Note: A fan generally turns slower than a blower and therefore cannot reach the same pressure differential (inlet / outlet). Fluidizing a batch of coffee requires more pressure than a fan delivers.
The easy, but wasteful, way to circulate hot air is to heat it after it passes through the blower. This means that all the air has to be heated from ambient temperature to final temperature before it is vented while, if the air is recirculated, only the lost heat needs to be replaced. The higher the temperature the more efficient the recirculation process becomes. The advantage is not a few percentage points but rather tens of percentage points.
Our prime interest is roasting coffee which becomes much more efficient when the heated air is recirculated. No off the shelf unit could be purchased. Initially we tried then modified an existing name brand blower. The MTBF (mean time before failures) improved from the initial few hours to over 1,500 hours but the maintenance and problems required a new solution. The concept was simple but mastering the quirks was not always a linear process.
The designed temperature ceiling for the current model is 300°C but it has only been operated up to 270°C. Since we did not find this to be an optimum roasting temperature the importance of establishing a higher operating threshold lost its urgency. There is reason to believe that the upper temperature limit can be raised significantly.Apart from surviving the high temperature the priority is to develop sufficient pressure to create a fluidized bed so that the coffee beans are evenly heated and continually mixed. The pressure, while roasting, is just under 1 psi (approximately 6 kPa). This is the flowing pressure not dead head pressure.
The current blower configuration is driven by a 3 Hp electrical motor connected to a three phase contactor based motor controller. The Belt Guard Cover was removed for the photo.
Have a freshly roasted coffee and think of another application for the unit.
