When you would need a Steam Tube Dryer vs. a Direct Heat Dryer

When you would need a Steam Tube Dryer vs. a Direct Heat Dryer

Jul 28, 2020 | Technology

Choosing the right drying system is a significant investment and critical to your process. It is essential to know the variations between the different dryer options. The heat source is one of the most common differences. Read on and consider the following reasons why we often recommend a steam tube dryer over a direct heat dryer.

 

Steam Tube Drying vs. Direct Heat Drying: What’s the Difference? 

 

Steam Tube Drying  

Steam tube dryers perform as if it were a rotating tube with a shell heat exchanger. The drum is fitted with a high-pressure chamber that distributes steam into tubes running the length of the drum. Because its applications are for low-temperature processes, the equipment required for thermal destruction is reduced or eliminated. 

 

Steam Tube Dryers Are the Most Efficient Industrial Dryer

The most efficient of all the dryers, the steam tube dryer has very little air being exhausted – making the stack loss significantly reduced. The air pollution control system is also much smaller in a steam tube dyer and, since only a minor amount of air is used to sweep out the water vapors, the exhaust volume is a fraction compared to that of a direct heat dryer.

Steam Tube Dryers are also one of the easiest dryer types to operate. Once the steam pressure is set, the unit will take what it needs to do the job. Think of it as having a set-it-and-forget-it technology.

A steam tube dryer is used extensively in processing high-moisture organic materials such as distiller’s grain, gluten feed from the wet milling industry, oilseeds, and a variety of sludges.

steam tube dryer

 

How Does an Industrial Steam Tube Dryer Work?

Material is fed into the drum and tumbles over the outside of the inner tubes. Inside those tubes, steam is collapsing to condensate. The latent heat from the steam—about 850 BTUs per pound—provides the heat energy for drying. The condensate from the steam is removed through a rotary joint. It is then taken back to the boiler for regeneration.

 

Direct Heat Drying

 

How Does A Direct Heat Rotary Dryer Work?

A direct heat rotary dryer uses a hot gas which is induced into a rotary drum. Its heat source may be as simple as a steam coil in low-temperature applications, or it can have a burner for higher temperatures. The burner may combust into a chamber or it may fire directly into the dryer drum. We use the term direct-fired when there is no combustion chamber.

direct heat dryer

 

Direct heat dryers typically work best when the burner system can operate with a minimal amount of excess air or near stoichiometric balance of the gas and air. This allows the dryer to minimize the air required to carry the thermal energy.

 

 

frac sand dryer

 

Wet material is fed into the same end of the drum (parallel flow) or the opposite end of the drum (counterflow) and is picked up and showered (or veiled) into the hot gas stream. The thermal energy is transferred from the hot gas to the material, which heats the solids and the water, ultimately evaporating the water. The primary heat transfer is by convection.

 

frac sand dryer

If your product is not affected by heat from the burner flame or the products of combustion, then a direct-fired unit will provide an efficient drying solution through the full utilization of all three heat transfer methods: radiation, convection, and conduction.

Heat-sensitive materials such as biomass and organic products could be processed with either a direct heat dryer (with an air heater to control the temperature) or a steam tube dryer.

 

 

Why Choose a Steam Tube Dryer over a Direct Heat Dryer? 

 

Developed more than 100 years ago, the Louisville Steam Tube Dryer has always been a quality choice for drying high moisture organic by-products such as brewers’ and distillers’ spent grain. Today, this same dryer technology serves thousands of applications. 

 

What Materials Does an Industrial Steam Tube Dryer Process?

A steam tube drying system is our recommended choice for drying and processing inorganic and organic chemicals and other bulk solid materials including, but not limited to: 

  • Hemp
  • Dried distillers grains (DDG, DDGS)
  • Wet corn (fiber, grits, meal)
  • Wood chips
  • Soda ash
  • Paper mill sludge
  • Oil Seed (soy, sunflower, canola)
  • Soy meal
  • Chemical products
  • Petrochemical products (TA, PTA)
  • Lithium processing
  • Copper concentrate

All our dryers, including the steam tube system, are built in our state-of-the-art factory using proven technologies. Our in-house design team of engineers have vast experience in every type of process and industry; thus, our customers always receive a drying system that is custom fit for their operation. 

 

5 Benefits of A Steam Tube Drying System  

 

#1: More Cost-Effective

Louisville Steam Tube Dryers have been proven superior to other types of indirect-heat dryer installations in which hot gas, rather than steam, is used as the heat source. We provide solutions with the Lowest Cost per Revolution throughout our industries. Maintenance savings alone have often prompted the replacement of other dryer systems with the steam tube dryer.

 

 

#2: Better Quality

A steam tube dryer is classified as an ASME vessel. Thus, the requirements for its construction are more stringent than for other dryers. 

 

 

 

#3: More Efficient 

The steam tube dryer generally operates at a lower temperature than other dryers and rotates at a slower speed. Material tumbles gently around tubes that rotate with the shell, eliminating the friction forces that other types of dryers require.

 

 

 

#4: Easier Maintenance

Louisville Steam Tube Dryers enjoy a longer lifespan than most other dryers and require very little maintenance.

 

#5: Environmentally Friendly

Steam Tube Dryers are environmentally friendly. Since the steam tube dryer utilizes the latent heat of steam to drive the drying process, only a small amount of sweep air is required to remove the water vapor driven off the product generated in the drying process. Typically, the method uses less than about 30% of the exhaust gas required for a direct heat dryer. 

 

Purchasing A Steam Tube Dryer 

 

Because choosing the right drying system is a major investment and critical to your process, you should be sure you are getting the best equipment available. The Louisville Dryer Company manufacturing facilities hold esteemed accreditation by the American Society of Mechanical Engineers (ASME) and The National Board of Boiler and Pressure Vessel Inspectors. 

We extend personal attention and service to each customer, ensuring that their rotary processing equipment needs are efficiently met. If you are interested in learning more about the Louisville Dryer Company Steam Tube Drying System and its applications, please reach out directly to Hank Lawson, our Senior Process Engineer. He would be happy to listen and discuss the best options available for your application.

 

Call (800) 735-3163 or fill out our online contact form to get started.

 

Can a Rotary Dryer Operate Under Positive Pressure?

Can a Rotary Dryer Operate Under Positive Pressure?

Jul 14, 2020 | Technology

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 Effects 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 effect 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.

To Learn More

Learn about our high-pressure steam tube dryers and other steam tube dryers, and then to request help designing your industrial drying solution, contact our team today.