Document LzL2gMJ1VMG07kM3MzOLmbJ5

American Society of Heating and Ventilating Engineers Guide, 1926-27 division point of two sections because the Section H-A conveys the heat for all three radiators, whereas Section A-1 carries only the heat for Radiator 1. For a similar reason B must be a division point. The reason for selec ting 1 as a division point is that it'is often necessary to select a different pipe size for Section A-l than for Section 1 -B, in order to secure the proper friction head for Radiator 1. If it were certain that the pipe size for Section A-l could be the same as that for Section 1-5 the pipe line from A to B could be called one section; since it is impossible to know in advance what the pipe sizes leading to and from the radiator must be, it is best to make the radiator the division point of two sections. 2. Having divided the several circuits into sections prepare a table, like Table 48, and record there the designations of the several sections, the quantity of heat conveyed by each section, and the number of feet of pipe and the number of elbows or elbow equivalents in each section. All of this information should be obtained from the plans for the heating system. In Fig. 35 is shown only a diagrammatic repre sentation of the system to illustrate the general arrangement. In this case a number of elbows, valves, air valves, and the expansion tank are omitted in order to simplify the drawing. For the actual design, a complete drawing, such as that shown in Fig. 36, must be available or the designer must be sufficiently familiar with the proposed installation to be able to insert the correct pipe length and the correct number of elbows or elbow equivalents in the description of each section, as recorded in Table 48. 3., Calculate the pressure head available for the circuit by means of Fig. 32 and record it in the table. 4. Calculate the pipe sizes for the several sections by means of Figs. 33 and 34 so that the sum of the friction heads in all the sections com posing any one circuit is equal to the pressure head available for that circuit. Table 48. Circuit I* Pressure head = 7 X 90 = 630 m.i. Section B.t.u, Feet Elbows Assumed Diameter In. Unit Total Friction Friction Selected Diameter In. Unit Total Friction Friction H-A 30,000 8.67 m 12 104 3.5 122 A-l 10,000 1.2 IK 35 4 5.5 3.5 41 1-B 10,000 2.67 1 7.5 33 7.5 12.5 183 B-H 30,000 10.7 ilA 24.5 128 4.5 12 157 35 Total.... 772 IK 104 122 IK 4 41 l K 3.5 9 7.5 56 IK- 128 157 621 *The calculations for Circuits II and III are made and recorded in a similar manner. 84 American Society of Heating and Ventilating Engineers Guide, 1926-27 FORCED CIRCULATION In designing the pipe lines for forced-circulation systems, the same methods and formulas are used to determine pressure heads and friction heads which are used in the design of gravity-flow systems, with the one exception that in forced-circulation systems the force producing the necessary pressure head is supplied partly or entirely by a pump. The designing engineer should determine, in every case, what portion, if any, of the force necessary to produce the required pressure head Central .Station mr^pjmp 6" -- i.ooqooo -4- 1 A " 6" 'if 1,000,00b l--i 1 -* B 1.50 O'OOt 6h SSi. 2. ~1 if Section Length 1000 BtU* Pipe size Velocity rrictionhead feet per* he inches ft per sec in.of water O-A 339 12,500 6 715 64.75 A-B 346 1 1,500 6 655 69.20 B-C 339 10.500 6 6.00 6441 C-D 339 9.000 5 744 105.09 D-E t-r F-G G-H 346 3139 339 346 . 6000 7000 5500 4.500 5 5 4t 4i 660 570 360 476 96.60 71.19 74.50 GZJE> h- I l-J J-K K-o 339 339 >46 339 3500 2,000 1.000 12.500 1 4 3i zi 6 451 334 345 7.15 61.02 47.46 6304 64 75 AI y 904.65 x )Z.50qOOO - || 9 - horse power 1 12 a 40x 3600x550 4i`l y 1 -----1 I500000l- 4' --: 1- 1.00000c 5' At' G - ; )~ IOOOOOD 5' At' \ r\ 1500000 Fig. 37. Central Hot Water Heating System Supplying Group of Eleven Buildings will be supplied by gravity. For example, in a central heating system, the pump may supply the entire pressure head for the distributing mains while gravity may supply all or part of the pressure head for the Service lines in the several buildings. In forced-circulation systems, the pressure-head caused by the differ ence in density of the water, and which maintains circulation in gravityflow systems, is generally small when compared with the pressure head 85