During the initial planning of an industrial dryer process, so many of the details become critical factors for the success of the final installation. Equipment and processes are calculated and designed specifically based upon the details provided by the equipment operator. So, providing accurate information during the design and planning phases of an industrial dryer is important to ensure capacity and performance. Once the equipment is installed, available remedies are often costly and limited. Further, addressing situations reactively creates delays, which can ultimately impact the schedule for process startup.
Planning a startup can be intimidating, making it easy to overlook small details. Taking the time upfront to ensure every detail is correct will save time, money and stress in the future. Here are four common situations where important details can be overlooked during the design and specification process. Understanding the perils can help you avoid them.
1. Secure the Instrumentation Needed During Industrial Dryer Startup and Commissioning
Having the proper instrumentation during startup operation is critical for having accurate measurements and data regarding the process. Instrumentation includes any device noted in the process diagram (thermal sensors, pressure indicators, speed sensors, etc.). Information from the instrumentation is important for making the correct adjustments to equipment and validating process performance.
Because instrumentation can be relatively minor compared to larger critical items for a process installation, it often is overlooked in scope of supply. Spare items are necessary as replacements for faulty instrumentation, and they should be readily available onsite during a startup. Spare items are also necessary if it is determined that additional measurements are needed to ensure process stability while adjustments are made during the startup process.
Lack of required instrumentation for equipment startup will increase the cost of the process commissioning by increasing labor hours for startup technicians. Lack of instrumentation also can delay the startup schedule, which can be extremely costly in terms of lost revenue and profit.
To avoid these pitfalls, plan to have all necessary instrumentation to take any critical process measurement, and be sure it is comprehensively included in a scope of supply. Also, be sure to order spare instrumentation with the scope of supply.
2. Optimize the Size and Capacity of Downstream Equipment
Having equipment downstream of a dryer with capacity in excess of design requirements is advantageous for unexpected process disruptions. If an operation needs to stop during a production run, the capacity to continue extracting material from the dryer will prevent maintenance and possible safety issues. Because the dryer is a thermal process, a buildup or stoppage of material in the dryer will result unstable process conditions. Depending upon process temperature and material, stopping material flow inside a dryer will typically result in damaged or lost product and even a risk of fire. A complete buildup of material in a fluid-bed dryer requires a manual clean out (confined space) with a process shutdown.
If the equipment immediately downstream of the dryer can handle additional material, this additional capacity can act as a buffer during unplanned line stoppages. As a general rule, having excess capacity of 30 percent for downstream equipment will provide flexibility and contingency against process disruptions.
3. Carefully Select Upstream Equipment, Especially the Industrial Dryer Feed System
Having good product enter the dryer is a critical step in getting good product out of the dryer. And, having reliable equipment upstream of the dryer that provides a consistent product — both in terms of moisture content and consistent feed rate — is essential to successful dryer performance.
The system that feeds material into the dryer is critical for consistency in product flow. Inconsistent product flow into the dryer creates inconsistent drying results. The material characteristics of the product — flowability, angle of repose and particle size and shape, among others — should be considered carefully before selecting the feed system for an industrial dryer application.
4. Ensuring the Inlet Moisture Content of the Product
The most critical and most significant factor for dryer specification is the amount of moisture the dryer must be designed to remove. After installation, the opportunity to adjust or increase the amount of moisture the dryer is able to handle is limited. The worst-case scenario for dryer installation is a calculation or estimation of moisture-removal requirements during the design and specification phase that proves deficient.
Because the dryer and related equipment (burner, exhaust system, etc.) is application specific, all of the equipment is sized according to the inlet moisture specification. The limitation to respond to a product moisture level beyond the design criteria is not only because of the size of the dryer unit itself. It also is due to the design limitations of the associated equipment that support the dryer.
Product-moisture content levels above the dryer design specification can result from any number of possible causes. One example is moisture-content information derived from product samples not representative of the worst-case scenario. An example of this scenario is raw material that is stored outside, exposed to changes in weather. Rain and changes in the ambient air temperature, which fluctuate by season, affect the moisture-content levels of the raw materials. Samples of raw materials used to develop design criteria should be representative of the product and include worst-case scenarios. The design specification of the dryer should include the maximum moisture content. Changes in raw-material suppliers, changes to the process and issues with equipment upstream in the process from the dryer are also possible causes for moisture-content levels that exceed design conditions.
Once the equipment is installed, as noted, few remedies exist for addressing moisture levels beyond the specified design range. Typically, there are three possible options to increase dryer system performance after installation:
- Increase temperature.
- Increase airflow.
- Increase residence time.
The first option is to increase temperature. Some products are sensitive to heat, and for those, the maximum temperature is limited. Also, increasing temperature depends on there being excess capacity in the heat supply system. If the dryer system was undersized initially, the burner system would likely be undersized in parallel.
The second option is to increase the amount of airflow. By increasing the volume of air, more dry air is able to evaporate and remove moisture from the product. This option is also limited based upon the excess capacity of the air supply system (fans).
If the product is being dried in a fluid-bed dryer, increasing airflow also increases the pressure drop across the product bed. Smaller particles of otherwise good product will be carried into the exhaust system. This will create unnecessary product loss. In addition, it will increase the dust loading of the exhaust air beyond the design specification of the exhaust filtration system.
A third option is to increase residence time of the product in the dryer system. Examples of dryer system adjustments that typically increase residence time include increasing the height of a weir on a fluid-bed dryer, adjusting the angle of flights/lifter paddles inside a rotary drum dryer or reducing the drum’s rotations per minute within a rotary-drum dryer. These adjustments will increase the cycle of time product through the dryer and, ultimately, reduce the throughput of the overall process.
Ideally, the maximum moisture condition of the product is studied and well known during the design and specification phase of the equipment. To avoid designing a dryer that cannot achieve the moisture-content levels needed for your process, take multiple moisture samples to ensure a representative analysis of inlet moisture content during the design phase. Also, design for the maximum worst-case condition of moisture in product, including all associated equipment such as the heating system, controls and exhaust air treatment.
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