Howden System. The Howden system of forced draft with closed ash-pit has been used to a considerable extent in both mercantile and naval service. The air supplied to the ash-pit is first heated by passing through a heater in the uptake. Waste gases pass through tubes; and the air, passing among them before entering the furnace, is heated to a high temperature. A consumption of 60 lbs. of coal per square foot of grate is easily obtained with this system; and care must be taken that the fire is not forced too hard, as there is more danger of burning out the grate than if the air-supply is not heated.
Ellis and Eaves System. Heating the air does not necessitate its being forced into the closed ash-pit, for it is quite feasible to heat the air in connection with d raft induced by an exhaust fan at the base of the funnel. Such is the Ellis and Eaves system. This system was first tried in the boiler shops at the works of the John Brown Company, in Sheffield, England, and was later adopted on many vessels. The Ellis and Eaves heater is fixed on top of the boilers, and is divided into two parts separated at the front by a smoke-box and at the back by a funnel. The hot gases, therefore—which pass outside the tubes— have to take a somewhat circuitous course; while the passage of the air to be heated, on the contrary, takes a direct course. The distribution of air to the ash-pit is similar to that of the Howden system.
The advantages of this system lie in the general convenience of the induced draft and the absence of jets of hot air shooting out into the boiler-room. The draft need not be shut off when stoking the fires, unless it is desired to prevent the inrush of air already referred to under the general discussion of "closed stoke-holds." The air in the fire-room being of a relatively higher temperature than would obtain with closed stoke-holds, and the quantity being much less, this objection has no great weight. With the Howden system it is necessary that the doors should be tight; otherwise hot air will be blown out into the fire-room. With this system a few leaks are of no consequence, and the fire-room will be somewhat cooler than with the Howden System. The objections to the Ellis and Eaves system are the se inherent in any system of draft induced by a fan—that is to say,, a poor efficiency of the fan working in heated gases, and lost work in drawing air through tortuous passages.
Steam Jets. Steam jets may be used for inducing a draft. They may be placed either in the smokestack, or below or above the grate; but in general they are not so economical as a fan used for the same purpose. In locomotives and fire-engines, where the exhaust steam is at high pressure, an intense draft may be induced by exhausting this up the smokestack. In both these cases, the saving of weight due to the use of a small boiler running at high tension, is of greater practical importance than the economy of fuel; and for such purposes this arrangement is entirely satisfactory.
A steam jet may be used directly in the furnace, either above or below the grate. The steam enters through a small pipe, and expands through a nozzle surrounded by an annular, funnel-shaped tube. The escape of steam from the inner nozzle draws in a large volume of air through the outer tube, and produces an intense draft. If steam is blown into the ash-pit in this manner, it forms a sort of producer gas by mingling with the incandescent fuel, and materially aids in the combustion of cheap and apparently worthless fuel. Almost as poor fuel can be successfully used with this arrangement as can be used in the grates of the down-draft furnaces. Such arrangements have given excellent satisfaction, and the production of smoke is materially lessened.
Some tests made in the French Navy some years ago, showed that, with the use of the steam jet above the grate, the coal consumption per square foot of grate area could readily be doubled; but this result would be attained at the expense of fuel economy; for, while with natural draft one pound of coal produced approximately eight pounds of steam which could be used by the engine, with a steam jet less than 6-3/4 pounds of steam per pound of coal was available for like purposes. The total evaporation per pound of fuel was approximately the same in each case, the difference being the quantity of steam used in the jet. If a steam jet is used on board ship, it consumes a considerable amount of fresh water, which must be replaced by evaporators, or by the use of salt water, which is decidedly objectionable.