Document r6erB11ZLn0Yo8K2gV91JaLMq

560 CHAPTER 21 1957 Guide its total equivalent length by the actual friction loss per foot at design flow. The circuit friction pressure drop is the summation of individual section pressure drops. Calculation of Actual Friction Loss for Example 1 Sec tion Flow Mbh gpm Friction Loss Pipe' FT/100 FT Sub in. mi/ft (Fig. 3) (Fig. 3) Fittings Type Equiv FT** Pipe Length ft Total Equiv Length of Pipe Ft Friction Loss Calculation MI FT BC 43.5 4.35 CD 36.0 3.6 DE 30.0 3.0 EF 25.0 2.5 FG 18.0 1.8 GN 0.8 0.08 NO 41.8 4.18 OB 50.0 5.00 1.5 180 1.0 120 2.5 300 1.8 210 1.0 120 -1.2 150 1.4 170 1.9 230 1 ell 2.4 . ---- H 1 ell 1.9 , . S4 -- -- - H- -- -- M 3 ells 3.6 1 H tee 3.4 1 1 in. tee 4.8 1 Rad. V 3.6 1 ell 2.5 1 3 ells 7.6 2 tees 10.0 2 Gate 2.5 Valve 1 Flow 50.0 Valve 1 Boiler : 7-s 14 15 8 10 14. 8 31 38 16 0.16 X 1.5 0.24 16 X 180 2880 15 0.15 X 1.0 0.15 15 X 120 1800 10' 0.10 X 2.5 0.25 10 X 300 3000 10 0.10 X 1.8 0.18 10 X 210 2100 14 0.14 X 1.0 0.14 14 X 120 1680 23 6.23 X 1.2 0.26 23 X 150 3500 34 0.34 X 1-4 0.48 34 X 170 5780 116 1.16 X 1.0 2.20 116 X 230 26700 Convector friction Iosb*. .. 1200 0.10 Total Friction Loes * Data obtained from manufacturers' catalogs. ** See also Tables 2 and 3 and Fig. 7. Ml e milinnhty If the total friction loss as determined does not agree with the pump head, some pipe size changes will be necessary to obtain closer agreement. The slight differ ence in head calculated by milinches and by feet of head is due to use of only 2 sig nificant figures for the latter. This is well within the accuracy required. As would be expected the calculated head of 4.02 ft is different from the 4.7 ft esti mated. This is due to two factors: (1) the difference between the actual and esti mated number of fittings and (2) the differences between actual and estimated losses in the different sections as the friction losses were generally below the average fric tion loss assumed. The lrin, pump initially selected would be entirely satisfactory for the installation and would produce a flow slightly in excess of 5 gpm. A procedure to determine the actual flow and head capacity of the pump and system is discussed under Circulating Pumps. One-Pipe System, Description and Design The one-pipe system, Fig. 10, has a single main for both supply and re turn. Special fittings installed at the connections to radiation risers or runouts generate the head necessary to produce flow through the heating unit. In this system the main or circuit loop does not change size,from the first to the last radiation unit. The amount of water flowing in the main is constant, except at points where some of it is by-passed through radiation. Since the water temperature in the main drops progressively, some designers consider it necessary to increase the size of heat-transmitting surface ac- .Hot'Water Heating Systems ' 561 cordingly by using a progressively lower design water temperature in sizing the heating units. Most one-pipe heating systems are designed, however, on the basis of a single design water temperature. Generally,, the theo retical increase, in heat-transmitting surface,is heeded only when the sys tem is operating at full design output. A water temperature drop in the main of 20 deg or less minimizes the capacity reduction effect. In comparison with a two-pipe system, the one-pipe system: 1. Permits simpler piping due to the use of one main of uniform size, . 2. May be more acceptable when pipe is exposed in occupied spaces. 3. Is very adaptable to small systems or to individual loops in large systems, either horizontal or vertical, as a part of a two-pipe system, or to obtain zone control in large combination systems. 4. May be lower in installation cost. 5. Has a Bimpler problem in regard to water flow adjustment. Continuing the comparison with two-pipe systems some disadvantages of one-pipe systems are: 1. The amount of water which can be by-passed by a one-pipe fitting or fittings is limited by the economics of design and the head applied. The fittings are not gen erally used to provide the water flow necessary for large air-handling units having a high pressure drop. 2. The pump head required is slightly higher than, that required for a two-pipe reversed-retum system of similar flow requirements. 3. If the progressive increase of radiation size as temperature drops is taken into account, a design complication and additional cost factor are encountered. , 4. The main flow design temperature drop should not exceed 20 deg. Design of a One-Pipe System The basic design premise of a one-pipe system is that the pressure drop of the one-pipe fitting, or fittings, at the required main flow, be equal to or greater than the radiation circuit pressure drop at its required flow. The system may be divided into two or more loops for smaller main pipe' sizes Md pumps. Main and radiation circuit temperature drops may be dif-