Our process engineers were recently asked if a rotary dryer could operate under positive pressure. We thought it would be helpful to share the answer and provide a concise review of the distinct roles of positive and negative pressure within a Rotary Dryer system.
Atmospheric pressure is the downward force from the weight of air
Air Pressure, How Does It Work?
First, let’s consider a non-industrial description of air pressure. In the context of barometric pressure as reported by a weather forecaster, we essentially live at the bottom of a sea of air. That air is pushing down on our bodies at a force of about 14.7 pounds per square inch (atmospheric pressure at sea level).
In a closed system such as one with an attached pressure gauge that is set to zero, when suction is applied to the attached line in that system, air would be withdrawn from the system, and the gauge would then read less than zero. Pressure that is lower than barometric pressure is negative pressure. Whereas if air was blown into the system, the gauge would read positive.
There are positive and negative air pressures
Similarly, in terms of pressure within a building, when you have a fan blowing air out of your house, you can open a window and then fresh air will enter the house – because the pressure in your house would be negative (lower than atmospheric pressure).
If you were to turn that same fan around – and blow air into your house, then the air already in the house would exhaust through the open window because the building is being positively pressurized.
How Air Pressure Affects the Drying Process
As it applies to a rotary drying vessel, a positive pressure is created at the burner from the combustion blower system while the rotary dryer is kept at a slight negative pressure to maintain burner flame envelope and to facilitate evacuation of evaporated moisture and dust particles in a controlled manner.
Dryer Pressures affect burner flame shape and efficiency
That negative pressure is controlled by the induced draft fan which can be adjusted to increase the air velocity through the dryer to allow more of the material dust particles to be directed out of the product stream to the Dust Collection system which reduces fines in the finished product. Conversely, the air velocity through the dryer can be reduced to allow more of the finer particles to drop out of the air stream prior to leaving the dryer.
A Switchgrass Dryer and Dust Collection System by Louisville Dryer Company
With each of those factors in mind, the system’s fan should be properly configured so that the dryer is always operating under a slight negative pressure. Under this condition, combustion gases and noxious vapors can be directed to the air pollution control system and not escape through the seams.
A thorough understanding of how pressure dynamics relate to heat management, moisture control and dust flow is integral to the proper design and efficient operation of industrial drying equipment. So, rotary dryer owners and operators should wonder – and confirm – how well their dryers operate under pressure.
A three-stage classification system by Louisville Dryer Company. The knockout boxes and baghouse integrated with the rotary dryer effectively evacuate dust, fines and moisture from the drying process.
Louisville Dryer Company engineers have extensive knowledge and proven experience providing air/exhaust-handling solutions and complete dryer systems for customers throughout the U.S. and the world – from material assessment/product testing to the custom design, fabrication, installation, commissioning and servicing of all components – all of which helps you accomplish your business goals and our professional commitment to your Lowest Cost per Revolution.