Brad Cochran, CPP, Inc.
Enthalpy wheels provide a great opportunity to capture thermal energy in laboratory exhaust air before it is vented to the atmosphere. However, mechanical designers have been reluctant to use an enthalpy wheel for laboratory exhaust systems due to the potential for cross-contamination of chemicals within the exhaust stream.
The potential for cross-contamination was examined using the manufactured specified leakage rate of 0.045 percent (or a dilution of roughly 1:2200). The resulting chemical concentrations calculated to leak through the enthalpy wheel were compared to published health and odor limits for chemicals commonly used in a research laboratory environment. These concentration values were also compared to typical design limits applied to exhaust systems to evaluate re-entrainment at nearby air intake locations and the ASHRAE 110 fume hood exhaust standard for a manikin standing in front of the fume hood. The calculations indicate, assuming the manufacture's stated leakage rate, that chemical cross-contamination through the enthalpy wheel is, in many instances, negligible when compared to the potential re-entrainment through nearby air intakes or the acceptability limit defined in ASHRAE 110.
A variety of questions considered include: 1) Are the manufacturers'
stated leakage rates accurate enough to be the basis for design in potential
life critical applications? 2) Does the stated leakage rate equally apply
for all chemical emissions, or is it a function of the molecular size
of the gas particles? 3) If the leakage rate is a function of the molecular
size of the gas particle, is 0.045 percent a conservative value? And 4)
are pressure and flow sensors across the enthalpy wheel sufficient to
detect leakage rates in excess of 0.045 percent, or should sensors be
placed in the supply plenums to detect the presence of potential harmful
chemicals passing through the enthalpy wheel?
Brad Cochran, an associate at CPP, has over 15 years of experience conducting wind tunnel and numerical modeling studies related to laboratory exhaust design. Over the years, Brad has managed several hundred exhaust dispersion projects for such clients as Northwestern University, University of California at Los Angeles, the National Institutes of Health, University of Texas Medical Center, Loyola University, Bayer Pharmaceuticals, University of California at Irvine, University of California at Davis, and University of California at Berkeley. Recently, Brad has conducted air quality assessments for several large laboratory research buildings that are evaluating the potential to utilize enthalpy wheels for energy recovery in their laboratory exhaust. Brad has a Bachelor of Science degree in mechanical engineering from New Mexico State University, a Master of Science degree in mechanical/aeronautical engineering from the University of California at Davis, and a Master of Business Administration degree from Regis University.