Document bB40g87mKbgkkQN6oDdy3nvk6

HEATINC VENTILATING AIR CONDITIONING GUIDE 1944 METHOD OF DESIGNING FORCED-AIR HEATING SYSTEMS6 1. Determine heat loss from each room in Btu per hour. (See Chapter 6.) 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 at least 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 between 145 to 165 F. Use lower value if larger number of air recircu- Table 2. Factors Corresponding to Register Temperature for Equation 2 Register Temperature Deg F Factor 110 120 130 140 .150 160 . 170 . . 0.0221 0.0184 0.0158 0.0140 0.0125 0.0114 0.0105 lations is desired, per hour. The number of air recirculations.should range from three to eight 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 ah approximate value of the register air temperature for each register. 8.. Determine the required air volume for each room from Equation 1, or from the values listed in Table 2: where Q 60 X 0.24 X d (k - 65) 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. 1 . 65 = return air temperature, degrees Fahrenheit. (1) For any given register temperature the solution of this equation simplifies to: Q = H X Factor in which the values of the Factor may be obtained from Table 2. (2) 9. Determine register size from the air volume delivered to each room: Gross area of register, square feet = (3) UrCTeocnhdnitiicoanlinCgoAdes,soFcoiautirothn, E1d45itioPnu.bJicanSuqaurayre1.' C1l0e4v9elannndh.Oiiohtio^ .by .t.he N..atl.onal Warm Air Heating and 376 CHAPTER 20. MECHANICAL WARM AIR FURNACE SYSTEMS where 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. Allowable register velocities to be used in Equation 3 are given in Table 3. In residential applications it is not advisable to handle more than 150 cfm through any single register. 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 outlinedf in Chapter 32, except that for residence applications the velocities in the main duct ancl in the various parts of the system should approximate the values recommended in Table 3. 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 " ..... .1--*- a reasonable velocity usine Table Table 3. Approximate Design Velocities through Ducts and Registers Description Low Velocity System (ppm) Medium Velocity System (ppm) High Velocity System (ppm) Baseboard registers (Down deflecting) Wall registers above 5 ft (min.).-...... 500 450 350 300 > 500 . 750 600 500 400 550 1000 750 600 500 600 3 as a guide. From friction chart in Chapter 32 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 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: a. 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. d. Accessories such as washers or air filters, from manufacturer's data. e. Inlet and outlet registers arid grilles' r /"v, I____ - -t!______ *l,, C.------- ~ mormfni'tliror'i: Hata. 'Pressure Losses in Registers and Stockheuds in Forcrf tVarm Air Hesting, by A. P. KraU and S. Konzo 377