CAUSE OF CIRCULATION OF HOT WATER. 



265 



diameter comes increased friction also ; 

 so that, with the boiler of the same height, 

 it is impossible to heat a given length of 

 2-inch pipe to the same mean tempera- 

 ture with an equal length of 4-inch pipe. 

 But there is yet another disadvantage to 

 be taken into account on the score of the 

 2-inch pipe, which is this : that as a given 

 boiler is calculated to supply, say 100 feet 

 of radiating surface — if this 100 feet of 

 surface is exhibited in the form of 200 

 feet of 2-inch pipe, instead of 100 feet of 

 4-inch pipe, you have not only the in- 

 creased friction due to the diminished bore 

 to contend with, but also the increased 

 friction due to a double length of pipe ; 

 so that, in order to bring the radiating 

 surface of 200 feet of 2-inch pipe to the 

 same mean temperature as 100 feet of 

 4-inch pipe, you require a greater increase 

 of moving power — that is, a great eleva- 

 tion of the pipes above the boiler." One 

 foot of rise, Mr Rogers says, is sufficient 

 in all ordinary cases, " even with 2-inch 

 pipes, but the higher the better ; for the 

 higher the pipes are above the boiler, the 

 greater will be the economy of fuel, inas- 

 much as the heat will be carried off more 

 rapidly from the boiler. It is a possible 

 condition that small pipes might be pro- 

 perly placed, and perfectly free from 

 air or other impediment, and yet that 

 the water in the boiler should boil to 

 waste in steam, although the mean tem- 

 perature of the pipes fall far short of 200° 

 — simply because in a very long length 

 of pipe, if it be laid level with, or only 

 just above, the top of the boiler, the 

 moving force would not be sufficient to 

 overcome the friction." 



It was some time after the application 

 of hot water in heating hothouses, before 

 close-topped boilers, and carrying the 

 pipes to any extent of altitude, were prac- 

 tised. The advantages of departing from 

 the original plan are many, such as an in- 

 crease of circulation, adapting this mode 

 of heating to situation and circumstances 

 where the horizontal method could not 

 be applied, &c. It is not easy to say to 

 what altitude water may be raised by this 

 means, but certain it is that it can be 

 elevated to any extent directly connected 

 with horticultural buildings, provided the 

 boiler be made sufficiently strong to resist 

 the pressure. 



According to the calculations of Hood, 



VOL. I. 



"the pressure of water on each square 

 inch of surface increases at the rate of 

 about ^ lb. for every foot of perpendi- 

 cular height : if the height from the 

 bottom of the boiler to the top of the pipe 

 be 6 feet, the pressure on the bottom will 

 be 3 lb. on every square inch of surface ; 

 but, if the boiler be 2 feet high, the pres- 

 sure on the top— which will be a pressure 

 upwards — will be only 2 lb. on every 

 square inch of surface, because it will 

 only have 4 perpendicular feet of water 

 above it. If the height of the pipe be 

 increased to 28 feet, and the depth of the 

 boiler be 2 feet as before, making 30 feet 

 together, the pressure will be 15 lb. on 

 each square inch of the bottom, and 14 lb. 

 on each square inch of the top, and an 

 average pressure of 14^ lb. on each square 

 inch of the sides of the boiler. Suppose, 

 now, a boiler to be 3 feet long, 2 feet 

 wide, and 2 feet deep, with a pipe 28 feet 

 high from the top of the boiler, when the 

 apparatus is filled with water there will 

 be a pressure on the boiler of 66,816 lb., 

 or very nearly 30 tons." 



It is seldom such a pressure as this 

 will be applied to garden purposes, if we 

 except cases where hot water may be 

 supplied from a boiler to heat baths, &c, 

 in higher parts of attached buildings, or 

 in the case of a whole garden being- 

 covered with glass, (a thing by no means 

 in the future improbable,) and where 

 part of the columns which would be re- 

 quired for the support of the roof might 

 be used as pipes for radiating heat. The 

 expansive force of water is so trifling, that 

 under no circumstance could it burst an 

 ordinary well- constructed boiler; the only 

 effect would be a slight leakage : but the 

 pressure upon the boiler is quite a differ- 

 ent thing. The amount of the friction of 

 water passing through pipes has never 

 been accurately determined ; but it must, 

 of course, be greater in pipes of a rough 

 or uneven inner surface than in such as 

 are smooth and even. Hence glass pipes 

 would have less friction than any others. 

 Difficulties frequently present themselves 

 when it becomes necessary, from local cir- 

 cumstances, to carry hot-water pipes to a 

 higher or lower level than that of the 

 horizontal line of the main pipes and 

 boiler ; but this latter should be avoided 

 as much as possible. If 16 inches be 

 taken as the minimum distance at which 



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