40 



FURNACES 



FURNES 



Furnaces. Furnaces perform one of the most 

 important of functions, and on them largely 

 depend the power and economical efficiency of the 

 steam-engine. Great care and skill, combined 

 with an intimate knowledge of the laws which 

 regulate combustion, must be exercised in the 

 designing and construction of furnaces for steam- 

 boilers. They may be considered as divided into 

 three parts. ( 1 ) The fire-chamber, where combus- 

 tion begins, the fuel is split up into its constituent 

 gases, and the remainder consumed. (2) The com- 

 bustion-chamber, where combustion of the gases 

 is completed, and the heat applied. (3) The 

 arrangements for the supply of air, and its mixture 

 with the heated gases. In the combustion of fuel 

 there are two leading conditions to be observed 

 viz. to obtain as complete combustion of the fue 1 

 with as little waste of heat as possible, and to 

 apply as much of the heat as is practicable to 

 those parts of the boiler where evaporation will 

 be greatest. These two conditions are somewhat 

 difficult to realise in a furnace, and, while the best 

 method of applying heat is well known, the portion 

 available out of a given quantity bears but a very 

 small proportion to what is lost or wasted under 

 the most favourable circumstances. The supply of 

 air is a most important factor ; too much has the 

 effect of chilling and diluting the gases, reducing 

 the temperature of the furnace, and diminishing 

 the force of the draught ; while too little causes 

 the gases to escape unconsiimed, and results in 

 great waste. The proper supply of air is therefore 

 a very difficult matter to accomplish, especially 

 when there is an ever-varying demand for it, as is 

 the case with solid fuel. Liquid or gaseous fuel 

 does not present the same variation. It has 



been found that the best effect is obtained from 

 furnaces with forced draught i.e. sending a steady 

 flow of air under pressure through the incandescent 

 fuel by means of a fan or other mechanical contriv- 

 ance. With the ordinary chimney draught, the 

 heated products of combustion must be allowed to 

 escape at a high temperature, say 600, and at a 

 speed of about 30 feet per second, in order to main- 

 tain an effective draught. With artificial draught, 

 the heat can be retained in the furnace a much 

 longer time, and a balance established between the 

 pressure of the atmosphere and the heat inside. 

 Also the waste heat, instead of rushing away at 

 great velocity, may be made to do work in heating 

 the air for the furnace or the feed-water for the 

 boiler ; and is thus allowed to escape only when 

 deprived of its power of doing useful work. The 

 difference in efficiency is said to exceed 25 per cent. 

 in favour of artificial draught. 



A good furnace ought to be able to burn a large 

 quantity of coal on a small area of fire-grate. The 

 amount of fuel consumed in different kinds of 



furnaces varies greatly, and shows the power that 

 forced draught gives. A land-boiler furnace burns 

 about 14 Ib. of coal, a marine furnace 16 to 24 lb., 

 and a locomotive, with the draught increased by 

 the escaping steam, from 80 to 200 lb. on the square 

 foot of fire-grate in one hour. The great objects to 

 be desired in furnace management are the exact 

 apportionment of air to the varying wants of the 

 fuel, so as to convert all the carbon to carbonic acid 

 and the hydrogen to water, an equal and high 

 temperature of the furnace, and that the grate- 

 bars be always covered with fuel. Granted these 

 conditions, and we obtain the best effect from 

 the furnace, without smoke. Smoke may be 

 caused by too much as well as too little 

 air, especially with a low temperature in the 

 furnace. Too much air reduces the heat of the 

 furnace and gases below the temperature for 

 combustion, and so smoke is formed. The same 

 result comes from a deficient supply of air to take 

 up all the carbon, a portion of which escapes as 

 smoke. At the same 

 time, with a high tem- 

 perature in the furnace, 

 insufficient air does not 

 cause smoke ; carbonic 

 oxide instead of car- 

 bonic acid is formed, 

 and one-half of the heat 

 is wasted. In practice, 

 deficient boiler power 

 is a fertile cause of 

 smoke, from having to 

 urge the fire beyond 

 its capacity. Self- 

 feeding furnaces are 

 more economical and 

 efficient than those 

 which are fed by hand. 

 Fig. 1 shows one of A- 

 the most successful. 

 A large hopper fixed 

 in front of the boiler 

 contains a supply 

 of fuel for a stated 



Fig. 2. Plan of Furnace. 



period, and requires no 

 further attendance until its contents are consumed. 



There is an opening at 



the level of the grate, 

 through which the coals 

 are thrown on to the 

 bars. It is claimed for 

 this self-feeding furnace 

 that it more nearly ap- 

 proaches in regularity 

 firing by hand than any 

 other in use, but there 

 is no smoke when once 

 in operation, and a sav- 

 ing of 10 per cent, in 

 fuel. Figs. 2 and 3 



Fig. 3. 

 Section through AB, fig. 2. 



show the best arrangement of flues. The flame 

 on leaving the grate passes through the central 

 tube, descends and returns along the bottom to 

 the front, where it splits and passes on both sides 

 to the chimney. For Blast-f urnaces, c. , see GLASS, 

 IRON, COPPER, LEAD, STEEL, REVERBERATORY 

 FURNACE, ELECTRIC FURNACE ; also BOILER, 

 HEAT, OVEN, POTTERY, STEAM-ENGINE. 



FunieailX Islands, a group of barren islands 

 in Bass Strait, between Australia and Tasmania, 

 Flinders Island being the largest. About 300- 

 people of mixed breed capture seals and sea-birds. 

 Tobias Furneaux, one of Cook's captains, discovered 

 the group in 1 773. 



Flirnes, a town of Belgium, in West Flanders, 

 16 miles by rail E. by N. of Dunkirk, has tanneries 

 and linen manufactures. Pop. (1890)5604. 



