Cd30, the in-range light will energize.
The Controller l ogic is designed s o the suction
modulation valve will begin to close as the set point is
reached. The modulation valve will close to restrict
refrigerant flow until the capacity of the unit and the
load are balanced, unless the compressor reliability
enhancement logic on the first compressor start
prevents closure.
If the temperature drops below the set point, the
compressor will remain running for afew minutes. This
is to accommodate any initial undershoot which might
occur. After this time, and at 0.2_C (0.4_F) or greater
below the set point, the compressor will beturned OFF.
The heaters will b e en ergized ifthe temperature dropsto
0.5_C (0.9_F ) below the set point. The heaters will
de-energize when the temperature rises to 0.2_C
(0.4_F) below the set point. The compressor will not
restart until the temperature rises to 0.2_C (0. 4_F)
above the set point and a three minute time delay since
the last compressor turn off has been satisfied.
b. Operation in the dehumidification mode
(Code 33 value selected) -- Optional
The dehumidification mode is activated by selecting
Code 33, choosing a desired relative humidity value,
and pressing the ENTER key. The control probe LED
(supply 1) will flash ON and OFF every second to
indicate that the dehumidification mode is active. Once
the Mode is active and the following conditions are
satisfied, the Controller will activate the heat relay to
begin dehumidification.
1. The humidity sensor reading i s above the set point
and valid (AL67).
2. The pulldown mode is NOT active. (ie., The SSV
valve is closed, and the control temperature is less
than 5_C above set point.)
3. The control probe (i.e.; Supply 1) temperature is
less than set point, plus 0.25_C.
4. Temperaturecontrol set pointis greater than -- 1 0 _C
(+14_F), or --5_C(+23_F) optionally, in the
perishable range, and the compressor is running.
5. The heater debounce timer (three minutes) has
timed out.
6. Heater termination thermostat (HTT) is closed.
7. The Controlled Atmosphere (CA) option VENT or
Pre-Trip mode is not initiated.
8. Humidity sensor alarm is not active (AL67).
9. High pressure switch (HPS) is not open.
If the above conditions remain true for at least one hour
the evaporator fans will switch from high to low speed
operation (on units so equipped). The evaporator fan
speed will switch every hour thereafter as long as all
conditions are met (see Bulb Mode section for different
evaporator fan speed options). If any condition except
for item (1.) becomes false OR if the relative humidity
sensed is 2% below the dehumidification s et point, the
high speed evaporator fans will be energized.
Thedehumidificationmodeappliespowerto thedefrost
and drain pan heaters. This added heat load causes the
Controller to open the modulating valve to match the
new total heat load while still holding the supply air
temperature very close to the set point.
Opening the modulating valve reduces the temperature
of theevaporator coil s urface, wh ich i ncreases therate a t
which water is condensed from the air passing through
the coil. Removing water from the air reduces the
relative humidity. When the relative humidity sensed is
2% below the set point (function code Cd33), the
Controller de-energizes the heat relay. The Controller
will continue to cycle heating to maintain relative
humidity below the selected set point.
Two timers are provided in the Dehumidification mode
to prevent rapid mode switching and consequent
contactor wear. They are:
S Heater debounce timer (three minutes).
S Out-of-range timer (five minutes).
The heater debounce timer is activated whenever the
heat contactor status is changed. The heat contactor
remains energized (or de-energized) for at least three
minutes even iftheset pointcriteriaaresatisfied. This is
to prevent rapid cycling of the heat contactor when the
humidity set point is satisfied. If themode is terminated
by a condition other than the humidity sensor, e.g., an