The liquor to be evaporated is run in at the end nearest the chimney from the tank placed above the chamber c. A number of cast-iron fanners i, dip into the liquor and revolve rapidly, usually at the rate of about 300 revolutions per {183} minute, producing and filling the chamber with a very fine spray, thus presenting a very large evaporating surface.
Between the furnace and the evaporator are placed the chambers c and f. In c a number of brick walls d are so placed that the flames from the furnace are intercepted and broken up. The object of this is to give time for all the products of combustion to be thoroughly burnt up, which would not be the case without the “smell-consumer,” as these chambers are called. This part is an addition to the original evaporator, and was devised by Messrs. Menzies and Davis. The liquor after having been concentrated in the chamber k runs into a trough placed alongside the doors h and flows into one or other of the furnace beds b where it is still further concentrated, and the residue ignited by the flames from the fires a. The draught can be regulated by the damper g, and also by one placed near the shaft j. The doors e, in the smell-consuming chamber, are for the purpose of cleaning out. The fanners i are worked by a small steam engine, not shown in the drawing. Under properly regulated conditions very excellent results can be obtained with this evaporator. The temperature of the gases near the chimney should not be higher than about 85°. By running the fanners at a very high speed the temperature of the gases may be even further reduced, thus showing the completeness of the evaporation.
This form of evaporator is open to the objection that the whole of the sulphur in the coal employed for the furnaces, finds its way into the recovered soda. It combines with the alkali to form sulphite of soda, part of which is decomposed in the furnace with formation of sodium sulphate, sulphide, and other sulphur compounds. The same objection, of course, applies, though perhaps in a less degree, to all systems of evaporation in which the flame is in contact with the liquor to be evaporated.
The Porion evaporator can be erected at very small cost, and costs but little for maintenance. It is capable of producing 3⁄4 ton of recovered soda per ton of coal with liquors {184} of the usual strength. It has proved itself to be perhaps the most economical evaporator existing.
Some time ago there was erected in Lancashire an evaporator invented and patented by Mr. Alfred Chapman. It is shown in Figs. 71, 72, 73, and 74. The evaporation is effected at a low temperature in three vacuum pans E, and with the unusual result that the concentrated liquor gelatinises after leaving the third vacuum-pan, instead of taking the ordinary form of the concentrated products of other evaporators. It is said that this apparatus gives an excellent product, with great economy of labour and water, and with no drainage of foul liquor from the buildings. Observations extending over three months have proved that it evaporates 22 lb. of water from the liquor per lb. of coal used under the boiler. It is however very costly to erect.
FIG. 71.
FIG. 72.
FIG. 73.