STEAM PRESSURES AND TEMPERATURES Steam pressures and temperatures have a certain definite relation to each other, the temperature increasing with the pressure, but not as rapidly for a given increase with high-pressure as with low-pressure steam. For example, with an increase in pressure from 10 pounds to 20 pounds, the temperature rises about 19° F.; whereas with an increase of 10 pounds from 90 to 100 pounds the temperature increases only about 7° F. From atmospheric pressure to 10 pounds' gauge pressure, the increase in temperature is nearly 28° F; a slight difference in the pressure in radiators making a marked difference in their tem perature.
In the case of a partial vacuum, so called—expressed generally in inches of mercury—the decrease in temperature as a condition of perfect vacuum is approached is very marked, as shown in table VII, which gives also steam temperatures corresponding to various pressures. The latter are given in each case of a pound in excess of the gauge pressure, as practically all tables of the proper ties of steam give the absolute pressure—that is, the pressure above a vacuum—the absolute pressure corresponding to 5.3 pounds' gauge pressure, for example, being 20 pounds absolute.
The atmospheric pressure at sea-level is practically 14.7 pounds absolute, and the boiling point of water is 212°. As the pressure decreases, due to altitude or to the removal of air from a vessel by artificial means, the boiling point falls.
Amount of Expansion. An allowance of was of an inch per 100 feet of pipe for each degree rise in temperature, is a fair allowance in computing the amount of expansion that will take place in a line of pipe.
One must assume the temperature at which the pipe will be put up—say anywhere from 0° to 40° in an unfinished building in winter— and, knowing the pressure to be carried, look up in a table of the prop erties of saturated steam the temperature corresponding. See table VII.
Example. Find the expansion that will take place in a line 100 feet long put up in 30-degree weather, when it is filled with steam at 80 pounds' pressure. The temperature corresponding to 80 pounds' steam pressure is 324°; the increase from 30° is 294°, which multiplied by Taus gives 2 N inches expansion, or, expressed in decimals, 2.35 inches.
In low-pressure work 100 feet of pipe heated from 30° to 230° will expand about 1.6 inches.
Provision for Expansion. The expansion of mains can generally be provided for by offsets and swivels, as shown in Fig. 29. All that is necessary is to have the two vertical nipples placed far enough apart, as determined by the length of the horizontal offset, to permit the expansion to take place without too much turning on the threads. The less the turn, the less will be the lihood of leakage. The shorter the offsets, the greater the number that must be used.
A pretty conservative rule would be to allow 4 `feet of offset to each inch of expansion to be taken up on the line. In the case of underground work a good deal of the ex pansion can be taken up where pipes enter buildings by the same kind of swings as shown in Fig. 29, making them longer and tnus re
ducing the number of expansion joints or offsets in the tunnel or duct.
Expansion of Risers. In providing for the expansion of risers, considerable skill must be used, especially in tall buildings. In buildings of not over 6 to 8 stories, or possibly 10 floors at the outside, if they are not high-studded, the expansion may all be taken up in the basement, using swings like those shown in Fig. 30, similar swings being used in the attic also if the overhead-feed system is used, the connections being taken from the bot tom of the main, as previously stated.
In higher buildings than those mentioned, either slip-pattern expan sion joints or swivels made up of pipe and fittings are commonly used. One of these to every six to eight floors is generally considered sufficient, depend ing on the length and arrangement of the radiator connections. One must be sure the pipes above and below slip joints are in proper alignment; otherwise, binding and leakage will occur. If the risers are concealed, such joints must be made accessi ble through proper openings in the walls, as the packing will have to be taken up from time to time and replaced.
Expansion joints made up of pipes are illustrated in Fig. 31. Such joints are unsightly if exposed; but they may generally be con cealed either in specially provid ed pockets in .the floor or in spaces furred down below the ceilings and near the walls.
When expansion joints are used, the risers should be an chored about midway between them. These anchors consist merely of clamps around t h e pipes fastened to the beams, one type being shown in Fig. 32.
Where expansion takes place, swivels must be provided in the radiator connections, to allow for same. Fig. 33 shows a convenient way of taking off radiator connections from risers, any expansion being taken up by the turning of the horizontal connection in the parallel nipples. The connection should of course pitch back toward the riser, to drain freely. Where the expansion is considerable, this is difficult to accomplish unless the radiator is slightly raised.
When risers must be located along the same wall as that on which the radiator is placed, the swivels may be arranged as shown in Fig. 34.
Radiators on the first floor have their con nections made by angle valves with the pipes in the basement, to avoid running along the base-board. It is well to take the branch to the first-floor radiators from riser connections in the basement, rather than to cut into the mains for these branches. See Fig. 35.