The Performance of Membranes in a Newly Proposed Run-around Heat and Moisture Exchanger

by Larson, Michael David

Abstract (Summary)
The growing cost of energy combined with the increasing energy demand has driven the need for more efficient energy use. Air-to-air energy recovery in buildings has been shown to provide substantial energy savings in many cases. A new type of air-to-air energy recovery system, known as a run-around energy exchanger (RAEE), and which has excellent potential for the retrofit market, has been proposed and numerically modelled for heat and moisture exchange by Fan et al. (2006). This thesis focuses on the material properties of semi-permeable membranes required for each RAEE exchanger core.

Two commercially available membranes are considered in this thesis: a spunbonded polyolefin manufactured by DuPont with the trade name Tyvek®, and a two layer polypropylene laminate material manufactured by the 3M Company with the trade name Propore.

The moisture transfer effectiveness of the RAEE system depends mostly on the ability of its membrane to transfer water vapour. This effectiveness is investigated by measuring the vapour diffusion resistance of Tyvek® and Propore using a dynamic moisture permeation cell (DMPC). For Tyvek®, the average vapour diffusion resistance is 440 s/m, which corresponds to an expected typical RAEE energy recovery effectiveness of 52%. For Propore, the average vapour diffusion resistance is 140 s/m, which corresponds to an RAEE effectiveness of 62% in the same exchanger system.

The air permeability is also measured using the DMPC with Tyvek® having a Darcy air flow resistance of 27 nm-1 and Propore having a Darcy air flow resistance of 111 nm-1. The lower air flow resistance of Tyvek® is undesirable since air transfer is undesirable in the RAEE system.

The liquid penetration pressure is determined using a modified standard method that resembles the geometry of a membrane in the RAEE exchanger. It is found that the Propore has a liquid penetration pressure beyond the measurement capabilities of the apparatus (276 kPa); while the Tyvek® membrane has a liquid penetration pressure of 18 kPa which agrees well with published values.

The elastic moduli of the membranes are required to predict the membrane deflection under typical operating pressures and to properly size a support screen. The elastic modulus is determined using two tensile standards and a bulge test. The bulge test results are used in the design since the geometry of the bulge test better represents the situation of a pressurized membrane in the RAEE. The elastic modulus of Propore is found to be 20 ± 3 MPa and the elastic modulus of Tyvek® is found to be 300 ± 45 MPa. The values are used in subsequent calculations for sizing the square screen, where it is found that a screen with square openings of 12.7 mm (0.5 in.) is required to support the membrane.

The degradation of Tyvek® and Propore with UVC exposure is also investigated. It is found that both materials deteriorate when exposed to UVC radiation, and that the degradation is primarily a function of the exposure time and not the exposure intensity.

Considering all material properties tested, it is concluded that the Propore membrane is a better membrane choice for the RAEE than the Tyvek® membrane.

Bibliographical Information:

Advisor:Yannacopoulos, Spiro; Torvi, David A.; Simonson, Carey J.; Besant, Robert W.

School:University of Saskatchewan

School Location:Canada - Saskatchewan

Source Type:Master's Thesis

Keywords:elastic modulus uvc degradation bulge test liquid penetration vapour diffusion resistance membrane


Date of Publication:12/19/2006

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