Spray dryer comprising a drying cylinder provided at one end with a spray nozzle and hot primary air injecting tube surrounding said nozzle has been known.
In such a drying equipment, the products tend to deposit on the cylinder wall and recycle into the high temperature zone. In order to do away with such defects, various methods have been devised, but they all failed. One of the most commonly employed methods is the introduction of the hot secondary air concurrently to the main flow, but it is not so successful as expected.
  According to this, the above mentioned defects may be effectively overcome by introducing tangential air currents from the inlets arranged arround the discharge end of the cylinder. Within the cylinder, said air currents spirally flow along the cylinder wall and envelope completely the main flow consisting of the particles and hot primary air whereby the deposit of the products on the cylinder wall may be completely avoided. As said air currents fill the vacant space adjacent the spray zone the back eddying of the particle may also be effectively avoided.
  Finally, said spiral air currents turn their courses and flow back to the discharge end of the cylinder together with the main flow. 
  The above mentioned function of the spiral air currents were observed through peep holes provided in the cylinder wall with the introduction of paper strips.
  It is to be noted that not whole of the said countercurrents follow the above mentioned courses, but small portion of said countercurrents flows back to the discharge end of the cylinder together with the main flow before reaching the spray zone.