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 there is always a certain amount of water in the supply mains and risers due to the radiation from the pipes themselves. If the pipes are thoroughly covered with a good non-conductor of heat there is but very little water from this cause; but little as it is, the mains must be so run that it will flow to certain points, where it must be drained into the return or into proper receptacles. If the steam pipes are arranged so that water can accumulate at any point, trouble is sure to follow. It is a fundamental principle in steam heating that pipes shall be so graded that water of condensation will tend, by the action of gravity, to flow with the current of steam to certain points, where it can be properly drained off.

The various systems of piping are sometimes more or less combined in the same installation, and when radiators are of very large size they should, if possible, be given both a supply and return connection, as the principal advantage of the double connection lies in the internal circulation which tends toward the more rapid removal of air and water. More will be said on the subject of radiator connections in a subsequent chapter on radiators.

Gravity systems.—In the preceding discussion and the accompanying diagrams we have assumed that the water of condensation returns through the return pipes directly to the boiler, there to be re-evaporated into steam by the fire on the grate. This is what is known as the "gravity system" of steam supply, and is self-regulating as to water consumption, except for such small amounts of steam and water as may be lost by leaks. It is but one of the many methods of steam supply, though the one now most employed where the plant is used only for heating and there is no steam power.

In the gravity system the water stands in the return pipes and risers at practically the same level as that in the boiler, though in the remote parts of the system it rises above the boiler level by a height equivalent to the pressure required to effect the circulation through the system. For this reason gravity systems should be designed for free circulation, with pipes of ample size, the difference of pressure required between the steam mains and the most extreme point of the returns never exceeding a pound or two per square inch. Gravity systems are therefore generally run at very low pressures, though frequently in very cold weather as much as 15 pounds is carried for the sake of the higher temperature of steam. The operation of this system is the same at any pressure.