Document qaQ6ooXr6MQVn3N0OdyXVoyox

American Society of Heating and Ventilating Engineers Guide, 1937" 3. A protective limit control located in the bonnet to shut down the system inde pendently of the thermostat if the bonnet temperature exceeds 225 F. 4. On oiland gas burner installations, a control is usually included which will shut down the system if the fire goes out or if there is a failure of the ignition system. 5. A humidistat to regulate the moisture supplied to the rooms. 6. On automatic stoker installations, a control is usually included which will start the operation regardless of thermostat settings whenever the bonnet temperature indicates that the fire is dying. METHOD OF DESIGNING FORCED-AIR HEATING SYSTEMS 1. Determine heat loss from each room in Btu per hour. (See Chapter 7). 2. Locate warm air registers and return registers on plans of house, beginning with the upper story rooms. 3. Sketch in duct layout to connect all registers and grilles with the central unit. 4. Determine equivalent length of duct for each register, allowing 10 diameters of straight pipe as equivalent to each 90 deg elbow having an inner radius not less than the diameter of the round pipe or the depth of the rectangular pipe. 5. Select a value for temperature of the air at the furnace bonnet. It is customary to use some value lying between 150 to 165 F. Use lower value if larger number of air recirculations is desired. 6. Determine approximate value of temperature reduction in each duct caused by heat loss from the ducts. A value of from 0.3 to 0.6 F per foot of duct has been obtained from tests conducted in the Research Residence installation for uninsulated duct lengths up to approximately 60 ft. 7. Subtract this temperature reduction from the assumed bonnet air temperature to obtain an approximate value of the register air temperature for each register. 8. Determine the required air volume for each room from the following equation, or from the values listed in Table 1: where ^_ n. y ~ 60 X 0.24 X d (tr - 65) (1) Q = required air volume, cubic feet per minute. H = heat loss of room, Btu per hour. d = density of air at register temperature, pounds per cubic foot. tr = register temperature, degrees Fahrenheit. 0.24 = specific heat of air. 65 = return air temperature. For any given register temperature the solution of this equation simplifies to the following form: Q = H. X Factor (2) in which the values of the Factor may be obtained from Table 1: 9. Determine register size from the air volume delivered to each room by the following formula: Free area of register, square feet = where tG-ross area ocf regisat.er, square f,ee.t = -r--r-e-e-=r-t-r-e--a K . Q = required air volume, cubic feet per minute. V = velocity at register face, feet per minute. R = ratio of free area to gross area of register. 434 (3) I W,,, if ? Chapter 24---Mechanical Warm Air Furnace Systems Table 1. Factors Corresponding to Register Temperature for Equation 2 ROISTER TEMPERATURE .. Factor 110 0.02210 120 0.01840 130 0.01585 140 0.01397 150 0.01253 160 0.01140 170 0.01049 Allowable register velocities to be used in Equation 3 are approximately as follows: Baseboard, non-deflecting type, maximum = 300 fpm. Baseboard, deflecting toward floor, maximum = 500 fpm. Baseboard, deflecting and diffusing = up to 800 fpm. . High sidewall = not less than 600 fpm. 10. Duct systems for forced-air installations,may consist of either trunk systems or individual duct systems. Trunk Systems. Determine duct sizes and friction losses as outlined in Chapter 20, except that for residence applications the velocities in the main duct and in the various parts of the system should approximate the values recommended in Table 2. Individual Duct Systems. An individual duct system is one having separate ducts extending from the heating unit to each register. In designing such a system select first the duct having the greatest equivalent length. Select a reasonable velocity using Table 2 as a guide. From friction chart on p. 366 determine unit friction loss per 100 ft of run, and from this the total friction loss in the duct selected. If this total friction loss exceeds a reasonable value a lower velocity should be used. The remaining ducts are proportioned so that the total pressure in each duct is the same as that calculated for the longest duct. The added resistance necessary in the shorter ducts is accomplished by increasing the velocity in these ducts. No duct should, be less than 6 in. in diameter, nor should the velocity in any duct exceed approximately 1200 fpm. The final adjustment in a duct system may be made by employing dampers. Instead of proportioning the ducts as outlined in the preceding paragraph it is more usual in practice to proportion all the ducts so that they have the same velocity as that used in the longest duct and to balance the system by employing dampers in the shorter ducts. Return duct systems are designed making use of the same principles as those used in the design of supply duct systems. In this case the design may be based on the volume of aircorresponding to the1 density of air existing in the return ducts, or in order to provide a factor for air leakage, it may be based on the same volume as used for the supply ducts. Table 2. Recommended Velocities Through Ducts and Registers Description Low Velocity System (fpm) Medium Velocity System (fpm) High Velocity System (mi) Main ducts............... ................. 500 750 1000 Branch Hurts 450 600 750 Wall stacks. .. ...... ........ ....... 350 500 600 300 350 400 Wall registers above 5 ft (min.)_____ 500 550 600 435