Flow of Gases through Porous Plates

Recently, Sameshima(24) has measured the rates of flow of various simple gases through a compact unglazed earthenware plate. The rates of flow definitely did not obey the Knudsen formula t = k JM, where t denotes the time required for the effusion at constant pressure of a volume F of a gas of molecular weight M, and where fc is a constant. On the other hand, the law t = k M was accurately obeyed when the gases effused through a platinum plate with a single orifice. For the earthenware plate Sameshima found a formula t = to apply. If the wall was very thin n approached zero, and the simple behaviour of the perforated platinum plate was found. If the wall was thick n approached unity and the equation became t = kij (ij denotes viscosity). 

Adzumi(25) has elaborated a semi-empirical theory for flow processes through porous plates, based on the assumption that the plate is perforated by numerous fine holes, the diameters of which can vary down their lengths, so that effectively each capillary is a number of capillaries of very numerous diameters arranged in series and each such composite capillary is in parallel with all the other capillaries. At high pressures the Poiseuille law — ap p — p applies (p. 55), and at low pressures the law (P- 

For medium pressures, or for a porous plate, the two laws may be considered to overlap  

The treatment of Adzumi allows one to estimate a mean j)ore size, and the number of pores per unit volume. Should all the n pores be of radius f, 

For further examples of flow through porous plates one may study the permeability of various refractories, the properties of which, in relation to the diffusion problem, are given in the next section. 

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