At this point let us put back the clock a little that we may understand why tallness in chimneys is much less in vogue for steam plants than formerly, and why this change is found to be well worth while. A device at least two centuries old is the smoke-jack, of which a specimen lingers here and there in the museums and curiosity shops of England. The rotary motion of its vanes, due to the upward draft from a kitchen fire, was employed to turn a joint of meat as it roasted in front of the coals. To-day the successors of this primitive heat-mill are the cardboard or mica toys which, fastened to a stove-pipe, or close to a lamp chimney, set at work a carpenter with his saw, a laundress with her sad-iron, and so on. These playthings show us the simplest way in which heat can yield motive power; because simplest it prevails almost universally, and yet it is wasteful in the extreme. Nobody for a moment would think of putting a wheel like that of a smoke-jack in a chimney so that the rising stream of hot gases might drive a sewing-machine or a churn, and yet for a task just as mechanical, namely, the pushing upward a chimney current itself, the heating that current to an extreme temperature is to-day the usual plan. Under good design the gases of combustion are obliged to do all the work that can be squeezed out of them; then and only then they are sent into the chimney. What if their temperature be so low, comparatively, that their rise in the stack, if left to themselves, is slow as compared with the rise in another stack of gases 300° hotter? One hundredth part, or even less, of the saved heat when applied through an engine to a fan will ensure as quick a breeze through the grate-bars as if the chimney gases were wastefully hot, and this while the chimney is but one eighth to one fourth as tall as an old-fashioned structure. This is the reason why mechanical draft is now adopted far and wide in factories, mills and power-houses. The advantages which follow are manifold: the plant is rendered independent of wind and weather, inferior fuels are thoroughly and quickly consumed, at times of uncommon demand a fire can be easily forced so as to increase the duty of the boilers. To-day in the best practice the feed water for the boilers is heated by the furnace gases just before they enter the stack; the piping for this purpose, formed into coils known as economizers, checks the chimney draft. This checking is readily overcome by mechanical draft, leaving the engineer a considerable net gain as fan and economizer are united. One incidental advantage in modern plants of sound design, and good management, is that they send forth but little smoke or none at all. With thorough combustion no smoke whatever leaves the stack.
Smoke-jack.
Automatic Stoking.
The avoidance of smoke is promoted by the use of well designed mechanical stokers: two of the best are the Roney and the Jones models. The Jones apparatus forces its fuel into the fire from beneath, so that its gases, passing upward through blazing coal, are thoroughly consumed.
Boilers.
In large plants the boilers are usually of the water-tube variety, working at high pressures which may be increased at need. Mr. Walter B. Snow says:[38]—“Until the recent past the steam generator or boiler and the manner of its operation received far less attention than they deserved. Although under the best conditions over 80 per cent. of the full calorific value of the fuel may be utilized in the production of steam, this high standard is seldom reached in ordinary practice. Mr. J. C. Hoadley showed an efficiency of nearly 88 per cent. in his tests of a warm-blast steam-boiler furnace with air-heaters and mechanical draft, while Mr. W. H. Bryan has reported eighty-six tests conducted under common conditions with ordinary fuel, upon boilers of various types, which indicate an average efficiency of only 58 per cent., and have a range between a minimum of 34.6 per cent. obtained with a small vertical boiler, and a maximum of 81.32 per cent. with a water-tube boiler of improved setting. The possibilities of increased economy in ordinary boiler practice are thus clearly evident.”
[38] In his “Steam Boiler Practice.” New York, John Wiley & Sons, 1904. $3.00.
POWER HOUSE, INTERBOROUGH RAPID TRANSIT CO.
11th Avenue and 59th Street, New York.