Читать книгу Mechanics of the Household - E. S. Keene - Страница 26

—This system of heating is not greatly different from the steam plants already described but it is operated under conditions which do not permit the steam in the boiler to rise beyond a few ounces of pressure. Since the plant is intended to work at a pressure that is scarcely indicated by an ordinary steam gage, it has been termed a vapor system to distinguish it from the pressure systems which employ steam, up to 5 pounds or more to the square inch. The heat is transmitted to the radiators in the same manner as in the pressure systems. The heat of vaporization of steam is somewhat greater at the boiling point of water than at higher pressures, and the lack of pressure, therefore, increases its heating capacity. This is shown in the table, properties of steam, on page 3. The successful operation of such a plant rests in the delivery of the vapor to the radiators at only the slightest pressure and the return of the condensate to the boiler without noise or obstruction to the circulation at the same time ejecting the contained air.

The excellence of the system depends in the greatest measure on good design and the employment of special facilities that allow all water to be discharged from the radiators and returned to the boiler without accumulation at any part of the circulating system. It requires, further, the discharge of the air from the system at atmospheric pressure. The system is, therefore, practically pressureless.

Various systems of vapor heating are sold under the names of their manufacturers. Each possesses special appliances for producing positive circulation that are advocated as features of particular excellence. The vapor system of heating has met with a great deal of favor as a more nearly universal form of heating than either the pressure-steam plant or the hot-water method of heating.

Fig. 31a is a diagram illustrating the C. A. Dunham system of vapor heating. It will be noticed that there are no air vents on the radiators. The air from the radiators is ejected through a special form of trap that is indicated in the drawing. These traps permit the water and air to pass from the radiators but close against the slightly higher temperature of the vapor. This assures the condensation of the vapor in the radiators and excludes it from the return pipes. The water returns to the boiler in much the same manner as in the pressure systems already described but the air escapes through the air eliminator as indicated in the drawing. The system is, therefore, under atmospheric pressure at this point and only a slight amount greater in the boiler.

Fig. 31 a.—Diagram showing the C. A. Dunham Co.’s system of vapor heating.

The water of condensation is returned to the boiler against the vapor pressure, by a force exerted by the column of water in the pipe connecting the air eliminator with the boiler. The main return is placed 24 inches or more above the water line of the boiler. It is the pressure of this column that forces the water into the boiler through the check valve, against the vapor pressure in the boiler.

It might be imagined that the water in the boiler and that in the air-eliminator pipe formed a “U-tube,” the vapor pressure on the water surface in the boiler, and the atmospheric pressure on the water in the eliminator standpipe. The slight vapor pressure in the boiler is counterbalanced by a column of water in the eliminator pipe. It is this condition that fixes a distance of 24 inches from the water line to the return pipe; that is, the force exerted by a column of water 24 inches high is required to send the water into the boiler.

The vapor pressure is controlled by means of the pressurestat, which is an electrified Bourdon spring pressure gage, connected up by simple wiring to the damper motor, which may be any form of damper regulator. In residential work, the pressurestat is so connected with a thermostat, that both pressure and temperature conditions operate and control this damper regulator, which in turn controls the draft and the fire.

The two instruments are so connected that if the pressure mounts to 8 ounces and the pressurestat caused the draft damper to close and the check to open, the thermostat cannot reverse the damper, regardless of the temperature in the room, until the pressure drops below the limiting 8-ounce pressure. Just so long as the pressure is below 8 ounces, the thermostat is the master in the control of the dampers. The minute that the pressure goes up to 8 ounces then the pressurestat takes control.