Document b5rEx189M12b60meanZ1gn4v3

682 CHAPTER 38 i------------------------------------ 1948 Guide VAPOR BARRIERS The walls enclosing a dehumidified space are subjected to a vapor pressure differential. The pressure of the vapor outside the walls tends to force moisture through the walls into the dehumidified zone of relatively low vapor pressure. As this process can be an unnecessary load on the dehumidifying equipment, provisions should be made for keeping the vapor transfer to a minimum. Also, if the space is cooled below the ambient dew-point there is a possibility that condensation may occur within the walls unless vapor transfer is controlled. For those reasons Fig. 6. Performance Data for Typical Commercial Solid Adsorbent Dehumidifier a vapor barrier should be located within the wall construction as near to the high vapor pressure, side as feasible. To be effective a barrier must be continuous and should be so located within the structure that it will be protected from rupture. Some types of insulation are supplied with the vapor barrier attached. Asphaltic paints, asphalt impregnated and surface coated papers, lami nated papers, and combinations of metal foil and kraft papers are among the types of available barriers. Sheet metal linings have been Used in rooms where moisture transmission must be eliminated. In existing construction, barriers may be added to the surface of the walls by appli cation of proper paints or the addition of foil or laminated impregnated paper. Dehumidification by Sorbent Materials' 683 When vapor permeability is expressed as grains per (hour) (square foot) (inch of mercury pressure differential) a value not exceeding unity indicates a proper vapor barrier. Moisture vapor transmission rates for various materials are listed in Table 24, Chapter 15. The formula for the amount of moisture vapor transmitted through a wall is: W = vA (P, - P,) where W = total moisture vapor flow, grains per hour through the wall. p. = permeability, grains per (hour) (square foot) (unit vapor pressure differential). A -- area of the wall, square feet. l\ = vapor pressure on the'humid side of the wall, and Pi = vapor pressure on the other side of the wall, both in units consistent with the pressure units of the transmission coefficient. The over-all moisture transfer coefficient for a wall consisting of a combination of several materials in series may be calculated by combining the permeabilities (pi, p2, p*, etc.) of the individual materials according to the formula: 1 Pl Pl P Pn Example 2. Find the moisture transfer through a wall of 1000 sq ft area if the per meability of the wall is 0.45 grains per (square foot) (hour) (inch Hg) and the air con ditions are 85 F dry-bulb with 65 per cent relative humidity outside and 90 F dry-bulb with 20 per cent relative humidity inside. Solution. The vapor pressures are therefore 0.65 X 1.21 = 0.787 in. Hg outside and 0.20 X 1.42 = 0.284 in. Hg inside. The vapor transfer of the wall is therefore 1000 X 0.45 (0.787 -- 0.284) =* 226 grains per hour. Allowance should be made in design load calculations for imperfections in application of vapor barriers and consequently some judgment is necessary in determining the quality of the workmanship. BIBLIOGRAPHY The Adsorption of Gases and Vapors, Vol. 1, Physical Adsorption, by Stephen Brunauer (Princeton University Press, 1943). Vapor Adsorption, by Edward Ledoux (Chemical Publishing Co., 1945). Adsorption, by Charles Mantell (McGraw-Hill Book Co., 1945). Gas Summer Air Conditioning (American Gas Association Research Bulletin No. 18). Conditioning of Gases and Air, A Symposium (Chemical and Metallurgical Engineering, May, 1940, p. 286). Chemical Dehumidification Agents, by F. R. Bichowsky (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, October, 1940, p. 627). Dehumidification Protects U. S. Navy's Inactive Fleet, by T. H. Urdahl and E. R. Queer (Heating, Piping and Air Conditioning, March, 1946, p. 71).