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Evaluating Cross-Contamination in Enthalpy Wheels Serving Laboratory Exhaust Systems

Brad Cochran, CPP, Inc.

Enthalpy wheels provide a great opportunity to capture thermal energy out of a laboratory exhaust system 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.

During this roundtable the potential for cross-contamination will be examined using the manufacturer-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 will be compared to published health and odor limits for chemicals commonly used in a research laboratory environment. These concentration values will also be compared to typical design limits applied to exhaust systems to evaluate re-entrainment at nearby air intake locations.

These calculations indicate, assuming the manufacturer'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.

The questions then become, are the manufacturer's stated leakage rates accurate enough to be the basis for design in potential life-critical applications? Does the stated leakage rate equally apply for all chemical emissions, or are they a function of the molecular size of the gas particles? If the leakage rate is a function of the molecular size of the gas particle, is 0.045 percent a conservative value? And, are pressure and flow sensors across the enthalpy wheel sufficient to detect leakage rates in excess of 0.045 percent?

Biography:

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.

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October 8, 2007