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Due to its inherent characteristics, the thermal processing of powders (including calcination, sintering, carburization, etc.) is primarily performed in rotary tube
furnaces. Rotary furnaces are usually the preferred processing unit for flowable powdered or granular materials. The rotary tube furnace, under the proper conditions, is significantly more energy efficient than the pusher tunnel furnace.
The energy requirement of heating the rigid furniture of the pusher tunnel furnace is replaced by the stationary but rotating tube of the rotary, which sets the rotary apart with significant lower total energy consumption. What is less widely known about the rotary, however, is its capability of becoming even more energy efficient with the use of tube internals.
The use of tube internals in a rotary furnace can increase heat transfer surface area, improve radial mixing, improve gas-solid interactions, control gas flow direction, and even control material dwell time or velocity through the tube. All of these can result in a reduction of residence time required or increase unit capacity for a given volume of furnace, while also improving product quality and the control of quality and uniformity.
In its most elementary function, a tube internal for a rotary furnace increases the surface area for heat transfer. Through its mere presence in the heating section of the rotary furnace, the internal will be maintained at temperature (lower than the temperature of the tube wall but higher than the process material, thermal conductivity of steel vs. bulk material) and, in turn, provide another surface (other than the inner wall of the process tube) to transfer heat to the product.
This increased heat transfer area can be thought of as the fins or plates of a heat exchanger; an increased number of fins or plates of a heat exchanger increases the total heat transfer of the exchanger. When considered in the preliminary designs of the rotary furnace, the added heat transfer surface area can reduce the required total processing length of the rotary furnace. If the intention is to add internals to an existing rotary furnace, it is conceivable that the added heat transfer surface area may reduce the required residence time for processing and subsequently increase the mass throughput capabilities of the unit.
Increasing surface area not only aids in the heating of the process material, but it can also help with cooling the product post-processing. Many rotary tube cooling
Increasing surface area not only aids in the heating of the process material, but it can also help with cooling the product post-processing.
chambers do not contain internals in the overall design. Instead, they typically include a simple, water-cooled jacket around the outside of the rotary tube—which is not always efficient. By introducing more surface area inside the tube, the material encounters more cold surfaces and cools more efficiently.
Some designs not only use the increased surface area of a tube internal, conducting heat just through the tube wall, but also allow the cooling water to flow inside the internals to yet again increase the total cooling efficiency (see Figure 1). In this design, the pipes running through the process area are open to the outside of the tube such that cooling water can flow through without wetting the process material, thus allowing for an increase in cooling surface without relying solely on the heat transfer through conduction with the rotary tube wall.
Product homogeneity and consistency are vital for all high-value, advanced materials. When processing in a rotary furnace with specific tube internals, such as flat bar lifters (see Figure 2), consistency can be greatly improved by increasing the radial mixing of the material.
The flat bar lifter, when sized below the height of the bed of material, causes agitation in the bed of material as it is conveyed by the rotary tube and increases envelopment. If the lifter were to be sized larger than the bed of material, a greater bed turn-over is achieved—but possibly at a cost. A greater chance of reduced yields due to entrainment and dusting can occur if the material particle size is too small and the processing gas velocity is too high.
Reaction gas is vital for many processes, such as aluminum nitriding and boron nitriding. The need for consistent contact of the material bed with the reaction gas determines whether or not the reaction takes place. Knowing this, it’s not always necessary to reduce the tube fill percent in order to ensure gas penetration through the entire bed to obtain the needed gas-solid reaction.
The addition of uniquely designed lifters in the rotary furnace can bring material up along the tube walls to a curtain point of discharge where the material is dropped through the process gas, thus increasing the time exposure and amount of material exposed to the process gas.
Depending on the reaction, contact of the process off-gas(es) with the final product can hinder quality. This commonly occurs while off-gassing moisture and trying to prevent the vapor from condensing on the dry, reacted product.
In some cases, off-gas contact with the raw feed material can also hinder processing. If this is an issue, strategic placement of a solid core helical flight or Archimedes screw may be the solution (see Figure 3). The helical flight or Archimedes screw can create enough of a barrier to prevent unfavorable gas migration.
A rotary furnace is a great piece of thermal equipment for large agglomerates or irregularly shaped materials. Without internal flights in the rotary tube, irregularly shaped materials and agglomerates may segregate themselves due to the tumbling flow through the tube. This phenomenon can be controlled with the use of an Archimedes screw that is continuous throughout the entire tube length. In this case, the rotary tube angle does not play as large of a role in determining the residence time of the material. Instead, the pitch of the flight paired with the tube rotation determines the exact residence time.
In many cases, when processing in the rotary, a short section of a helical flight is installed at the entrance of the tube. This is done to ensure that material does not build up at the entrance, causing the material to spill forward into the entrance chamber or worse, plug up the tube.
Custom thermal processing equipment manufacturers have recently designed a rotary tube furnace used in the demilitarization of excess, obsolete and unserviceable munitions, done so for safety and environmental purposes. One of the key features that made this project successful was the specific design for tube internals.
This particular design had to consider the controlled migration of material through the tube while acting as containment, preventing chain reactions due to rapid exothermic reactions, controlling gas flow through the reaction area and—most significantly—forcing a specified key reaction location within the rotary tube. Functional tube internals can improve processing efficiency and product quality, but may make the difference between success and failure.
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