Document k6wO12aNbYyX2LxZknLGDnzvD

ft American Society of Heating and Ventilating Engineers Guide, 1925-26 division point of two sections because the Section H-A conveys the heat for all three radiators, whereas Section 4-1 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 differentpipe size for Section 4-1 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 4-1 could be the same as that for Section 1-B the pipe line from 4 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 inserf 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 iuad -- 7 X 90 = 630 771.j. Section b.t.u. Feet Elbows Assumed Diameter In. 'sUnit Total Friction Friction Selected Diameter In. Unit Total Friction Friction H-A 30,000 8.67 m 12 104 IK 104 3.5 122 122 A-1 10,000 1.2 IK 35 ' 4 : . IK 4 5.5 . 3.5. 41. ,41 1-B 10,000 2.67 i 7.5 33 :M . 3.5 9 7.5 12.5 183 7.5 ' 56 ' B-H 30,000 10.7 IK 24.5 128 IK .128 4.5 12 167 157 35 Total.... 772 621 The calculations for Circuits II and III are made and recorded in a similar manner. 84 American Society of Heating and Ventilating Engineers Guide, 1925-26 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 will be supplied by gravity. For example, in a central heating system; Central Station r*pimp fC v I fc> irr*o *" ,ooqocx5 --L4-- A ipocpab 6' 1 2! B 1.50 5,00C 6' 3* c! ooo'oool I s' 1 1 1 11 K ti 1 6* o !: 1 J 1 5ectior Length 1000 Btub Pipe size Vfelocity rriction Head feet per hr. inches ft per a* in.af water O-A A-B 339 \ 1,500 346 1 1.500 6 6 7A5 655 34.75' 69.20 e>-c. C-D D-E E.-F F-G G-M 339 >39 346 339 339. 346 10.500 9,000 6.000 7.000 3300 A500 6. 6.00 5 7.44 5 660 5 573 At 560 At 476 6441 10 5.09 96.63 71.19 7456 6246 H-1 I-J J-K K-O 339 339 346 339 3300 2,000 1,000 12,500 4 3i zt '6 . 451 3.45 7.15 61,02 47.46 6304 6475 Total friction head 904.65 1 I 1 3f 1 1 6 . Horse power - 904.65 x I2.50Q000.1,0 40 x 3600x550 I l 1 1 !' . S H 3' G :Goal 4* 1000000! At' 1- )1394 1.00000! .4*' 4' 5' Dj 5' OO e! "___OOOO 4a s LSS0600 .1 I00Q00C Fig. 37. Central, Hot Water Heating System Supplying Group of Eleven Buildings 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 produced by the pump. It may be neglected in the design if it is less than 20 per cent of that produced by the pump. To illustrate, let it be required to determine the pipe size for the central hot-water heating system shown in Fig. 37. 85