In selecting a boiler, the most efficient design will be found to be that in which the greatest amount of shell surface is exposed to direct heat. It is the direct heating surface that does the bulk of the work and every tendency to reduce it, either in the construction or setting of the boiler, should be avoided. The smaller the amount of surface enclosed by or in contact with the setting, the better results will be obtained.
A boiler with a bad circulation is the bane of an engineer’s existence. Proper circulation facilities constitute one of the chief factors in the construction of a successful and economical boiler. In tubular boilers the best practice places the tubes in vertical rows, leaving out what would be the centre row. The circulation is up the sides of the boiler and down the centre. Tubes set zig-zag to break spaces impede the circulation and are not considered productive of the best results.
The surface from which evaporation takes place should be made greater as the steam pressure is reduced, that is to say, as the size of the bubbles of steam become greater. To produce 100 lbs. of steam per hour at atmospheric pressure this surface should not be less than 732 square feet, which may be reduced to 146 square feet for steam at 75 lbs. pressure, and to 73 feet for steam at a pressure of 150 lbs. It is for this reason that triple-expansion engines can be worked with smaller boilers than are required with engines using steam of lower pressure. The amount of steam space to be permitted depends upon the volume of the cylinders and the number of revolutions made per minute. For ordinary engines it may be made a hundred times as great as the average volume of steam generated per second.
A volume of heated water in a boiler performs the same office in furnishing a steady supply of steam as a fly-wheel does to an engine in insuring uniformity of speed; hence the centre space of a boiler should be ample, in order to take advantage of this reserve force.
QUALITY OF STEEL PLATES.
Steel for boilers is always of the kind known as low steel, or soft steel, and is, properly speaking, ingot iron, all of its characteristics being those of a tenacious, bending, equal grained iron, and quite different from true steels, such as knife blades, cutting tools, etc., are composed of. Steel is rapidly displacing iron in boiler construction, as it has greater strength for the same thickness, than iron; and, except in rare instances, where the nature of the water available for feed renders steel undesirable, iron should not be used for making boilers, careful tests having shown it to be vastly inferior to steel in many important features.
Good boiler steel up to one-half inch in thickness should be capable of being doubled over and hammered down on itself without showing any signs of fracture, and above that thickness it should be capable of being bent around a mandrel of a diameter equal to one and one-half times the thickness of the plate, to an angle of 180 degrees without sign of distress. Such bending pieces should not be less in length than sixteen times the thickness of the plate.
On this test piece the metal should show the following physical qualities:
Tensile strength, 55,000 to 65,000 pounds per square inch.