The submerged combustion of a gas, such as natural gas, has been used for the
concentration of very corrosive liquors, including spent pickle liquors, weak phosphoric
and sulphuric acids. A suitable burner for direct immersion in the liquor, as developed by
SWINDIN(25)
, is shown in Figure 14.16. The depth of immersion of the burner is determined
by the time of heat absorption and, for example, a 50 mm burner may be immersed by
250 mm and a 175 mm burner by about 450 mm. The efficiency of heat absorption is
measured by the difference between the temperature of the liquid and that of the gases
leaving the surface, values of 2–5 deg K being obtained in practice. The great attraction
of this technique, apart from the ability to handle corrosive liquors, is the very great
heat release obtained and the almost instantaneous transmission of the heat to the liquid,
typically 70 MW/m3.
14.7.3. Natural circulation evaporators
Whilst each of the previous types of evaporator is of considerable importance in a given
industry, it is the steam-heated evaporator that is the most widely used unit in the process
industries and this is now considered in detail. In Chapter 9 of Volume 1, it is shown that
the movement of the liquid over the heating surface has a marked influence on the rate
of heat transfer, and it is thus convenient to classify evaporators according to the method
of agitation or the nature of the circulation of the liquor over the heating surface. On this
basis evaporators may be divided into three main types:
(a) Natural circulation units.
(b) Forced circulation units.
(c) Film-type units.