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XT-70 Control Panel 90
CO
2
buildup to levels higher than the control system setpoint is detrimental to the health of stored potatoes and
can be caused by a number of things. Lack of fresh air is the primary factor, while temperature and product con-
dition are contributing factors. Corrective action involves flushing or purging the storage with fresh air until CO
2
levels return to acceptable levels. For energy conservation reasons, it is important to initiate CO
2
control before
levels get too high.
COOLING CO
2
CONTROL:
How CO
2
buildup is alleviated depends on the mode of operation of the XT control system. During Cooling
mode, CO
2
buildup becomes a problem when outside air temperatures drop to lower extremes and fresh air in-
take is minimal. Typically, more fresh air is required to purge the excess CO
2
at times when less outside air is
required to maintain system temperature at setpoint.
The XT control system addresses this problem in the following manner. In Cooling mode, when CO
2
levels rise
above setpoint, a return air heater is activated to raise the return air temperature. Simultaneously, fresh air
doors are pulsed open to introduce cooler outside air. This accomplishes two things: the cooler air counteracts
the temperature rise caused by the heater while maintaining system temperature at setpoint and, at the same
time, provides fresh air to purge the excess CO
2
. The return air heater will remain on until the CO
2
level is 200
ppm below the system CO
2
setpoint. When that occurs, the heater will shut down and the fresh air doors will
pulse closed to maintain system temperature at setpoint. This cycle could last for up to 20 minutes and could
repeat every three to four hours for as long as outside temperatures are at lower extremes.
If the storage has been shutdown for an extended period of time, and CO
2
levels are very high, fan speed may
need to be increased for a period of time to expedite flushing of the CO
2
to a more controllable level.
REFRIGERATION CO
2
CONTROL:
CO
2
levels will climb during periods of prolonged refrigeration which necessitates a different flushing strategy
than that used in the cooling mode. The XT control system was designed with the ability to control fresh air door
position concurrent with refrigeration control. This allows management of CO
2
by modulating fresh air doors to
control CO
2
and modulating refrigeration to control temperature.
REFRIGERATION FALSE LOADING:
For a number of years, the conventional strategy was to use evaporator hot gas bypass to false load the coil and
keep the refrigeration system running during periods of light loads. This is an effective way of loading the refrig-
eration system but it does nothing to alleviate CO
2
buildup. The XT control system has the ability to control false
loading by introducing warmer fresh air through modulation of the fresh air doors while simultaneously purging
the storage of CO
2
.
Pertinent system parameters are:
Refrig. Min. Run Time When outside air and evaporative cooling is no longer effective due to outside air tem-
perature and/or humidity, the control system will switch to refrigeration mode. This is
the minimum time that the refrigeration system must run before the control system can
revert back to Cooling mode.
Pulse Width Amount of time, in milliseconds, for each door open or close pulse.
Update Time Amount of time, in seconds, between each door open or close pulse.
APPENDIX F:
CO
2
MANAGEMENT
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