Document 2RxzDVOXx6OJErn8aroDyD36g

264 CHAPTER 15 ' 1948-Guide Cooling Load . Table 8. Values of K for Vertical Planes in North Latitudes . . During, the Period May 2-August 10 265 - The azjmuth angle at is always measured dockwise from- the south' to the exterior side of .the wall in question. energy to the sky and to the surroundings. The heat balance on an outer surface;-for d unit time interval, may be expressed as follows: - : : ' Heat entering) _ /Incident direct. ' . /Incident sky' . _ /Reflected direct outer surface / ' [solar radiation ' ' \ radiation ' ~ I -radiation i ' ,/Reflected sky ' -i. /Radiation emitted ^ /Airrto-surface' : ... . -I.- radiation . \to,sky by.surface-- /convection (2) The convective heat transfer .may be either to or from the surface, depending upon whether, the air temperature is higher or lower than the surface 'temperature. ; '.Np./tfejrm; has' be&i 'included to represent radiation exchange with other, objects in the ' surfoundings.as this, effect is hot usually of significant maghitilde relative to'the others. !. , In practical calculations, the differencebetweein incident and reflected radiation, is computed .by multiplying the incident radiation by a factor called the absorptivity or the emissivity; see, Table 6 and 'Equation 3, Chapter 5. Convection is calculated as explained in Chapters 5 and 6.' The daytime radiation emitted.to the sky by a surface.is not usually calculated as a separate item in practical air-cbiiditioniiig work. 'Its effect may be roughly accounted for by ah adjustment of thh magnitude .of the absorptivity factor for solar radiation, and this approach is often followed in' making 'emissivity- measurements for . building materials exposed to :: Tab?.e7; Values oF/iTriflt .Horizontal Places in NorthXatitudes3 ' During the Period May 2-August 10 `' Local Mean Sun Time**. North Latitude--Decrees A.M. P.M. . 25 30 35 40. 45 50 5 '7 ' 6 6 'V .145 7 5. .365 8 ....;-4 . .570 9 r 3. .747 10 ' -2 , .882 11 1 .967 12 s .996 .171 .382 .578 .746 .876 .957 .985 .034 .196 .220 .395 .406 .581 .580 .740 .729 .863 .843 .940 .915. .966 . . .940 .070 ..242;. .414 .574 ...712 v:817 .884 .906 , " ' ' " '' .106 .262 .418 .564 .689 .785 .845 .866 .Figured for solar declination of 20 deg. bThe relation between local mean suit time and'dvil time may be obtained from Weather Bureau- offices in various localities. *> > * Values for 30 deg latitude may be employed over the range 25 deg to 35 deg. Values for 45 deg latitude may be employed over the range 40 deg to 50 deg. Interpolation will yield values for the range 35 deg to40 deg latitude. ' bThe relation between local mean sun time and civil time may be obtained from Weather Bureau offices in various localities. `Values in ordinary type are for azimuths 0 to 180 deg. Values in italic type are for azimuths 180 to 360 deg. the sun and sky. During the night,- however, the emitted radiation becomes very important. Radiation Exchange with the Night Sky When a building surface is exposed to a clear night sky, the net exchange of radiant energy isfrom, the surface to the sky. This acts to decrease the load on an air-conditioning system. For practical calculations, it will serve to cite Brunt's equation as given by Haurwitz 4, 2s = 0.173 e (0.56 - 0.47 VPd) (3) where qs = Net radiation exchange between a surface and the night sky, Btu per (hour) (square foot). s = Emissivity of the surface, dimensionless (for the surface temperature in volved, and not for solar radiation). T = Surface temperature, (Fahrenheit degrees absolute). . - Pd = Partial pressure of the atmospheric water vapor at the earth's surface, inches of mercury. Equation 3 is not applicable to either extremely low or extremely high values of P&\ (Pa is the saturation pressure corresponding to the dew point). Clouds will decrease appreciably the magnitude of this radiation exchange. Calculations from Brunt's equation should be reduced by a factor of % to ^ where cloudiness is involved. For vertical surfaces the resultant should be reduced about The Sol-Air Temperature A convenient way of combining the effects of solar and sky radiation, solar absorptivity, temperature, and air movement is. to use the concept of sol-air temperature.,. The sol-air temperature, te, is the temperature of