Document vVN2p8gXGZMwLgMvb6Kodyy3R

American Society of Heating and Ventilating Engineers Guide, 1937 Referring to the column for an equivalent total length of 300 ft for Sections AB and KA, each supplying 117.6 Mbh, it will be found that a lj^-in. pipe is too small and a2-in pipe is too large. Consequently, a 1 J-in. pipe is selected for the flow line AB, and a 2-in. pipe for the return line KA. For Sections BC and JK, each supplying 88 Mbh, a lJdj-in. pipe is only slightly too small and it is selected. The remaining pipe sizes are selected in a similar manner and recorded in Fig. 5. For a temperature drop of 10 p 24.5 gpm of water must be circulated. The pump to select is one which has its highest efficiency when it is delivering 24.5 gpm against a 6-ft head. To secure a.correct distribution of hot water among the several risers it is necessary, Table 4. Capacities of Pipes in Mbh (1000 Btu per Hour) and Velocities of Water in Pipes in Inches per Second for Forced Circulation Systems with a Total Friction Head of 18 ft and for a Maximum Temperature Drop of 10 P3 1 Pots 8m (Inches) 21 Equivalent. Length op Pdh (Febt*0 3|4 5 6 1 '? Equivalent Total Length op Pipe in Feet in Longest Circuit 200 400 600 800 1000 Unit Friction Head, m Milinches 1080 540 360 270 216 1.0 12.7 32 8.6 23 7.2 18 6.2 15 5.5 13 2.0 27.5 40 18.7 28 15.1 22 13.7 19 11.5 17 1 2.5 55.0 48 36.8 34 30.0 27 26.4 23 22.6 20 IH 3.0 122.0 59 81.5 42 66.0 33 58.3 28 50.5 25 m 4.0 182.0 122.0 98.2 86.2 74.2 66 46 37 31 27 2 5.0 371.0 252.0 201.0 180.0 151.0 80 56 45 38 33 2H 7.0 598.0 407.0 323.0 287.0 240.0 91 65 51 43 38 3 9.0 1110.0 790.0 598.0 527.0 443.0 107 76 60 51 44 remain unchanged7........ ......... -- muiupuea Dy a. l ne veloauej th1veiodtyimate length of pipe in feet valent to one elbow in friction head. This value varies with as previously stated, to introduce special resistances to balance the several risers, as follows: The first riser is 80 ft nearer the boiler than the fifth riser. In order that the two may be balanced, i.e., that they may operate under equal pressure heads, resistance must be added to the first riser equal to the friction head in the 80 ft of flow main from B to t plus that in the 80 ft of return main from C to K. It will be noted from Table 3 that the unit friction head is about 240 milinches per foot. The total friction head in the flow and return mains between the first and fifth risers is therefore 160 X 240 or 38,400 milinches, or a little more than 3 ft, which must be supplied by additional resistance in the first riser to prevent its having an advantage over the fifth riser. This resistance can be supplied by a calibrated and adjusted modulating valve or by 610 Chapter 33--Hot Water Heating Systems and Piping an orifice resistor in a union. If the orifice resistor is to be used, its size may be selected from Table 5 as follows: Thedower part of the first flow riser supplies 28.8 Mbh. According to Table 3, it should be a 1-in. pipe and would have a velocity of 22 in. per second, if it were supplying 24 Mbh. Since it is supplying 28.8 Mbh, the velocity will be about 26 in. per second. From Table 5 it will be found that for a 1-in. pipe and a velocity of 24 in. per second, an 0.45-in. orifice will produce a loss of head of 37,000 milinches. For a velocity of 26 in. per second, the loss of head will be somewhat more, probably about 43,000 milinches; the difference between it and the required resistance will be about 10 per cent, which is per missible, and the 0.45-in. orifice is selected. The sizes of the orifice resistors for the second, third, and fourth risers are selected in a similar manner and found to be 0.45 in., 0.50 in., and 0.55 in., respectively. If the design of the system of Fig. 5 is to be extremely refined, the gravity pressure heads produced by the risers should be taken into con sideration. With water at 220'F and 210 F, respectively, in the risers, the w *>' w gravity head is 50 milinches per foot of water column or 25 milinches per foot of flow and return pipe. The pump pressure head in this case is 240 milinches per foot of pipe, and the gravity head, being only one tenth as large as the pump head, may be neglected without serious error. This is generally done. Temperatures of 220 F and 210 F would be used only during the coldest weather for which the system is designed. At other times the tempera tures would be lower, the temperature drop smaller, and the gravity heads smaller. The pump pressure head remains constant throughout the season if the pump is operated at a constant speed and, consequently, the gravity head is generally less than one-tenth of the pump head. Effect of Variations in Pipe Sizes The pipe sizes for the several parts of 'the system selected from thetables are only approximately correct but the resulting error should be negligible as may be seen from the following study. Assume, as an extreme case, that the error in pipe size is so large that the water flows twice as fast through one of the radiators as through the others. This would make the friction head through this radiator almost four times as 611