Concluding my discussion, here are some additional concepts that can be implemented to improve your dryer's performance and potentially increase its output.
Recover Dry Product Energy. Another way to harness the heat in your dryer is to recover some of the dry product energy. Cooling the dry product with atmospheric air will increase that air's temperature. This gain can be used effectively as the inlet air to a prior zone or to preheat the feed product. One limitation is that this approach only serves as a benefit if the product needs to be cooled by specification. The cost of the fan and the energy required to operate it may prove more expensive than the potential savings from recovering the heat if cooling is not required.
Increasing Air Velocity. Increasing air velocity increases the drying rate for many applications. The increase is most significant where the moisture to be removed is free, or unbound, moisture. Obviously, in applications where air velocity is critical to the operation -- flash or fluid bed dryers, for example -- this option must be evaluated carefully as it may impair the process.
Provide Relief on the Final Moisture Content Specification. In many instances, the product's final moisture content can be increased without any detrimental effect on the post process. Easing this requirement can significantly improve the overall production. As an alternative, it may be advantageous to install a second dryer to remove the last, small fraction of moisture. Frequently referred to as two-stage drying, this approach offers benefits in both energy consumed and production due to reduced airflows and heat requirements for such a small fraction of moisture removal.
Implement Alternate Energy Sources. In many processes, an additional or alternate energy source can be introduced to improve the dryer performance. For example, many existing installations can benefit from a pre- or post-drying zone employing infrared, convection or other heating methods to increase production.
Increase the Feed Residence. For certain products, it can be beneficial to increase feed residence time by increasing solids loading and reducing throughput. This will allow a longer soak time in the unit, effectively increasing the drying rate per unit mass.
Reduce Moisture Loading. Moisture is introduced to the dryer by the feed, the process air and, in certain instances, by reaction, such as combustion. Reducing this loading allows the energy to be better utilized on the drying process.
For example, mechanically dewatering the feed to reduce its initial moisture content is far less expensive per unit of moisture removed than thermally evaporating it. Wherever possible, mechanical dewatering techniques -- filtration (vacuum, pressure, membrane, etc.), concentration, air knives, centrifugation, etc. -- should be employed. Also, it may be advantageous to change your current mechanical dewatering system to a more efficient method. For instance, concentrates can be dewatered on vacuum filters to approximately 25% moisture (wet basis). Membrane pressure filters can achieve final moistures below 10% for the same concentrate.
Using Dry Air. Using dry air for the process air reduces the quantity of moisture in the air that requires heating and vaporization. For small volumes of air, using desiccant or dehumidifying techniques will reduce air moisture levels effectively, but for larger volumes, this becomes impractical. In very humid environments, however, conditioning of the air will reduce the energy requirements.
Implement Mechanical Enhancements. Other mechanical enhancements can further improve dryer performance.
- Reduce Losses. Clearly, energy losses to the atmosphere -- whether caused by surface radiation, leakage of process air, product discharge temperature being too high, or exhaust temperature being too high -- are unfavorable.
Prevent Leakage. Leaks reduce the operation's effectiveness. Ingressive leaks dilute the air and expend valuable energy on heating up this additional air and any moisture in it. Egressive leaks result in the loss of process air and will decrease the unit's performance.
- Insulation. Insulation will contain the energy for the process. All surfaces should be insulated appropriately -- with the correct material, thickness and installation quality -- to restrain heat from being lost.
- Maintain Utility Supply Lines. Utilities such as steam, fuel, compressed air, etc., should be regularly maintained to control losses. These losses are unrecoverable and will contribute to the overall operating cost of the system.
- Inspect and Repair Dryer Fabric. Check the internal surfaces of the dryer for buildup of foreign materials, clogging and product buildup, which reduce the surface areas of the dryer and alter the operating or design condition. Buildup also acts as insulation, inhibiting heat transfer to the feed.