131. Common Boiler.
132. Improved Boiler.
Experience has proved that the best material from which these boilers should be constructed is wrought iron or copper; the wrought iron should be of 5/16 in. or ⅜ in. substance, and copper can be of a little less substance except the front where the external wear and tear takes place (chiefly by the poker). The principal of these remarks apply to independent boilers also, which, however, are generally set in brickwork and are of larger capacity and strength in proportion. Fig. 132 represents a more powerful form of boiler for domestic purposes, and is found a very rapid heating and efficient shape, in instances where a large number of draw-offs are in use, or a coil or heating pipes are in connection. Care should be taken to avoid boilers with narrow water-ways where the heat is applied; there are occasionally made boilers with 2 projecting horns or cheeks which occupy the place of the fire-bricks in the range firebox; these narrow parts, which are subjected to almost the most intense heat, will fur up solid and crack in 6 months with London water. The advantages of a copper boiler are quicker heating, greater durability, and greater expansion and contraction, which prevents the incrustation adhering to the surface so firmly as it does inside an iron boiler; and although a copper boiler is of greater first cost, yet when worn out it is of almost sufficient value to purchase a new iron one.
These boilers are recognised by several names, viz. “high pressure,” “Bath,” and “circulating,” &c., boilers. The first term is generally known.
Cast-iron high-pressure boilers have now gone almost entirely into disuse on account of the unsatisfactory results experienced.
Safety valves have been generally treated of in an earlier part of this chapter, but it might be impressed upon the reader that the valve should be as near as possible or directly in the boiler; they are sometimes put in the circulating pipes or in the cylinder or tank, but this is away from the seat of danger; it would be a rarity to hear of a cylinder or tank exploding.
The reason that cylinders are used when the No. 1 system is adopted, and tanks with No. 2 system, is that the cylindrical shape is better adapted to bear the greater pressure; otherwise a tank would answer as well to all intents and purposes; to show the pressure-resisting strength, a tank of ⅛ in. plate is tested and warranted to bear 5 lb. pressure to the square inch. This is equal to the cistern being 10 ft. above the tank, whereas a ⅛ in. plate cylinder is tested and warranted to bear 25 lb. pressure to the square inch, equal to 50 ft. In speaking of pressure by feet this alludes to the vertical height between the cylinder or tank and the cold cistern; it does not matter what size pipe connects them. A cylinder or tank can generally be depended upon to bear a little greater pressure than they are tested to. There are 3 or 4 strengths made to meet the various requirements, and the makers’ lists show what pressure they are tested to in lb., and every lb. can be calculated as equal to 2 ft. of vertical pipe.
The general sizes of tanks and cylinders for domestic purposes vary from 30 to 60 gal. according to requirements; the disadvantage of too large a tank is the time taken in heating its contents and completing the circulation, and in some instances the space it occupies; tanks of 100 gal. capacity are sometimes fixed in residences where there are only 4 or 5 taps (hot water). This is much too large for any but large mansions or for business purposes; for say 5 taps, including bath, a 50 gal. tank or cylinder will be found large enough and will admit of 3 baths in succession about 1 hour after lighting the fire.
The pipes or tubes commonly used are iron steam tube, galvanised iron steam tube, iron gas tube, or barrel, and lead pipe; the two latter, though commonly found, should be avoided as being totally unsuitable for this work. Gas barrel is sometimes used on cheap work and in small speculative property, and answers well, when no real work is put upon it; but no respectable firm would undertake to use it, as satisfactory results cannot be relied upon except under very favourable circumstances.
Steam tube, commonly known as “red steam” tube on account of its colour externally, is much like gas barrel, but especially strong for engineering purposes, and the utmost reliance can be put in it. Galvanised steam is the same quality tube, but galvanised to prevent rust; this is much liked, and is absolutely necessary in some districts where unprotected iron rusts at an alarming rate; but care should be exercised to see that it is galvanised inside as well as out; preference, however, is on the side of the “red steam” as it is considered that the tube is deteriorated in strength by the process of galvanising.