Engines furnished with Mr. Hall's apparatus have not yet, so far as I am informed, been tried with reference to the power exerted by the consumption of a given quantity of fuel. The mere fact of a good vacuum being sustained in the condenser cannot by itself be regarded as a conclusive proof of the efficiency of the engine, without the water or air introduced by a condensing jet. Mr. Hall, nevertheless, uses as large an air-pump as that of an ordinary condensing engine, and recommends even a larger one. For what purpose, it may be asked, is such an appendage introduced? If there be nothing to be removed but the condensed steam, a very small pump ought to be sufficient. It is not wonderful that a good vacuum is sustained in the condenser, if the power expended on the air-pump is employed in pumping away uncondensed steam. Such a contrivance would be merely a deception, giving an apparent but no real advantage to the engine.
Having mentioned these advantages, which are said to arise from Mr. Hall's condenser, it is right to state that it is in fact a reproduction of an early invention of Mr. Watt. There is in possession of James Watt, esquire, a drawing of a condenser laid before parliament in 1776, in which the same method of condensing without a jet is proposed. Mr. Watt, however, finding that he could not procure by that means so sudden or so perfect a vacuum as by injection, abandoned it. I believe he also found that the tubes of the condenser became furred with a deposite which impeded the process of condensation. It would seem, however, that Mr. Hall has found means to obviate these effects. It is right to add, that Mr. Hall, in his specification, distinctly disclaims all claim to the method of condensing by tubes without jet.
There is another part of Mr. Hall's contrivance which merits notice. In all engines, a considerable quantity of steam is allowed to escape from the safety valve. Whenever the vessel stops, the steam, which would otherwise be taken from the boiler by the cylinders, passes out through this valve into the atmosphere. Also, whenever the cylinders work at under-power, and do not consume the steam as fast as it is produced by the boiler, the surplus steam escapes through the valve. Now, according to the principle of Mr. Hall's method, it is necessary to save the water which thus escapes in vapour, since otherwise the pure water of the boiler would be more rapidly wasted. Mr. Hall accordingly places a safety valve of peculiar construction in communication with a tube which leads to the condenser, so that whenever, either by stopping the engine or diminishing its working power, steam accumulates in the boiler, its increased pressure opens the safety valve, and it passes through this pipe to the condenser, where it is reconverted into water, and pumped off by the air-pump into the cistern from which the boiler is fed.
The attainment of an object so advantageous as to extend the powers of steam navigation, and to render the performance of voyages of any length practicable, so far as the efficiency of the machinery is concerned, has naturally stimulated the inventive genius of the country. The preservation of the boiler by the prevention of deposite and incrustation is an object of paramount importance; and its attainment necessarily involves, to a certain degree, another condition on which the extension of steam voyages must depend, viz. the economy of fuel. In proportion as the economy of fuel is increased, in the same proportion will the limit to which steam navigation may be carried be extended.
(116.) A patent has been obtained by Mr. Thomas Howard of London for a form of engine possessing much novelty and ingenuity, and having pretensions to the attainment of a very extraordinary economy of fuel, in addition to those advantages which have been already explained as attending Mr. Hall's engines. In these engines, as in Mr. Hall's, the steam is constantly reproduced from the same water, so that pure or distilled water may be used; but Mr. Howard dispenses with the use of a boiler altogether. The steam also with which he works is in a state essentially different from the steam used in ordinary engines. In these, the vapour is raised directly from the water in a boiling state, and it contains as much water as it is capable of holding at its temperature. Thus, at the temperature of 212°, a cubic foot of steam used in common engines will contain about a cubic inch of water; but in the contrivance of Mr. Howard, a considerable quantity of heat is imparted to the steam before it passes into the cylinder in addition to what is necessary to maintain it in the vaporous form.
A quantity of mercury is placed in a shallow wrought-iron vessel over a coke fire, by which it is maintained at the temperature of from 400° to 500°. The surface exposed to the fire is three fourths of a square foot for each horse-power. The upper surface of the mercury is covered by a very thin plate of iron, which rests in contact with it, and which is so contrived as to present about four times as much surface as that exposed beneath to the fire. Adjacent to this a vessel of water is placed, kept heated nearly to the boiling-point, which communicates by a nozzle and valve with the chamber or vessel immediately above the mercury. At intervals corresponding to the motion of the piston, a small quantity of water is injected from this vessel, and thrown upon the plate of iron which rests upon the hot mercury: from this it receives the heat necessary not only to convert it into steam, but to expand that steam, and raise it to a temperature above the temperature it would receive if raised in immediate contact with water. In fact, the steam thus produced will have a temperature not corresponding to its pressure, but considerably above that point, and it will therefore be in circumstances under which it will part with more or less of its heat, and allow its temperature to be lowered without being even partially condensed, whereas steam used in the ordinary steam engines must be more or less condensed by the slightest diminution of its temperature. The quantity of liquid injected into the steam chamber must be regulated by the power at which the engine is intended to work. The fire is supplied with air by a blowing machine, which is subject to exact regulation. The steam, produced in the manner already explained, passes into a chamber which surrounds the working cylinder; and this chamber itself is enclosed by another space, through which the air from the furnace must pass before it reaches the flue. In this way it imparts its redundant heat to the steam which is about to work the cylinder, and raises it to a temperature of about 400°; the pressure, however, not exceeding 25 lbs. per square inch. The arrangement of valves for the admission of the steam to the cylinder is such as to cause the steam to act expansively.
The vacuum on the opposite side of the piston is maintained by condensation in the following manner:—The condenser is a copper vessel placed in a cistern constantly supplied with cold water, and the steam flows to it from the cylinder by an eduction pipe in the usual way: a jet is admitted to it from an adjacent vessel, which, before the engine commences work, is filled with distilled water; the condensing water and condensed steam are pumped from the condenser by air-pumps of the usual construction, but smaller, inasmuch as there is no air to be withdrawn, as in common engines. The warm water thus pumped out of the condenser is driven into a copper pipe or worm, which is carried with many coils through a cistern of cold water, so that when it arrives at the end of this pipe it is reduced to the common temperature of the atmosphere. The pipe is then conducted into the vessel of distilled water already mentioned, and the water flowing from it continually replaces the water which flows into the condenser through the condensing jet. The condensing water being purged of air, a very small air-pump is sufficient; since it has only to exhaust the condenser and tubes at starting, and to remove whatever air may enter by casual leakage. The patentee states that the condensation takes place as rapidly and as perfectly as in the best steam engine, and it is evident that this method of condensation is applicable even where the mercurial generator already described may not be employed. The vessel from which the water is injected into the mercurial generator is likewise fed by the air-pump connected with the condenser. There is another pipe besides the copper worm already described, which is carried from the hot well to this vessel, and the water is of course returned through it without being cooled. This vessel is likewise sufficiently exposed to the action of the fire to maintain it at a temperature somewhat below the boiling point.
An apparatus of this construction was in the spring of the present year (1835) placed in the Admiralty steamer called the Comet, in connexion with a pair of 40-horse engines. The patentee states that these engines were ill adapted to the contrivance; nevertheless, the vessel was successfully worked in the Thames for 800 miles: she also performed a voyage from Falmouth to Lisbon, but was prevented from returning by an accident which occurred to the machinery near the latter port. In this experimental voyage, the consumption of fuel is stated never to have exceeded a third of her former consumption, when worked by Bolton and Watt's engines; the former consumption of coals being about 800 lbs. per hour, and the consumption with Mr. Howard's engine being under 250 lbs. of coke per hour.
After this failure (which, however, was admitted to be one of accident and not of principle) the government did not consider itself justified in bestowing further time or incurring greater expense in trying this engine. Mr. Howard, however, has himself built a new vessel, in which he is about to place a pair of forty-horse engines. This vessel is now (December, 1835) nearly ready, and will bring the question to issue by a fair experiment. The advantages of the contrivance as enumerated by the patentee are:—
First, The small space and weight occupied by the machinery, arising from the absence of a boiler.