PIPE COVERINGS. (See also BOILERS.) A form of pipe covering, Fig. 1, made by the United States Mineral Wool Co., consists of a metallic casing, made from steel plate, coated with lead, constructed with a lock which conceals the edge and en ables the two edges to be permanently fastened with wood screws, forming a cylinder. One end of each cylinder is crimped and beaded to facilitate the making of an end joint. Perforated disks are used to support the cylinder, and secure the equal distribution of the "rock wool " with which the casing around the pipe is filled, and holding it up against the pipe. The rock wool is a silicate of lime and magnesia, made from a magnesian lime rock by melting the same in a cupola with blast, and turning the molten rock upon a jet of dry steam at 80 lbs. pressure. The incited rock is thereby blown into the form of a fibrous substance con taining 97 per cent. of air. resembling wool in appearance. It is similar to mineral wool, or slag wool, which is made by blowing a jet of steam or air at high pressure into a stream of liquid slag as it flows from a blast furnace. Slag wool made from iron furnaces, however, generally contains sulphur, usually a lime sulphide, which tends to corrode iron pipes, and is therefore objectionable as a pipe covering. This objection does not hold in regard to rock wool.
Magnesium carbonate has recently come into extensive use as a non-conductor of heat. The substance referred to is the artificially prepared basic carbonate of magnesia, a com pound of the carbonate with the hydroxide. It is t-he " block magnesia" of commerce, the magnesia alba of the pharmacist. It is moulded to form coverings suitable for steam-pipes and their fittings, and sectional jackets for boilers and cylinders ; it is furnished also in forms suitable for lining refrigerators, walls and roofs of buildings, fire-proof safes, etc. It is a smooth, white. close-grained solid, in outward appearance resembling a block of Paris plaster. It possesses the lightness of cork, the porosity of sponge, and withal a degree of firmness and strength that, in view of its levity, is quite remarkable. To examine more closely the properties of this substance, H. Luttgen (Trans. Ant. Inst. Mining Eng-rs., Vol.
p. 614) made the following experiment: A number of 1-in. cubes were sawed from the commercial block carbonate ; also some bricks, that is, blocks measuring accurately 2 x 4x8 ins., the dimensions of an ordinary brick, The bricks were carefully measured and weighed,
and placed in vessels containing distilled water, in which they became gradually submerged, owing to the displacement by water of the air enclosed in the structure of the magnesium carbonate. After twenty-four hours the blocks were removed from the water, dried super ficially by contact with filter-paper, and weighed. From the increase in weight, the volume of the water absorbed, and consequently that of the air displaced by it, were obtained. The results showed that the air-cells occupied from 92 to per cent. of the volume of the blocks. Mr. Luttgen made sonic experiments on the non-conducting power of various pipe coverings; a brief abstract of the results is given below. The experiments were made on 6-ft. lengths of 2-in. steam-pipe, which were covered with the different coverings, with results as follows: With reference to the economy and cost of non-conducting materials, it may be said that the material which is in the greatest degree non-conducting, incombustible, and durable will prove the most economical, eren though its first cost be greater than that of an inferior arti cle. Experiments with naked pipes show that a 2-in. pipe carrying steam at 60 lbs. pressure will condense 0.397 lb. per ft. per hour. Covered with a good covering like magnesium carbonate, the condensation, according to Mr. Luttgen, will he but 0'084 lb. per ft. per hour, a saving of lb. per ft. per hour, or lbs. of steam per day of ten boors, for each foot of pipe covered. The covering of 100 ft. of pipe, then, will save in a year of 300 ten-hour days the coal necessary to convert 93.900 lbs. of water into steam. One pound of bituminous coal is capable of making about 8.5 lbs. of steam, so the saving of coal due to the 100 ft. of covering would be 5 tons per year, which, at $4 per ton, amounts to $22. The real saving will probably amount to more than this estimate in most eases; and it may be said in round terms that the 100 ft. of covering causes each year a saving of its own first cost ($25). Inasmuch as the material pays for itself in a year, and will last indefinitely under ordinary conditions, its advantageousness is beyond question.