The Steam Engine Familiarly Explained and Illustrated / With an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain axioms for railway speculators

by Dionysius Lardner

It is evident that it must be a matter of considerable importance to reduce the space occupied by the machinery on board a vessel to the least possible dimensions. The marine boilers, therefore, are constructed so as to yield the necessary quantity of steam with the smallest practical dimensions. With this view a much more extensive surface in proportion to the size of the boiler is exposed to the action of the fire. In fact, the flues which carry off the heated air to the chimney are conducted through the boiler, so as to act upon the water on every side in thin oblong shells, which traverse the boiler backward and forward repeatedly, until finally they terminate in the chimney. By this arrangement the original expense of the boilers is very considerably increased; but, on the other hand, their steam-producing power is also greatly augmented; and from experiments lately made by Mr. Watt at Birmingham, it appears that they work with an economy of fuel compared with common land boilers in the proportion of about two to three. Thus they have the additional advantage of saving the tonnage as well as the expense of one third of the fuel.

One of the most formidable difficulties which has been encountered in applying the steam engine to the purposes of navigation has arisen from the necessity of supplying the boiler with sea water, instead of pure fresh water. This water (also used for the purpose of condensation) being injected into the condenser and mixed with the condensed steam, is conducted as feeding water into the boiler.

The salt contained in the sea water, not being evaporated, remains in the boiler. In fact, it is separated from the water in the same manner as by the process of distillation. As the evaporation in the boiler is continued, the proportion of salt contained in the water is, therefore, constantly increased, until a greater proportion is accumulated than the water is capable of holding in solution; a deposition of salt then commences, and is lodged in the cavities at the bottom of the boiler. The continuance of this process, it is evident, would at length fill the boiler with salt.

But besides this, under some circumstances, a deposition of lime[35] is made, and a hard incrustation is formed on the inner surface of the boiler. In some situations, also, sand and mud are received into the boiler, being suspended in the water pumped in for feeding it. All these substances, whether deposited in a loose form in the lower parts of the boiler or collected in a crust on its inner surface, form obstructions to the passage of heat from the fire to the water. The crust thus formed is not unfrequently an inch or more in thickness, and so hard that good chisels are broken in removing it. The heat more or less intercepted by these substances collects in the metal of the boiler, and raises it to a temperature far exceeding that of the water within. It may even, if the incrustation be great, be sufficient to render the boiler red-hot. These circumstances occasion the rapid wear of the boiler, and endanger its safety by softening it.

The remedy which has generally been adopted to remove or diminish these injurious effects consists in allowing a stream of hot water continually to flow from the boiler, and supplying from the feed-pipe a corresponding portion of cold water. While the hot water which flows from the boiler in this case contains, besides its just proportion of salt, that portion which has been liberated from the water converted into vapour, the cold water which is supplied through the feed-pipe contains less than its just proportion of salt, since it is composed of the natural sea water, mixed with the condensed steam, which latter contains no salt. In this manner, the proportion of the salt in the boiler may be prevented from accumulating; but this is attended with considerable inconvenience and loss. It is evident that the discharge of the hot water, and the introduction of so considerable a quantity of cold water, entails upon the machine a great waste of fuel, and, consequently, renders it necessary that the vessel should be supplied with a much larger quantity of coals than are merely necessary for propelling it. In long voyages, where this inconvenience is most felt, this is a circumstance of obvious importance. But besides the waste of fuel, the speed of the vessel is diminished by the rate of evaporation in the boiler being checked by the constant stream of cold water flowing into it. This process of discharging the water, which is called blowing out, is only practised occasionally. In the Admiralty steamers, the engineers are ordered to blow out every two hours. But it is more usual to do so only once a day.

This method, however, of blowing out furnishes but a partial remedy for the evils we have alluded to: a loose deposite will perhaps be removed by such means, but an incrustation, more or less according to the circumstances and quality of the water, will be formed; besides which, the temptation to work the vessel with efficiency for the moment influences the engine men to neglect blowing out; and it is found that this class of persons can rarely be relied upon to resort to this remedy with that constancy and regularity which are essential for the due preservation of the boilers. The class of steam vessels which, at present, are exposed to the greatest injury from these causes are the sea-going steamers employed by the Admiralty; and we find, by a report made by Messrs. Lloyd and Kingston to the Admiralty, in August, 1834, that it is admitted that the method of blowing out is, even when daily attended to, ineffectual. "The water in the boiler," these gentlemen observe, "is kept from exceeding a certain degree of saltness, by periodically blowing a portion of it into the sea; but whatever care is taken, in long voyages especially, salt will accumulate, and sometimes in great quantities and of great hardness, so that it is with difficulty it can be removed. Boilers are thus often injured as much in a few months as they would otherwise be in as many years. The other evil necessarily resulting from this state of things is, besides the rapid destruction of the boilers, a great waste of fuel, occasioned by the difficulty with which the heat passes through the incrustation on the inside, by the leaks which are thereby caused, and by the practice of blowing out periodically, as before mentioned, a considerable portion of the boiling water."

It would be impracticable to carry on board the vessel a sufficient quantity of pure fresh water to work the engine exclusively by its means. To accomplish this, it would be necessary to have a sufficient supply of cold water to keep the condensing cistern cold, to supply the jet in the condenser, and to have a reservoir in which the warm water coming from the waste pipe of the cold cistern might be allowed to cool. Engineers have therefore directed their attention to some method by which the steam may be condensed without a jet, and after condensation be preserved for the purpose of feeding the boiler. If this could be accomplished, it would not be necessary to provide a greater quantity of pure water than would be sufficient to make up the small portion of waste which might proceed from leakage and from other causes; and it is evident that this portion might always be readily obtained by the distillation of sea water, which might be effected by a small vessel exposed to the same fire which acts upon the boiler.

(115.) Mr. Samuel Hall of Basford, near Nottingham, has taken out patents for a new form of condenser, contrived for the attainment of these ends, besides some other improvements in the engine.

The condenser of Mr. Hall consists of a great number of narrow tubes immersed in a cistern of cold water: the steam as it passes from the cylinder, after having worked the piston, enters these tubes, and is immediately condensed by their cold surfaces. It flows in the form of water from their remote extremities, and is drawn off by the air-pump, and conducted in the usual way to a cistern from which the boiler is fed. In the marine engines constructed under Mr. Hall's patents, the tubes of the condenser being in an upright or vertical position, the steam flows from the cylinder into the upper part of the condenser, which is a low flat chamber, in the bottom of which is inserted the upper extremities of the tubes, through which the steam passes downwards, and as it passes is condensed. It flows thence into a similar chamber below, from whence it is drawn off by the air-pump.

It is evident that at sea an unlimited supply of cold water may be obtained to keep the condensing cistern cold, so that a perfect condensation may always be effected by these tubes, if they be made sufficiently small. The water formed by the condensed steam will be pure distilled water; and if the boiler be originally filled with water which does not hold in solution any earthy or other matter which might be deposited or encrusted, it may be worked for any length of time without injury. The small quantity of waste from leakage is supplied in Mr. Hall's engine by a simple apparatus in which a sufficient quantity of sea water may be distilled.