Notes

1. Based on average wheel temperature of 70 deg F

2. Effectiveness ratings are accurate # 0.03

TABLE 3 - CORRECTED EFFECTIVENESS FOR UNEQUAL AIR FLOWS

CONDENSATION AND FROSTING

In special situations condensation and frosting can occur. For this reason, the VHR package includes a full drain pan and a frost-alert circuit complete with sensor, indicating light, and fan operating controls.

Condensation occurs when the exhaust air stream is cooled below its dew point.  If this temperature also falls below freezing , ice will form.  This ice adds pressure loss which reduces the airflow through the heat wheel.  In extreme cases involving accumulated frost, the airflow could be blocked off entirely.

A psychromatic chart can be used to predict condensation and frosting.  For model VHRS, if a line connecting exhaust and supply air entering condition crosses the saturating curve, the exhaust temperature must be kept above 34 deg f to prevent frosting.  If this line does not cross the saturation curve, the exhaust temperature can drop to 20 deg F without the likelihood of icing

For model VHRE, if a line connecting the exhaust and supply air condition crosses the saturation curve, the exhaust temperature at which icing will occur is a t a point where the entering supply air specific humidity line intersects with a line from the entering exhaust air drawn tangent to the saturation curve.  Frosting will not occur if the entering condition line does not cross the saturation curve.