Volatile Organic Compounds (VOCs) constitute an important class of indoor air contaminants and they may cause adverse health effects for occupants in buildings. Indoor generated contaminants may be transferred between the supply and exhaust air streams of the building’s Heating, Ventilation and Air-conditioning (HVAC) system when air-to-air energy recovery devices are used. The run-around membrane energy exchanger (RAMEE) is a novel exchanger, which uses aqueous magnesium chloride (MgCl2) salt solution (34-35 wt%) as a liquid desiccant to transfer heat and moisture between remote supply and exhaust air streams. In the RAMEE, a gas-phase porous membrane is placed between the air stream and the liquid desiccant stream in each exchanger and the membrane prevents the salt solution from entering the air stream but still allows the transfer of water vapor through the semi-permeable membrane. In the RAMEE, VOCs may transfer between the exhaust and supply air streams due to (i) air leakage or (ii) due to dissolution of VOCs into the liquid desiccant in the exhaust exchanger and their subsequent evaporation into the air stream of the supply exchanger. These two transfer mechanisms were tested in the laboratory using two counter-cross-flow RAMEE prototypes (Prototype #4 and Prototype #6). Tests were conducted at different air and desiccant flow rates at AHRI standard summer and winter operating conditions. Sulfur hexafluoride (SF6) was used as a tracer gas to test air leakage and toluene (C7H8) and formaldehyde (HCHO) were used to test VOC dissolution and transfer. From an external source, a known concentration of VOC was injected into the exhaust air inlet stream and the transfer fraction of VOC to the supply air stream was calculated. This transfer fraction or Exhaust Air Transfer Ratio (EATR) defined by ANSI/ASHRAE Standard 84 (2012) at steady state conditions was used to quantify and compare the transfer fraction of contaminants in both prototypes. The uncertainty in the transfer fraction was calculated and all the uncertainty bounds were calculated for 95% confidence interval. The transfer fraction of sulfur hexafluoride was 0.02 +/- 3.6% for both prototypes tested, which means that the air leakage between the air streams is negligible. The transfer of toluene, which has a low solubility in water, was less than the uncertainty in the measurement. EATR* values for toluene were 2.3-3.4% and the uncertainties were 3.4-3.6%. The transfer of formaldehyde between the exhaust and the supply air streams was the highest and the EATR* values just exceeded the uncertainties in the EATR* measurement. The highest EATR* values for the transfer of formaldehyde in Prototype #4 and Prototype #6 were 6.4 +/- 3.6% and 5.3 +/- 3.6%, respectively. At steady state, the measured EATR* values for both prototypes were insensitive to changes in the air flow rate, the liquid desiccant flow rate, the latent effectiveness and the environmental conditions but time delays to reach steady state were significant. These results imply that there is a negligible transfer of contaminants due to air leakage between the air streams, a negligible transfer of low water soluble VOCs (such as toluene), but possibly a small detectable transfer of very water soluble VOCs (such as formaldehyde) between the exhaust and supply air streams of the RAMEE.

In this study, a novel counter-cross-flow run-around membrane energy exchanger (RAMEE) system was designed and tested in the laboratory. The RAMEE system consists of two (2) counter-cross-flow Liquid-to-Air Membrane Energy Exchangers (LAMEEs) to be located in the supply and exhaust air streams in the building Heating Ventilation and Air-Conditioning (HVAC) system. Inside each exchanger, a micro-porous membrane separates the air and liquid streams and allows transfer of the sensible and latent energy from the air stream to the liquid stream or vice-versa. The system exchanges sensible and latent energy between supply and exhaust air streams using a desiccant solution loop. The supply and exhaust air streams in the RAMEE can be located far apart from each other or adjacent to each other. The flexibility of non-adjacent ducting makes the RAMEE system a better alternative compared to available energy recovery systems for the retrofit of HVAC systems.<p> Two counter-cross-flow exchangers for the RAMEE system were designed based on an industry recommended standard which is to obtain a target overall system effectiveness of 65% for the RAMEE system at a face velocity of 2 m/s. The exchanger design was based on heat exchanger theory and counter-cross-flow design approach. An exchanger membrane surface aspect ratio (ratio of exchanger membrane surface height to exchanger length) of 1/9 and the desiccant solution entrance ratio (ratio of desiccant solution entrance length to exchanger length) of 1/24 were employed. Based on different heat transfer case studies, the energy transfer size of each exchanger was determined as 1800 mm x 200 mm x 86 mm. ProporeTM was used as the membrane material and Magnesium-Chloride solution was employed as the desiccant solution.<p> The RAMEE performance (sensible, latent and total effectiveness) was evaluated by testing the system in a run-around membrane energy exchanger test apparatus by varying the air stream and liquid solution-flow rates at standard summer and winter operating conditions. From the test data, the RAMEE effectiveness values were found to be sensitive to the air and solution flow rates. Maximum total effectiveness of 45% (summer condition) and 50% (winter condition) were measured at a face velocity ¡Ö 2 m/s. A comparison between the experimental and numerical results from the literature showed an average absolute discrepancy of 3% to 8% for the overall total system effectiveness. At a low number of heat transfer units, i.e. NTU = 4, the numerical and experimental results show agreement within 3% and at NTU = 12 the experimental data were 8% lower than the simulations. The counter-cross-flow RAMEE total system effectiveness were found to be 10% to 20% higher than those reported for a cross-flow RAMEE system by another researcher.<p> It is thought that discrepancies between experimental and predicted results (design and numerical effectiveness) may be due to the mal-distributed desiccant solution-flow, desiccant solution leakage, lower than expected water vapor permeability of the membrane, uncertainties in membrane properties (thickness and water vapor permeability) and heat loss/gain effects. Future research is needed to determine the exact cause of the discrepancies.

In this study, a novel counter-cross-flow run-around membrane energy exchanger (RAMEE) system was designed and tested in the laboratory. The RAMEE system consists of two (2) counter-cross-flow Liquid-to-Air Membrane Energy Exchangers (LAMEEs) to be located in the supply and exhaust air streams in the building Heating Ventilation and Air-Conditioning (HVAC) system. Inside each exchanger, a micro-porous membrane separates the air and liquid streams and allows transfer of the sensible and latent energy from the air stream to the liquid stream or vice-versa. The system exchanges sensible and latent energy between supply and exhaust air streams using a desiccant solution loop. The supply and exhaust air streams in the RAMEE can be located far apart from each other or adjacent to each other. The flexibility of non-adjacent ducting makes the RAMEE system a better alternative compared to available energy recovery systems for the retrofit of HVAC systems.<p> Two counter-cross-flow exchangers for the RAMEE system were designed based on an industry recommended standard which is to obtain a target overall system effectiveness of 65% for the RAMEE system at a face velocity of 2 m/s. The exchanger design was based on heat exchanger theory and counter-cross-flow design approach. An exchanger membrane surface aspect ratio (ratio of exchanger membrane surface height to exchanger length) of 1/9 and the desiccant solution entrance ratio (ratio of desiccant solution entrance length to exchanger length) of 1/24 were employed. Based on different heat transfer case studies, the energy transfer size of each exchanger was determined as 1800 mm x 200 mm x 86 mm. ProporeTM was used as the membrane material and Magnesium-Chloride solution was employed as the desiccant solution.<p> The RAMEE performance (sensible, latent and total effectiveness) was evaluated by testing the system in a run-around membrane energy exchanger test apparatus by varying the air stream and liquid solution-flow rates at standard summer and winter operating conditions. From the test data, the RAMEE effectiveness values were found to be sensitive to the air and solution flow rates. Maximum total effectiveness of 45% (summer condition) and 50% (winter condition) were measured at a face velocity ¡Ö 2 m/s. A comparison between the experimental and numerical results from the literature showed an average absolute discrepancy of 3% to 8% for the overall total system effectiveness. At a low number of heat transfer units, i.e. NTU = 4, the numerical and experimental results show agreement within 3% and at NTU = 12 the experimental data were 8% lower than the simulations. The counter-cross-flow RAMEE total system effectiveness were found to be 10% to 20% higher than those reported for a cross-flow RAMEE system by another researcher.<p> It is thought that discrepancies between experimental and predicted results (design and numerical effectiveness) may be due to the mal-distributed desiccant solution-flow, desiccant solution leakage, lower than expected water vapor permeability of the membrane, uncertainties in membrane properties (thickness and water vapor permeability) and heat loss/gain effects. Future research is needed to determine the exact cause of the discrepancies.