Service manual

3-15
T-294-01
Thesupplyprobe is usedforcontrolandis so indicated
by the “SUPPLY” LED on the display module. The
Perishable temperature range demands high accuracy.
The unit is capable of maintaining supply air
temperature to within ¦0.25_C(¦0.5_F) of the set
point temperature setting. In Perishable range above
-- 1 0
_ C(+14 _ F),or--5_C(+23_F)optionally,controlis
maintained by controlling the position of the stepper
motor suction modulation valve (SMV) with the
compressor energized.
When pulling down from a control temperature that is
more than 5_C(9_F) above set point, the SMV valve
will beopento reducethe pulldown time. Thepressure
andcurrent limit functionsmayrestrict the valve, ifthe
pressure or current is above the selected value.
When the controlling probe temperature enters the
in-range temperature tolerance as selected at function
code Cd30, the in-range light will energize.
TheControllerlogicisdesignedsotheSMV willbegin
to close as the set point is reached. TheSMV willclose
to restrict refrigerant flow until the capacity of the unit
and the load are balanced, unless the compressor
reliability enhancement logic on t he first compressor
start prevents closure.
If the t emperature drops below the set point, the
compressorwillremainrunningforafewminutes.This
is to accommodate any initial undershoot which might
occur. After this time, and at 0.2_C(0.4_F) or greater
belowthesetpoint,thecompressorwillbeturnedOFF.
Theheaterswillbeenergizedifthetemperaturedropsto
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 tim e delaysince
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
indicatet hat thedehumidificationmodeis 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 is above the set point
and valid (AL67).
2. The pulldown mode is NOT active. (ie., 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. Temperaturecontrolsetpointisgreaterthan-- 1 0
_ C
(+14
_ F), or --5_C(+23_F) optionally, i n the
perishable range, and the compressor is running.
5. The heater debounce timer (three minutes) has
timed out.
6. Heat termination thermostat (HTT) is closed.
7. TheControlledAtmosphere(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.
Iftheabove conditionsremain trueforatleast onehour
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
conditionsarem et(seeBulb Modesectionfordifferent
evaporator fan speed options). If any condition except
for item (1.) becomes false OR if the relativehumidity
sensed is 2% below the dehumidification set point, the
high speed evaporator fans will be energized.
Thedehumidificationmodeappliespowertothedefrost
and drain pan heaters. This added heat load causes the
Controller t o 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
oftheevaporatorcoilsurface,whichincreasestherateat
which water is condensed from the air passing through
the coil. Removing water from the air reduces the
relativehumidity.Whentherelativehumiditysensedis
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.
Twotimersareprovided intheDehumidificationmode
to prevent rapid mode switching and consequent
contactor wear. They are:
S Heater debounce timer (three mi nutes).
S Out-of-range timer (five minutes).
The heater debounce timer is activated whenever the
heater contactor status is changed. The heat contactor
remains energized (or de-energized) for at least three
minutesevenifthesetpointcriteriaaresatisfied.Thisis