Non-Conductors of Heat.

In a vast manufactory of steel cars, of steel structural forms, steam has to be conveyed long distances from the boilers. Here, as in similar huge establishments, or in the heating of towns and cities from central stations, it is desirable to lose as little heat as possible by the way, for undue waste means enormous inroads upon profits. There are other reasons for wishing to keep heat within a steam pipe; much damage may be done to fruit, flour and other merchandise unduly warmed. Furthermore there is a risk of setting fire to woodwork, paper, cotton and the like; it has been observed that after a month’s exposure to heat from steampipes, wood takes fire at a temperature which at first would not have led to ignition, because then the wood contained a little moisture. To guard against loss and danger it has long been the practice to cover steampipes with jackets of non-conducting material, such as mineral-wool,—furnace-slag blown into short glassy fibres by a sharp blast of air. Felt, loosely folded, also serves well. Many advertised claims for asbestos are not well founded; this mineral is incombustible and is therefore useful in thick curtains to separate a stage from the auditorium of a theatre. But it is a fairly good conductor, and for steampipes should be used as a direct covering of the metal simply to keep an outer and much thicker coat of felt from being charred. Whatever the material chiefly employed, one point is clearly brought out by experiment, namely, that the air detained by the fibres of a covering greatly aids in obstructing the passage of heat. Hence it is well to keep the materials from becoming compacted together, as do ashes when moistened. Asbestos fibres, which are smooth and glassy, do not take hold of air as do cork and wool.

Professor J. M. Ordway, of the Massachusetts Institute of Technology, Boston, tells us that non-conductors should be of materials that are abundant and cheap; clean and inodorous; light and easy to apply; not liable to become compacted by jarring or to change by long keeping; not attractive to insects or mice; not likely to scorch, char or ignite at the long-continued highest temperature to which they may be exposed; not liable to spontaneous combustion when partly soaked in oil; not prone to attract moisture from the air; not capable of exerting chemical action on the surfaces they touch. No material combines all these desirable qualities, but a considerable range of substances fulfil most of the requirements.

Tests of steam-pipe coverings at Sibley College, Cornell University, and at Michigan University, have resulted as follows:—

Kind of Covering	Relative Amount of Heat Transmitted
Naked pipe	100.
Two layers asbestos pipe, 1 inch hair felt, canvas cover	15.	2
The same, wrapped with manila paper	15.
Two layers asbestos paper, 1 inch hair felt	17.
Hair felt sectional covering, asbestos lined	18.	6
One thickness asbestos board	59.	4
Four thicknesses asbestos paper	50.	3
Two layers asbestos paper	77.	7
Wool felt, asbestos lined	23.	1
Wool felt with air spaces, asbestos lined	19.	7
Wool felt, plaster paris lined	25.	9
Asbestos molded, mixed with plaster paris[188]	31.	8
Asbestos felted, pure long fibre	20.	1
Asbestos and sponge	18.	8
Asbestos and wool felt	20.	8
Magnesia, molded, applied in plastic condition	22.	4
Magnesia, sectional	18.	8
Mineral wool, sectional	19.	3
Rock wool, fibrous	20.	3
Rock wool, felted	20.	9
Fossil meal, molded, ³⁄₄ inch thick	29.	7

In general the thickness of the coverings tested was one inch. Some tests were made with coverings of different thicknesses, from which it would appear that the gain in insulating power obtained by increasing the thickness is very slight compared with the increase in cost.[19]

[19] Rolla C. Carpenter, “Heating and Ventilating Buildings,” p. 229. New York, John Wiley & Sons, 1905.

Some properties of matter seem to have family ties. Tenacity and conductivity for heat, as an example, go together; all the tenacious metals as a group are conducting as well. Conversely, the non-conductors,—felt, gypsum, and the rest, are structurally weak. If the inventor could lay hands on a material able to withstand high pressure and, at the same time, carry off wastefully but little heat, he would build with it cylinders for steam engines much more economical than those of to-day He would also give cooking apparatus of all kinds a covering which would conduce to the health and comfort of the cook, while, at the same time, heat would be economized to the utmost. One of the advantages of electric heat is that it can be readily introduced into kettles and chafing dishes surrounded by excellent non-conductors; the result is an efficiency of about ninety-five per cent., quite unapproached in the operations of a common stove or range.