CRUST IN BOILERS.] 



APPLIED MECHANICS. 



851 



tubes being cut of the proper lengths and put in their 

 places, the ends are forced open by means of a conical 

 tool driven by hammering into their mouths. Other 

 methods of fixing tubes and stays are employed ; and 

 there are numerous other details of boiler-making of a 

 technical character, upon which we need not enter. The 

 furnace or fire-grate of a boiler is generally made of 

 numerous bars of wrought or cast-iron, laid side by side 

 so as to form a grating, on which the fuel is placed, 

 leaving spaces about inch wide between the bars for 

 the passage of air upwards to support the combustion, 

 and of the ashes or incombustible refuse downwards 

 into the ash-pit. In front of the bars, there is generally 

 placed a dead-plate or surface without openings, to 

 receive the fresh fuel, which, lying there for some time 

 exposed to the radiation of the fire beyond, parts with a 

 portion of its gases, and is partially coked before it is 

 pushed onwards to the fire-grate. The gases are ignited 

 in their passage over the hot fire, and produce flame, 

 which plays on the surfaces of the flues. When the 

 supply of air is deficient, large volumes of these liberated 

 gases, having numerous particles of carbon suspended 

 in them, pass through the flues without ignition, and 

 thence through the cliimney as black smoke. When this 

 happeus, not only is a large and valuable part of the 

 fuel wasted, but the air is inconveniently polluted. The 

 :t of smoke-consuming apparatus is to prevent this 

 evil ; and the general principle on which all such appara- 

 tu.s U constructed, is either to manage the production of 

 thesx gases in such a continuous regular manner, as that 

 sufficient air may be supplied for their combustion when 

 they are sufficiently heated to ignite, or to supply heated 

 air in some part of the flues, so as to turn into flame 

 there, the gases that would otherwise escape unconsumed ; 

 or, finally, when the production of the gases is irregular, 

 to supply air to burn them only at the proper times and in 

 suitable proportions. 



The chimney of a steam-boiler should be of sufficient 

 area and height to produce a good draught or quick 

 current of the heated products of combustion. When 

 the draught is insufficient, the fresh air supplied to the 

 fire is too small in volume, the combustion is retarded, 

 and the flues are filled with smoke instead of flame. A 

 chimney of 30 or 40 feet in height, ami having an area 

 of 1 square foot for 10 horse-power, generally gives a 

 sufficient draught. Where height cannot bo obtained, 

 as in locomotive engines, the waste steam is made to 

 rush up the chimney with considerable force, and thus 

 to create an artificial draught. The chimney or Hue 

 leading to it is provided with a damper or slide, by 

 which the area of passnge may be diminished, and the 

 draught reduced at pleasure. At convenient places in 

 the flues, soot-doors are fixed for giving access to clean 

 them from soot or ashes deposited there. 



INCRUSTATION. As all water supplied to boilers 

 is more or less impure, or contains ingredients that be- 

 come deposited while the water itself is driven off in the 

 form of vapour, it is essential to provide for the frequent 

 clean&ing of boilers. In marine boilers particularly, 

 where sea-water is necessarily employed, the deposit of 

 saline ingredients, none of which are volatile, or pass off 

 with the steam, is very large, and of a very troublesome 

 character. It generally settles in the form of a hard, 

 stony crust upon the interior surface of the boiler ; and 

 as this crust is a very bad conductor of heat, not only is 

 there a waste of fuel where it exists, owing to its arresting 

 the passage of the heat from the flues to the water, but 

 there is a positive danger from the circumstance that the 

 interior surfaces of the flues are thus over-heated, and 

 the iron of which they are made becomes rapidly oxidised, 

 scaling off in flakes, losing thickness and strength, and 

 becoming liable to disruption from the pressure within 

 exceeding the strength left to sustain it. As salt-water 

 does not deposit rapidly until it becomes super-saturated 

 with salt, it is usual to permit, frequently, the escape 

 of a considerable quantity of the excessively salt-water 

 in the boiler, and to replace it with now water from the 

 sea, less salt and less liable to deposit. For effecting 

 this object and also for emptying the boiler when re- 



quired, a pipe and cock, called technically the blow-ojf, 

 is fitted to the lower part of the boiler. This cock 

 should be frequently opened, especially where the water 

 is very dirty or of a saline character, and the worst part 

 of the contents of the boiler, which, being heaviest, lie 

 near the bottom, thus blown out. In order to save the 

 loss of heat occasioned by frequently blowing off the hot 

 impure water from marine boilers, and replacing it by 

 cold, but purer water, an apparatus calluJ tue change- 

 water apparatus is sometimes employed. It consists of 

 a casing, with numerous small tubes arranged in it as in 

 a tubular boiler. A portion of the contents of the boiler 

 being always permitted to flow through the casing, and 

 thence into the sea, a corresponding quantity of purer 

 water is made to pass through the tubes to the boiler ; 

 and the latter is thus made to absorb in its passage 

 through the tubes, a considerable portion of the heat 

 given out by the former, in its passage round them. 



The large amount of incrustation in marine boilers, 

 and the consequent ineffective expenditure of fuel, have 

 for many years proved a source of great cost and incon- 

 venience in this application of steam-power. When it 

 is recollected that a steam-vessel has to carry its fuel in 

 a place which might otherwise be available for the stowage 

 of cargo, or that fuel has often to be purchased at distant 

 depots, where its price is enhanced by the cost and 

 difficulty of transport, it is a wonder that owners should 

 have submitted so long to a large waste of fuel, which 

 might have been avoided. A good many years ago, Mr. 

 Hall introduced a surface-condenser, by which the steam, 

 after working the engine, was recoudensod into pure 

 distilled water, and in that condition returned into the 

 boiler to be again converted into steam ; and so on, in 

 continual circulation. Notwithstanding the great ad- 

 vantage of this arrangement in saving incrustation and 

 deposit in avoiding the necessity for blowing-off in 

 economising fuel, and saving tear and wear of boilers 

 it is not until very recently that engineers have practically 

 adopted the principle of surface-condensation. As yet, 

 very few engines have been fitted with surface-condensers ; 

 but it is probable, that before many years have elapsed, 

 their adoption will become universal. Hereafter we 

 shall have occasion to enter more fully into their con- 

 struction. We only allude to them, at present, in connec- 

 tion with the subject of boilers, in order to show that the 

 deposit and incrustation, which form the grounds of so 

 many complaints and of so much danger, are by no 

 means necessary evils. 



Mud-holes are small holes provided in the lower parts 

 of boilers, and fitted with tight covers, which may be 

 removed, when the boiler is not in use, for the ad- 

 mission of a rake to draw out the mud deposited from 

 the water. 



The man-hole, an opening sufficiently large to admit 

 a man, is provided in the upper part of a boiler, and fitted 

 with a tight cover, which may be removed, when the 

 boiler is not in use, for the admission of a man for clean, 

 ing or repairs. 



In boUers having considerable flue surface, but not 

 great height of steam-room above the water-level, there 

 is always a danger of priming that is to say, the water 

 in a rapid state of ebullition is often made to boil over, 

 or blow in considerable quantities into the steam-pipes, 

 and thence into the engine, where it is not only useless, 

 but highly detrimental to the action of the machinery. 

 It is usual, therefore, to provide a boiler with a steam- 

 chest, or dome, in its highest part, to give greater space 

 for steam, and greater height for the mouth of the steam- 

 pipe above the surface of the boiling-water. In general, 

 there should not be less than six inches of water above 

 the flues, nor less than three feet of steam-room above 

 the water. In marine boilers, where the water is surged 

 about by the rolling of the vessel, there should be at 

 least double the height named for water and steam. 

 Should the level of the water be so low that the surface 

 of a flue is no longer covered by it, the flue is liable to 

 become over-heated, sometimes to redness ; the iron is 

 softened and weakened by the excessive heat, and rapidly 

 deteriorated by oxidation and scaling off. But this is 



