Direct-Fired Dryers – How Do They Work?
Direct-fired dryers operate in one of two modes: co-current or counter-current. In either of the above arrangements, the operation of a direct-fired rotary dryer is based on the combustion system (burner) being directly coupled to the rotating shell. Therefore, the combustion process (flame) occurs inside the rotating shell.
The wet material is fed into one end of the rotating drum and travels to the other end where it discharges as dry product onto a transfer conveyor. As the material travels the length of the drum it is picked up by lifting flights attached to the inside of the shell’s circumference. It is then “showered” across the full diameter of the drum. This allows the transfer of heat to the product and allows the moisture to be released.
Due to this inherent design the unit operates in a high-performance mode. This allows the dryer to use the heat generated by the burner to its maximum efficiency utilizing all three forms of heat transfer. This is especially prevalent with an application involving various particle sizes.
It has the further benefits of being a forgiving machine as it can comfortably handle swings in percent moisture along with varying particle size and process rates. In addition, it does not require the monitoring assistance of a skilled technician.
Components in a direct-fired rotary dryer include:
1. Burner to provide the heated gas.
2. Rotating drum to tumble and lift the material into the gas stream.
3. Air-exhaust system to remove the vapor-laden gas.