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XT-Micro Control Panel
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The OUTSIDE AIR CONTROL screen parameter determines when cooling air will be available. The OSA Diff, or
outside air differential, can be plus or minus and provides an offset from the selected reference. The references
is SP (setpoint), With the most basic system with no evaporative cooling, the reference is equal to SP, the start
temperature is equal to SP + OSA Diff. For example, if setpoint is 44.0° and the OSA Diff = -4°, the start tem-
perature would equal 40.0° (44.0 – 4). Any time the outside air temperature falls below 40.0°, cooling air would
be available.
If the storage has evaporative cooling or cell deck installed, this section will explain how the start temperature is
calculated.
Theory:
Evaporative cooling can drop the temperature going through the cell by saturating the air. If the
incoming air is very dry (<30 %), drops of up to 20° can be achieved. If the incoming air is very
humid (>70 %), there is very little room to saturate the air and 3 to 5 degrees of cooling may be
all that is available.
Wet_D:
Wetbulb depression is the theoretical temperature drop across the cell media. This is a
calculation based on the air being 100 percent saturated when coming out of the cell.
Cell_eff:
Cell Efficiency (0-100). The cell material, thickness, amount of water, and design can greatly
affect how well the evaporative cooling will work. The cell efficiency variable will help to
compensate and give an accurate start temperature. For example, most cells produce a
temperature drop of about 80 percent of the actual calculated wetbulb. Thus, if the calculated
wetbulb is 20° and the cell_eff was set to 80%, the adjusted wetbulb would be 16°.
Wet_A:
Wetbulb adjusted. This is the theoretical wetbulb multiplied by the cell efficiency variable. This
value should closely match the actual measured drops on the cell medium.
Start_T :
Start temperature.
Start_T = (Diff_ref + OS Wet_A)
Example:
Diff_ref = SP, SP = 45.0°, OSA_Diff = -1, Wet_A = 12
Start_T = (45.0 - 1) + 12 = 56°.
If the outside air was 56° or less, cooling air would be available.
Short cycling of the system can happen when the outside air is very close to the start temperature. Clouds or
other weather conditions can cause the outside air temperature to fluctuate very quickly around the start tem-
perature. The XT panel uses a number of unique ways to prevent this condition from short cycling the system.
MCR
Minimum Cooling Run. This variable is in minutes and by default is set to five minutes. Once
the outside air drops below the start temperature, the system will go into Cooling mode. The
MCR time will guarantee that the system will stay in cooling for the duration of the MCR timer.
Thus, if the MCR is set to 5, the system would have to run in Cooling mode for a minimum of
five minutes.
OUTSIDE AIR CONTROL:
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