82 McQuay OM 920-1
Operator’s Guide
Figure 8: Exhaust Air Psychometric Chart
The first point on this line is the outdoor air temperature at 95% relative humidity (point 1 in
Figure 8) and the second point on the line is the return air temperature at the return air relative
humidity (point 2 in Figure 8). One exhaust air process line showing frosting conditions and
another showing no frost condition is shown in Figure 8. The controller makes a continuous
calculation to determine if and at what temperatures the saturated air and exhaust air process
lines intersect. When they do not intersect, the enthalpy wheel runs at full speed. When they
do intersect, the enthalpy wheel is controlled to a slower speed to maintain the dry bulb
temperature of the exhaust air leaving the enthalpy wheel above the higher of the two
intersecting dry bulb temperatures (point ST2 in Figure 8). This is referred to as the
“Intersection Point.” This prevents the wheel from operating under frosting conditions.
The following describes the details involved in the frost protection function that affect the
speed and start/stop of the enthalpy wheel:
When the enthalpy wheel has been operating at maximum speed for at least the Enthalpy
Wheel Stage Time and the exhaust air temperature leaving the wheel drops below the
Intersection Point plus the Minimum Temperature Difference, the enthalpy wheel will be
slowed to its minimum speed.
If the enthalpy wheel has been operating at minimum speed for at least the Enthalpy Wheel
Stage Time and the exhaust air temperature leaving the wheel is still below then Intersection
Point plus the Minimum Temperature Difference, the enthalpy wheel will be stopped. If the
exhaust air temperature leaving the wheel then rises above the Intersection Point plus the
Maximum Temperature Difference and the enthalpy wheel has been off for longer than the
Enthalpy Wheel Minimum Off Time, the wheel will be restarted and will run at its minimum
speed.
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