The cooling of small particle-containing hot gases obtained, e.g., from the gasification of coal, in which the presence of small fly ash particles is unavoidable, involves a heat transmission problem and other problems, as will be explained hereinafter. 
When for cooling purposes the gases are passed through tubes which are cooled with a cooling medium on the outside, the walls of the tubes acquire a high temperature owing to transmission of heat from the hot gases to the tube metal which heat is further transmitted to the cooling medium.  For reasons of space saving helically coiled tubes are preferably used.  From British Pat.  No.  1,332,809 a method and a heat exchanger for the cooling of gases originating from a process for the partial combustion of hydrocarbons which gases contain soot-particles are known in which such helically coiled tubes are applied.  In this patent specification the problem of the deposition causes fouling of the tubes and consequently a decrease of the heat transmission. 
The fouling problem is probably caused by thermophoresis, which is a phenomenon whereby small particles suspended in hot fluid move toward locations with a lower temperature under the influence of a temperature gradient.  It is known that particles suspended in hot fluids tend to deposit on the cold walls of piping in chemical plants causing fouling of the heat transfer surface.  For further information on this phenomenon reference is made to Ind. Eng.  Chem.  Process Des.  Develop.,  Vol.  13, No.  4, 1974, pp.  408-415.  The particles are fine particles of a size which is in the range of about 1-5 micron. 
Besides the problem of fouling by deposition of particles there is another problem which arises if the hot gases transport small particles of which a part is very fine having a size&lt;1 micron.  If those particles, which are mostly soot-containing ash particles, deposit on the surface of inner tubes of a heat exchanger due to thermophoresis not only the heat transmission will be reduced but the particles will also carburize the tube material by diffusion of carbon atoms into the wall. 
The life of heat-exchanger tubes may be governed by the brittleness due to carburization of the tube. 
Still another problem is created if a part of the small particles present in the hot gases are coarser, i.e., having a size&gt;5 micron.  These coarser particles may cause erosion of the heat exchanger tubes if the small particle-containing gases pass through the tubes with velocities which exceed certain limits. 
In order to avoid deposition of particles on tube walls under influence of thermophoresis quite high velocities of the small particle-containing gases are necessary which however may create the above erosion problem if coarser particles are also present in the gases to be cooled.