Greenhouse Heating 



apparatus. As is well known, these air valves are more or less compli- 

 cated, requiring considerable attention, and the greenhouse operator never 

 feels sure that they are operating properly until he goes around and exam- 

 ines and tests them. 



In the vacuum system the operator would not have this trouble, for 

 the purpose of one of the pumps, which we call the vacuum pump, is to 

 exhaust all air from the apparatus, which must be effectively done. The 

 operator having accomplished this before turning on the steam, knows that 

 the pressure of his boiler must fill the most remote part of the radiation 

 surface. 



There is another advantage in this system which is most commend- 

 able. Steam at a very low pressure, a mere vapor, in fact, without 

 pressure, can be admitted into the radiation pipes and the quantity throt- 

 tled at the valve, a feature which makes the heat as controllable as hot 

 water, as in either case the controlling power of the valve is practically 

 the same. The water of condensation is returned in practically the same 

 manner as described in the positive system. In fact, the vacuum system 

 may be termed a positive system without air valves. There are numerous 

 modifications of the vacuum system. A description of them will, how- 

 ever, have but little interest for florists. This method of steam heating 

 probably more efficiently meets the requirements of large florists' estab- 

 lishments than any other. 



The Single-Pipe Method 



The so-called single-pipe system is largely used in the heating of 

 buildings. It is not very popular among florists. We think it possesses 

 so few features commending it to the florist that it is not worth while to 

 devote space to a description of it here. 



Types of Steam Boilers 



The type of boiler which is adapted for steam use should differ in 

 construction from the hot-water boiler ; that is, a boiler which may give 

 excellent results in hot-water practice is not necessarily as efficient in steam 

 practice. The circulation of a hot-water boiler is entirely, or should be 

 entirely, a continuous flow from the bottom toward the top. There should 

 be no return current in the interior of the hot-water boiler. The steam 

 boiler has a circulation almost entirely within itself. The water should 

 be in constant motion, rising upward in the hottest part and returning 

 down the coolest portions. This gives the steam bubbles a chance to escape 



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