202
MODERN GASWORKS PRACTICE
bricks together when the plant is fired for the first time. So far as texture is con-cerned, the main, point to emphasize is that, whether the grain be open or close, uniformity is essential.
THE TESTING OF REFRACTORY MATERIALS
In addition to the ordinary laboratory tests for the determination of ultimate analysis, refractory materials should be subjected to certain practical tests in order to determine their suitability for retort-benoh purposes. The tests usually applied, and specified by the Refractory Materials Committee of the Institution of Gas Engineers, are as follows :—
(«) Refractoriness.
(6) Apparent porosity.
(c) Contraction or expansion. (tZ) Crushing strength..
(e) Refractoriness under load.
It must be mentioned at the outset that refractoriness is not merely dependent upon the maximum temperature which the material is capable of withstanding before fusion, takes place. The rate of change of tem ■ perature has also some considerable influence, and for this reason it is specified that for tests for refractoriness the temperature of the experimental furnace shall be increased at the rate of 50° C. (122° F.) during five minutes. There are various furnaces suitable for the purpose, but the electrical apparatus of Hirsch, or the Méker air-gas type, are to be prc-ferred.
The Hirsch furnace (Fig. 106) is of the resistance type, the resistance material used being chiefly carbon. The current is led into the furnace by means of the two electrodes shown, these being of iron and em-bedded in a large area of carbon, which tends to keep them cool. The volume of carbon in the centre of the furnace is made smaller, so that the heat is con-
centrated. here. To attain the maximum temperature the furnace requires a current of 130 amperes at 70 to 80 volts. The clay to be tested is mounted oa the top of the cylindrical support and introduced into the centre tube of the furnace, this tube having an internal diameter of 2| inches. The appearance of the sample undergoing test can. be watched by looking down. the centre tube from the top. A weak current of air constantly passes through, the centre tube, the °b] eet of this being to overcome the reducing atm.osph.ere always associated with carbon resistance furnaces.
The Méker furnace (Fig. 107) is capable of giving a temperature up to 1,850° C.
Electrode
Fro. 106.—Hirsch’s Electric Furnace.