Document 82Z0K1dp6nJ4kLOBMbrG0m445

404 CHAPTER 28 1960 Guide MAO IN MIUNCHCS H * FT. OP WATCH COLUMN KCIOHT fig. 1....Heads Resulting from Temperature Difference {Gravity Systems) K*re*j> fig. 2.... Mass Flow and Specific Heat of Water radiation should be located at points of maximum heat loss or along outside walls, unless this is impracticable. Determination of Water Flow Rate The rate of water, flow in a system is determined by the heat carrying capacity of the water. This capacity is the product of the rate of flow, the specific heat, and the tem perature drop of the water as it passes through the beating units. The equation generally used to describe this relation ship is: H = WcUi - **> 0) where H = heat carrying capacity, Btu per hour. < W = mass flow or weight of water, pounds per hour. c " specific heat of water. it a temperature of water entering heating unit, Fahren heit. (i temperature of water leaving heating unit, Fahrenheit. The equation may be rewritten as follows for convenient use with Fig. 2. H = Gw{tx - t,) (2) where G ** rate of water flow, gallons per minute. u> = rate of water flow per gallon per minute, pounds per hour. The value of to should be obtained from Fig. 2 for the tem perature at which the rate of flow is determined, and the value of e should be determined at the average temperature. For example: If water enters a heating unit at 210 F and leaves at 190 F, and a water flow of 1 gpm is measured at 190 F, the heat carrying capacity, found by substituting values from Fig. 2 in Equation 2, is If * (1)(484)(1.006) (210-190) - 9728.4 Btu per hr or 9.73 MBh At 60 F the mass flow rate is 500 lb per hr at 1 gpm and the specific heat is very close to 1 Btu per (lb) (F deg). Accord ingly, at a 20-deg temperature drop, 1 gpm will release 10 MBh. This value, 10 MBh per gpm of water at a 20-deg tempera ture drop, is extensively used throughout the hot water heat ing industry in the calculation of data for low temperature systems. It is considered well-within the limits of accuracy of heating system design. If 1 gpm will release 10 MBh at a 20-deg drop, it will release 15 MBh at a 30-deg drop, and 20 MBh at a 40-deg drop. Most low temperature systems installed today are designed on the basis of a 20-deg temperature drop. The temperature drop selected in the design of a system is used to determine the water flow necessary to supply the heat required under conditions of maximum heat loss with supply water tem perature at a maximum value. If the supply water tempera ture is increased at any time, the capacity of a heating unit and the temperature drop of water through the unit is in creased accordingly. When the supply water temperature is modulated according to outdoor temperature, the tempera ture drop of the.water in the system is considerably less f.hnn the design value during the greater part of the heating sea son. The use of a design temperature drop greater than 20 deg should be considered where large quantities of required ven- Hot Water Heating Systems tilation air are heated by extended-surface coils, where the required pumping capacity is high, where pumping costs are a critical item, and where extended systems make a saving by the reduction in pipe size possible. Factors considered in selection of a proper temperature drop for a system are (1) the effect of the design average water temperature and water velocity on the capacity and cost of the hunting units, (2) the size and cost of the piping, and (3) the initial and operating cost of the pump. Design Water Temperature The design supply water temperature is the maximum temperature of the water supplied to the system for the purpose of developing the selected rated capacity of the heating units. The design average water temperature of a hot water hating system is the design supply water temperature less Vi the design temperature drop. Capacities of heating units are generally determined on the basis of the design average water temperature for cast-iron radiation and for baseboard or finned pipe radiation. Where a greater degree of precision is required to establish the capacity of heating units, for example, for contract and competitive bidding work, it may be desirable to determine beating unit capacities on the ha-gig of df-agn supply water temperature and temperature drop through the unit. This i3 particularly true where the secondary heating surface in the heating element of the unit is a large percentage of the total heating surface, as in con vectors. Design supply water temperatures for radiation systems vary from 180 to 250 F, a common temperature be ing 210 F. Factors considered in the selection of a design supply water temperature are (1) the effect on the capacity of the heating unit, (2) the effect on occupants who may be near the unit, and (3) the greater possibility of boiling as the pressure varies. Pipe Sizing Methods The theoretical basis of calculations of head loss due to friction of fluids in pipe is the Darcy or Fanning equation, discussed in some detail in Chapter 4, Fluid Flow. Pipe sizes for hot water heating systems are determined most conveniently from charts such as Figs. 3 to 6 and Table 1 showing pressure loss in relation to rate of flow and size of pipe. Allowance for fittings is generally expressed in elbow equivalents which is the number of 90-deg elbows that would have equivalent friction loss at the same rate of flow. Fig. 7 and Table 2 are useful in determining the number of elbow equivalents which then may be converted to equivalent length of pipe by means of Table 3. Figs. 3 to 7 and Tables 1 to 3 are based chiefly on empirical equations developed by F. E. Giesecke in cooperative research with the Society at the University of Texas and the Engineering Ex periment Station of the Agricultural and Mechanical College of Texas.*-TM The friction loss selected for sizing pipe for a system de pends upon the type of pump used and the economics of small pipe size and high pump head versus large pipe and low pump head. Noise is also an important factor. Velocities are usually limited to a maximum of 4 fps for pipe sizes less than 2-in. to reduce the possibility of noise. Velocities up to 10 fps are frequently used in piping larger than 6-in. Average friction drops of IB to 4.0 ft of water per 100 ft of pipe (120 to 480 milinches per ft of pipe) are commonly used in sizing piping systems. The actual friction loss for a 405 isoo-f: : Uio tfioo- :a S00-+-40 400- " "30 100- - 280^ 100.000'+--topoo 80,000-4--WOO - "*00 20,000- ;------ '0-i IQO0- 600- -5 500- I.4 400- K : r* S3 S100!^ Mso-j i 2 5ioo:--.ao* S 50o s ko00-| -800 X000-^ -100 -200 u - . i .ff* . 40-: gtpoo^ r | w. -- . i ,. -ao ,, 3 "3 | 8 500-1 70 g I ZS z 1~ 1- --2 * tc X< 8 300200- -30 t20 zPH0ZJH*u 3L 2- it ? - 3 1 a " 8,si; ! ? -i-o'j- iI SO-t-S i 2 - -3 b <*4 J. 80^2 I I rt835 *i 8-1-0.2 BASED ON CLEAN PIPE WTTM 1*0 T WATER fig. 3.... Friction loss Due to Flow of Water in Iron Pipe1*1 LOOOt Lb oo-f 6004 -8 500400- ' pSO-; *00- 5 8 5 jS T 50-1 f 5 so- 5 30.000-+-VX 80000- -2fi<X> ISO' rio e0- 4 -6 10,00 1000 500 Sajooo-J-vjo 9- : so- .4 g ifiOOr -too 40-! fso:: 2 oo- r#o " o . + i < 300* -30 n * w -Off , ?o 'OOr rio S5 " I * ] ,.ott io- " I so- r 3 -- 2 5 g'--is W* ` 5 304-3 * P,JL 8 -wj ^ 2 I-0.9 S._ -0 6 Based on clean name.With mo f water -ao -OlS fig. 4.... Friction Loss Due to Flow of Water in Type l Copper Tube4 t