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274 CHAPTER 13 1954 Guide tion of heat to the enclosure by any one or all of the mechanisms of conduc tion, convection, and radiation. A gain of latent heal is considered to occur when there is an addition of water vapor to the air of the enclosure. For example, when the humidity in an enclosure is increased by water vapor emitted by human occupants, or by water vapor resulting from a process such as cooking, the heat required to vaporize the water does not come from the air. Maintenance of a constant humidity ratio in a sealed enclosure requires the condensation of water vapor in the cooling apparatus at a rate equal to its rate of addition within the enclosure. The rate of heat removal from this condensing vapor would be substantially equal to the product of the rate of condensation and the latent heat of condensation; this product, expressed in Btu per hour, would be called a latent heal load. As a further example, the infiltration of outdoor air with a high drybulb temperature and a high humidity ratio, and the corresponding escape of room air at a lower dry-buld temperature and a lower humidity ratio, would increase both the sensible heal load and the latent heal load. SOLAR RADIATION Magnitude of Solar Radiation If a plane surface were set perpendicular to the sun's rays (i.e., for normal incidence) outside the earth's atmosphere, it would receive solar radiation of about 420 Btu per (hr) (sq ft). A similarly oriented surface; at the surface of the earth, would receive considerably less solar energy because a large part of the radiation is scattered in passing through the air, moisture, smoke, and dust which comprise the earth's atmosphere, and also, because some of the atmospheric constituents, notably water vapor, ozone, and carbon dioxide, absorb solar radiation. The intensity of solar radiation varies with wave length, reaching a peak at about 0.5 microns (a micron equals 1/1000 of a millimeter) and, for practical pur poses, is confined to the radiation spectrum between 0.3 and 2.3 microns: The effects of scattering and absorption vary with the wave length, but to make an exact analysis of these phenomena is impracticable in air; conditioning estimates. The important principle to remember is that the . total radiation It, received by a surface at the earth, is the sum of Id and; Id, where t Id ~ K /do = the direct or beamed solar radiation, Btu per (hour) (square foot of receiving surface). Ino = the direct solar radiation normal to the sun's rays, Btu per (hour) (square.; foot of receiving surface). Id -- the sky or diffuse solar radiation, Btu per (hour) (square foot of receiving surface). This comes principally from the atmosphere itself as a consent quence of scattering. Vertical surfaces also receive solar radiation by ) reflection of direct and diffuse radiation from the ground and other objects.^) Such radiation is usually diffuse. The diffuse radiation strikes at othf angles. /Ft /, = total incident solar radiation, Btu per (hour) (square foot of receiving.), surface). K - cosine of the angle of incidence, 8. For a vertical surface, 8 is defined Fig. 1. Standardized, practical-purpose values of the direct solar radiation I^j, incident upon a plane 'perpendicular to the sun's rays at the earth's surface,'.^ Cooling Load 21 Fig. 1. AZIMUTH, f SOLAR AZIMUTH, f UHC PtRRHWCULAR TO VERTICAL WALL Definition of Solar Angles have been proposed by Moon.4 Table 4 gives these values. They a; representative of a clear summer day at sea-level elevation, and are near! identical with values derived from suggested design sol-air temperatun for Lincoln, Nebraska.6 Values typical of a humid industrial area derive from sol-air data for New York City6 are also given in Table 4. Day-tt day changes in the amount of dust and water vapor in the atmospher cause large differences in solar intensity values observed on cloudless day at a given locality. For example, it has been observed in Cleveland tha values of the order of those given for industrial atmospheres are usually associated with dry-bulb and wet-bulb temperatures near the desigi values of 95 F and 75 F (67 F dew-point). On the other hand, value; Table 4. Values of Id,, Dibect Solar Radiation Received atKormal Incidence AT THE EaBTH'S SURFACE, AND VALUES OF. Id, DIFFUSE OR SKY SOLAR RADIA TION, Received by Vabiously Oriented Subfaces Solar j Btu per (hour) foqt) Altitude [ - - d" j te; ^ j ~ | ; noRiz. j[ Jj N W | S E Hob 2 8 b m "g paPrrtoiaplopsreedsssutarendoaf rodzofnoer.sea level, 20 mm precipitable water vapor, 300 dust particles per cu c deg north latitude on about August 1. ^ornjap6^-observations by ASHVE Laboratory at Cleveland on cloudless days during which the obser d n'. enCe values closely approximated the normal incidence values tabulated. absorptivity^ *j^I^reoommended design sol-air temperatures4 for New York City fora horizontal surface v 1 ;li