Document 4vOxapNBo18nBqjM4bNw23dx1

528 CHAPTER 27 7 1965 .Guide. And-Data Book Table 33 .... Shade line Factors , OiradiM tofifod*, D*0re* Worfow 25 30 35 40 45 SO E 0.8 0.8 0.8 0.8 0.8 0.8 SE 19 1.6 1.4 1.3 1.1 1.0 8 10.1 5-4 3.6 2.6 2.0 1.7 SW 1.9 1.6 1.4 1.3- 1.1 1.0 W 0-8 0.8 0.8 0.8 0.8 . 0.8 - 55 0.8 ` 0.9 1.4 0.9 0.8 . 'Table- 34 .... Sensible Cooling load. Due to Infiltration `. and Ventilation Otaga Temporator*, f 85 90 95 too 105 no Infiltration, Btuh per sq ft of gross exposed wall area 0.7 1.1 1.5 1.9 2.2 2.6 Mechanical Ventilation, Btuh per cfm u.o 16.0. 22.0 27.0 32.0 38.0 Note: DistiAM ihadov i-m fait* below tbe ed$e ol tbe orvrttti equeis t -ka. fator multiplied bj width cd orerbaa*. Values era amaze* fat fire bocte of peetot aoW mtensity an Auco*t L Heat release per .occupant of a residence is usually as sumed to be 300 Btuh of sensible heat. The number of occu pants must be estimated as accurately as possible, with of the overhang for each foot of overhang width..The tabu* s lated values are the average of the shade line values for the-. 5 hr of maximum solar intensity on 'each wall orientation shown, and for a solar declination of 18 deg north-(August 1). ' Northeast and northwest-facing windows are not effectively - protected by roof overhangs, and in most cases no credit t. should be token for shading of them. -_ special attention being paid to the owner's use of living or recreation rooms for entertaining large groups of guests. When the owner does not indicate the need for special provisions for entertainment, and the exact number of occupants is not known, a convenient rule-of-thumb is to assume that there will be approximately twice as many occupants as the struc ture has bedrooms. The occupancy load should then be dis tributed about equally among the living rooms of the resi- Infiltration and Ventilation Natural air leakage in residential structures is much smaller: in summer than in winter. Winter air leakage in a specific structure is caused by two weather factors, the wind direction - and velocity and the inside-outside temperature difference '1 which causes the so-called chimney effect. This chimney effect is, of course, absent in summer, and wind velocities are lower, in many localities. . v Factors for calculating natural infiltration are shown in'; Table 34. These are based upon a leakage rate of one-half air ! change per hour, but, because of the effect of house con figuration on area exposed to wind, they are tabulated in terms of Btuh per square foot of gross exposed wall area.' .. - Mechanical ventilation may be provided in residential air conditioning systems. It is not a code requirement in most localities, and only general guidance can be given in determin ing its desirability. Held experience indicates thatnaturalair leakage is adequate to provide sufficient ventilation in many, ingfcftllftt.inns. Nevertheless, the. need for [native-ventilation must be evaluated for each installation-> Considerations in clude owner desires, entertaining by the owner, etc. In larger homes, ventilation should be provided. This ventilation air' ' Hence. This is suggested because the period of maximum load occurs when* most of the people residing in the house are occupying the living areas rather than the sleeping areas. In any case, care should be token not to grossly overestimate the number of occupants in a residence, since this can lead to oversmng the cooling equipment. Appliance loads should be limited to the kitchen in most'cases: A value of 1200 Btuh .of sensible heat release.by kitchen appliances has been found to be satisfactory for residential cooling load calculation. It is recognised that this does hot equal the load that can'be im posed by even one top burner or element of a domestic range. Nevertheless, such factors as intermittent use of appliances, flywheel effects, and use of kitchen ventilating fans make this a reasonable and practical value to use.-- It may be necessary to include the loads imposed by other household equipment, particularly by laundry equipment if it is located within-the conditioned'space.-These can be esti mated from information included in Table 28 of this chapter. Appliances which are major sources-of sensible and latent heat must be vented. It is all but impossible, satisfactorily and economically, to overcome the load' imposed by ah unvented domestic clothes dryer, for example. - < is usually introduced at the rate of about one air change per hour. It is considered desirable to install a manually operated, locking-type damper in ttie outdoor air `duct'iso'the. owner can have the option of ventilating positively or relying on natural air leakage. The ventilation factors of Table 34 are expressed as Btuh of sensible heat per cfm of ventilatioh'air. Generally, ventila= Total Sensible Heat Gain ............. The total sensible heat gain of the'structure is the sum of the sensible heat gains of all rooms. If the heat gain has been calculated treating the house as a'whole rather than on a room-' by-room basis! the sensible heat gain is the sum of the sensible heat gain of each component of the structure. tion will impose a greater load than will infiltration. Further-: more, positive introduction of ventilation air `during the Latent Load - summer can substantially eliminate natural air leakage. The latent portion of a residential cooling load is usually Hence, appropriate factors for either natural infiltration or estimated as equaling thirty percent of the calculated sensible ventilation should be'used in residential load calculation in. load. It is-recognized that this is an approximation, yet mois most instances. f' .j! ture sources in a residence are both,numerous and, if taken Occupancy Loads individually, difficult to evaluate precisely. Furthermore, / residential cooling equipment is controlled by one or more Even though occupant density is usually lower in residences than in many other types of structures, occupancy loads must room thermostats, and. only. rarely does-humidity directly affect a control device. be considered in residential cooling load calculations. .Loads are generated both by the occupants themselves;. and by, Total Cooling Load household appliances. Such loads must usually be treated in The total cooling load is the sum of:the sensible load and an approximate-manner, since occupancy and occupant;ae- the latent load. Inasmuch as latent load does not cuter tivities are varied and unpredictable. . directly into residential load calculation, total load is usually. Air-Conditioning Cooling Load 529 calculated as 1.3 times the calculated sensible'load. Design 0f the distribution system and equipment'selection;, if-all ducts are located within the conditioned space, is based upon tfje total calculated cooling load of each room. > Whenever the distribution system is located outside of the conditional space--in attics; crawl spaces, or unconditioned rooms--heat guns to the ducts or pipes must.be included as an equipment load - and must be considered in equipment selection!" - Another factor affecting equipment selection is the.effect of outside' conditions on the Btuh -capacity of unitary cooling equipment. The rated capacity of'unitary equipment' is affected by. standard test conditions, and .the properties and quantities of the fluids passed through the evaporator, and condenser. One standard combination of test conditions cannot; be considered to provide a. capacity rating that'is meaningful for all outside design conditions of dry-bulb tem perature and. daily temperature range. Furthermore, ` the inside design temperature swing materially, affects.-the equipment capacity requirement.- [ The load calculation procedure is predicated on'an inside temperature. swing of three degrees. ' Engineers' are not tpianirqmi_q on the maximum acceptable, temperature swing,; but 3 deg is viewed as a practical minimum. Others, including 4} deg and 6 _ deg, have been responsibly recommended.; Despite apparent reported,acceptance of greater indoor tem perature swings,'for engineered jobs a three.degree swing and equipment capacity,selection on this basis'are recommended. Equipment capacity installed must, therefore, be based upon' three considerations: calculated heat gain of, the'structure, beat gain to the distribution system,'and the effects of incido temperature swing and outside design conditions: Equipmentcapacity multipliers which relate outside design conditiohs'tb total equipment load (structure plus distribution'system) and inside- temperature swing are given in' the publications of-- Reference 57- for unitary. equipment.-utiliting 'air-cooled, eyaporatively cooled, or water-cooled condensing units. These-' are equipment capacity multipliers which! when,multiplied by the calculated equipment load, determine the; equipment capacity requirement in terms of its standard rating.*7 LETTER SYMBOLS USED IN CHAPTER. 27 ." = fraction of incident solar radiation absorbed,' dimen- . !. sioolen; subscripts D, d, and t refer to direct, diffuse . and total, respectively. . .0 m solar altitude, degrees. T w wall solar azimuth, degrees. 4 * incident angle, degrees. 4 w solar azimuth, degrees. ^ " wall azimuth, degrees. " . - A area across which heat is being transferred, square feet. - 6 * fraction'of air passing through mD a-hirh , tact surfaces,- coil bypass factor. ' ^ of the tolar beat gain to the incident solar'radiation . . for direct and diffuse solar radiation,.respectively, di mensionless. , , ?o usage factor - 0.50. "'radiant heat factor - 032. rrt " flue loss factor -- 1.6. / " unit surface conductance, Btu per (hour) (square foot) (Fahrenheit degree). Subscripts e, r, o, and i refer to . nvection, radiation, outdoor, and indoor, respectively.' # ** enthalpy of air per pound of dry air, Btu per pound. . Subscripts i, o, and * refer to indoor, outdoor, and sup ply air,-respectively.. 1 < -i . ... / " incident solar radiation, Btu per (hour) (square foot)! Subscripts'll, d, Dn, and {refer to direct/diffuse, direct . normal, and total tolar radiation, respectively. .7 K - cosine of angle of incidence for direct solar radiation ` ; striking a surface, dimftnBwntom if - the permeance of the specimen in perm* or grains per' . (square foot) (hour) (Inch of mercury vapor pressure difference). Q * rate of entry of outdoor air, cubic feet per minute. *' QTM " required air quantity through conditioning equipment, cubic feet per minute. ` t ?/4 = instantaneous rate of total heat transfer, Btu per (hour) (square foot). .. ? * instantaneous rate of heat transfer, Btu per hour. .4 " heat gain from appliaoce/Btu per hour. f' ' q, - instantaneous latent heat load, Btu per hour. . '.. r 9* " instantaneous space latent ventilation - load,. Btu' per hour. ?i " instantaneous rate of heat grun from electric lights, Btu per hour. ' .r instantaneous rate of heat gain from electric motor*, Btu per hour. ?* rr instantaneous latent ventilation load which does not become a part of space load, Btu. 5i " rated input of appliance, Btu per hom. 1 ? TM latent beat load due to moisture - transmission through materials, Btu per (hour) (square foot). q, -- instantaneous sensible heat load, Btu'per hour.' . " instantaneous space sensible ventilation load; Btii oer - hour. .. t,( . ?** instantaneous sensible ventilation load which does notf become a part of space'load,-Btu per hour. ?"? + ?* also jw + 4 + jrf qtr, Btu per hour. . .. . S shading coefficient for fenestration being considered. . tk TM temperature of air in adjacent space, Fahrenheit. t, " sol-air temperature, Fahrenheit. -- l " indoor air temperature, Fahrenheit. U * outdoor air temperature, Fahrenheit. < , t, = room supply air dry-bulb temperature, Fahrenheit! 7 f/ * overall coefficient of heat transfer of a structural see-!'' tion, Btu per (square foot) (hour) (Fahrenheit degree). W humidity ratio, pounds moisture per pound of dry air.' Subscripts i, o, and refer to indoor, outdoor, and siip- ' ply air, respectively.' REFERENCES 1 Victor Paachkia:-Periodic beat flow in building'walls deter mined by electrical analog method (ASHVE 'Runsacitoks. Vol. 48,1942. d. 75). / : . . * T. N. Wilfeox et al: Analog computer analysis of residential coding loads (ASHVE Teansactions, Vol'60.-1954; p.-605).' '* C. O. Mackey and N. R. Gay: Cooling loads from sunlit glow and wall (ASHVE Tbansactjons, Vol 60! 1954, p. 469). i * H. B. Nottage and G. V.-Pannelee: Cireuit analysis apptied to load estimating (ASHVE Transactions, Vol 60, 1954, p. 59). i. *. H. B. Nottage and G. V. Parmelee: Cireuit analysis applied to load estimating. Part II (ASHAE Transactions, Vol 6L 1955, p. 125). i * Harry Buchberg: Electric analogue prediction of the thermal behavior of an inhabitable enclosure (ASHAR-T*MWftrTT<>WH: VoL 61, 1955, p. 339). izuyBAcnonB, 7 Harry Buchberg: Electric analogue studies of amzle'walls (ASHAE Transactions, VoL 62, 1956. p. 177). ' P. L.' Pfenningwerth and Meri-Baker: Intermittent heating mid cooling of buddings (ASHAE Transactions. Vol 62. 1956.p. 4S9). ' * W. R. Brisken and G. E. Reque: Thermal circuit and mining computer methods. Thamsl response (ASHAE Transactions.' VoL 62, 1956, p. 391). ' 11 G. V. Parmelee, P.- Vance, and A. N: Cemy: Analysis of an sir -conditioning thermal circuit by an electronic differential* analyzer (ASHAE Transactions, VoL 63, 1957, p. 129).-- . ` u 9. F. Gtilman and O. W. Qausen: Thermal circuit analysis for developing application'engineering information (ASHAE Trans actions, Vor.'63, 1957, p. 313). ' ' -> - - \