Heat Recovery Wheel Cross Contamination / Leakage

Cross contamination is a measure of the amount of unwanted compounds or impurities that are transferred from the return air stream to the supply air stream. This is usually not an issue in normal commercial heating and cooling applications although it can sometimes be important to some consulting engineers when specifying an energy recovery unit. In the effort to reduce the levels of undesirable impurities in the air,

certainly heavy cross contamination can limit the effectiveness of ventilation. It is important to avoid cross contamination in conditions where there is a high volume of polluted indoor or outdoor air. It is extremely important to prevent cross contamination in medical applications. These applications include laboratories, heavily industrialized areas, paint or solvent storage areas, morgues and health care facilities.

There are three different sources of cross contamination:

I.) carryover from return to supply stream due to
wheel rotation

2.) seal leakage from return to supply stream

3.) adsorption by the desiccant with later release
into air stream

A purge section will minimize carryover from return air to supply air. Desert Aire offers an optional purge unit that adjusts from 0-15 degrees. This provides ample latitude to prevent the carryover attributed to the wheel’s rotation.

The best way to prevent seal leakage is to use quality seals and to ensure that the supply wheel face has a positive pressure differential between supply and return. Our Desert Aire cassettes seal peripherally on the wheel band, and diametrically along the centerline of the wheel face using full-contact nylon
bush seals which provide a good proximity seal. Tests have shown this proximity seal to limit air leakage to around 2% at
1 inch water column of differential between the air streams.

In addition to the molecular selectivity feature of molecular sieves (as explained under Desiccant Selection), it is also
important to know that co-adsorption of commonly found impurities is limited. This is due to the fact that molecular adsorption rates are largely dependent on partial pressures. Since the partial pressure of water (i.e. moisture) is over 100 times greater than any of the commonly found impurities in a return air stream, adsorption of an impurity by the molecular sieve is highly unlikely. Also, molecular sieves have a natural affinity for polar molecules. Because water has the highest molecular polarity, it is readily absorbed by a molecular sieve.