In the Shola - V heater, hot flue gases drawn by an induced draught created over the grate by the ID Fan & Chimney pass through 2 vertical rows of Tube - bundles on either side of the grate and over the cast surfaces. Ambient fresh air drawn from the sides, back and top by the dryer fan, picks up heat from the heated surfaces and enters the drying chamber. The principle of both 3 pass and 5 pass is the same except that in 5 pass, hot air is made to pass five times in the tube banks as against 3 times in the 3 pass. So, heat transfer area is increased by more than 1.5 times & this improves the heater efficiency.