SMOKE AND DUST ABATEMENT — COTTRELL. ()61 



THE REMOVAL OF INCOMBUSTIBLE SOLIDS AND LIQUIDS. 



The removal of the suspended particles of solids or liquids which 

 make up the visible clouds of dust and fume may be considered a 

 purely mechanical problem, no matter how fine these particles are, 

 and as such presents interesting general features easily understood 

 without recourse to highly technical considerations. 



The chief methods employed for removing such dust and fume 

 from gases may be classed under the heads of washing, filtering, 

 centrifuging. and electrical precipitation. 



WASHING METHODS. 



Where washing is employed it is usually accomplished either by a 

 sj'stem of fine sprays of water or by bubbling the gases through 

 water or by churning them up with water in various forms of agita- 

 tors, or by a combination of these. For relatively small volumes of 

 gases having a distinct commercial value — e. g.. in the cleaning of 

 fuel gas for domestic use or the scrubbing of iron blast-furnace gas 

 preparatory to use in gas engines — these methods have become well 

 established, but the thoroughness of the scrubbing required with the 

 consequent amount of power consumed, together with the fact that, 

 where the gases contain acid constituents, very corrosive liquors are 

 generated requiring difficult and expensive acid-proof construction 

 of the apparatus, has thus far set rather sharp limitations to the 

 general extension of this method. A vast number of schemes based on 

 these methods have been proposed and many patents have been taken 

 out, but few who have not actually worked with such methods on a 

 really large scale seem to realize the great difficulty they present from 

 the practical and economic point of view when it comes to dealing 

 with very large gas volumes. 



riLTHATION METHODS. 



The filtration of gases through fabrics, usually in the form of bags, 

 has found a somewhat wider application to large scale work,^ notably 

 in the zinc and lead industry. The bag house has the advantage over 

 a washing system in that the material is collected in a dry state, and 

 for equal volumes of gas treated the expense of installation and main- 

 tenance is generally far less than for water scrubbing. The chief 

 limitations of the bag house have arisen from the impracticability 

 up to the present of securing a suitable fabric which would continu- 

 ously withstand high temperatures, acids, and other corrosive agents 

 in the gases. Cotton bags are very satisfactory for moderately cool 

 gases free from acids, such as met with in zinc-oxide manufacture. 

 Wool bags, though more expensive, will withstand a somewhat 



1 See also pp. 671, 674 below and figs. 8 and 19. 



