Document 375JxGZ2kqa2yaYYXdvzMrLda

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 Therefore, calculations based on the latter assumption are likely to be somewhat in error. A formula for calculating attic temperatures is: where ACU& + to (ATUr + AcUc + AgUR) 13 AtUr + AWU,, + Agi/g + ACUC /a = attic temperature, degrees Fahrenheit. t\ = inside temperature near top floor ceiling, degrees Fahrenheit. to = outside temperature, degrees Fahrenheit. Ac = area of ceiling, square feet. At = area of roof, square feet. /lw = area of net vertical wall surface, square feet. As = area of glass, square feet. U) Uc = coefficient of transmission of ceiling, based on surface coefficient of 2.20 (upper surface, see Chapter 4). Ut -- coefficient of transmission of roof, based on surface coefficient of 2.20 (lower surface, see Chapter 4). Uv -- coefficient of transmission of vertical wall surface. Vg = coefficient of transmission of glass. Example 1. Calculate the temperature in an unheated attic, assuming the following conditions: U = 70; to = 10; Ac = 1000; A, = 1200; 4W = 100; Ac - 10; Ux = 0.50; Uc = 0.40; Uc = 0.30; UB = 1.13. Solution: Substituting these values in Equation 1: k = (1000 X 0.40 X 70) + 10 ((1200 X 0.50) + (100 X 0.30) + (10 X 1.13)1 (1200 X 0.50) + (100 X 0.30) + (10 X 1.13) + (1000 X 0.40) 34,413 fa = loir -331F- High Ceilings: Research data concerning stratification of air in build ings are lacking, but in general it may be said that where the increase in temperature is due to the natural tendency of the warmer or less dense air to rise, as where a direct radiation system is installed, the temperature of the air at the ceiling increases with the ceiling height. The relation, however, is not a straight line function, as the amount of increase per foot of height apparently decreases as the height of the ceiling increases, ac cording to present available information1. Where ceiling heights are under 20 ft, it is common engineering practice to consider that the Fahrenheit temperature increases 2 per cent for each foot of height above the breathing line. This rule, sufficiently accurate for most cases, will give the probable air temperature at any given level for a room heated by direct radiation. Thus, the probable temperature in a room at a point 3 ft above the breathing line, if the breathing line temperature is 70 F, will be [1.00 + (3 X 0.02)] 70 = 74.2 F. With certain types of heating and ventilating systems, which tend to oppose the natural tendency of warm air to rise, the temperature differ ential between floor and ceiling can be greatly reduced. These include, unit heaters, fan-furnace heaters, and the various types of mechanical ventilating systems. The amount of reduction is problematical in certain instances, as it depends upon many factors such as location of heaters, 'A.S.H.V.E. Research Report No. 968--Temperature Gradient Observations in a Large Heated Space, by G. L. Larson. D. ,W. Nelson and O. C. Cromer (A.S.H.V.E. Transactions, Vol. 39,. 1933. p. 243). iAn.gS. .bHy.VG.E. .LR. eLsaersaornc,hDR.eWpo.rNteNlsoo.n1a01n1d--JoThenstsJaomf eTshr(eAe.SH.Hea.Vti.nEg. STyrstaenmssacintiaonnsI,nVduols.tr4i1a,l T19y3p5e. op.f B18u5i)ld. 128 CHAPTER 6. HEATING LOAD air temperature, and direction and velocity of air discharge. In some cases it has been possible to reduce the temperature between the floor and ceiling by a few degrees, whereas, in other cases, the temperature at the ceiling has actually been increased because of improper design, instal lation or operation of equipment. So much depends upon the factors enumerated that it is not advisable to allow less than 1 per cent per foot (and usually more) above the. breathing line in arriving at the air tem perature at any given level for anyof these types of heating and ventilating systems, unless the manufacturers are willing to guarantee that the par ticular type of equipment under consideration will maintain a smaller temperature differential for the specific conditions involved. Temperature at Floor Level: In determining mean air temperatures just above floors which are next to ground or unheated spaces, a tempera ture 5 deg lower than the breathing line temperature may be used, pro vided the breathing line temperature is not less than 55 F. OUTSIDE TEMPERATURES The outside temperature used in computing the heat loss from a build ing is seldom taken as the lowest temperature ever recorded in a given . locality. Such temperatures are usually of short duration and are rarely repeated in successive years. It is therefore evident that a temperature somewhat higher than the lowest on record may be properly assumed in making the heat loss computations. The outside temperature to be assumed in the design of any heating system is ordinarily not more than 15 deg above the lowest recorded tem perature as reported by the Weather Bureau during the preceding 10 years for the locality in which the heating system is to be installed. In the case of massive and well insulated buildings in localities where the minimum does not prevail for more than a few hours, or where the lowest recorded temperature is extremely unusual, more than 15 deg above the minimum may be allowed, due primarily to the fly-wheel effect of the heat capacity of the structure. The outside temperature assumed and used in the design should always be stated in the heating specifications. Table 2 lists the coldest dry-bulb temperatures ever recorded by the Weather Bureau at the places listed. Recommended design temperatures are given in Column E: If Weather-Bureau reports are not available for the locality in question, then the reports for the station nearest to this locality are to be used, unless some other temperature is specifically stated in the specifications. In computing the average heat transmission losses for the heating season in the United States the average outside temperature from October 1 to May I should be used. WIND VELOCITY EFFECTS The effect of wind on the heating requirements of any .building should be given consideration under two heads; 1. Wind movement increases the heat transmission of walls, glass, and roof, affecting poor walls to a much greater extent than good walls. 2. Wind movement materially increases the infiltration (inleakage) of cold air through the cracks around doors and windows, and even through the building materials them selves, if such materials are at all porous. 129