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HEATING VENTILATING AIR CONDITIONING GUIDE 1942 Table 6. Capacities of Mains in Mbh, foe One-Pipe and for Two-Pipe Direct Return Gravity Circulation Systems with a Total Friction Head of 0.6 In., a Temperature Drop of 35 F, when the Mains are 4 Ft above the Center of the Boiler 1l 2 Pipe StEB (Inches) Equivalent Length or Pipe (Feet*) IK ' 2 2K 3 3K 4 3.0 4.0 4.3 5.0 5.5 6.0 13 75 | 4 131 6 1 8 1 9 ' 10 Equivalent Total Length or Pipe in Feet in Longer*r Circuit [ 100 | 125 | 150 175 200 I 250 300 11 350 C7nit Friction Head, in Milinches 8.0 6.0 4.8 4.0 3.4 3.0 2.4 2.0 i.r 43.0 37.5 33.0 30.0 87.0 85.0 88.8 80.8 18.7 83.0 78.0 63.0 57.0 51.0 48.0 . 48.0 38.0 35.0 140.0 116.0 100.0 90.0 81.5 75.4 67.8 61.0 66.0 834-0 804.0 175.5 160.0 143.0 133.0 110.0 107.5 100.0 347.0 300.0 860.0 836.0 814.0 800.0 177.0 160.0 146.0 490.0 488.0 370.0 334.0 897.0 878.0 848.0 883.0 805.0 lenEth of pipe in feet etluiTaIent to '<bow in friction head. This value varies with are used, the capacity will be 8.0 Mbh; if both risers are % in., the capacity will be 14.0 Mbh. The K-in. pipe is selected for both risers. To design the radiator connections, use Table 8 and note that for the first floor radiator connections the capacity of a K-in. flow and 1-in. return is 9.1 Mbh, and that of a 1-in. flow arid a 1-in. return is 12.5 Mbh. The former is more nearly the correct size, but since it is difficult to secure a good flow through first floor radiators, the 1-in. flow and return connection is selected. For the two upper floors, the capacity of a J4-in. flow and return connection is 10.5 Mbh, and that size is used. Table 7. Maximum Capacities of Risers3 in Mbh, and Velocities of Water in Pipes in Inches Per Second for One-Pipe and for Two-Pipe Direct Return Gravity Circulation Systems with a. Drop of 35 F through Each Radiator Pips So* (Inches) Flow Return K KK KK Kl ll l IK IK IK IK IK m IK Eocivacbnt Length op Pips (Feetc) 1.0 1.5 2.0 3.0 3.5 1HT FLOOBfc Vel (In. per Seed) Mbh Flow Return 9 2.3 2.3 is 3.2 2.0 18 2.5 2.5 SI 3.0 2.0 S6 3.0 3.0 34 4.0 2.5 48 3.0 3.0 2nu Flooh Mbh 5.0 6.4 10.1 1S.8 50.0 S5.S 43.0 3rd and 4th Floors e.e 8.0 14.0 17.1 86.0 34.0 55.0 aThls table is based on pressure heads of 450, 1800, 3150, and 4500, respectively, for the first, second, third, and fourth floor radiators, and on friction heads of 200 milinches for the first floor radiators and. con nections, and 700 milinches for all-other radiators and their connections. bThe riser branches, the piping which connects the risers to the mains, are to be one size larger than the risers. eApproximate length of pipes in feet equivalent to one elbow in friction head. This value varies with the velocity. ^Velocities apply to the riser branches. 322 CHAPTER 16. HOT WATER HEATING SYSTEMS AND PIPING Table 8. Maximum Capacities of Radiator Connections in Mbh, for One-Pipe a ion for Two-Pipe Direct Return Gravity Circulation Systems with Pipe Son Flow Return K K K 1 1 lK IK l K or Pipe (Feet*) x.o 1.5 2.0 3.0 1st Floor Mbh 4.1 6.8 7.0 9.1 18.6 17.6 83.3 2nd, 3rd, and 4th Floors Mbk 5.9 7.5 10.5 13.0 17.8 83.8 33.8 .Approximate length of pipe In feet equivalents one elbow in friction head. This value vanes with the velocity. As explained in the design of the forced circulation system of Fig. 5, the two-pipe direct return system of Fig. 8 will not function correctly unless its four sets of risers are balanced among themselves. This neces-. sary balancing is accomplished by adding resistances to all risers, except the one farthest from the boiler, equal to the excess boiler pressure heads available for those risers above the boiler pressure head available for the farthest riser. For example, the first set of risers is 60 ft nearer the boiler than the last set. Since the flow and return mains are designed for a friction head of 3 milinches per foot (see Table 6, Column 8), the boiler pressure head available for the first set of risers is 360 milinches in excess of that available for the fourth set. The velocity in the riser branch is 3 in. per second (see Table 7) and, therefore, according to Table 5, an 0.65-in. orifice in a lj^dn. union should be used. This will provide a resistance of about 420 milinches. In the same manner it is found that for the second set of risers a resistance of 240 milinches is required and * '1 / ' ---------A NAcir4-onf'O nf 323