Document rerKvJKRegaD5DLVoxxegxQbv

American Society of Heating and Ventilating Engineers Guide, "1935 To secure a correct distribution of hot water among the several risers it is necessa as p. reviously stated, to introduce special resistances to balance the several riser?^' follows The first riser is 80 ft nearer the boiler than the fifth riser. In order that the two m be balanced, i.e., that they may operate under equal pressure heads, resistance must^ added to the first riser equal to the friction head in the 80 ft of flow main from B tot 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 nor 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 bv an orifice resistor in a union: If the orifice resistor is to be used, its size mav be seleotJi from Table 5 as follows: , Elected The lower 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 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 Fa - X|2 Pip* Size (Inches) Equivalent Length (Frarb) 3 * 5 67 PmEquivalent Total Length of in Feet in Longest Circuit 200 400 600 800 1000 Unit Friction Head, in Milinches , 1080 540 360 270 216 1.0 12.7 8:6 7.2 6.2 5.5 32 23 18 15 13 Va. 2.0 i 2.5 27.5 40 55.0 48 18.7 28 36.8 34 --15.122 30.0 27 13.7 19 26.4 23 11.5 N17 22.6 20 m 3.0 122.0 81.5 66.0 58.3 60.5 59 42 33 28 25 ih . 4.0' 182.0 66 122.0 46 98.2 37 86.2 31 74.2 27 2 5.0 371.0 252.0 201.0 180.0 151.0 80 56 45 38 33 2^ 7.0 598.0 407.0 323.0 287.0 240.0 s 91 65 51 43 38 3 9.0 1110.0 790.0 598.0 527.0 443.0 107 76 60 51 44 "For other temperature drops the capacities of pipes are to be changed correspondingly. For example, for a'temperature drop of 30 F, the capacities shown in this table are to be multiplied by 3. The velocities remain unchanged. - bApproximate length of pipe in feet equivalent to one elbow in friction head. This value varies with the velocity. 566 Chapter 33--Hot Water Heating Systems and Piping a*rnce between it and the required resistance will be about 10 per cent, which is periisrible, and the 0.45-in. orifice is selected. WT, 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 <rravity pressure heads produced by the risers should be taken into confkleration. With water at 220 F and 210 F, respectively, in the risers, the 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 *- w w 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 the tables 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 large as those through the other radiators. The result would be that the water, in flowing through the radiator, would cool 5 F instead of 10 F. The mean water temperature in the radiator would then be 217J4 F in stead of 215 F, and the mean temperature difference, water to air, would be 14734 F instead of 145 F. The heat dissipated by the radiator would therefore be about 2 per cent more than calculated. It is evident that this difference in heat dissipation is smaller than the difference between