Document YjBnLd7ypjX3608kqqBV8q4YK

266 CHAPTER 12 1951 Guide 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 heal 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 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- Cooling Load 267 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 airconditioning 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 I&, where = K /do = the direct or beamed solar radiation, Btu per (hour) (square foot Ip of receiving surface). = the direct solar radiation normal to the sun's rayB, Btu per (hour) (square 5. orTable Values /dh, Dibect Solab Radiation Received at Normal Incidence at the Earth's Subface, and Values of Diffuse ob Set Solas Radia tion, Received by Variously Oriented Surfaces Solab Altitude AM; 5 10 15 35 40 45 50 60 70 Btu per (houb) (square foot) Fob Cleab Atmospheres Direct* Normal Radi ation Diffuse ob Sky Radiation* Fob Industrial Atmospheres Direct* Normal Radi- Diffuse or Sky Radiation* . N E s w Hoaxz. 67 123 166 197 218 7 14 19 34 58 80 103 121 4 11 5 3 8 22 9 7 11 28 13 9 13 36 17 12 16 43 21 16 9 18 24 31 38 235 248 258 28, 30 31 136 148 158 165 172 18 47 24 18 44 19 50 27 21 . 48 20 50 30 23 52 21 49 31 25 55 22 47 34 27 58 34 35 22 41 37 30 63 195 200 22 34 41 34 __ -- -- -- -------- 69 * -- -- . Hobiz. ` N W S E Hobiz. Moon s* proposed BuoiiuoAiA ivi 2.8 mdums HUg parutiali pressure omf ouzaovnuec. BFaosre4d0odnegobnsoerrtvhaltaiotnitsudbey oAsSaHbVoEut LAaubgoursatto1r,y at Cleveland os cloudless days during which the ob served normal incidence values closely approximated the normal incidence values tabulated. a from mftunmended design sol-sir temperatures4 for New York City for a horizontal 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 conse 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 all angles. '. It = total incident solar radiation, Btu per (hour) (square foot of receiving surface). K = cosine of the angle of incidence, 9. For a vertical surface, 9 is defined in Fig. 1. Standardized, practical-purpose values of the direct solar radiation /d,, incident upon a plane perpendicular to the sun's rays at the earth's surface, have been proposed by Moon.2 Table 5 gives these values. They are