336 VENTILATION 



The controlling thermostat, (Fig. 238) consists of an iron 

 saucer-shaped capsule, L, with its edges turned in. Across 

 the face of this capsule is a stout rubber diaphragm. A smaller 

 saucer-shaped metal plate, P, presses against this rubber 

 diaphragm. 



When the compressed air enters the capsule it forces the 

 diaphragm and plate, P, outward as shown by the dotted lines. 

 A system of levers transmits this motion to the vanes V and 

 V, forcing them more nearly into a vertical position. Thus 

 the flow of heated air is lessened and the flow of tempered air 

 is increased. 



When the schoolroom is too cool the wall thermostat, T, 

 closes the tube K, thus preventing the compressed air from 

 passing to the controlling thermostat, S. The pressure upon 

 the diaphragm and the plate, P, lessens and the vanes tend to 

 fall back into a more nearly horizontal position. When 

 the vanes are in a horizontal position more heated air and less 

 tempered air passes to the room in which the thermostat, T, is 

 located. 



PROBLEM 



If the plate, P, Fig. 238, is 4 in. in diameter and the pressure gauge 

 shows an air pressure of 15 Ib. per sq. in., with what force will the 

 plate, P, be forced outward? 



369. The Suction System of Heating and Ventilating. In 



the suction system the fan, or blower, is usually placed in the 

 attic of the building. The purpose is to suck the air out of the 

 building (see Art. 374). The air within the room is, then, 

 under less than 1 atmosphere of pressure. The suction system 

 is not often used alone ; it is often used in connection with the 

 plenum system to secure more perfect ventilation of labora- 

 tories, toilet rooms, kitchens, or other rooms. 



III. DUST AND ITS DANGERS 



370. Live Dust and Dead Dust. All dust may be classified 

 as LIVE DUST or DEAD DUST. While all dust looks alike to the 



