Processing Materials with Forced Air Convection

November 1, 2001
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A new forced air convection system can provide a lower-cost, energy-efficient, more environmentally friendly alternative for many material processing applications.

Material manufacturing involves several complex processing steps, many of which use heat to accelerate drying, firing, sintering and other operations. These processing steps are usually performed with gas or oil-fired burners in kilns or other types of equipment that often require a great deal of time and energy to operate.

A new forced air convection system* has been developed that can provide a lower-cost, energy-efficient, more environmentally friendly alternative for many material processing applications. The new system uses a special class of heating elements to heat ambient air, and the hot air is delivered through a host of replaceable nozzles to perform the desired operation. The system can achieve temperatures ranging from room temperature to 1400?C (2552?F) with infinitely variable volume flow rates and no harmful emissions, providing a beneficial new method for drying, firing, sintering, melting, heat treating, and modifying the surface and shape of ceramic and metallic materials.

Figure 1. Simulated heat treating uniformity.

Convective vs. Radiative Heating

Unlike conventional burners and heat sources, which typically use gas or other fuels to heat the product through radiative, line-of-sight methods, the forced air convection system is an electrical device that uses molybdenum disilicide resistance heating elements as its heat source. These elements, which are powered with anywhere from 0.5 kW to 60 kW of energy (depending on the application and required temperature), are specially designed to deliver high temperature under convective load heating conditions. Convective heating ensures that the desired temperature uniformly surrounds the materials or parts being processed, allowing heat treaters to process parts faster with substantially less part failure caused by non-uniform processing (see Figure 1).

Figure 2. The higher temperature forced air convection system with a plasma stream.
Additionally, because the system is powered by electricity, it is up to 70% more efficient than conventional gas-powered systems.

For applications below 1000?C (1832?F), such as drying, preheating, curing and melting, the elements heat the ambient air. A blower then directs the heated air through nozzles into the processing chamber, heating it and the workload by forced convection. The exit temperature of the air or inert gas is controlled by adjusting the input of electric power or the gas flow rate through the blower. The heating provided by the unit is controllable from a very gentle convection to high-flow convection by adjusting the blower speed.

For applications that require temperatures from 1000-1400?C, such as sintering, curing, debonding, fusing and heat treating, hot air convection systems are available that catalytically ionize the hot air and emit an environmentally benign plasma stream (see Figure 2). The plasma enables the system to reach higher temperatures while still using uniform convective heating and air or inert gas as the heating method.

Environmental Benefits

The air, inert gas or plasma emitted by the forced air convection system is pollution-free. Unlike flame-based technologies, which can cause explosions, generate noise pollution and produce carbon soot and other harmful emissions, the forced air convection system operates silently and does not produce any harmful, pungent or contaminant gases that could harm the user or the part being heated.

User-Friendly Operation

Sophisticated electronics, with features such as soft starting, programmable heating and data acquisition, are integrated into the system to make processing operations user-friendly.1

A lot of flexibility is also built into the system. For instance, production engineers can use argon, compressed air, steam, nitrogen or ambient air (using a blower) for the outlet gas, depending on the application and atmosphere required.

Nozzles, too, can be customized to the end use application. They can be obtained in different sizes and configurations for operations such as plasma, bulk mass heating, drying and melting.

The system can be used as either an integral component of a convection oven or a stand-alone heating device (in place of conventional burners) in batch, conveyor or continuous pusher furnaces. In many cases, the system can be used to augment a conventional system, such as infrared radiation, that is already in place. Additionally, because the forced air convection system operates with a broad temperature range, a single unit can often be used for multiple processing operations, such as drying, sintering and firing, meaning that less operating space and equipment are required.

Figure 3. Typical forced air convection system with a blower mounted on top. The accompanying chart is a temperature versus flow rate, indicating an inverse relation.

Factors Affecting Operation

The forced air convection system can be successfully used in almost any heat treating application within the ambient-to-1400?C temperature range (see sidebar). However, a number of factors influence the system’s performance in demanding applications, including the required volume flow rate, exit temperature, the angle of incidence, nozzle design configurations and the velocity of the hot air.

Volume flow rates and exit temperatures have an inverse relation, as seen in Figure 3. As the required cubic feet per minute (cfm) of flow increases, the temperature that can be achieved by the system drops. For applications that require both a high flow rate and high exit temperatures, multiple systems may be needed. Three-phase systems, comprising three forced air convection units operated with one controller, can provide an efficient solution in these situations.

The heating rate depends on the direction of airflow (angle of incidence). For optimum performance, the forced air convection system should be installed at a 180?, 90? or 45? angle. Other angles, including inverted, will also work.

The temperature of the exit air or gas also affects the speed and success of the system. In general, the higher the temperature of the exited air, the faster the materials or parts can be processed. However, such temperatures are usually limited by the materials’ or parts’ characteristics.

The nozzle configuration also affects the system’s operation by affecting the velocity of the hot air delivered, which, in turn, influences the rate of drying. In general, the higher the air velocity, the faster the drying rate will be. However, as with temperature, the application typically determines the required air velocity.

Competitive Manufacturing

High energy prices and the need to control production costs are making it necessary for ceramic and glass manufacturers to re-evaluate their operations. Equipment that was once considered essential in thermal material processing operations must now be reconsidered in light of new technologies. With an efficient heat transfer, flexible and highly controllable operation, low equipment cost and non-polluting operation, the new forced air convection system is an alternative worth considering as manufacturers strive to remain competitive in the global market.



Figure A. A continuous kiln for drying ceramic materials using the forced air convection system coupled with steam.

SIDEBAR: Successful Applications of Forced Air Convection

Drying.
Improper drying conditions often lead to the formation of blisters on coatings and cracks on the product’s surface. In conventional industrial drying systems, convection heating of the material is limited by the heat transfer coefficient, hb, for the static boundary layer between the moving air and the static liquid on the surface. Infrared radiation, another type of commercial drying technology, does not efficiently penetrate deeply into the material because the waves are scattered by the particle system.

To prevent these problems, the forced air convection system allows tight control of the volume flow rate of hot air for a given drying temperature. This flexibility in fine-tuning the system’s parameters helps the operator accurately and efficiently control the drying process.

For accelerated drying, hot air from the forced air convection system can be coupled with super-heated steam to permit safe, efficient drying of sensitive materials over 100¿C (see Figure A). Because the specific heat capacity of steam (2010 J/Kg/¿C) is twice that of air at 100¿C, the steam transfers more heat for the same mass flow.1 Steam also promotes an increased ability to percolate through a moist ceramic body, as the viscosity of steam is approximately half that of air at the same temperature, thus reducing the drying cycle. Hot air coupled with steam will eliminate the moisture’s surface tension while reducing its viscosity to half. The absence of surface tension helps the surface moisture evaporate rapidly, while the lower viscosity forces the internal moisture to migrate rapidly to the body surface.

In contrast, the new forced air convection system promotes higher velocity, reduces the thickness of the boundary layer, and increases the transportation of heat and the apparent evaporation rate. The rate of evaporation (RE) using the forced air convection system can be expressed as:

RE = hb(TA-TL)/LE

where TA and TL are temperature of air and liquid, respectively, and LE is the latent heat of evaporation. The new forced air convection system operates at a higher air velocity and produces higher (TA-TL) values, and both of these parameters promote high evaporation rates, as seen from the equation.

While infrared emitters radiate heat that can be used for flat surfaces with minimal losses due to reflection, infrared radiation heats only those surfaces of the part that are in the line of sight of the device.2 As a result, the part attains non-uniform temperature, which, in turn, causes differential expansion of thin film coatings and substrates. Combining the forced air convection technology with infrared radiation heating offers the advantage of heat convection, leading to uniform part temperature even in deeply contoured and hidden surfaces of parts, thus eliminating spalling and enhancing the drying rate as it rapidly drives off moisture.

Sintering Hard Ceramics
The new forced air convection technology can be coupled with continuous belt operation to manufacture higher density silicon nitride machine tool inserts, which can be used to machine extremely hard materials. Tests revealed that the new system provided lower humidity and improved uniform heating during the sintering operation, and these characteristics yielded inserts with 3% higher density, maximum shrinkage and no warping or micro-cracks. The increase in density is believed to improve tool life.

Heat Treating Carbonaceous Materials
When used with continuous belt operation, the forced air convection technology’s cyclic “heating-cooling” type of loading has enabled it to be used to dry or continuously treat the surface of fibrous, carbonaceous materials in a host of heat loss environments. In conventional flame-based systems, burning, oxidation and non-uniform heating often occur, which adversely affect the integrity of the fibrous material. Combining the new technology with steam for convective heat transfer applications has made it possible to attain the intended goals of drying and heat treating without adverse effects due to the added ability to closely control and monitor the temperature and flow rate.

Melting Alloy Ingots
The higher temperature plasma forced air convection technology has been used to melt and cast aluminum alloy ingots in a fireclay crucible with graphite wash. The plasma acts as a protective or inert atmosphere over the melt and prevents oxidation, resulting in cleaner alloy ingots. Ingots made using the plasma technology had a bright shining surface, and losses due to dross formation were less than 0.5%, compared to furnace melted and cast ingots, which are generally 3%.

Shaping Composite Glass
Forced air convection technology coupled with infrared heating can also be used for shaping fiber reinforced composite glass in situations where infrared heating alone is not sufficient. Glass shaped with this technology has exhibited low residual stresses and higher impact resistance, resulting in improved life.

References
1. Frank Sims, Engineering Formulas, Industrial Press Inc., 1996, p.185.
2. Michael Grande, “Combining Infrared & Convection Heating,” Process Heating, September 2001, pp. 26-29.

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