114 THE PRINCIPLES OF FLORICULTURE 



head in the same way as previously described in hot-water 

 overhead mains. This system is not as objectionable for 

 steam as it is for hot-water for the reason that there 

 is little loss of pressure in the length of the coils, and 

 consequently very little change of temperature in the 

 length. 



" The steam mains may be run with the return mains 

 in a trench, or the steam main may be overhead. It is 

 only necessary to keep the steam main free from water." 



87. Low pressure steam heating with steam return 

 trap. " In many cases it is not practicable, perhaps 

 not possible, and maybe not desirable to excavate a pit 

 of sufficient depth for the boiler, so that the water of con- 

 densation may be returned by gravity. In such cases, 

 the water is returned to the boiler by means of a return 

 steam trap. There are a number of different designs of 

 return traps, but they all work on about the same prin- 

 ciples. They are usually placed at a height of about 

 three feet above the boiler, and connected to the boiler 

 by two pipes, a steam pipe to the top of the boiler, and 

 by a water feed pipe to the bottom of the boiler. They 

 are also connected by a vertical pipe to a receiver, into 

 which the returns drain. The operation of these return 

 traps is, briefly, as follows : the condensation of steam in 

 the vessel of the trap causes a partial vacuum to be formed, 

 and as there is some pressure against the water in the 

 receiver, the water is forced up into the vessel which is 

 so fixed as to tilt when it has received its load. This 

 tilting opens a valve, which turns the steam into the trap, 

 thus putting the full boiler pressure of the steam upon the 

 bucketful of water, and closing the check-valve in the 

 pipe connected with the receiver, so that the whole charge 

 of water in the return trap falls into the boiler by gravity." 



