Spray dryers basically have three essential functions of (1) spraying feed liquid, (2) drying resulting microdrops and (3) separating and recovering a resulting fine powder product, and ordinarily have an atomizer, a drying chamber and a fine powder recovering unit, respectively, corresponding to the above functions.
As examples of these spray dryers, there have hitherto been known those shown in FIGS. 9 and 10 (see Japanese Patent Publication No. 32601/1983 and Japanese Utility Model Laid-Open No. 26950/1983).
In the spray dryer of FIG. 9, a feed liquid is sprayed into a drying chamber 1 from a nozzle 5 and is momentarily heated by hot air from an inlet 2, whereby the liquid component of the feed liquid is evaporated and the solid component is made into a fine powder. The majority of the fine powder is taken out as a product via a rotary valve 6. Part of the fine powder accompanies hot air, passes a waste gas outlet 3 and enters a cyclone 7, and is recovered from a rotary valve 8.
The spray dryer of FIG. 10 is a type using, as an atomizer, a rotary spraying disc (a rotary disc) 10 in place of the nozzle 5.
In the spray dryers as shown in FIGS. 9 and 10, a fine powder product is recovered separately in the main section of the dryer and in the cyclone. In this case, a powder of lighter particles having smaller particle diameters is recovered in the cyclone and a powder of heavier particles having larger particle diameters is taken out from the main section; accordingly, slight compositional variations in the product occurs.
In the past, this compositional variation posed little problem depending upon the type of product.
It has been required recently that spray dryers can perform, instead of conventional few grade, large quantity type production, many grade, small quantity type production especially for medicines, fine ceramics, etc.
Also, it is increasingly required that spray dryers produce a product of higher purity.
Under these new situations requiring a spray dryer capable of providing many grade, small quantity type production and a powder of the higher purity, the abovementioned compositional variation of product which posed little problem in the past has entered a new phase. Moreover a new problem has arisen.
That is, with the increase in the frequency of product switch-over, the rapid and complete washing of the deposit adhering to the inner wall of the drying chamber has become necessary. It is necessary because if washing takes a long time it is detrimental to the efficient execution of many grade, small quantity type production and also because the complete washing of the deposit adhering to the inner wall of the drying chamber, which is required in the product switchover when a product of higher purity and higher quality is needed, is very difficult. Moreover, when a product harmful to humans is produced, washing is extremely difficult because the operator is prohibited from making direct contact with the product.
Further, the requirement for a product of higher purity has turned the above-mentioned slight compositional variation among products into a problem.