Document gbVjEKJVpOoomyxg8wb1Y24rQ

272 CHAPTER 13 1953'Guide tion of heat to the enclosure by any one or all-of the mechanisms of conduc-. tiun, convection, and radiation. A gsm of latent he/at 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 riot.come from the air. Maintenance of a constant humidity ratio in a sealed enclosure requires the condensation of water vapor in the cpoling 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-buld 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 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 Id, where Id = K ID,, = the direct or beamed solar radiation, Btu per (hour) (square foot of receiving surface). IDn = 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 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, 6. For a vertical surface, 8 is defined in . - Fig. 1. I: Standardized; practical-purpose values of the direct solar radiation 7du incident upon a plane perpendicular to the sun's rays at the earth's surface, Cooling Load 273 Fig. 1. Definition of Solar Angles have been proposed by Moon,4 Table 4 gives these values. They are representative of a clear summer day at sea-level elevation, and `are nearly identical with values derived from suggested design sol-air temperatures for Lincoln, Nebraska.6 Values typical of a humid industrial area derived from sol-air data for New York City6 are also given in Table 4. Day-today changes in the amount of dust and water, vapor in the atmosphere cause large differences in solar intensity values observed on cloudless days at a given locality. For example, it has been observed in Cleveland that values of the order of those given for industrial atmospheres are usually associated with dry-bulb and wet-bulb temperatures near the design values of 95 F and 75 F (67 F dew-point). On the other hand, values Table 4. Values of /d,,, Direct Solar Radiation Received at Normal Incidence at the Earth's Surface, and Values of Id, Diffuse or Sky Solar Radia tion, Received by Variously Oriented Surfaces Btu per (hour) (square foot) Solar Altitude ^ 0> Degrees For Clear Atmospheres Direct b Normal Radi ation Diffuse obBky Radiation-6 For Industrial Atmospheres Direct*1 Normal Radi ation Diffuse obJSkt Radiation* c AM -* l NE s w Horiz. n E s w Horiz. 5 10 67 6 11 4 4 7 123 11 20 8 7 14 34 4 11 6 3 6 58 8 . 22 9 7 18 15 20 25 166 14 27 11 10 19 197 15 32 13 12 23 218 16 35 15 13 26 80 11 28 13 . 9. 24 : 103 13 36 17 12, 31 121 16 43 21 16 38 30 35 40 45 50 235 17 36 17 15 248 17 36 19 16 258 18 36 21 17 266 19 35 23 18 273 19 33 25 19 28 30 31 32 33 136 18 47 24 18 44'' 148 19 50 27 . 21- 48 158 20 50 30 23 52 165 21 49 31 25 ' 55 172 22 .47 34 27. . ,58 . _ _ _60 70 80 283 21 28 27 21 34 289 22 23 29 23 35 292 _. 181 22 41 37 30 : 63 188 22 34 41 34 69 195 -- -- -- -- -- 90 294 -- -- -- -- -- 200 -- -- -- pL -* N W S E Horiz. N W S E Horiz. a Moon's* proposed standard for sea level, 20 mm precipitable water vapor, 300 dust particles per cu cm, 2.8 mm Hg partial pressure of ozone. For 40 deg north latitude on about August 1. 0 Based on observations by ASHVE Laboratory at Cleveland on cloudless days during which the observed . normal incidence values closely approximated the normal incidence values tabulated. ^ Derived from recommended design sol-air temperatures4 for New York City for a horizontal surface with absorptivity of 1.0.