Document Gmg20mpDNGj8XGwjqm184Qdpq

American Society of Heating and Ventilating Engineers Guide, 1929 Table 1. Air Velocities and Register Temperatures for Fan Furnace Systems VELocmss in Feet res Minutb Ttpe or Building Thru In Free Area Horizontal of Heaters Heat Ducts In Heat Risers Into Room In Vent Outlets In Vent Risers In Register Horizontal Tempera Vent Ducts ture and Recir culating Ducts Schools 800 800 500 300 300 500 600 90 to to to to to to to to 1000 1000 600 400 400 600 800 120 Churches 800 700 400 300 300 400 500 80 to to to to to to to to 1000 900 600 500 500 600 700 120 Auditoriums 800 800 500 300 300 500 600 80 and to to to to to to to to Convention Halls 1000 1000 600 500 500 600 800 120 Garages and Industrial Buildings 1000 to 1400 1000 to 1400 600 to 1000 400 to 1000 400 to 600 600 to 1000 800 to 1200 80 to 140 consumption. In practice, velocities through the free area of the heaters are used as follows: ' 1. Schools, Churches, Auditoriums, Theatres, etc., from 800 to 1000 ft. per minute. 2. Industrial Buildings, Factories and Garages, from 1000 Jo 1400 ft. per minute. The importance of power costs effects the decision as to what free area and static resistance are allowable. The thermal efficiency of the various makes of heaters is a consideration deserving some thought in case the fuel to be burned is relatively high in cost. First cost and fuel economy are items that also bear on the type of casing to be employed. One of the most important considerations is the durability or life of the apparatus. Investigation of the several, makes of heaters contemplated submitted to him should include such -data as the thickness of the metal in each heater, the provisions made for expansionand contractions, and the protection afforded the firebox at the point of most intense heat. A heater made of thicker metal than others will naturally give greater length of service, all other things being equal. . " Proper provisions for expansion and contraction are imperative. With% out such provisions a cast-iron heater is likely to crack and a steel heater is likely to warp and buckle. Most furnaces have linings around the periphery of the fuel bed to protect the firebox from the action of the hottest part of the fire. Thelinings are removable and are as easily installed as new grates. 194 Chapter IX--Fan Furnace Heating SUGGESTED METHODS OF PROCEDURE IN DESIGN OF FAN FURNACE SYSTEMS 1. Calculate Heat Loss of each room in building. See Chapter I. Example.--Assume school room with cubic content 6550. Heat loss = 45,269 B.t.u. 2. Determine volume of air per minute. ' Based on number of occupants. Based on number of air changes. Example.--Assuming 33 occupants; 30 C.F.M. per occupant. 33 X 30 = 990 C.F.M. 6550 content = 6}4 minutes air change or approximately 9 changes per hour. 3. Figure diffusion temperature of air for each room.* Example. Diffusion Temperature = gox" = 60 X 990 = 42,3 deg` 4. Determine required register temperature. Register temperature equals room temperature plus diffusion temperature. Example.--Assume 70 deg. room temp. 42.3 deg. plus 70 deg. = 112.3 deg. 5. Set final temperature of air leaving heater. Final temperature equals register temperature plus allowance for loss in ducts. Example.-- 112.3 deg. register + 10 deg. Loss in ducts -- 122.3 deg. 6. Determine temperature rise through heater. Temperature rise equals final temperature minus initial tem perature. Where part outside and part recirculation is used. Initial temperature approximately equals PoFo + PrTz. P0 = percentage outside air. T0 = temperature. Pz = percentage recirculated air. Tt = temperature recirculated 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 55 h h is the heat loss in B.t.u. per hour. ttt-- where v is the volume of air in cubic feet per minute wv . measured at 70 deg. fahr. for the roomin question. 195