Document 2NdyBOwoMVZNG6dKbGdN8Mg87

HEATINC VENTILATINC AIR CONDITIONING CUIDE 1943 The methods just described for the design of the two elementary systems are fundamental and apply to the design of all hot water heating systems. In every system, however large and complicated, the pipe, system must be such that the ph forcing the water from the boiler to any one radiator is equal to the fh in that radiator's circuit when the radiator is receiving its proper quantity of hot water and the system is functioning at a steady rate. Two-Pipe Gravity Circulation System Example S. In the system shown in Fig. 10, water leaving the boiler may flow to any one of the three radiators. If the system is designed correctly, each radiator will receive, its proper share of the hot water. Since Radiator 3 has the largest load and is also farthest from the boiler, it is the least favorably located with reference to circulation, and its circuit should be designed first. If the pipes leading to it are large enough, it will be easy to secure sufficient circulation for the.other two radiators. The system is to function with a 40 F flow-return temperature difference.' The ph for each radiator.is 7 x 175 (Fig. 4), or 1225 milinches; the fh for each radiator circuit must, therefore, also be 1225 milinches. * Solution. In order to design a radiator circuit accurately and systematically, the circuit should be divided into sections. The division points of sections must be where the pipe sizes change or may change, and where the volume of water flowing in' the pipe changes. The data relating to the several sections may be recorded as shown in Table 5. Data recorded in Table 5 show that Circuit 3 consists of 70.4 ft of pipe and 21.5 elbow equivalents. Assuming that the average size of the pipe will be 1J4 in., the 21.5 elbow equivalents may be.replaeed by 21.5 x 2.6 (%s 1.25x1.0), or 56 ft of pipe, which would make the total equivalent length of the circuit 70.4 plus.56 or 126.4 ft of pipe, and the average fh, 1225/126.4, or about 10 milinches per foot. For this unit fh and a temperature difference of 40 F (see Fig. 3), a 1 in. pipe will convey 18 Mbh, a 1in. pipe,-37 Mbh, and a in. pipe, 56 Mbh. The pipe sizes for the several sections of Circuit 3 may be selected as indicated in Table 5. Haying selected the pipe sizes, the unit friction heads .may be found from Fig. 3 and the total friction head calculated and recorded as shown in Table 5. If the grand total, in the present case 1192 milinches, differs materially from the available ph, 1225 milinches, one or more of- the pipe sizes must be changed and the calculation repeated until the total fh is practically equal to the available ph of 1225 milinches. It is not necessary, in the design of hot water heating systems, to be ektremely careful . to have thefh exactly equal to the available ph, because a hot water heating system has the ability to adjust itself to varying conditions of considerable magnitude. For example, in the present case, the calculated fh is 1192 milinches, or about 3 per cent less; than the calculated available ph; the water would, therefore, circulate a little faster than con templated and the return temperature would be a little higher than 160 F. This would 322 CHAPTER 16. HOT WATER HEATINC SYSTEMS AND PIPING immediately lower the available ph and the ph and fh would come into balance at a value higher than 1192 and lower than 1225. Since it is generally not necessary to make extremely refined calculations. Table 1 may often be used instead of the chart of Fig. 3 to determine pipe sizes. For example, in the nresent case, Table 1 shows that for an fh of 10 milinches a 114 in. pipe would convey 28 Mbh with a temperature difference of 20 F or 56 Mbh with a temperature difference of 40 F. Since the 1)4 in. pipe in Fig. 10 is to convey only 45 Mbh with a temperature difference of 40 F, or only 22.5 with a temperature difference of 20 F, it is evident from the table that the fh will be between 6 and 8 milinches. For 6 milinches the heat con veyed is 21.2 and for 8 milinches, it is 24.8; for 22.5 Mbh, the fh would be estimated to be about 6.5, which would be sufficiently accurate for the present calculation. < Table 5. Tabulated Data for Example 3 Circuit'No. 8. Available ph 1886 milinches Section 0-4 . 4-5 6-3 3-7 7-8 8-9 9-0 Total Load Mbh 45 ' 35 20 20 20 20 35 45 . Pipe 'Length Ft 3.5 10.0 12.0 6.5 9.5 12.8 13.8 2.3 - 70.4 Elbows No. 2.5 2.0 oq 5.3 5.5 1.0 2.5 2.5 21.5 ' Pipe Size . In. 1H IK IlX 1 1 IK m Equivalent Length, Ft 11.3 15.2 12.0 17.5 20.5 14.9 20.3 10.1 121.8 Unit Friction Milinches per Ft Total Friction, Milinches 6.3 71 9.2 140 3.4 41 13 228 13 267 13 194 9.2 187 6.3 64 1192 0-4 4-5 5-2 2-8 8-9 9-0 Total 45 35 15 15 35 45 Circuit No. 8. Available ph 1886 milinches 6.2 6 9.5 7 1 . 18.7 K 20.4 7.5 24.0 71 140. 140 490 187 1092 0-4 4-10 10-1 1-11 11-9 9-0 Total .45 10 ... 10 . 10 10 45 Circuit No. !.- Available ph 1886 milinches .9.0 5.0 9.5 . 11.5 Sv1.5 11.3 ' 5.5 10.7 ii 46 5:5 . K 2.5 K. 18.1 - 11 15.4 ' 11 71 124 434 .. 199 169 64 1 1067 . Having completed the design of Circuit 3t it is simple to. design Circuit-2 because it has four sections in. common with Circuit 3 and it is only necessary to design Sections 5-2 and 2-8, as shown in Table 5, so that* the total fh of Circuit'2 will be practically equal to the total fh of Circuit 3. Having completed the design of Circuit 2r it is necessary to design Circuit 1, as shown in Table 5, so that its total fh will be approximately equal to the total fh of the other two circuits since all three circuits have equal pressure heads. Two-Pipe Forced Circulation System Example 4; In the design of a system for Fig. 11; as in the design of Example 2, there are three unknowns^-pressure.head, pipe size, and.flow-return temperature difference, any two of which may be assumed and the third found. In the design-of a system for 323