Document KJKVm3NOxN5N7kBkva8LdmpDw

318 >O (gS oos. go 0(B a lo3 P5 h i *< S5hS5ph3 W . go *, CHAPTER 13 g ss i Sa 3pa f <=> S ss s <s> 5 >ro . " QQO o s s 1956 Guide T a b l e 26. R ate of H e a t G a in F rom A pplian c es I 5000 1 5& gao a ** in i : ^Sffl3ffl3^SSCQ3 J3* ?. I =3=3J J i a ' S313 .o1 | \li Cooling Load 319 heating appliances, but it is believed that when they are properly hooded with a positive fan exhaust system through the hood, 50 percent of the heat will be carried away and 50 percent dissipated in the space to be condi tioned. The same effectiveness of the hood should be figured for both latent and sensible heat. LOAD FROM MOISTURE TRANSFER THROUGH PERMEABLE BUILDING MATERIALS The diffusion of moisture through all common building materials is a natural phenomenon which is always present to a greater or lesser degree. The permeability and permeance values for various building materials are given in Table 2 of Chapter 10, together with an explanation of mois ture transmission through these materials. In the usual comfort air-conditioning application, it is common prac tice to neglect moisture transfer through walls, for the actual rate is quite small and the corresponding latent-heat load is hardly significant. Socalled vapor barriers are frequently employed in modem construction for the purpose of keeping moisture transfer to a minimum, and reducing the deteriorating and insulation-destroying effects of moisture. Industrial jobs,', on the. other hand, frequently call for a low moisture content to be maintained in a conditioned space. Here the matter of moisture transfer cannot be neglected; indeed, it is quite possible to have the latent-heat load accompanying this transfer be of greater magnitude than any other latent-heat load. The equation for computing this load is: (qAm\) = WM X (D( iffVeareponrceP,reIsns.uHreg)\ X W6' BtU per (, hr.) <, Sq ft) where (17) M = permeance of the specimen in perms, or grains per (square foot) (hour) (inch of mercury vapor pressure difference). 7000 = grains per pound. The factor 1076 is defined in list of symbols at Equation 9. (Sensible cooling of the water vapor is included in the factor 1076 ) The only means of preventing moisture transfer is to use a vapor proof wall, or to apply a special lining, which is vapor proof. All openings in moisture proof construction must be equipped with special gaskets to pre vent entrance of moisture. When moisture transfer contributes an appreciable part of the latentheat load, it is recommended that estimates should be made intentionally liberal in order to avoid later difficulties with insufficient dehumidifying capacity. Storage spaces, for example, would require sufficient dehumid ifying capacity to handle the moisture brought in with goods to be stored, m addition to moisture leaking in subsequently. MISCELLANEOUS HEAT LOADS This designation is intended to cover the various small heat gains from exposed piping, ducts, work done by circulating fan, and unforeseen conhngencies. Where sufficient data are available, these various heat gains may be estimated individually. In the majority of cases, however, common Practice is to lump these factors together and combine them with a safely