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280 CHAPTER 15 -1949. Guide , Table 5. Proposed Standard Values op/b , Direct Solar Radiation Received at NORMAL INCIDENCE, at the Earth's Surface*' b Solar .Altitude, 0 Dec 5 10 15 20 25 Btu per (Hr) (Sq Ft) 65 122 165 196 219 Solar Altitude, 0 Deg 30 35 40 45 50 /n Btu per (Hr) (Sq Ft) 234 245 253 260 266 Solar Altitude. 0 Dbg 60 70 80 90 Btu per (Hr) (Sq Ft) 276 283 289 294 * Calculated using the following assumptions: barometric pressure of 760 mm Hg (29.921 in.); depth of precipit&ble water of 20 mm (0.787 in.); dust particles, by counting, 300 per cc; partial pressure of the ozone ayer'in the atmosphere of 2.8 mm Hg (0.110 in.). This is representative of a dear summer day. b Far sea level. As an approximate altitude correction, add 1 per cent for each 1000 ft altitude. determined once the indoor and outdoor design conditions are fixed. Cal culations will be discussed subsequently. INSTANTANEOUS HEAT LOAD - The total cooling load is frequently divided for convenience into two components; sensible heat and latent heat. While this subdivision is not imperative, past practice has found it convenient. A gain of sensible heat is considered to occur when there is a direct addi-. tion of heat to the enclosure by any one or all of the mechanisms of conduc tion, convection, and radiation. A gain of latent heat 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 heat 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-bulb temperature and a lower humidity ratio would increase both the sensible heat load and the latent heat load. SOLAR AND SKY RADIATION AND TRANSMISSION LOSSES Magnitude of Solar Radiation--Calculation Tables If a plane surface were set perpendicular to the rays of the sun {e.g., for normal incidence) outside the earth's atmosphere, it would receive solar Table 6. Approximate Ratio op Direct Solar Radiation to Set, , Radiation Received on a HORIZONTAL SURFACE on Clear Days in Eastern States* Solar Altitude, 0 Dec 0 10 20 30 Ratio e 0 1.40 2.30 3.10 Solar Altitude, 0 . Deg 40 50 60 Ratio e 3.84 4.55 5.20 Solar Altitude, 0 Deg 70 80 90 * Ratio e 5.63 . 5.90 6.10 * For rough estimates aasnmo that the sky radiation on a vertical surface is one half of that on a horizontal surface. Sky radiation may be assumed independent of vertical-surface orientation. ^ BE^r BBBFk -Cooling Load 281 Fig. 1. Definition of Solar Altitude Fig. 2. Definition of Angle of Incidence radiation of about 420 Btu per (hr) (sq ft). A similar receiving surface on the surface of the earth would receive radiant energy at a considerably lower rate because a large part of the radiation entering the atmosphere is scattered in passing through the air, moisture, smoke, and dust which comprise the earth's envelope. Also, some of the atmospheric constitu ents, notably water vapor, carbon dioxide, and ozone, absorb radiant energy. Thisf absorption and scattering cause different proportionate reductions fromyouter-atmosphere radiation intensity with different wave-lengths. An exact analysis of these phenomena is beyond the practical purposes of air-conditioning load estimates; the important principle to remember is that the radiation reaching the surface of the earth is the sum of Ia and I,. In --- The direct radiation (at normal incidence), which is the transmitted fraction of the net sun radiation received by the outer atmosphere, and J# . The. sky or diffuse radiation coming from the atmosphere itself as a conse quence of the scattering and absorption which give rise, in part, to a reradiation to the earth. The diffuse radiation does not strike only at normal incidence; it strikes at all angles from which the sky sees the surface in ques tion. Standardized, practical-purpose values of the direct solar radiation inci dent upon a plane perpendicular to the sun's rays at the earth's surface have been proposed by P. Moon.1 Table 5 gives these data; they are repre sentative of a clear summer day at about sea-level elevation. (For indus trial areas, /,, values will be slightly less than Table 5, with the greatest decrease occurring at'low solar altitudes towards evening.) Practical design-data on sky radiation are meager. Table 6 presents a basis of estimates for dear summer days in terms of the direct solar radia tion to sky radiation ratio. The solar altitude is the angle (see Fig. 1) between the sun?s rays and the horizontal. In the usual application the receiving surface {e.g., building roof or wall) will not be perpendicular to the rays of the sun. The intensity ,of the direct radiation incident upon a surface, Btu per (hour) (square foot of absorbing surface), which is oriented with an angle of incidence 6 for the sun's rays, is Id=E/n, (1) where Is = Intensity of incident direct radiation, Btu per (hour) (square foot of absorbing surface). In = Intensity of direct radiation on a plane normal to the sun's rays, Btu per (hour) (square foot), (from Table 5). K = Cosine of the angle of incidence 9. The angle of incidence (see Fig. 2) is the angle between the sun's rays and the normal to the absorbing surface. ; i.; u,- ' .Un J: :i , i,; 5 t X