Document NeJ682XGg9z8BwYQB81Om4V7R

194 CHAPTER 13 1960 Guide Table 11.... Summer Coefficients of Heat Transmission U of Hal Roofs Covered with Built-Up Roofing* Bh per (baud (square fool) (F dog difference between the air oa (be two dn) taadatioa oa Top of Doc* (Coveted With Built Up tooting) Tfpo of Heat CM (Ceding'<f dxswnj Flat metal roof deck Precast cement tile Tkkknea of bo/ Dock (/ocM No Ceding Underside of Roof Exposed furred Ceding with Air Spocc, Meted Loth end Plaster 4 Ply Felt Roof Ditto + X in. Slag 4 Fly Felt Roof IX No irmriofioo fredating board* thidaum, in. H 1 1M 2 No Insulation (mooting board4 thickness, in. X IK 2 0.73 0.35 0.23 0.17 0.13 0.40 0.25 0.18 0.14 0.12 0.54 0.30 0.20 0.16 0.13 0.34 0.22 0.16 0.13 0.11 0.67 0.33 0.22 0.17 0.13 0.38 0.24 0.18 0.14 0.12 Concrete Ditto + X in. Slag m 4 Ply 2 -- / -* Gypsum and wood fiberb on K* gypsum board ---- Ditto 2 + x in- Slag 6 4 Ply Felt Roof . W Ditto 2M + in. Slag 3* 0.50 0.65 0.59 0.54 0.49 0.46 0.42 0.28 0.20 0.15 0.12 0.33 0.22 0.16 0.13 0.31 0.21 0.16 0.13 0.30 0.20 0.16 0.13 0.28 0.20 0.15 0.12 0.27 0.19 0.15 0.12 0.26 0.19. 0.14 0.12 0.32 0.37 0.36 0.33 0.31 0.30 0.29 0.21 0.17 0.13 0.11 0.24 0.18 0.23 0.17 0.22 0.17 0.13 0.11 0.21 0.16 0.21 0.16 0.20 0.16 0.13 0.10 0.34 0.28 0.29 0.25 0.23 0.17 0.13 0.12 0.20 0.15 0.12 0.11 0.20 0.16 0.13 Oil 0.18 0.14 0.12 0.10 0.25 0.21 0.22 0.19 0.18 0.14 0.16 0.13 0.11 0.094 0.16 0.15 0.13 0.10 0.090 Wood* 4 Ply Roof IX 1 Ditto IK + X in- Slag 0.43 0.33 0.29 0.22 0.35 0.29 0.28 0.20 0.26 0.22 0.20 0.16 0.19 0.17 0.16 0.13 0.15 0.13 0.13 0.11 0.12 0.11 0.11 0.09 0.23 0.20 0.19 0.18 0.17 0.15 0.14 0.12 0.14 0.12 0.12 0.10 0.11 0.10 0.10 0.09 0.29 0.24 0.22 0.17 0.25 0.21 0.20 0.16 0.20 0.18 0.16 0.13 0.14 0.13 0.12 0.11 0.10 0.094 0.085 0.18 0.17 0.15 0.13 0.14 0.13 0.13 0.11 0.12 0.10 0.09 0.10 0.090 0.081 . .------- ~' ----------> ~ wwi wnn u otnaoor wim rotooif oi K apt Far toouxia isnaa ntne * been o*duWtemdo! lie rt*^xl.6S rslus. In *S of these root* 4 pl7 felt roof ins been mourned H ia. thi-V, thermal epndoetivitr - 143. J h*v ben iniMd uaaedditioaal tbickocspf H in. which hsebeen Mticned therfla] coadoctivity - 14. laboth cues thermal toodocimiy refer* to o b 87M pwtent cypmm, UM percent wood fiber. TUckaen radiated sadadce H in. typeasa bond. Tfafea peered rerf. * Nataiasl tfctekaena t wood is spedfied, botactual thiekms m aed la celeutetlcns. 4 lfoortbordifi*ultioiu*ed.theoeflfcieat surface, is exposed to the sun. The location is the central part of the United States. Design temperatures are: outdoor 95F* daily range 20 deg; indoor temperature 80 F. Find the heatnow rate at 2:00 p.m. for a day In July. StAulion: For the purpose of selecting the equivalent temperature differential, this construction is assumed to be equal approximately to an uninsulated 6-in. concrete roof, for which the equivalent temperature is found to be 38 deg in the 2:00 p.m. column of Table 9. Calculate the overall heat transmission coefficient U (see Equation 3 of Chapter 9) of the roof as follows: j_~ A 4 Q.375 T5 T " 033 1.2 + 12 + 4.9 + 1.33 + I. + 4.0 The heat-flow rate is then 38 X 0.33 equals 12.5 Btu per (hr) (sqft). TABLES FOR CALCULATING SOLAR HEAT GAIN THROUGH GLASS AREAS Basic Principles In order to set forth the principles involved in calculating heat flow through glass areas, the general instantaneous heat balance relation will be presented. It will be shown sche matically in Fig. 2. The net heat gain for toe indoor space is the result of several contributing factors. The following observations concerning the behavior of Cooling Load 195 gloCT with respect to radiant energy will lead to a better nndonfotnding of the heat balance relation. 1. Ql transmits, in varying degrees, radiation having wavelengths between 0.29 and 4.75 microns. The percentage of each wavelength transmitted is dependent upon the chem ical and physical characteristics of the glass, and upon the angle of incidence of the radiant energy. Of the energy not transmitted, part is absorbed and part reflected. 2. Glass is opaque to radiant energy emitted from sources below 450 F. 3. Because of the above principles, it is convenient to group radiant energy into two classifications, solar radiant energy and low temperature radiant energy. The complete heat balance for a glass section can be ex pired for a unit time interval as follows: Id , le = incident direct and diffuse solar radiation, respec tively, Btu per (hour) (square foot). an , <*4 -- absorptance of glass for direct and diffuse solar radiation, respectively. * emissivity of glass at temperature l, R, * low temperature radiant energy failing oa glass from outdoor surroundings, Btu per (hour) (square foot). Rt, a low temperature radiant energy emitted by a sur face with emissivity equal to 1.0 at temperature tM . /,, > outdoor convective conductance, Btu per (hour) (square foot) (Fahrenheit degree). 11 -- temperature of outdoor surfaceof glass, Fahrenheit. -- temperature of outdoor air, Fahrenheit. S =* rate at which glass stores energy, Btu per (hour) (square foot). CTotal heat flow, T through glass sectionj I" Transmitted, "I Lsolar radiationJ [~Heat flow by convective-} - (2a) I and radiative exchanges at ] [.the indoor surface J The second term of the right side of Equation 2a can also be expressed by a heat balance equation as follows: PHeat flow by convective-} Absorbed"] I and radiative exchanges I = j solar I {_at the indoor surface J !_radiationJ Transmissivity and absorptivity vary with both wave length of toe incident radiation and incident angle. Normal incidence transmittance values for some commonly used - types 'and combination are given in Table 14. Some varia tion in these values can be expected in practice due to vari ations in manufacture and in solar energy distribution. . However, a change in transmissivity causes a compensating .change in absorptivity. Generally, the total heal flow is not greatly altered. Transmittance data for a number of types of glass and various patterns of 8-in. glass block are given in ASHRAE research papers.1** As stated earlier in this chapter, present data as to the value of Rt are inadequate, so for the present it is suggested that ft* be increased to include radiation, and the term tcJl, -- tfjig, be disregarded. It b not practicable to give values of S in this chapter. However, for ordinary glass, the value of S b smallFig. 3 b a graphical solution, for single glass, of Equation 2b. Only absorbed solar radiation b considered, although low temperature radiation exchange and heat storage can be added algebraically to atIt if such data are available. The small thermal resistance of the glass has been neglected. The heat-flow rates are for 80 F indoor temperature, an in- Radiative exchanges be -] [-tween outer surface of glass I and outdoor surrounddiings J (2b) Convective exchanges ~| J"Heat storage-} [between outer surface of I =t= [ within the I glass and outdoor air J Lglass sectionj Equations 2a and 2b can be combined and expressed in symbolic terms by Equation 2c. Tabular values of the two bracketed terms of Equation 2a are presented later in this section for various types of glass for specific design conditions. (?/A) " Irplo + T4I4] -f (apfp + <ul* + <* -- **R where - -U)-Si (2c) (q/A) = instantaneous rate of heat flow, Btu per (hour) (square foot). td ,T4 = transmittance of glass for direct and diffuse solar radiation, respectively. Fig. 3___ Convection and Radiation Heat Row for Vertical Single Glass