Modern Gasworks Practice

228 MODERN GASWORKS PRACTICE cells. Accordingly, care is required during manufacture in order to ensure that the cell structures are not broken during crushing or mixiug. E. Griffiths 1 states that the material has an extremely low thermal conductivity, and its refractory nature pennits of its use at high, temperatures if it is protected from the cutting action of flames and abrasion. From a series of investigations on the insulating property of diatomaceous tiles Griffiths obtained the following results:— Temperature. Hot face. (°c.) Temperature. Cold face, ec.) Conductivity in. C.G.S. units. 186 24 •000310 287 25 •000331 505 28 ■000359 622 32 ■000387 939 64 •000461 It is interesting to note that although it is generally accepted that the thermal conductivity of insulation materials is greater the less porous the material, some authorities are opposed to the view that a porous substance forms the best insulator, pointing out that the transmission of heat by radiation and convection has not been taken into account. Millard and Lichty,2 as a result of investigations in America, particularly point to the fact that though air-space construction is of decided value at low temperatures it is not so at high, temperatures. In all cases the heat transmission across an air space will rapidly increase with an increase in temperature of the enclosing walls, and also with the same difference of temperature if the absolute values of the wall temperatures are increased. Mellor3 holds the opinion that at ordinary temperatures the quantity of heat travelling through porous bricks by conduction is mach greater than the amount travelling by radiation across the pore spaces. By making a calculation for material with pores cm. across, with a temperature of 1,400° C., it works out that the amount of heat carried per second across the air space and through the solid material is the same. Smaller pores would make the temperature higher, and larger pores would make it lower. With pores J cm. across, the insulating properties begin. to break down above 730° C.—that is, within ordinary kiln temperatures. Mellor 4 clearly states, however, that his opinion is one which may be refuted. by facts when definite proof is available. The difficulty with. the measurement of high temperature thermal conductivity is that of finding a method sufficiently sensitive to show differences in the conductivity of bricks of different porosity. From several indications it appears that there may be a reversal in conductivity at about 1,000° C. in the sense that the more porous bricks become better conductors than the same bricks of less porosity. The considerable amount of literature which. has been published on thermal conductivity is indecisive. 1 Proe. Faraday Society, November, 1916. 2 University of Illinois, Bulletin No. 102. 3 Proe. Ceramic Society, April, 1919. 4 In a letter to the author, June, 1920.