198 GREENHOUSES 



special devices to be described later must 

 be employed to return the water of conden- 

 sation. As with hot water, two or more 

 boilers should be provided, rather than one 

 large one, to allow for repairs in case of ac- 

 cident and for use in severe weather to avoid 

 the necessity of forcing. 



Steam Pumps and Traps. As suggested 

 in the preceding paragraph, it is sometimes 

 impossible or inconvenient to place the boil- 

 er below the level of the heating coils. This 

 is especially true in large establishments, re- 

 quiring large boilers using large quantities 

 of fuel. In order to return the water of con- 

 densation in such cases steam return traps 

 and steam pumps are used. Their use is al- 

 so necessary where a higher pressure is car- 

 ried at the boiler than in the coils. 



The return trap is a contrivance which is 

 automatic in its action, and which overcomes 

 the back pressure from the boiler by an in- 

 genious method of equalizing the difference 

 in pressure between the boiler and the coils. 

 Being automatic in its action and requiring 

 but little attention it has been quite gener- 

 ally used. Steam pumps, on the other hand, 

 require considerable attention, though they 



