Document L6BJQkQoZqEDgbx12NqGq6Mb

Siiilgggs American Society of Heating and Ventilating Engineers Guide, 1931 -| > The air temperature at the mean height between floor and ceiling is ; the breathing-line temperature, Table 1, for rooms not over 10 ft. in height. For rooms above this height, the temperature should be increased according to the difference between the mean height of the vertical wall, glass, roof or ceiling surface, and the height of the breathing line. The amount of the increase per foot of height will depend on the inside breath ing-line temperature; the outside temperature; the wall and roof con struction, and the type and location of heaters and other factors. 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 information. Furthermore, the temperature of the air at any level in a building such as at the mean height of walls, is not necessarily proportional to the distance from the floor or breathing line, although in general the tempera ture increases as the height increases. It is the common practice of engi neers to allow 2 per cent per foot of height above the breathing line in determining the probable air temperature at any given level for a direct radiation system, and this value is, no doubt, sufficiently accurate in most cases. ' 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, air temperature, and direction and velocity of air discharge. In sorrie cases it has been possible to reduce the temperature between the floor and ceiling to 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 any of these types of heating and ventilating systems, unless the manufacturer^ are willing to guarantee that the par ticular type of equipment under consideration will maintain a smaller temperature differential for the specific conditions involved. Rietschel 's Formula: Rietschel states that the average mean tempera ture tm never exceeds 1.15 t for the highest ceilings. His formula is: . <m = / [ 1. + 0.017 (A - 10) ] in which h is the height of the ceiling or outside walls. Substituting tm = 1.15 t, it is found that h -- 19 ft., so that for heights over 19 ft. a value of tm = 1.15 t may be used. Temperature at Floor Level: In determining mean air temperatures just above floors which are next to ground or unheated spaces, a tempera- 84 Chapter 5--Heat Loss Calculations 5 deg lower than the breathing-line temperature may be used, prot'^ded the breathing-line temperature is not less than 55 deg. fahr. Summer Where buildings are being artificially cooled for human comfort during , summer a different scale of inside temperatures is required and this ` ide temperature must depend to a large extent upon the variation in mStside temperatures. It is manifestly undesirable and impracticable to maintain a temperature of 70 deg. inside when it is 95 deg. outside, nor can it be said that the inside temperature should be as much as 15 deg. Sow the outside temperature. At 95 deg. outside, and dry air, a tem perature somewhat lower than 80 deg. might be desirable. On the other hand with 85 deg. outside a 70-deg. temperature inside would be entirely too cold. In other words, the spread between the inside and outside temperature must be varied as the outside temperature varies. Table 2 is based on average moisture conditions outside and upon the assumption of relative humidities inside, between 50 and 60 per cent, which is usual in such installations. OUTSIDE TEMPERATURES Winter 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 must not be more than 15 deg. fahr. above the lowest recorded temperature as reported by the Weather Bureau (Table 3) during the preceding 10 years for the locality in which the heating system is to be installed. The outside temperature assumed and used in the design should always be stated in the heating specifications. 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 1, shall be used. This is to be that reported by the U. S. Weather Bureau during the preceding 10 years, for the locality in question. Summer The maximum cooling load in summer may be calculated for an outside dry-bulb temperature not to exceed 95 deg. and a wet-bulb temperature not to exceed 77 deg. While higher outside dry-bulb temperatures are frequently observed they are either of short duration or accompanied by low relative humidities. Weather Bureau reports are frequently mis leading in this respect as they report the maximum temperature for the day with the relative humidity, which occurs during a different period and 85