Steam:
Its Generation and Use

marked “gold” and “steel” show the relation to heat and temperature and the melting points of these metals. All the inclined lines would be slightly curved if attention had been paid to the changing specific heat, but the curvature would be small. It is worth noting that, with one or two exceptions, the curves of all substances lie between the vertical and that for water. That is to say, that water has a greater capacity for heat than all other substances except two, hydrogen and bromine. In order to generate steam, then, only two steps are required : First, procure the heat, and, second, transfer it to the water. Now, you have it laid down as an axiom that when a body has been transferred or transformed from one place or state into another, the same work has been clone and the same energy expended, whatever may have been the intermediate steps or conditions, or whatever the apparatus. Therefore, when a given quantity of water at a given temperature has been made into steam at a given temperature, a certain definite work has been done, and a cer- tain amount of energy expended, from whatever the heat may have been obtained, or whatever boiler may have been employed for the purpose. A pound of coal or any other fuel has a defi- nite heat-producing capacity, and is capable of evaporating a definite quantity of water under given conditions. That is the limit beyond which even perfection cannot go, and yet I have known, and doubtless you have heard of, cases where in- ventors have claimed, and so-called engineers have certified to, much higher results. The first step in generating steam is in burning the fuel to the best advantage. A pound of car- bon will generate 14,500 British thermal units (luring combustion into carbonic dioxide, and this will be the same, whatever the temperature or the rapidity at which the combustion may take place. If possible, we might oxidize it at as slow a rate as that with which iron rusts or wood rots in the open air, or we might burn it with the ra- pidity of gunpowder, a ton in a second, yet the total heat generated would be precisely the same. Again, we may keep the temperature down to the lowest point at which combustion can take place, by bringing large bodies of air in contact with it, or otherwise, or we may supply it with just the right quantity of pure oxygen, and burn it at a temperature approaching that of dissocia- tion, and still the heat units given off will be neither more nor less. It follows, therefore, that great latitude in the manner or rapidity of com- bustion may be taken without affecting the quan- tity of heat generated. But in practice it is found that other considera- tions limit this latitude, and that there are certain conditions necessary in order to get the most available heat from a pound of coal. There are three ways, and only three, in which the heat de- veloped by the combustion of coal in a steam boiler furnace may be expended. First, and principally, it should be conveyed to the water in the boiler, and be utilized in the production of steam. To be perfect, a boiler should so utilize all the heat of combustion, but there are no perfect boilers. Second.— A portion of the heat of combustion is conveyed up the chimney in the waste gases. This is in proportion to the weight of the gases, and the difference between their temperature and that of the air and coal before they entered the fire. Third.—Another portion is dissipated by radi- ation from the sides of the furnace. In a stove the heat is all used in these latter two ways, either it goes off through the chimney or is radi- ated into the surrounding space. It is one of the principal problems of boiler engineering to render the amount of heat thus lost as small as possible. The loss from radiation is in proportion to the amount of surface, its nature, its temperature, and the time it is exposed. This loss can be almost entirely eliminated by thick walls and a smooth white or polished surface, but its amount is ordin- arily so small that these extraordinary precau- tions do not pay in practice. It is evident that the temperature of the escap- ing gases cannot be brought below that of the absorbing surfaces, while it may be much greater even to that of the fire. This is supposing that all of the escaping gases have passed through the fire. In case air is allowed to leak into the flues, and mingle with the gases after they have left the heating surfaces, the temperature may be brought down to almost any point above that of the atmosphere, but without any reduction in the amount of heat wasted. It is in this way that those low chimney temperatures are sometimes attained which pass for proof of economy with the unobserving. All surplus air admitted to the fire, or to the gases before they leave the heat- ing surfaces, increases the losses. We are now prepared to see why and how the temperature and the rapidity of combustion in the boiler furnace affect the economy, and that though the amount of heat developed may be the same, the heat available for the generation of steam may be much less with one rate or tem- perature of combustion than another. Assuming that there is no air passing up the chimney other than that which has passed through