Document Mo7J3Z5bEOeqm8a97k1mGEaVa
Heating Ventilating Air Conditioning Guide 1938 too large. The solution is to use some 2 in. and some 234 in. pipe. Since the 234 in. is nearer the correct size than the 2 in., select 2 in. pipe for the first 50 or 60 ft from the boiler and 234 in. for the remaining-pipe back to the boiler.
Tables 8 and 9 may be used to design the radiator risers and connections. According to Table 8, for 12 Mbh the flow riser should be 34 in. and the return riser 1 in., and the riser branches should be 1 in. and 134 in., respectively. Note that according to Table 9, both radiator tappings should be 1 in. To simplify the construction, select 1 in. flow risers with 1 in. riser branches and 1 in. radiator tappings. Also select 134 in. return risers with 134 in. riser branches, and 134 in- radiator tappings. Similarly, for 18 Mbh, select 134 in. flow and return risers and riser branches, and 134 in. radiator tappings.
Fig. 7. A One-Pipe Gravity Circulation System
Fig. 8. A Two-Pipe Direct Return Gravity Circulation System
To develop a rule for determining radiator sizes, assume a system similar to that of Fig. 7, in which the total temperature drop is to be 35 F and which is equipped with 7 radiators, all radiators dissipating equal quantities of heat. The mean temperature of the water in the radiators will be reduced 5 F for each successive radiator. If the mean temperature of .the water in the first radiator is 200 F, the mean temperature of the water in the seventh radiator will be 170 F, and, according to Table 4, Chapter 14, the heat dissipation of these two radiators will be to each other as 1.62 is to 1.15, or as 140 is to 100, and therefore if the last radiator is to dissipate as much heat as the first, its size must be 40 per cent larger.
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Chapter 17. Hot Water Heating Systems and Piping
Table 7. Capacities of Mains in Mbh, for One-Pipe and for Two-Pipe Direct Return Gravity Circulation Systems with a Total Friction Head of 0.6 In., a Temperature Drop of 35 F, when the Mains are 4 Ft Above the Center of the Boiler
11
2
Pipe See (Inches)
Equivalent Length op Pipe (Feet3)
134 2 234 3 334 .4
3.0 4.0 4.5 5.0 5.5 6.0
53 r4
6 | 7 | -8 | 9
10 11
Equivalent Total Length op Pipe in Feet in Longest Circuit
12S | ISO | 175 | 200 | 250
75 100
300
350
Unit Friction Head, in Milinches
8.0
'43.0
6.0
37.5
4.8
33.0
4.0
30.0
3.4
27.0
3.0
25.0
14
22.2
2.0
20.2
1.7
18.7
83.0 72.0 63.0 57.0 51.0 48.0 42.0 38.0 35.0
lJfl.0 115.0 100.0 90.0 81.5 75.4 67.2 61.0 56.0
234-0 204.0 175.5 160.0 143.0 133.0 110.0 107.5 100.0
347.0 300.0 260.0 236.0 214.0 200.0 177.0 160.0 146.0
490.0 422.0 370.0 334-0 297.0 278.0 248.0 223.0 205.0
^Approximate length of pipe in feet equivalent to one elbow in friction head. This value varies with the velocity.
Example 4- Design a two-pipe, direct return, gravity circulation system for the lay out shown in Fig. 8. Assume that the main circuit from the boiler to the farthest flow riser and from the farthest return riser back to the boiler consists of 160 ft of pipe, 6 elbows, and 1 boiler.
Solution. Replacing the boiler by 3 elbow equivalents and assuming that the largest size of the main will be about 3 in., the total equivalent length of the main will be 160 plus 45. or 205 ft. Assuming that the center of the boiler will be about 4 ft lower than the horizontal portion of the main, and that the temperature drop will be 35 F for the system, the pressure head caused by the difference in weight between the water in the flow and return risers joining the mains to the boiler will be about 0.6 in. of water.
Table 7 may be used to determine the size of the main as follows: Refer to Column 8 and note that for Sections AB and IA, which supply 105.6 Mbh, a 3 in. pipe is too large and a 234 in- pipe is too small; hence, select 234 in. rather than 3 in! as noted in Fig. 8 for Section AB and 3 in. for Section IA. For Sections BC and HI, which supply 76.8 Mbh, a 234 in. pipe is almost exactly the correct size and is selected for both sections.
Tables 7 and 8 are based on the assumption that the boiler pressure head must be equal to the friction head in the mains, and that the several radiator pressure heads must be equal to the respective radiator and riser friction heads.
To design the radiator risers, use Table 8 and begin with the set nearest the boiler. The first floor risers must supply 28.8 Mbh. According to the table, 134 in. flow and return risers will supply 26.0 Mbh; if the return riser is increased to 134 in., the capacity will be increased to 34.0 Mbh. This is considerably larger than necessary, and 134 in. flow and return risers are selected. However, it must be remembered that the riser branches, which are the connections from the flow and return mains to the flow and return risers, are to be one size larger than the risers.
The second floor risers must supply 19.2 Mbh. According to the table, the capacity of 1 in. flow and return risers is 20.0 Mbh, and that size is selected.
The third floor risers must supply 9.6 Mbh. If a 34 in. flow and a 34 in. return riser is used, the capacity will be 8.0 Mbh; if both risers are 34 in-, the capacity will be 14.0 Mbh. The 34 in- pipe is selected for both risers.
To design the radiator connections, use Table 9 and note that for the .first floor radiator connections the capacity of a 34 in. flow and Tin. return is 9.1 Mbh, and that of.
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