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_.