Document kDo3NEGyaVJDa1mrBZnmr71Rq

Heating Ventilating Air Conditioning Guide 1939 . 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 th * diameter of the round pipe or the depth of the rectangular pipe. e 5. Select a value for temperature of the air at the furnace bonnet. It is customarv to use some value lying between 150 to 165 F. Use lower value if larger number of ai recirculations is desired. It is recommended that the number of air recirculations should be in excess of 5 per hour. 6. Determine approximate value of temperature reduction in each duct caused bv 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: ' rH V 60 X 0.24 X d (<r - 65) (1> where 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. ix = 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: 2, Free area of register, square feet = - - (3) Gross area of register, square feet = Free Area ,. ..K ' where - . .. r, -. . 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. (4) Table 1. Factors Corresponding to Register Temperature for Equation 2 Register Temperature . , . Factor . 110 120 130 140 150 i 160 170 0.0221 0.0184 0.0158 0.0140 0.0125 0.0114 : 0.0105 406 Chapter 20. Mechanical Warm Air Furnace Systems AH abIe register velocities to ** 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. Duct systems for forced-air installations may consist of either trunk systems or individual duct systems. Systems. Determine duct sizes and friction losses as outlined in Chapter 29, t that for residence applications the velocities in the main duct and in the various e*rts of t|,e system should approximate the values recommended in Table 2. ^individual Duct Systems. An individual duct system is one having separate ducts 1"Tj" from the heating unit to each register. In designing such a system select first having the greatest equivalent length. Select a reasonable velocity using Table o 6 a vuide From friction chart on page 584 determine unit friction loss per 100 ft of 2 3S and from this the total friction, loss in the duct selected. If this total friction loss TMceeds a reasonable value a lower velocity should be used. 611 The remaining ducts are proportioned so that the total pressure in each duct is the me as that calculated for the longest duct. The added resistance necessary in the Sorter ducts is accomplished by increasing the velocity in these ducts. No duct should he less than 6 in. in diameter, nor should the velocity in any duct exceed approximately 1200fpm The final adjustment in a duct system may be made by employing dampers. Table 2. Recommended Velocities Through Ducts and Registers.... Description Low Velocitt Ststbh (ppm) Medium Velocitt Stbtbm (ppm) High Velocitt Ststsm ' (ppm) .. Wall registers above 5 ft (min.).-...... 500 450 350 300 500' 750 600 500 350 550 itioo' : 750 : 600 400 600 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 air corresponding to the'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: 11. Determine frictional resistance in: o. Supply side of system as outlined in item 10. b. Return side of system as outlined in item 10. . c. Furnace units, casing or hood, which is usually considered as equivalent to 0.03 to 0.10 in. of water. v d. Accessories such as washers or air filters, from manufacturer's data. e. Inlet and outlet registers and grilles, from manufacturer's data. ; /. Other accessory equipment such as cooling coils, from manufacturer's data. Choose a fan which, according to its manufacturer's rating, is capable of delivering ^ volume of air, expressed in cubic feet per minute, against a frictional resistance, expressed in inches of water, computed by adding together the items listed in the preceding discus sion. In practice it is recommended that liberal allowances should be made so that the