THE IMPORTANCE OF THE DESICCANT RESULTS OF CROSS-CONTAMINATION TESTING ON DESICCANTS

THE IMPORTANCE OF THE DESICCANT RESULTS OF CROSS-CONTAMINATION TESTING ON DESICCANTS

Preface This report summarizes the results of a performance testing program conducted in accordance with ASHRAE Standard 84 entitled Method of Testing Air-to-Air Heat Exchangers. The primary purpose of this report is to provide independent verification of performance, in accordance with the ASHRAE standard, for the different desiccants used today. This report was prepared in conjunction with the Georgia Tech Research Institute (GTRI) which observed and collected all performance data for each desiccant tested. SEMCO Incorporated 1999-2006. All rights reserved. The information in this technical report is furnished for informational use only, is subject to change without notice, and should not be construed as a commitment by SEMCO Incorporated. SEMCO Incorporated assumes no responsibility for any errors that may appear in this report. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, recording, or otherwise, without the prior written permission of SEMCO Incorporated. SEMCO and the SEMCO logo are registered trademarks of SEMCO Incorporated. 1

THE IMPORTANCE OF THE DESICCANT IN TOTAL ENERGY WHEEL CROSS-CONTAMINATION FINAL REPORT ON PROJECT A-5849-000 Prepared for: SEMCO Inc. 1800 East Pointe Drive Columbia, MO 65202 Phone: 573-443-1481 Fax: 573-443-6921 Prepared by: Charlene W. Bayer, Ph.D. and Robert J. Hendry Electro-Optics, Environment & Materials Laboratory Georgia Tech Research Institute Georgia Institute of Technology Mail Code 0820 Atlanta, Georgia 30332-0820 May 17, 1999 2

THE ROLE OF THE DESICCANT IN TOTAL ENERGY WHEEL CROSS-CONTAMINATION Charlene W. Bayer, Ph.D., and Robert J. Hendry Georgia Tech Research Institute EXECUTIVE SUMMARY INTRODUCTION The primary function of desiccant-based energy wheels is to exchange the sensible (temperature) and latent (moisture) energy from the exhaust air, exiting a building, to the building supply air stream. The latent transference has the potential to concurrently transfer pollutants from the exhaust air into the supply air. A research study was undertaken to determine the potential for pollutant transference, based on the type of desiccant coating utilized by the desiccant wheel. Three desiccants applied to commercially available desiccant wheels were investigated: silica gel, 4Å molecular sieve, and 3Å molecular sieve. This project is the second phase of a project initially reported in June 1991. METHODOLOGY The energy wheel test facility, designed in accordance with ASHRAE Standard 84-91, is located at the SEMCO laboratories in Columbia, MO, and was setup and operated by SEMCO personnel during the testing. All samples were collected, and analyzed by researchers with the Georgia Tech Research Institute (GTRI). The three tested energy recovery wheels were manufactured by SEMCO to be identical in every way except for the desiccant coating. Methyl isobutyl ketone (MIBK), p-xylene, isopropanol (IPA), acetaldehyde, sulfur hexafluoride (SF 6 ), propane, carbon dioxide (CO2), and methanol/ipa were injected into the wheel system return air stream. Samples were collected from the outside air stream, supply air stream and return air stream to determine the level of chemical transference by the energy recovery wheels. The compounds were selected by GTRI to represent a range of chemical classes of typical indoor air contaminants with varying molecular sizes and polarity, with the exception of SF 6. SF 6 was used as a tracer gas to determine purge efficiency and seal leakage as described in ASHRAE Standard 84-91 for testing the performance of air-to-air energy recovery devices. 1

DISCUSSION AND RESULTS SILICA GEL WHEEL Under the condition where no moisture was transferred between the return and supply air streams (i.e. the humidity content of the return and outdoor air streams were equal), the silica gel wheel had the highest transfer rate for six of the eight compounds used in this study. The silica gel wheel transferred a much higher percentage of p-xylene and MIBK, over 30%, compared to the molecular sieve wheels (< 5%). The silica gel wheel also transferred greater than 28% IPA, 47% acetaldehyde and 44% methanol/ipa. Under the condition where moisture was transferred between the return and supply air streams (i.e. the humidity content of the return air stream was humidified to 30% RH, providing an absolute humidity difference between the return and outdoor air streams and resulting in latent transfer), the transfer rate for IPA, acetaldehyde, and methanol/ipa mixture by the silica gel wheel increased approximately 5.9 to 6.5% for each contaminant, or to approximately 34%, 54% and 49%, respectively. The silica gel wheel was the only wheel to transfer propane, 3%. The data show that increasing compound polarity and water solubility increased the transfer rate through the silica gel-coated energy wheel. 4Å MOLECULAR SIEVE WHEEL The 4Å molecular sieve wheel was tested with both moisture being transferred by the desiccant wheel and without moisture transfer taking place (with the humidity content of the outdoor and return air streams equal). The 4Å molecular sieve wheel transferred the largest percentage of IPA, 46.6%, when the wheel transferred no moisture. The 4Å molecular sieve wheel transferred approximately 30 to 35% of IPA, acetaldehyde and methanol/ipa, regardless of moisture transference. Each of these is a polar compounds and less than or equal to 4Å in size. The 4Å molecular sieve wheel transferred less than 5% of the more non-polar p-xylene and MIBK, both greater than 4Å in size. The transfer of SF 6, propane and CO 2 was less than 1%. When larger than 4Å, polarity appeared to be the primary influencing factor for the transfer rate. 3Å MOLECULAR SIEVE WHEEL The 3Å molecular sieve wheel was tested only with moisture being transferred by the desiccant wheel. This was done because the 4Å molecular sieve wheel carryover data showed little difference with and without moisture transference, and since data without moisture transference for the 3Å molecular sieve wheel previously was collected and reported in the June 1991. No transference of the compounds was detected with the 3Å molecular sieve wheel when tested with moisture being transferred between the return and supply air streams. Likewise, no transference was detected during the June 1991 tests, when no humidity difference existed between the outdoor air and return air streams. 2

CONCLUSIONS The greatest transfer rate for both polar and non-polar compounds was measured with the silica gel wheel. The 4Å molecular sieve also transferred the polar compounds used in this project that were smaller than approximately 4Å, yet showed little transference of the polar and non-polar compounds that were larger than 4Å. No transference was detected with the 3Å molecular sieve-coated wheel. Humidity levels resulted in little or no impact on the transfer rate by both the 4Å and 3Å molecular sieve-coated desiccant wheels. Increasing humidity levels increased the transfer rate for silica gel, and increasing water solubility also appeared to increase the transfer rate. This research demonstrates the importance of selection of the desiccant coating on an energy wheel, particularly for applications where no pollutant transference is acceptable (such as bathroom exhausts, smoking areas, hospitals, and laboratories) and/or operations that release large amounts of moisture (such as bathrooms). Pollutant transference can be minimized by proper desiccant coating selection. In instances where recirculation of airborne contaminants is considered acceptable, the outside air quantity delivered by the energy wheel where the desiccant carry-over exists must be increased by the amount necessary to achieve the same quality of air that would be achieved with a desiccant-coated wheel that had no transference. Based on the research, this increased amount of outdoor air required may be significant. 3

60 45 30 15 0 1.4 40.1 32.1 4 3 2.4 0.5 0 0.4 0.9 0 0 0 0 0 33.9 28 46.6 32.1 0 47.5 54 49.4 43.2 30.6 32 30.9 0 29 0 SF6 Propane CO2 p-xylene MIBK IPA Acetaldehyde Methanol/IPA Silica Gel (No humidity transfer) Silica Gel (With humidity transfer) 4A Molecular Sieve (No humidity transfer) 4A Molecular Sieve (With humidty transfer) 3A Molecular Sieve (With humidity transfer) Compound Investigated % Transference 6

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