An estimate of the waste of fuel in neglecting to cover steam-pipes has been made by M. Le Bour, who, referring to experiments made by M. Walther Meunier. gives the following as the quantities of steam condensed per hour and per year of 300 working days of 10 hours, per square foot of surfac-c for different metals, with steam at about 260° F.
Assuming that. it requires an expenditure of fuel of 1 lb. of coal for every 7 lbs. of steam, the annual waste of fuel will be as given below for every square foot of the surface of the steam-pipe. and! taking coal at $4 per ton. the loss per square foot of surface will be as in the second column.
A few years since, an investigation was made at the instance of the Boston Manufact urers' Mutual Fire Insurance Co., by Prof. John M. Ordway, of the Niat‘saehusetts Institute of Technology, upon the non-heat-conducting properties of various materials, some of which may be used for covering steam-pipes and boilers, while others, owing to their liability either to become' carbonized or to take fire, cannot be directly applied to such use. The results of this investigation are given as follows in a circular (No. 27, December, 1889t, issued by the insurance company to its members : •‘ In order that the relative merits of the different substances which are offered for pre venting the escape of heat from boilers and stenni-pipes, or as substitutes for wire lathing and plastering, or for tin plates in the protection or elevator shafts, or of woodwork nailed closely to walls, the following tables are submitted. These tables and extracts are taken from a report made by Professor Ordway. It will be observed that several of the incom bustible materials are nearly as efficient as wool, cotton, and feathers, with which they may be compared in the following table. The materials which may be considered wholly free from the danger of being carbonized or ignited by slow contact with pipes or boilers are printed in solid black type. Those which are more or less liable to be carbonized are printed in italics.
"Professor Ordway's report is as follows : Careful experiments have been made with various non-conductors, each used in a mass 1 in. thick, placed on a flat surface of iron kept heated by steam to 310° F. The preceding table gives the amount of heat transmitted per hour through each kind of non-conductor 1 in. thick, reckoned in pounds of water heated 10° F., the unit of area being 1 sq. ft. of covering.
" The first column of figures of results gives the loss by the measure of pounds of water heated 10°. The second column gives the amount of solid matter in the mass 1 in. thick. The third column gives the amount or bulk of included or entrapped air.' "There are some mixtures of two materials which may be quite safe, although consisting in part of substances which mar be carbonized. It must also be considered that a covering for a steam-pipe or boiler should have some strength or elasticity, so that, when even put on loosely and holding a great deal of entrapped air, it may not be converted into a solid con dition by the constant jar of the building, then becoming rather a quick conductor. This
warning may be applied especially to what is called slag wool,' which consists of short, very fine threads of a brittle kind of glass. The following table has been submitted by Prof. Ordway. with the following explanation: '• • The substances given in the following table were actually tried as coverings for two-inch steam-pipe, but, for convenience of comparison, the results have been reduced by calculation to the same terms as in the foregoing table.' " ' Later experiments have given results for still air which differ little from those of Nos. 3, 4, and 6. In fact, the bulk of matter in the best non-conductors is relatively too small to have any specific effect, except to entrap the air and keep it stagnant. These substances keep the air still by virtue of the roughness of their fibres or particles. The asbestos of 18 had smooth fibres, which could not prevent the air from moving about. Later trials with an asbestos of exceedingly fine fibre have made a somewhat better showing. but asbestos is really one of the poorest non-conductors. By reason of its fibrous character it may be used advan tageously to hold together other incombustible substances, but the less the better. We have made trials of two samples of a "magnesia covering" consisting of carbonate of magnesia with a small percentage of good asbestos fibre. One transmitted heat which, reduced to the terms of the first of the above tables, would amount to 15 lbs.; the denser one gave 20 lbs. The former contained 250 thousandths of solid matter; the latter 396 thousandths.' " ' Charcoal, lamp-black, and anthracite coal are virtually the same substance, and Nos. 5, 6, 7, 8, and 9 show that non-conducting power is determined far less by the substance itself than by its mechanical texture. In some cases when a greater quantity of a material is crowded into the same thickness the non-conducting virtue is somewhat increased, because the included air is thereby rendered more completely fixed. But if the same quantity is compressed so as to diminish its thickness, its efficiency is lessened; for the resistance to the transmission of heat is nearly—though by no means exactly—in proportion to the thickness of the non-con ductor. Hence, though a great many layers of paper—as in No. 23—prove to be a tolerably good retainer of heat, one or two layers arc of exceedingly little service. Any suitable sub stance which is used to prevent the escape of steam-heat should not be less than an inch thick.' " • Any covering should be kept perfectly dry, for not only is water a good carrier of heat, but it has been found in our trials that still water conducts heat about eight times as rapidly as still air.' "