Pipe System." In applying both tables to the same two-pipe system a slight difference in the results may occur, because the tables represent the successful practice of different engineers rather than absolute values based on purely theoretical considerations. The differences that may be found are not material ones, however,

SIZE OF MAIN, DIRECT RADIATION, ONE-PIPE

SYSTEM.

Length of Main. Feet.

Surface. 20 40 80 100 200 300 400 600 1,000 Square Feet.

Nominal Diameter of Pipe. Inches.

Radiating

Influence of Available Drop in Pressure on Sizes of Mains.-The distance between the boiler water-line and the lowest point in the steam supply main often determines the size of pipe that must be used. To obtain satisfactory results the piping system must be designed according to the available drop in pressure. In operation, the height of the water level in the

returns above the water-line of the boiler represents the available drop in pressure, which is commonly expressed in inches of water column. Generally speaking, on gravityreturn jobs, the higher the main the smaller it may be; and the nearer the lower end is to the level of the boiler waterline the larger must be the main. For instance, the above

illustration shows how a radiator may be connected up with different sizes of pipe and yet the same heating results be obtained in each case under a 5-lb. pressure. At (a), the main has been run so low that it is necessary to use a 2-in. main to prevent such a drop in pressure as would cause the water in the drip a to rise into the main and thereby flood the inlet to the pipe b. The end of the main is only 3 in. above the boiler water-line, shown dotted, making necessary the use of a large main.

At (b), the available height to which the water in a may rise is 12 in. This is due simply to raising the main. A 1-in. main will then serve the radiator as easily as the 2-in, main

in (a).

pro

The main shown at (c) is raised to a height of 36 in. above the boiler water-line, and a 1-in. pipe will serve the radiator as easily as the larger pipes required under the conditions indicated in (a) and (b). The only difference is that a pressure of about 5 lb. may be carried in (c) and about 4 lb. in (b), while (a) will work satisfactorily at a pressure of 1 lb., vided, of course, that in each case the boiler water-line does not rise above the dotted line shown. This shows how heating engineers may use small pipes on some jobs, but must use large pipes on others. It shows, too, how economy may be exercised in piping a steam job. There are many places where the fitter may reduce the sizes of the pipes by raising them, but he must use good judgment, otherwise there will be times when the water will back up in the mains and cause considerable trouble. The pipe sizes given in the illustration are the smallest that should be used. To provide a margin of safety, it is advisable to make all the pipes one size larger than shown. This will provide for the increased flow of steam when it is turned on a cold radiator.

The factors given in the accompanying table may be used in calculating the size of piping for various pressure drops in mains 100 ft. long. They represent values obtained by Prof. R. C. Carpenter and are applicable to ordinary house-heating installations, where the amount of radiation does not exceed 2,000 sq. ft.

FACTORS FOR BASING SIZES OF MAINS ON AVAILABLE DROPS.

Sizes of Returns.-The sizes of return pipes depend on the size of the system and the character of the returns, whether wet or dry. With wet returns, it is considered good practice to make the area of the return piping equal to about onefourth of that of the steam piping in plants where the steam pipe is larger than 3 in. Where the steam pipe is less than 3 in. it is customary to make the return one or two sizes smaller than the corresponding steam pipe.

With dry returns, it is considered good practice to make the area of the piping equal to about one-half that of the corresponding steam pipe. It is good practice never to use a return smaller than -in. pipe. The table, "Sizes of Mains, Direct Radiation, Two-Pipe System," previously given, indicates sizes of return pipes, with corresponding steam mains, that have been found to work satisfactorily in practice. The diameter of return pipes from indirect heating surfaces should be about 50 per cent. greater than for a similar amount of direct radiation.

Sizes of Drip Pipes for Steam Mains.-Since the amount of water of condensation to be handled by drip or relief pipes is practically an unknown quantity, no general rule can be given for proportioning them. It has been found in practice,

however, that the sizes given in the accompanying table are ordinarily ample for draining covered mains of various lengths. Drip pipes should be connected to the return main in such a manner as not to impede the circulation, but rather to increase the velocity with which the water of condensation returns to the boiler.

SIZES OF DRIP PIPES FOR COVERED STEAM MAINS.

9

10

11

12

14

16

Diameter of Drip Pipe. Inches.

1414121

14

22223

Sizes of Drip Pipes for Risers.-The sizes of drip pipes at the foot of risers of one-pipe systems depend on the amount of radiation supplied from the risers. Ordinarily the sizes given for return pipes in the second column from the left in the table "Sizes of Mains, Direct Radiation, Two-Pipe System," previously given, are suitable and will give satisfactory results by using them in conjunction with the figures given in the radiation column under the 5-lb. pressure. If the drips are connected to a dry-return main without being sealed, steam will flow through them into the return main and may cause water hammer.