163. Vacuum Steam Heating.—In steam heating, air valves (Fig. 145) are placed on the radiators to allow the air they contain to escape when the steam is turned on. When all the air is driven out the valve closes. Automatic vacuum valves (Fig. 146) are sometimes used. When the fire is low and there is no steam pressure in the radiators the pressure of the air closes the valve, making a partial vacuum inside. The boiling point of water falls as the pressure upon it is reduced. As water will not boil under ordinary atmospheric pressure until its temperature is 100°C. (212°F.), it follows that by the use of vacuum systems, often called vapor systems, of steam heating, water will be giving off hot vapor even after the fire has been banked for hours. This results in a considerable saving of fuel.
Fig. 147.—Plenum hot-blast system with temperature regulation.
164. The Plenum System of Heating.—In the plenum system of heating (see Fig. 147) fresh air is drawn through a window from outdoors and goes first through tempering coils where the temperature is raised to about 70°. The fan then forces some of the air through heating coils, where it is reheated and raised to a much higher temperature, depending upon the weather conditions. Both the hot and tempered air are kept under pressure by the fan in the plenum room and are forced from this room through galvanized iron ducts to the various rooms to be heated. The foul air is forced out of the room through vent ducts which lead to the attic where it escapes through ventilators in the roof.
Fig. 148.—A thermostat. (Johnson System.)
A thermostat is placed in the tempered-air part of the plenum room to maintain the proper temperature of the tempered air. This thermostat operates the by-pass damper under the tempering coils, and sometimes the valves on the coils. The mixing dampers at the base of the galvanized-iron ducts are controlled by their respective room thermostats. Attic-vent, fresh-air, and return-air dampers are under pneumatic switch control. A humidifier can be provided readily for this system. This system of heating is designed particularly for school houses where adequate ventilation is a necessity.
165. The Thermostat.—One of the many examples of the expansion of metals is shown in one form of the thermostat (Fig. 148) in which two pieces of different metals and of unequal rates of expansion, as brass and iron, are securely fastened together.
The thermostatic strip T moving inward and outward, as affected by the room temperature, varies the amount of air which can escape through the small port C. When the port C is completely closed (Fig. 148a) the full air pressure collects on the diaphragm B which forces down the main valve, letting the compressed air from the main pass through the chamber D into chamber E as the valve is forced off its seat. The air from chamber E then passes into the branch to operate the damper.
When port C is fully open (Fig. 148b) the air pressure on diaphragm B is relieved, the back pressure in E lifts up the diaphragm and the air from the branch escapes out through the hollow stem of the main valve, operating the damper in the opposite direction from that when C is closed.