Document k6qEvNw00qyNaQjzMn6XN0xbV

410 CHAPTER 28 1960 Guide connections are made so that water travels from top to bottom in one coil and from bottom to top in the other. The design of a two-pipe reversed-retura system will be illustrated by Example l. Example 1: Design a two-pipe reversed-returo hot water heating system for a 37 x 29 ft one-etory residence with basement having a heat los of 50 MBh (50,000 Btu per hr). The heating units are to be convectors operating with a supply water tem- C:rature of 210 F and a 20-deg temperature drop. The piping yout <nH location of convectors are shown in Fig. 8. Solution: A 50 MBh load with a 20-deg drop requires a water circulation of 5 gptn. The length of the reversed-return circuit is determined as 138 ft of pipe, by following the supply from the boiler to point G Fig. 8 and the return from point N back to the boiler. The estimated total equivalent length is 138 X 1.5 -- 207 ft. This, plus an allowance of the equivalent of 25 ft of pipe for one convector circuit results in a total equivalent length of pipe of 232 ft. Fig. 9 shows that a 1-in. pump has a capacity of 5 gpm at 4.7 ft available head. The preliminary average design friction loss is then 4.7 X 100/232 or 2 ft per 10Q ft of pipe (240 milinehpH per ft). This value can also be determined by refer ence to Table 1, where, the vertical column under 5 ft, the next larger equivalent length to 232 ft is 250 ft. To the right in thg linp read the average design friction loss as 2 ft per 100 ft or 240 milinches per foot. The piping may be sired initially from Table 1 as follows: In the friction los column find the value 2 ft per 100 ft. Proceed to right in the same line to the nearest gpm value and then read the pipe size at the top of the column. The pipe rises found by this method for the various sections of Fig. 8 are shown in the following table: Section A-B B-C C-D D-E E-F F-G G-H Supply MBh 50 >3.5 *36 30 25 18 8.2 G'" 5.0 4.35 3.6 3.0 2.5 1.8 0.82 Pipe Size inches l 1 X H X I-J J-K K-L L-M M-N N-0 O-A Return 6.5 14 20 25 32 41.8 50 0.65 1.4 2.0 2.5 3.2 4.18 5.0 X X 8 X l l The risers to the convectors may also be sized for the same average friction loss, 2 ft/100 ft or 240 milinches per foot, as follows: Convector No. I 2 3 4 s 6 7 Load MBh.... 6.5 7.5 6.0 5.0 0.65 0.75 0.60 0.5 Pip^ssze......... X X X X 7.0 9.8 8.2 0.7 0.98 0.82 XXX After the initial pipe rises have been selected the friction loa in the system should be recalculated as indicated in the following table. The highest presriire drop circuit should be marked off into sections; each of a different flow rate. The total equivalent length of each section should then be determined by adding its elbow equivalent length (see Tables 2 and 3 and Fig. 7) to its actual measured length. Total friction loss is. established from fig. 9.... Performance Chart for Circulating Pump charts or tables for each section by multiplying its total equiva lent length by the actual friction loss per foot at design flow. The circuit friction pressure drop is the summation of individual section pressure drops as shown in Table 4. If the total friction loss as determined does not agree with the pump head, some pipe rise changes will be necessary to obtain closer agreement. The slight difference in head calculated by milinches and by feet of head is due to use of only 2 significant figures for the latter. This is well within the accuracy required. As would be expected the calculated head of 4.02 ft is differ ent from the 4.7 ft estimated. This is due to two factors: (I) the difference between the actual and estimated number of fittings nnH (2) the differences between actual and estimated losses in the different sections as the friction losses were generally below the average friction loss assumed. The 1-in. pump initially selected would be entirely satisfac tory for the installation and would produce a flow slightly in excess of 5 gpm. A procedure to determine the actual flow and capacity of the pump and system is discussed under Cir culating Pumps. One-Pipe System, Description and Design The one-pipe system, Fig. 10, has a single main for both supply and return. Special fittings installed at the connec tions to radiation risers or runouts generate the head neces sary to produce flow through the heating unit. In this sys tem 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 bypassed through radiation. Since the water temperature in the main drops progressively, some designers consider it necessary to increase the size of heat-transmitting surface accordingly by using a progressively lower design water temperature in sizing the heating units. Most one-pipe heat ing systems are designed, however, on the basis of a single design water temperature. Generally, the theoretical in crease in heat-transmitting surface is needed only when the system is operating at full design output. A water tempera ture drop in the main of 20 deg or less minimizes the ca pacity reduction effect. In comparison with a two-pipe system, the one-pipe sys tem: 1. Permits simpler piping due to the use of one main of uni form size. 2. May be more acceptable when pipe is exposed in occupied spaces. 3. Is very adaptable to small systems or to individual loops Hot Wafer Heating Systems 411 Table 4 .... Calculation of Actual Friction loss for Example 1 Flow MBh Gpn friction test (fig. 3) Ft/100 Ft Mi/Ft Pipe Size te. (fig. 3) Type Equrv Flb Total Pipe length ft length of Pipe ft Friction lost Cokuiation Mi* BC 43.5 1.5 1 1 ell 4.35 180 2.4 14 16 0.16 X 15 16 X 180 2880 CD 36.0 1.0 1 3.6 120 -- - 15 15 0.15 X 1.0 15 X 120 1800 DE 30.0 2.5 X 1 ell 3.0 300 1.9 8 10 0.10 X 2.5 10 X 300 3000 EF 25.0 1.8 % 2.5 210 -- - 10 10 0.10 X 1.8 10 X 210 2100 FG 18.0 1.0 X -- -- 14 14 0.14 X 1.0 1.8 120 14 X 120 1680 GN 9.8 1.2 X 3 ells 0.98 150 1 % in. tee 1 1 in. tee 1 Rad. V 3.6 8 23 0.23 X 1-2 3.4 23 X 150 3500 4.8 3.6 NO 41.8 1.4 1 l ell 4.18 170 2.5 31 34 0 34 X 1 4 34 X 170 5780 OB 50.0 1.9 1 3 ells 7.5 38 116 1.16 X 1.9 5.00 230 2 tees 10.0 116 X 230 26700 2 Gate valve 2.5 1 Flow valve 50.0 1 Boiler 7.5 Convector friction loss4................................ 1200 Total friction lot 4 Dta obtained from manufacturers' catalogs. b Sm also Tables > and 1 end Fi*. 7. * Mi -- milinches. " 48590 Ft 0.24 0.15 0.25 0.18 0.14 0.26 0.48 2.20 0.10 4.02 in large systems, either horizontal or vertical, as a part of & twopipe system, or to obtain zone control in large combination sys tems. 4. May be lower in installation cost. 5. Has a simpler 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 bypassed by a one-pipe fitting or fittings is limited by the economics of design and the head applied. The fittings are not generally 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 re quired for a two-pipe reversed-return system of similar re quirements. 3. If the progressive increase of radiation cip. 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 re quired 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 and pumps. Main and radiation circuit temperature drops may be different. For example, a 10-deg temperature drop may be used in the main with a 20-deg drop through the radiation. Example g: A one-pipe system is to be designed for the resi dence discussed in Example 1. Fig. 10 shows the piping arrange ment for the one-pipe system. The size of the convectors is determined on the basis of a 210 F design supply water tempera ture and a 20-deg design temperature drop as in Example 1. Solution: For initial selection of pump use a 1-in. pump hav ing a capacity of 5 gpm at 4.7-ft head. The measured pipe length is 140 ft which establishes an equivalent length of 140 X li =