Document nOd1KJEwprMDzNk8MvZyG8M8

American Society of Heating and Ventilating Engineers GuideTIqj^ 's as given in Weather Bureau reports usually occur only from 1 per cent rft 4 per cent of the time, and they are therefore of such short duration tfc it is not practical to design a cooling system covering this range, -jv temperatures shown in Table 1 have been chosen after extensive stndv j the Weather Bureau records and are temperatures that are not exceed!! more than 5 to 8 per cent of the time during June, July, August, a3 September for an average year. 08 Solar Radiation | Fig. 1 shows the total amount of solar energy in Btu per square foot pd hour received during the day by a surface normal to the rays of the Smo by a horizontal surface, and by east, west, and south walls. The curves are drawn from A.S.H.V.E. Laboratory data obtained by pyrheliometer' are based on sun time, and are for a perfectly clear day on August 1 a5 j north latitude of 40 deg. Data from these curves may be used witf little error for most United States latitudes and for all of the hotter months of the year. f The absorption of solar radiation by a surface depends upon tht character of the surface and the angle of the surface with respect to tbr direction of the radiation. The heat absorption by a black oilcloth** surface perpendicular to the sun's rays was found to be as high as 273 Btu: per square foot per hour, based on tests conducted by the A.S.H.V.E,5 Research Laboratory in Pittsburgh1. Lamp black, red brick dust, and aluminum bronze. painted surfaces perpendicular to the sun's* rap' showed, respectively, 94.0, 63,4, and 28.2 per cent as high a rated; absorption as the black oilcloth. \ Unfortunately, the calculations for the transmission of heat from solar radiation through building walls are too complicated to be of much practical value to the heating and ventilating engineer. Approximate Tesults may be obtained by adding the number of degrees given in Table 2 ,o the outside design dry-bulb temperature in calculating the heat trans mission through a wall or roof which may be exposed to the sun for an appreaable length of time. Table 2 was obtained from a study of the data m A.S.H.V.E. research papers on solar radiation1- 3. Black and aluminum painted surfaces represent the extremes which are likelv t-n occur. * Ftoher otathbeler tcyapnesbeouf sseudr.faces, values intermediate between tuhlUosbee nivpn in Time Lag The calculation of heat transmitted through walls and roofs does not take into consideration the heat capacity of the structure and the con sequent time lag in the transmission of heat. In the thick walls used in modern office buildings the time lag may amount to 10 hours or more4. Thus in many cases the wall transmission cannot be added directly to the cooling load from other sources because the peak of the wall transmission load may not coincide with the peak of the total cooling load and may even occur after the cooling system has been shut down for the day. The data in Table 3 were taken from A.S.H.V.E. research papers3- 4 and while they result principally from a study of experimental slabs, they give j- W th he exnected in various structures. Table 3 Time Lag in Transmission of Solar Radiation through Walls and Roofs Table 2. Allowance for Solar Radiation on Roofs and Walls Approximate Number of Degrees to Add to Dry-Bulb Temperature for Different Types .of Surfaces Ttpe of Surface Roof, horizontal................. ......... East or west wall............. ........... South wall.......................................................... Black 45. ` 30 15 Red Brick or Tils ' Aluminum Pilm| 30 15 1 20 io 5 10 -0- ,,. . Ttpe and Thickness of Wall or Roof 2-in. iron and cork (equivalent to %-in. concrete and 2.15-in. cork)...-......... Time Lag, Hours -m -3 2H 2 2H 7K 19 10 v- Solar radiation is an important factor in the mechanism of heat flow] [ intb buildings. Research conducted at the A.S.H.V.E. Research Laboraf | tory2 has shown that a large error may be introduced into the calculations] *- by failure to consider the periodica} character of heat flow resulting froni y the diurnal movement of the sun and the heat capacity of the structure^ | which determine the timing and. magnitude of the heat wave flowing? T through the wall into a building on a hot, sunny day. - i| Absorption of Solar Radiation in Relation to the Temperature, Color, Angle, and Other Characteristia|fcft of the Absorbing Surface, by F. C. Houghten and Carl Gutberlet (A.S.H.V.E. Transactions, Vo!. 36.19301^F. *For further information on this subject see following A.S.H.V.E. research papers: Coefficients of Transfer. as Measured under Natural Weather Conditions, by F. C. Houghten and C. G. F. Zobel (A S-"1L,V V-EJ. Transactions, Vol. 34, 1928); Absorption of. Solar Radiation in Its.Relation to the Temperatarts*^ Color, Angle and Other Characteristics of the Absorbing Surface, by F. C. Houghteti and Carl Gutbenrti^ (A.S.H.V.E. Transactions, Vol. 36,.1930); Heat Transmission as Influenced by Heat Capacity and Sou^l Radiation, by F. C. Houghten, J. L. Blackshaw, E. M. Pugh and Paul McDermott (A.S.H.V.E. Tra^P actions, Vol. 38. 1932). . 148 In intermittently cooled buildings an excess cooling capacity must be provided to care for the additional load imposed by the necessity to cool down the furnishings and the material of the interior construction to the point of maintained temperatures. Transmission of Solar Radiation Through Glass In considering the transmission through glass several factors must be considered. As the sun's rays impinge against a pane of glass, most of the radiation passes through to'the other side, a small amount is reflected, and the balance is absorbed by the glass. The amount absorbed depends upon ''Heat Transmission as Influenced by Heat Capacity and Solar Radiation, by F. C. Houghten. J. L. Blackshaw, E. M. Pugh, and Paul McDermott (A.S.H.V.E. Transactions, Vol. 38. 1932). ---Field Studies of Office Building Cooling (A.S.H.V.E. Research Paper), by J. H. Walker. S. S. Sanford. and-E. P. Wells (A.S.H.V.E. Transactions. Vol. 38. 1932). 149