Document 0Nmk5m1Edp9mKr1oOJaN85Zm

American Society of Heating and Ventilating Engineers Guide, 1932 The length shown in the table nearest to this length is 200 ft. In the 200-ft column, a lj^-in. pipe is slightly too small and a 2-in. pipe is too large. The lj^-in. pipe will therefore be selected. For Sections CD and IJ, supplying 175 sq ft (42,000 Btu), a 1 J^-in. pipe is too small and a lj4-in. pipe is too large, so 1J4 in. will be selected for the flow and 1}^ in. for the return line. For larger systems, it will be economical to operate with higher friction heads, and tables similar to Tables 3 and 4, which are based respectively on total friction heads of 6 and 18 ft, may be prepared. Table 4. Capacities of Pipes in Equivalent Square Feet of Heating Surface2 and in 1000 Btu, 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 DEGb 1 Pipe Size (Inches) 34 % l IX 134 2 234 3 2 Equivalent Length op Pipe (Feetc) 1.0 -2.0 2.5 3.0 4.0 5.0 7.0 9.0 3 4 56 Equivalent Total Length op Pipe in Feet in Longest Circuit 200 400 600 800 . 1000- Unit Friction Head, in Milinches 1080 540 360 270 216 53 . 12.7 32 115 27.5 40 230 55.0 48 510 122.0 59 760 182.0 66 1550 871.0 80 2500 598.0 91 4600 1110.0 107 36 8.6 23 78 18.7 28 154 36.8 34 340 81.5 42 510 122.0 46 1050 252.0 56 1700 407.0 65 3300 790.0 76 30 7.2 18 63 15.1 22 125 80.0 27 276 66.0 33 410 98.2 37 850 201.0 45 1350 323.0* 51 2500 598.0 60 26 6.2 15 57 18.7 19 110 26.4 23 243 68.8 28 360 86.2 31 750 180.0 38 1200 287.0 43 2200 527.0 51 23 5.5 13 48 11.5 17 94 22.6 20 210 50.5 25 310 74.2 27 630 151.0 33 1000 .240.0 38 1850 443.0 44 Based on 240 Btu per square foot. bFor other temperature dro'ps the capacities of pipes are to be changed correspondingly. For example, for a temperature drop of 30 deg, the capacities shown in this table are to be multiplied by 3. The velocities remain unchanged. Approximate length of pipe, in feet, equivalent to one elbow in friction head. This value varies with the velocity. 118 Chapter 8--Hot Water Heating Systems and Pipe Sizes Example 2. Design a direct return two-pipe forced circulation system for the layout j shown in Fig. 5. For this system the length of the pipe line from the boiler to the ; highest radiator on the farthest riser and back to the boiler is about 250 ft. There are i about 16 elbow equivalents having an equivalent pipe length of about 50 ft, so the total equivalent pipe length is about 300 ft. Solution. The same pipe size tables may be' used as those developed for the reversed return system of Fig. 3. Since this system is somewhat larger than that shown in Fig. 3, Table 3 which provides for a friction head of 6 ft may be used instead of Table 2 which provides for a friction head of only 2 ft. Referring to the column for an equivalent total length of 300 ft for Sections AB and | KA, each supplying 490 sq ft (117,600 Btu), it will be found that a l)4-in. pipe is too small and a 2-in. pipe is too large. Consequently, a 134-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 sup- i plying 370 sq ft (88,000 Btu), a 134-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. This } system has 490 sq ft and must supply 117,600 Btu per hour. For a temperature drop of {. 10 deg, 23.52 gpm of water must be circulated. The pump to select is one which has its highest efficiency when it is delivering 23J4 gpm against a 6-ft head. ! To secure a correct distribution of hot water among the several risers it is necessary, I 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 F plus that in the 80 ft of return main from G 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 an orifice resistor in a union. If the orifice resistor is to be used, its size may be selected from Table 5 as follows: The lower part of the first flow riser supplies 120 sq ft (28,800 Btu). 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 100 sq ft. Since it is supplying 120 sq ft, 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, a 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 permissible, 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. 119