Document MGKng2RwQp7Xdze4dz6n8jRnz

American Society of Heating and Ventilating Engineers Guide, 1932 to Case D except that a portion of the air is recirculated. All of the air circulated is passed through the washer and heater. M = M0 + Mr = q~24 (ty -- t) Punc*s per hour (7) M0 is known from the ventilating requirements. Then Mr = M -- M0, weight of air recirculated. Assume 35 per cent relative humidity to be maintained in the rooms, which corresponds to a dew-point temperature of 41 F for t = 70 F, and a tempering coil to warm the entering outside air from zero to 35 F. The resulting temperature of the mixture of outside and recirculated air entering the tempering coil is: , _ M0 (35 + 460) + Mr (l + 460) A------------------------- ^--------------------- -460 (8) A washer supplied with a water heater will raise the temperature of the air passing through from <w to h = 41 F and saturate it at this temperature. Hi = 0.24 (Ij -- 41)jW, for heater Ht -- 0.24 (Iw -- tx)M, for tempering coil The heat to be supplied the washer per pound of air passing through' the washer is equal to the difference between the heat content of saturated air at a temperature h -- 41 F (15.7 Btu per pound) and the heat content per pound of the mixture at a temperature of and relative humidity to be calculated. Assuming the outside air temperature t0 = 0, and dry; the inside air temperature * = 70 and 35 per cent relative humidity corresponding to 0.00552 lb of moisture per pound of air, there are M pounds containing 0.00552AT lb moisture orA lb mois- , u.uu5v2m ture per pound of air entering the washer. Knowing the temperature f*, the relative humidity is readily determined and the heat content per pound. Case F. (Fig. 16) The temperature ly will ordinarily be different for each room. With H and M0 fixed, 0.24 (<y -- t) Ma = H, or H h 0.24 M0 +1 (9) In order to provide the proper temperature ty for each room, the so-called hot and cold or double plenum chamber system is employed. The weight of air drawn in from outside the building is the surn of the values for Ma as determined by the ventilating require ments for each room and is the total weight of air passing through the tempering coil and washer and entering the rooms. Each room is provided with an independent supply duct run to the heater plenum chamber; so that varying amounts of bypassed tempered air and hot air may be mixed to obtain the required temperature (iy) for each room independently (/m = ty -- Iz). The mixing dampers are ordinarily placed under ther mostatic control. Ducts and Outlets The design of the duct system should be based on data contained in Chapter 32. The total friction against which the fan must operate is the sum of the resistances of all of the elements of the'entire system. Fans and Motive Power The selection of Tan and motor should be based on data contained in Chapter 34. Centrifugal fans are generally used as they are well adapted for working against the frictional resistance of the system, and reach their maximum efficiency when wprking. against the resistance offered by the average central fan heating system. 450 Chapter 32 AIR DISTRIBUTION SYSTEMS Dynamic and Friction Losses; Friction Chart; Proportioning losses: Design of Ducts; General Rules; Procedure; Air Velocities; Propor tioning for Friction; Plenum Chamber and Individual Ducts; Main Trunk Ducts with Branches; Prevention of Noise; Duct Construction: Air Distribution; Stratification and .Diffusion; Downward and Upward Air Distribution; Inlets and Outlets; Grilles and Registers; Measure ment of Air Flow; Pitot Tubes; Anemometers; Kata Thermometers. THE laws governing the flow of fluids are based on the assumption that the density remains constant throughout the flow. In con sidering the flow of a gas such as air, these laws do not strictly hold. The velocity of flow in an air duct of uniform size through which a given weight of air is passing will vary with an increase or decrease in pressure which 'causes corresponding decrease or increase in volume; The flow of air due to large pressure differences is most accurately stated by thermodynamic formulae for air discharge under conditions of adiabatic flow, but such formulae are complicated and the error occasioned by the use of the same formulae that apply to the flow of fluids may be considered negligible when only such pressure differences are involved as occur in ordinary heating and ventilating practice. The pressure dif ference in this field seldom exceeds more than 2 in. water gage, and it is sufficiently accurate in considering the relations between pressure head and velocity of flow for air in ducts to apply the same formulae as are used for the flow of liquids. The basic formula is: where VV = 1096.5 _ W V = velocity in feet per minute. p := head or pressure in inches pf water. W = weight of air in pounds per cubic foot. (1) For standard air (70 F, and 29.92 barometer) W = 0.07495 lb per cubic foot. Sub ystituting this value in Equation 1: V = 1096.5 0.07495 = 4005 (2) An inspection of Equation 1 indicates that temperature affects the velocity of flow by changing the weight per cubic foot of air. Tables 5 and 6 (Chapter 39) give the velocities at various pressures and temperatures. 451