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496 CHAPTER 27 1965 Guide And Data Boole Ventilation Rate The introduction of outdoor air is necessary for the ventila tion of conditioned spaces. Chapter 7 suggests minimum out* door air requirements for representative applications, but it should be emphasised that minimum requirements are not necessarily adequate requirements for all psychological atti tudes and physiological responses. Where maximum economy in space and load is essential, as in submarines or other restricted spaces, as little as 1 cfm of outdoor air per person has been found to be sufficient, provided that satisfactory ventilation is simultaneously obtained by ah adequate de contamination of recirculated air.n local codes and ordinances frequently specify ventilation requirements for public places and for industrial For operating rooms, minimum requirements for safe practice are given in a National Board of Fire Underwriters' pam phlet. This pamphlet docs not require 100 percent outdoor air in operating rooms, although 100 percent outdoor air is nor mally used and recommended. Recommended and minimum ventilation- rates for the most common applications are gimmaritwi m Table 2. For further general applications, a basin of estimating the cfm per person may be taken as: 1- People not smoking........... 7\ Recommended b Minimum 2. People smoking.................. 40 Recommended 24 Minimum of the atmospheric constituents, notably water vapor, ozone, and carbon dioxide, absorb solar radiation and further redure the solar energy reaching the earth's surface. The intensity of solar radiation varies with wavelength, reaching a peak at about 0.5 microns (a micron equals 1/1000 of a millimeter) and, for practical purposes, is confined to the radiation spec-, trum between 0.3 and 2.5 microns. The effects of scattering and absorption vary with the wavelength, but making 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 /*, where ^ Id " A/c. - the direct or beamed solar radiation, Btn oer (hour) (square foot of receiving surface). Id* = the(direct solar radiation on a plane normal to the sun s rays, Btu per (hour) (square foot of receiving surface). U = the diffuse solar radiation, Btu per (hour) (square foot of receiving surface). This comes principally from the atmosphere itself as a consequence of scattering. Vertical surfaces also receive solar radiation by r flection of direct and diffuse radiation from the ground Sf?d ?"er objects. Such radiation is usually diffmThe dinuse radiation strikes at all nnglo* I, - total incident solar radiation, Btu per (hour) (square foot of receiving surface). K - cosine of the angle of incidence, 0. For a vertical sur face, 9 is defined in Fig. 1. The cooling load due to the introduction of outdoor air for ventilation is determined once the indoor and outdoor dnrign conditions are fixed. Calculations will be HisMica^ later! 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 addition of heat to the enclosure by any one or all of the mechanisms of conduction, convection, and radiation. A gain of latent heat is considered to occur when there is an addi tion of water vapor to the air of the" enclosure. For oxampfc 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 vaporise 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 dry-bulb temperature and a high humidity ratio, and the corresponding escape of room air at a lower dry-bulb.tem perature and a lower humidity ratio, would increase both th sensible heat load and the latent heat load. . Standardised, practical-purpose values of the direct solar radiation Id* incident upon a plane perpendicular to the sun's rays at the earth's surface have been proposed by Moon.** Table 3 gives these values. They are representative of a dear summer day at sea level elevation, and are nearly identical with values derived from suggested design sol-air temperatures for Lincoln, Nebraska." Values typical of a humid industrial area derived from sol-air data for New York City** are also given in Table 3. Day-to-day 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 tempera tures near the design values of 95 F and 75 F (67 F dew point). On the other hand, values approaching or exceeding those for a clear atmosphere are often encountered during Cleveland summers, but with dew-point and maximum dry-bulb tem peratures 10 to 15 deg lower. Considerable judgment, there fore, is required in selecting solar intensity values for design purposes. Data regarding the irradiation of vertical and horizontal surfaces by diffuse radiation are few. Suggested design values for a 40-deg latitude on August 1 (lS-deg declination, north) are given in Table 3 for the two types of atmospheres. 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 445 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 pasting through the air, moisture, smoke, and dust which comprise the earth's atmosphere. Some Air-Conditioning Cooling Load 497 Table 3 ... - Values of Id- Direct Solar RadiationReorived at Normal Incidence at the Earth's Surface, and Values of I* Diffuse or Sky Solar Radiaflon/Received by Variously Oriented Surfaces Ito per (beor) (ujuaie faof) Solar AJfitads (S, Deo Direct* normal radialion For Clear Atamphcra Define or dry radiation*** Dired* radtatioa For Industrial Atmospheres DHhse or dry radla&oo*** AM -- i 25 60 70 80 90 67 123 166 197 218 235' 248 258 266 273 283 289 292 294 NE 6 11 11 20 14 27 15 32 16 35 17 - 36 17 36 18 36 19 35 19 33 21 28 22 23 ---- ---- S. w Horix. 4 8 11 13 15 17 19 21 23 25 27 29 . -- -- 4 7 10 12 13 15 16 17 18 ' 19 21 23 -- -- 7 14 19 23 26 28 30 31 32 33 34 35. -- -- 34 58 80 103 121 136 148 158 165 172 181 188 195 ,200 NE 4 8 11 13 16 18 19 20 21 22 22 22 ` -- -- 11 22 28 36 43 47 50 so 49 47 41 34 -- -- $ w Horix. 53 97 13 9 17 12 21 16 ' 24 18 27 21 30 23 31 25 34- 27 37 30 41 34' -- '-- ---- 9 18 24 31 38 44 48 52 65 58 63 69 -- -- r PM -4 NW S E Hertz. N W S E Horix. * Mood'*" proposed iteodanl far ms lord, 20 mm precipitable wstsr Tspor, 800 dost particles per co cm, 2.8 mm H* partial pressure of etoae. * For <0 dec north latitsde on shoot August V ......................................... * Based ao observations by ASHRAB Laboratory at CloyeUnd oa cloudless dsys dona* winch the obaarrad normal mctdmne valuta closely approximated the d Drived from demsn sot-air temperatures* for New Talk City for a boriarxital mrfaea with absorptivity of IX. These are based upon observations made on cloudless days in Cleveland over a period of several summers. Since less ex tensive data were available for industrial atmospheres, there is more uncertainty regarding these values. In both instances,the values include an unknown amount of ground reflection, which may be expected to vary with location. It should be noted that clouds which do not obscure the sun tend to increase' diffuse radiation values. Nearby buildings may reduce diffuse irradiation by partial shading. Calculation Tables The irradiation of a surface by the sun is the product of Id*, the direct normal radiation (see Table 3), and K, the cosine of the incident angle 9. For horizontal surfaces,' the cosine K equals the of the solar altitude. For vertical wails, fC b a function of the solar altitude /3 and the wall solar azimuth 7: K -- cos 9 = cos fi cos y (1) These three angles are defined in Fig. 1. Values of K are given in Table 4 and values of f} and 7 are given in Table 5 for 18- deg north declination (August 1). To compute K values for orientations other than those given in Table 5, third angle <, the solar azimuth, is required. In this discussion, will be measured east from south in.the morning, and west from south in the afternoon. Hence, 6 values are equal to 90 deg minus the 7 values for an east or west facing wall, except when Table 5 shows the south' jralls to be in the shade. In this case $ equals 90 + 7, that is, ^ is greater than 90 The wall azimuth ^ is the angle, measured east from south to the perpendicular to the wall for walls winch have an ettsteriy component, and west; from south for those'having a westerly component. For example, 1p for a wall facing north east is 135 deg.. , ; . The wall solar azimuth 7 may be found according to the following schedule: For walls facing east of south: For walla faring west of south: y = 4 -- i> a.m. y = '4 + <i> a.m. y -- * + i> p..m. y -- * -- ^ p.m. Treat negative values of 7 as if they were positive. If 7 is greater than 90 deg, the wall is in'the shade. Values of K for other seasons and latitudes may be found in the literature,** or may be computed from data given in Hydrographic Office Bulletin No. 214, Tables oj Computed Altitude and Azimuth*1 and the Ephemerisof the Sun.** Table 6 shows the variation of solar declination during the months ordinarily requiring cooling; Bxample /.' Find the solar azimuth d at 6:30 p.m. at 40-deg' north latitude on August I. " Solution: From Table 5 in the column of y for a wall faring west for 6:00 p.m. is 90 -f 14 = 104 deg, and at 7:00 p.m. is 90 + 24.-- 114 deg. By interpolation, $-for.6:30 p.m. is.109 deg west of south (at 5:30 a.m. would be-109 deg east of south).' . .... Example t; Find K for a wall faring 18 deg east of south at 10:00 a.m. on August 1 at 50-deg north latitude. . Solution: The wall azimuth is.18.deg. The solar azimuth'is> 48 deg east (Table 5).The.wall solar azimuth is 48 -- 18 or 30 deg.,From Table 5, fi is 50 deg. Then:. A -- cos 0 cos 7 -- cos 50-X cos 30 -- 0.643 X 0.866 -- 0.557. Example 3: Find K for the'w&H m Example at 3:00 p.m.1 Solution: The solar azimuth is 65 deg west. .The wall solar, azimuth is therefore 65 + 18 -- 83 deg.'The angle A is 42 deg. K -- cos 42 X cos 83 - 0.743 X 0.122 = 0.091. Ji Example 4- Find the total solar irradiation for the wall for the^ conditions of Example S. Solution: Use clear atmosphere solar intensities. At 50-deg' altitude, the direct normal'radiation is'273 Btu'per*(hr):'(sq> ft)..Then: - - :* .U-*