Choosing the Right Dryer for Your Process

Drying systems are a major investment and a critical wood pellet process component.
By Nolan Johnson | August 24, 2021

Choosing the right drying system is a significant investment and critical to your process. Therefore, it is essential to understand the variations of different dryer options. One of the major differences is the heat source—i.e., steam tube drying vs. direct heat drying.  

Steam Tube Drying  
Steam tube dryers are essentially rotating tubes with shell heat exchangers. 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.

Very efficient, the steam tube dryer has little air being exhausted, significantly reducing stack loss. The air pollution control system is quite small in a steam tube dryer, and since only a minor amount of air is used to sweep out the water vapors, the exhaust volume is a much less than that of a direct heat dryer.

Steam tube dryers are 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. They are 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.

As for how industrial steam tube dryers 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 British thermal units per pound—provides the heat energy for drying. The condensate from the steam is removed through a rotary joint and then taken back to the boiler for regeneration.

Direct Heat Drying
A direct heat rotary dryer uses hot gas that 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 fire directly into the dryer drum (direct-fired meaning there is no combustion chamber).

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.

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.

 If the product is not affected by the burner flame heat 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.

Choosing a Steam Tube Dryer
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, DDGs, wet corn, wood chips, soda ash, paper mill sludge, soy meal and many more. The following are some of the benefits of this type of dryer.

Cost-effective. Steam tube dryers  use steam as the heat source. The exhaust gas from the dryer is about 80% less than what is required for direct heat dryers, which significantly reduces the size and cost of the air pollution control equipment.

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

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. Heat loss through the cylinder wall is minimal since the steam tubes are fully enclosed by the dryer.

Environmentally friendly. Steam tube dryers are environmentally friendly. Since they utilize 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 during the drying process. Typically, this method uses less than about 30% of the exhaust gas required for a direct heat dryer. The air pollution system is much smaller, and the permitted stack load is less.

Safe to operate. Due to a high ratio of water to air, the atmosphere inside the dryer is nearly inert, which greatly reduces the opportunity for fire.

In summary, drying systems are a major investment. Choosing the right system is critical to processes, so operators should be sure they are getting the best equipment available.


Author: Nolan Johnson
Manager, Application Engineering,
Louisville Dryer Co.,
www.louisvilledryer.com