Document mmmRBn9VBKmvbqVJVNkvpmeb4

American Society of Heating and Ventilating Engineers Guide, 1931 surface. When recirculation is employed, the air velocities through the heater may become excessive on account of the fact that a smaller heater capacity is required for the same quantity of air. s; The following general procedure is recommended for the design off fan-furnace systems: J 1. Calculate heat loss of each room in building: (See Chapter 5). Example: Assume schoolroom with cubic content 6,550. Heat loss = 45,269 Btu. 2. Determine volume of air per minute, based on number of occupants || and number of air changes: Example: Assuming 33 occupants; 30 cu ft per minute per occupant. 33 X 30 = 990 cu ft per minute. j.|6^ contents = 6H min. per air change or approximately 9 changes per hour. 3. Figure diffusion temperature of air for each room1: Example: Diffusion Temperature = 56 X H 60 X Qt 56 X 45,269 = 42.3 F. 60 X 990 t 4. Determine required register temperature: Register temperature equals room temperature plus diffusion temperature: Example: Assume 70 F room temp. 42.3 F plus 70 F = 112.3 F. 5. Determine final temperature of air leaving heater: Final temperature equals register temperature plus allowance for j| loss in ducts: Example: 112.3 F register + 10 F. Loss in ducts = 122.3 F. 6. Determine temperature rise through heater: Temperature rise equals final temperature minus initial tem- ;. perature. Where part outside air and part recirculated air is ; used, initial temperature approximately equals p0t0 + Pitt- po = percentage outside air. to = temperature of outside air. ps -- percentage recirculated air. tr = temperature recirculated air. Example: Temperature rise-122.3 F -- 0 F = 122.3 F with all outside air. !The diffusion temperature.may be defined as the number of degrees the volume of air as determined under Item 2 must be in excess of the room temperature, in order to offset the heat loss by cooling over that range. In other words it is the number of degrees representing, the difference between the-necessary supply register temperature and the room temperature for the conditions under which the heat losses are calculated in Item 1. It is readily calculated by using the following formula: Diffusion temperature- 5q h B is the heat loss in Btu per hour. ^ - where Qr is the volume of air in cubic feet per minute 00 vr measured at 70 F for the room in question. - 114 -Fan Furnace Heating ; air 0 F. lation 60 F. .. 50 X 60 = 30 F initial temperature, heater = 122.3 F - 30 F = 92.3 F. ea through heaters': h heaters suitable to problem, cuhic feet Der minute Example---Assume total cubic feet per minute of oil rnnmc ss 0.1 000 Register Temperatures for Fan-Furnace Systems Table 1. Air Velocities and Velocities in Feet pbb Mctutb Ttpb of Bcildiko In Thru Horisontal Id Supply Free Area Supply Risers of Heaters Docta Into Room In Vent Outlets In Vent Risers In Registeb Horizontal Tempera.* Vent Ducts TUBE and Recir culating Duets Schools 800 to 1000 Churches 800 to.' 1000 Auditoriums 800 . and to Convention Halls 1000 Garages and Industrial Buildings 1000 to 1400 800 to 1000 700 to 9 800 to 1000 1000 to 1400 500 to 600 400 to 6Q0 500 to600 600 to 1000 300 to 400 300 to 500 300 to 500 400 to 1000 300 to 400 300 to 500 300 to 500 400 to 600 500 to 600 400 to 600 500 to 600 600 to. 1000 600 to 800 500 to 700 600 to 800 800 to 1200 .90 to 120 80 to i2q 80 to 120 80 to 140 8. Select size and number of heaters: a. Take temperature rise from item 6; b. Take free area from item 7, or manufacturers' tables; c. Assume maximum allowable combustion rate for fuel to be burned and stack available; d. Note resistance of heaters for air volume;. ' . . . e. Select suitable heaters from ratings. 9. Design duct system: Ducts may be designed according to standard practice. ' (See Chapter.31). 'Some heaters are rated on the basis of the volume of air that may be warmed over any given temperature range for all heater sizes and list the static resistance in each instance. In .this case the free area need not be calculated, but the combustion rate should be investigated. 115