STEAM INSTALLATION 193 



turn pipes according to directions given in 

 a following paragraph. This is known as 

 the two-pipe system. The return pipe en- 

 ters the boiler below the surface of the water. 

 The coils should have a fall toward the boiler 

 of about i inch to 20 feet. It is not wise 

 to use the straight coils commonly used for 

 hot water in steam heating as they do not 

 allow for the unequal expansion of the pipes 

 when the steam is turned on quickly. In 

 steam heating special form of coils are com- 

 monly used among which are the corner coil 

 and mortise coil. 



Size of Supply and Return Pipes. Theo- 

 retically, the size of the flow and return pipes 

 in steam heating may be much smaller than 

 in hot- water heating. This is especially true 

 of the return pipe, since the water which it 

 carries occupies only 0.017 of the space oc- 

 cupied by the steam from which it is con- 

 densed. In practice, however, the flow or 

 supply pipe for steam is made nearly as large 

 as for hot water and the return pipe only 

 slightly smaller. 



The following table shows the flow of 

 steam in pipes of different sizes at a pressure 

 at the boiler of approximately five pounds. 



