104 GENERAL SCIENCE 



At the floor, just under the place where the hot air enters 

 the rooms, is another opening for the cool and impure 

 air to leave. It goes down through impure air ducts 

 to another large fan, which forces it to the outside of the 

 building, on the side opposite to that where the fresh air 

 enters. The illustration shows the flow of air in one of 

 the rooms. The air comes in at I and goes out at 0. 



74. Steam Heating System. The steam system of 

 heating is used extensively in large buildings and private 

 houses. The apparatus consists of a steam boiler or 

 boilers in the basement. The size and number of boilers 

 depend upon the size of the building to be heated. The 

 radiators in the rooms and halls contain a heating sur- 

 face which is about one forty-fifth of the numerical 

 cubic capacity of the rooms. The steam passes from the 

 boilers to the radiators through pipes, and the condensed 

 steam or water flows from the radiators through another 

 set of pipes which lead to the bottom of the boilers. 



The fire in the boiler changes the water to steam. 

 The pressure of the steam, which is never more than a 

 few pounds to the square inch, forces the steam from 

 the boiler through the pipes to the radiators. The steam 

 enters the radiators at a temperature of about 100 C. 

 It condenses and the water flows from the radiator at 

 about the same temperature. If this is true how does 

 the room become warm from the radiator? After the 

 steam is condensed it flows back to the boiler to be 

 changed into steam again. So the water keeps up a con- 

 tinuous motion, flowing from the boiler to the radiators 

 in the form of steam, and back to the boiler in the form 

 of liquid. Is this conduction, convection, or radiation? 



The flow of steam into the radiator is controlled by two 

 valves on the radiator: one large valve lets the steam 



