Paul System. What is known as the Paul system of heating is illustrated in Fig. 2. The general arrangement of the mains in the basement is similar to that of the systems described. From the boiler a steam is supplied to the engine b, from which the exhaust passes to the feedwater heater c and thence to the heating system, the feed-pump d returning the water of condensation to the boiler. A by-passed reducing valve e in the supplementary live-steam connection controls the supply of steam from the boiler in making up for any deficiency in the available amount of exhaust steam. The escape pipe to the roof is provided with a lightly weighted back-pressure valve f. The return pipe is connected to an automatic pump governor. The steam pipe g connects to the exhausting apparatus h, the discharge pipe from which is connected to the exhaust escape pipe to the atmosphere. The air pipes are connected to the suction opening in the exhausting apparatus h and have a check-valve i that serves to hold the vacuum in the air pipe, a gauge j indicating the amount of vacuum obtained. The exhausting apparatus and connections are auxiliary to regular gravity steam-heating systems to which the apparatus is commonly applied. The radiators may be arranged on the one-pipe, down-feed system, or may be connected up on the two-pipe, up-feed system. The system of steam and return pipes is practically the same, as far as the risers and connections to the radiators and mains are concerned, as for ordinary gravity systems. The air piping is practically the same as would be erected for gravity systems, with the exception that in the cellar the air pipe is connected to the exhausting apparatus of the system.

In Fig. 3 is shown the exhausting apparatus for a plant having a large number of radiators, the apparatus being arranged in duplicate so that either ejector may be operated should the other become disarranged. Live steam enters the exhausting apparatus through the pipe a and one or both of the valves b, b, passing to the ejectors c, c, pipes d, d, checkvalves e, e, and through ƒ to the exhaust escape pipe. The flow of steam through the ejectors c, c creates a partial vacuum in the air-line piping g, g, through which the air is

drawn from the radiators. The valves h, h provide for the independent operation of either half of the apparatus, while the check-valves i, i serve to prevent the establishment of a pressure in the air piping equal to or greater than that of

the atmosphere. The vacuum gauges j, j indicate the extent of the vacuum created by the steam in flowing through the ejectors c, c. The pipe k serves as a by-pass, by which one ejector may temporarily remove air from both air lines.

Mercury-Seal System.-The Trane system of vacuum heating belongs to a class of heating systems in which a partial vacuum created by condensation is maintained by a sealing device that prevents the entrance of air to the piping. In this system the main air pipe is connected to the top of a mercury sealing device. Automatic operation of the apparatus is obtained by using a thermostat in connection with a diaphragm draft regulator on the

boiler. The weight on the diaphragm regulator is adjusted to the requirements of the weather. Immediately on the air reaching the desired temperature, the thermostat closes the necessary electrical circuits for actuating the motor with which the thermostat and lever of the diaphragm regulator are connected. Thereupon the motor closes the drafts, and the thermostat assumes control of the fire, continuing in control until the temperature of the steam becomes too low to supply sufficient heat to maintain the required temperature in the room, when the thermostat transfers the control of the fire to the diaphragm regulator. By the joint action of the thermostat and diaphragm regulator, the steam pressure expels the air and the condensation of steam creates the partial vacuum wherewith the circulation of steam thereafter takes place.

FIG. 4.

d

The mercury trap, shown in Fig. 4, consists of an outer tube a and an inner tube b through which air from the heating system discharges through the mercury c. As long as the air pipe is cold and there is pressure above that of the atmosphere to drive the air from the system, it will be forced through the mercury and out of the orifice d in the side near the base of the seal. The use of

the mercury seal in connection with a low-pressure steam apparatus converts the latter into a vacuum heating system.

Each radiator is provided with a thermostatic air valve, such as is used with the Paul system, and as the air cannot return to the radiators because of the mercury seal, the steam may be maintained at a lower temperature than 2120 owing to the absence of air and the partial vacuum throughout the system due to condensation.

The sizes of air mains required are in. for 500 ft. of radiation, in. for 1,000 to 2,000 ft. of radiation, 1 in. for 3,000 ft. of radiation. The air pipe risers are in. to in., ordinarily.

In order to maintain the vacuum created by condensation, it is necessary that the valves and all joints shall be absolutely tight; and since the ordinary types of radiator valve with packed stuffingbox leak sufficiently to destroy the vacuum, it is desirable to use a special type of steam valve in which entrance of air to the piping system is prevented by substituting a flexible diaphragm for the ordinary stuffingbox.

CENTRAL STATION OR DISTRICT HEATING SYSTEM.

What is known as the district system of steam heating is employed in towns and cities, the steam being distributed from a central station by means of underground mains laid through the streets. A common arrangement of the connections from the street mains to the house pipes is shown in the accompanying illustration. The service pipe a is provided with a valve b inside the basement wall, so that the house system may be shut off when desired. The steam passes through a pressure-reducing valve c and thence into the distributing pipe or house main e. The water that may enter from the service pipe is led away by the drain pipe d. The returns are all connected into the return main f, which is below the water level fixed by the elevation given the steam trap t; thus, in the figure, it is at the line g The hot water from the trap should never be discharged directly into the house drains, because of its destructive effect on the pipes, but should be cooled before escaping to the sewers, by allow