BLOWING AND VENTILATING APPARATUS
A railway locomotive sends the exhaust steam up the funnel with sufficient force to expel all air from the same and to create a vacuum. The only passage for the air flying to fill this empty space lies through the fire-box and tubes traversing the boiler from end to end. Were it not for the "induced draught"—the invention of George Stephenson—no locomotive would be able to draw a train at a higher speed than a few miles an hour.
On shipboard the fresh water used in the boilers is far too precious to be wasted by using it as a fire-exciter. Salt water to make good the loss would soon corrode the boilers and cause terrible explosions. Therefore the necessary draught is created by forcing air through the furnaces instead of by drawing it.
The stoke-hold is entirely separated from the outer air, except for the ventilators, down which air is forced by centrifugal pumps at considerable pressure. This draught serves two purposes. It lowers the temperature of the stoke-hold, which otherwise would be unbearable, and also feeds the fires with plenty of oxygen. The air forced in can escape in one way only, viz. by passing through the furnaces. When the ship is slowed down the "forced draught" is turned off, and then you see the poor stokers coming up for a breath of fresh air. In the Red Sea or other tropical latitudes these grimy but useful men have a very hard time of it. While passengers up above are grumbling at the heat, the stoker below is almost fainting, although clad in nothing but the thinnest of trousers.
In the engine-room also things at times become uncomfortably warm. Take the case of the United States monitor Amphitrite, which went into commission in 1895 for a trial run.
Both stoke-hold and engine-room were very insufficiently ventilated. The vessel started from Hampton Roads for Brunswick, Georgia. "The trip of about 500 miles occupied five days in the latter part of July, and, for sheer suffering, has perhaps seldom been equalled in our naval history. The fire-room (stoke-hold) temperature was never below 150°, and often above 170°, while the engine-room ranged closely about 150°. For the first twenty-four hours the men stood it well, but on the second day seven succumbed to the heat and were put on the sick list, one of them nearly dying; before the voyage was ended, twenty-eight had been driven to seek medical attendance. The gaps thus created were partially filled with inexperienced men from the deck force, until there was only a lifeboat's crew left in each watch.... On the evening of the fourth day out our men had literally fought the fire to a finish and had been vanquished; the watch on duty broke down one by one, and the engines, after lumbering along slower and slower, actually stopped for want of steam.... At daybreak the next morning we got under way and steamed at a very conservative rate to our destination, fortunately only about ten miles distant. The scene in the fire-room that morning was not of this earth, and far beyond description. The heat was almost destructive to life; steam was blowing from many defective joints and water columns; tools, ladders, doors, and all fittings were too hot to touch; and the place was dense with smoke escaping from furnace doors, for there was absolutely no draught. The men collected to build up the fires were the best of those remaining fit for duty, but they were worn out physically, were nervous, apprehensive, and dispirited. Rough Irish firemen, who would stand in a fair fight till killed in their tracks, were crying like children, and begging to be allowed to go on deck, so completely were they unmanned by the cruel ordeal they had endured so long. 'Hell afloat' is a nautical figure of speech often idly used, but then we saw it. For a month thereafter the ship was actively employed on the southern coast, drilling militia at different ports, and sweltering in the new dock at Port Royal. One trip of twenty-nine hours broke the record for heat, the fire-room being frequently above 180°. All fire-room temperatures were taken in the actual spaces where the men had to work, and not from hot corners or overhead pockets."[16]
The ventilators were subsequently altered, and the men enjoyed comparative comfort. The words quoted will suffice to establish the importance of a proper current of air where men have to work. One of the greatest difficulties encountered in deep mining is that, while the temperature approaches and sometimes passes that of a stoke-hold, the task of sending down a cool current from above is, with depths of 4,000 ft. and over, a very awkward one to carry out.
On passenger ships the fans ventilating the cabins and saloons are constantly at work, either sucking out foul air or driving in fresh. The principle of the fan is very similar to that of the centrifugal water pump—vanes rotating in a case open at the centre, through which the air enters, to be flung by the blades against the sides of the case and driven out of an opening in its circumference. Sometimes an ordinary screw-shaped fan, such as we often see in public buildings, is employed.