This subject relates to the radiation of heat, which allows a reference to the laws of heat and tables of radiating power of various substances, as set forth on pages [212, 215].
The importance of a protection of exposed surfaces from radiation of heat is now undisputed, and many experiments have determined very closely the relative value of the various non-conducting substances.
Table of the Conducting Power of various substances.
| Substance. | Conducting Power. |
|---|---|
| Blotting Paper | .274 |
| Eiderdown | .314 |
| Cotton or Wool, any density | .323 |
| Hemp, Canvas | .418 |
| Mahogany Dust | .523 |
| Wood Ashes | .531 |
| Straw | .563 |
| Charcoal Powder | .636 |
| Wood, across fibre | .83 |
| Cork | 1.15 |
| Coke, pulverized | 1.29 |
| India Rubber | 1.37 |
| Wood, with fibre | 1.40 |
| Plaster of Paris | 3.86 |
| Baked Clay | 4.83 |
| Glass | 6.6 |
| Stone | 13.68 |
By the above table may be judged the comparative value of different coverings; blotting paper with its confined air, standing at one end of the list, stone at the other. It should be noted that the less the conducting power the better protection against radiation.
A non-conducting coating for steam pipes, etc., used for many years with perfect satisfaction, can be prepared by any steam user. It consists of a mixture of wood sawdust with common starch, used in a state of thick paste. If the surfaces to be covered are well cleaned from all trace of grease, the adherence of the paste is perfect for either cast or wrought iron; and a thickness of 1 inch will produce the same effect as that of the most costly non-conductors. For copper pipes there should be used a priming coat or two of potter’s clay, mixed thin with water and laid on with a brush. The sawdust is sifted to remove too large pieces, and mixed with very thin starch. A mixture of two-thirds of wheat starch with one-third of rye starch is the best for this purpose. It is the common practice to wind string spirally around the pipes to be treated to secure adhesion for the first coat, which is about l⁄5th of an inch thick. When this sets, a second and a third coat are successfully applied, and so on until the required thickness is attained. When it is all dry, two or three coats of coal tar, applied with a brush, protect it from the weather.
A very efficient covering may be made as follows: 1, wrap the pipe in asbestos paper—though this may be dispensed with; 2, lay slips of wood lengthways, from 6 to 12 according to size of pipe—binding them in position with wire or cord; 3, around the framework thus constructed wrap roofing paper, fastening it by paste or twine. For flanged pipe, space may be left for access to the bolts, which space should be filled with felt. Use tarred paper—or paint the exterior.
While a very efficient non-conductor, hair or wool felt has the disadvantage of becoming soon charred from the heat of steam at high pressure, and sometimes taking fire. The following table, prepared by Chas. E. Emory, Ph. D., shows the value of various substances, taking wool felt as a unit.
Table of Relative Value of Non-Conductors.
| Non-Conductor. | Value. |
|---|---|
| Wood Felt | 1.000 |
| Mineral Wool No. 2 | .832 |
| Do. with tar | .715 |
| Sawdust | .680 |
| Mineral Wool No. 1 | .676 |
| Charcoal | .632 |
| Pine Wood, across fibre | .553 |
| Loam, dry and open | .550 |
| Slaked Lime | .480 |
| Gas House Carbon | .470 |
| Asbestos | .363 |
| Coal Ashes | .345 |
| Coke in lumps | .277 |
| Air space, undivided | .136 |