GASEOUS FUEL 4 oi 



dependent on the order of the reaction, and combustion occurs equally throughout the 

 system as a whole, and secondly, heterogeneous combustion, in which the combustion 

 takes place in layers immediately in contact with a hot surface. This second condition 

 constitutes what is known as " surface combustion," and is governed by the rate of 

 combustion, the diffusion factors of the components of the gaseous mixture and the 

 products of combustion, the effect of the " activity " of the refractory surface, the physi- 

 cal texture of the surface, and the retarding effect of films of inert gas. 



The " activity " of a given surface can be varied by special treatment within wide 

 limits; but the nature of the causes of the variation is still somewhat obscure. For 

 instance, using a mixture of hydrogen or carbon monoxide with oxygen in conjunction 

 with a non-oxidisable metal or non-reducible oxide, if the surface be previously treated 

 by contact with the combustible gas, the "activity" of the surface is considerably 

 stimulated, whereas if treated with oxygen it is diminished. The activity of a surface 

 also increases rapidly with rise of temperature, the difference in activity possessed by 

 various substances being reduced until at bright incandescence it disappears. 



Diaphragm Process. If an explosive gaseous mixture be forced under pressure through a 

 porous refractory incandescent body, combustion in a greatly accelerated form takes place 

 within the pores, and the heat developed by this combustion keeps the surface in a state of 

 incandescence without the formation of flame. 



This principle may be employed in two ways. The first is to fit a slab of porous refractory 

 material in the front of a fuel chamber into which the gas and air, in proportions necessary 

 to give complete combustion, are forced under the pressure necessary to drive them through 

 the porous plate; the mixture is then ignited on the outer surface, which rapidly becomes 

 heated, and the mixture burns within the pores of the slab and close to its exterior surface, 

 which being raised to incandescence emits great radiant heat. The degree of porosity and 

 size of the pores of the slabs can be arranged to 'suit the quality of the gas, fine pores being 

 used with rich gases, whilst a coarser grade is necessary with gases of low calorific power, 

 the pressure also being regulated to ensure the rate of flow of the gas being sufficiently high 

 to prevent any risk of back-firing or explosion. This method of applying the principle is 

 generally known as the Diaphragm process, and as, being flameless, the slab can be fixed in 

 any position, it can be used to throw the radiant heat on the surface of evaporating liquids, 

 for the concentration of acids, and for many forms of industrial and domestic heating. 



Granular Bed Process. The second method, known as the Granular bed process, consists 

 in making a granular mass of the refractory material, and blowing the mixed gases on to 

 it or through it under pressure, so that the combustion takes place on the surface of the 

 granules, and a temperature of 2,000 C. can be obtained. The granules can be packed into 

 tubes directly immersed in water for steam raising, into the space round crucibles in processes 

 for the melting of metals, or formed into a bed for furnace work. In a special form of boiler 

 designed for this process an efficiency of 95 per cent has been attained, that steam can be 

 raised from cold in 20 minutes, and that the normal evaporation is 20 Ibs. per square foot of 

 combustion surface per hour, which can be increased to 30 with a slight loss in efficiency. 



Gas Stoves. -^-Ever since Bunsen in the middle of the last century gave to the world his 

 atmospheric burner, the use of gas for heating and power has progressed more and more, 

 until nearly as much gas is now used for these purposes as for illumination. 



The earlier forms of gas stove had many defects which created a strong prejudice in the 

 minds of the public against them, more especially as custom and comfort had created a 

 firm affection for the open coal fire. In the modern gas stoves however due regard has been 

 paid to overcoming the drawbacks found with the older forms, and their improved efficiency 

 together with the absence of smell, &c, have resulted in a steadily growing popularity. 



The greatest objection raised against the use of gas for domestic heating is that of cost. 

 If the open fire and the gas stove be compared on the price of fuel and the number of heat 

 units present, the use of gas is shown to be about five times as costly as coal but if the 

 question of efficiency be taken into consideration, the difference between the two methods 

 of warming is found to be but slight. With a coal fire in an open grate only about one fifth 

 of the heat units are utilised in warming a room, whereas with a modern gas stove nearly 

 four times as great an efficiency can be obtained. Moreover, in favour of gas one can place 

 cleanliness, absence of dust and labour, and convenience of lighting and extinction. 



Heating by radiation is the only true and hygienic method of warming a room, as in this 

 way the temperature of the air is not directly raised, but the walls, furniture and other solid 

 bodies in the room absorb the radiated heat and in turn slowly give off a portion of this heat 

 to the air around them ; consequently the walls &c are at a higher temperature than the 

 air. If however the room be heated by hot water pipes, steam or any other such system, 

 the temperature is raised by convection: the air around the pipes or radiator is heated, 

 expands and sets up convection currents until the whole mass of the air is at the one tempera- 



