steam being allowed to circulate in them. The steam mains, where necessary, are drained into the return pipes through thermostatic valves. The return mains being under suction, and having no direct connection with the steam pipes, can, a certain extent, be run independent of the usual necessity of draining by gravity, in some cases the water being lifted out of radiators placed below the return mains.
In the Union Depot at Columbus, Ohio, which is equipped with this system, the radiators in the basement are about 13 feet below the supply and return mains, which run parallel along the basement ceiling, and the return water is drawn up out of the radiators without any water-hammer or other inconvenience.
A modification of the Paul system was recently installed in a large office building in Chicago which has given decided satisfaction. Instead of the air valve on each radiator, a small tee with an aperture only 1/16 inch in diameter was screwed into the air hole of the radiator, and these connected together into a system of small air pipes running to an air pump or exhauster. This maintains a constant suction on the air holes. Although there is apparently a continual leakage of steam in this system,, it is not more than with the automatic air valves, as the latter are seldom maintained in perfect adjustment. The tees were made with a plug on the outside which could be removed for the purpose of cleaning the pin-hole by means of a wire.
Plants equipped with vacuum systems frequently operate slightly below the atmospheric pressure, and besides entirely doing away with back pressure on engines and removing the air from the system, there are many incidental advantages in the operation of plants of this character which will lead to a very extended adoption. The principal objection to vacuum systems lies in the fact that the exhausters or vacuum pumps take considerable live steam to operate them, and almost as much in moderate weather as on very cold days.
The recent development of vacuum pumps, however, has been of great value to steam-heating work. Pumps of this class are now made which will not run away when all the water is pumped out of the suction, the water end of the pump receiving only air and steam. They will run along slowly under such conditions, taking care of the water as it comes. If a pump of this description is connected to the lower point of the main return from
