McQuay OM 920-1 81
Operator’s Guide
Additional control is provided for units with a VFD controlling the speed of the Enthalpy
Wheel to prevent Frosting/Condensation on the Enthalpy Wheel. This control is based on an
Intersection Point described in paragraphs that follow.
The Enthalpy Wheel VFD speed is set to zero so it will operate at its internally edited
minimum speed when both of the following are true:
• The wheel has been operating at maximum speed for longer than the edited Wheel Stage
Time
• The exhaust air temperature leaving the Enthalpy wheel is below the Intersection Point plus
an edited Minimum Temperature Difference
To prevent Frosting/Condensation on the Enthalpy Wheel, the Enthalpy Wheel is turned off
when both of the following are true:
• The exhaust air temperature leaving the Enthalpy wheel is below the Intersection Point plus
an edited Minimum Temperature Difference
• The wheel has been operating at its internally edited minimum speed for longer than an
edited Wheel Stage Time
The Enthalpy Wheel is turned on at minimum speed when:
• The Wheel has been off due to Frosting/Condensation for longer than an edited Minimum
off Time
• The exhaust air temperature leaving the Enthalpy wheel is above the Intersection Point plus
an edited Maximum Temperature Difference
The Enthalpy Wheel speed will be increased to its maximum speed when:
• The exhaust air temperature leaving the Enthalpy wheel is above the Intersection Point plus
an edited Maximum Temperature Difference
• The Wheel has been operating at Minimum Speed for longer than the edited Wheel Stage
Time
When there is a threat of frost on the enthalpy wheel, the wheel is slowed down or stopped so
that less enthalpy transfer occurs and frosting of the wheel is avoided. Frosting can occur on
the enthalpy wheel when the exhaust air leaving the wheel is saturated. This condition occurs
when two lines intersect on a psychometric chart, and it does not occur when these two lines
do not intersect. One of these lines is the Humidity Ratio versus the dry bulb temperature for
saturated air. The other line is the exhaust air process line. The exhaust air process is defined
by two points on a psychometric chart (Figure 8).
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