Document 93rJXv67dZ4yvppELMVOxgZZR

Heating Ventilating Air Conditioning Guide 1938 .8 A.M.I P . M . 10 24 SUN TIME AUG I F ig . 1. C u r v e s G iv in g So l a r n t e n s it y N o r m a l t o . S u n , o n H o r iz o n t a l .I Surface and on W alls for A ugust 1 Chapter 8. Cooling Load manner similar to that described in Chapter 7 (except-that flow of heat is reversed) by means of the formula: , //t ^ AU (to -t) (1) where Ht = heat transmitted through the material of wall, glass, floor, etc., Btu per hour, A = net inside area of wall; glass, floor, etc., square feet, t = inside temperature, degrees Fahrenheit, U> " outside temperature, degrees Fahrenheit. u = coefficient of transmission of wall, glass, floor, etc., Btu per hour per square foot per degree Fahrenheit difference in temperature (Tables 3 to 13, Chapter 5). Solar Heat Transmission Calculations of the solar heatjtransmitted through walls and roofs are difficult to determine because of periodical character of heat flow and time lag due to heat capacity of construction. The variation in solar intensity normal to sun in Btu per square foot per hour-on a horizontal surface, and on east, west, and south walls is given in Fig. 1. The curves arfe drawn from A.S.H.V.E. Laboratory data obtained by pyrhelioiiieter, are based on sun time and apply for a per fectly clear day on August 1 at a north latitude of 40 deg.. A study of these curves discloses the periodic relationship and wide variation, in solar intensity on various surfaces. It will be observed that both the roof and south wall radiation curves are in exact phase relationship with each other and that whereas the east and west wall radiation curves overlap those for roof and south wall, they do not overlap each other. This phase relationship has an important bearing on the cooling load. .Failure to consider the periodical character of heat flow- resulting ;'from diurnal movement of the sun and the lag due to heat capacity of the structure, which determine'the timing and magnitude of the heat wave flowing through the wall, may result in a large error in load calculations........... The values of solar intensity appearing in Fig. 1 must not be confused with the actual heat transmission through the wall for much of this heat intensity-on an outside surface is in part reflected and in part wiped off by convection to the outside air. A mathematical solution for the deter mination of solar heat transmission has been developed but the equations involved are too complex for practical application.1, From results of this investigation and earlier studies,2 the Research Laboratory has prepared Tables 2, 3, 4 and 5 which give the solar intensity (7) for various hours of the day against walls of various orientations and horizontal surfaces and the solar radiation (Zg ) transmitted, through windows of various orien tations as well as skylights at various hours of the day. These values are shown for north latitudes from 30 to 45 deg; It should be noted that the values for (7) represent the rate of solar intensity impinging against and not transmitted through walls and roofs whereas values for (IG) represent actual rate of heat transmission through, windows and skylights. Since the amount of solar intensity actually `Heat Transmission as Influenced by Heat Capacity and Solar Radiation, by F. C. Houghten, J. L. Blackshaw, E. W. Pugh and Paul McDermott (A.S.H.V.E. Transactions, Vol. 38, 1932, p. 231). a** Absorption of Solar Radiation in its Relation to the Temperature, Color, Angle and Other Character`s-0? the Absorbing Surface, by F.'C. Houghten and Carl Gutberlet (A.S.H.V.E. Transactions, Vol. 36.1930. n ism . 151