The means adopted for the condensation of the steam is almost universally by the injection of cold water Into the condenser, as in land engines. But for the purposes of steam navigation, this method is attended by several serious evils, which we shall briefly notice. In the first place, the water being generally strongly impregnated with earthy or saline matter, and the steam consisting of pure water only, this extraneous matter continues to accumulate in the boilers, and at length to form incrustations, which if not removed, would eventually fill the boiler. To prevent this occurring, several methods are resorted to (some of which we shall notice), but none are completely successful, and all are attended with some objection. Another evil attendant upon this system of condensation when applied to marine engines, is that, as the supply of water to the boilers cannot be regulated by self-adjusting means, as in land engines, but must depend upon the care of the attendants, the fire-box and flues are liable to be burnt through in the event of the smallest inattention.
Great care also is required in regulating the injection flow of water, for the vacuum causes the water to enter the condenser as fast, when the engines are moving slowly, as when going at a great speed; and as it is exceedingly difficult to regulate the injection exactly to the irregularities in the speed of the engine, there is the risk of diminishing the power of the engines by admitting too small a quantity of water, or of choking the condenser and air-pump by an excess, which is frequently the occasion of serious accidents. The plan most commonly resorted to for preventing the deposition of earthy matter in the boiler, is to discharge a portion of the water from time to time (technically called "blowing out"), replacing it with water from the hot-well, with the view to prevent the water becoming saturated. Mr. Seaward has contrived an ingenious and extremely simple plan to guide the men in the performance of this operation. In the glass gauges, attached to the boiler to show the height of the water, are two glass bulbs of different specific gravities; when the water in the boiler approaches within a certain distance of the point of saturation, the heaviest bulb rises to the surface, whereupon the engineer should blow out water from the boiler, replacing it with water from the hot well, until the lightest bulb sinks.
This contrivance is doubtless of great utility, as it enables the engineer to conduct the "blowing out" process with much greater precision than he could otherwise do. Mr. Seaward's apparatus for this purpose is shown in the annexed cut. At a is the glass gauge, chiefly inclosed in its metallic case &,and connected by the pipes and cocks c c with the boiler d. At e and f are screwed plugs having vertical stems proceeding from them, which enter the ends of the glass tube; g and h represent the two glass balls, the lower one of which, being the heaviest, is shown as resting upon the top of the lower stem, while the upper one floats on the liquid. The process is, however, still dependent upon the attention of the engineer, and, therefore, liable to be neglected. To obviate this Messrs. Maudslay and Field, instead of periodically blowing out the water, maintain a constant stream of water through the boilers, by means of a small pump, worked by the engines, and so proportioned as to draw from the owest part of the boiler, at each stroke, as much salt as is deposited in the boiler by the steam consumed in that stroke. But the operation of blowing off, in whatever way it is effected, is seldom completely effectual.
It also causes a great waste of fuel by expelling from the boilers such a large quantity of water at the boiling point.
These and some minor objections to the system of condensing by injection, becoming more sensibly felt as steam navigation advanced, have caused numerous attempts to condense the steam without injection, by bringing it in contact with metallic surfaces, surrounded by cold water; and the system has at length been brought to such perfection, as scarcely to admit of further improvement. Some of the arrangements for this purpose, which have been put in practice, we shall now proceed to notice.
In 1822 Mr. M. I. Brunei obtained a patent for various improvements in marine engines; one of which improvements consisted in a method of condensing the steam without injection; with the view of keeping the water in the boiler constantly fresh, and thus preventing the corrosion and incrustation which the use of sea water occasions. The condenser consisted of a peculiar combination of pipes as represented by Fig. 1, which is a vertical section of the apparatus, a a a are a horizontal row of pipes, which the inventor calls "mains;" each of these mains has on the upper side a row of sockets, and from each socket rises a cluster of copper pipes b b, of small diameter. These pipes are closed at the upper ends, but open at the lower, which are inserted in holes in a sort of cups, fitted to the sockets on the mains, and secured thereto by a long screw-bolt c; which passes down the centre pipe of each cluster, and is screwed into the main, as shown in Fig. 2, which is a section of one of the clusters on a larger scale. The mains and small pipes are placed in a cistern, through which Fig.2 a current of cold water is maintained by a pump or other suitable means.
The steam from the engines enters the mains a by the pipe d, and ascending the small pipes b b, is condensed and restored to the form of water, which falling to the bottom of the mains, is withdrawn by a small force pump e, and returned to the boiler. We are not aware that this apparatus has been tried, but we think, that from the want of an air-pump, and from there being no thorough draft through the small pipes, it would be difficult to discharge the air from them; which would, therefore, prevent a good vacuum being obtained, and materially obstruct the condensation of the steam.