While product quality and consistency are key process requirements, operating costs and, in particular, energy consumption also are critical to the commercial success of any organization. The dryer/roaster is usually one of the most energy-demanding processes — and one of the most crucial to the product. Fortunately, forced convection air dryers are versatile and, therefore, are employed in myriad applications in many industries, including food, pharma, chemical, tobacco, biomass, textile, nonwoven fibers.
Sometimes these dryers are static or batch units. More commonly, however, they are conveyorized units with a perforated conveyor, or bed plates, to allow the passage of air through the plate and product. These types of dryers are capable of air temperatures from ambient to approximately 374°F (190°C).
The term “dryer” is somewhat of a misnomer because these units normally are required to carry out a number of functions within the same operation. Reduction of water or volatile content is one of the key functions. Others can include acceleration or instigations of chemical reactions as well as textural or other physical changes like color or logarithmic pathogen load reduction (more commonly referred to as “log kill”).
As an example, in the past, a “roaster” was simply a convection dryer capable of air temperatures of up to 356°F (180°C); able to impart color, texture and flavor to ground and tree nuts; and able to deliver a reduction of moisture for shelf life. Dryer buyers today typically demand these factors, within much tighter moisture/volatile losses, along with a certain log kill (pathogen logarithmic reduction), within the same process. Such requirements demand maximum flexibility and control within the unit as well as ultra-hygienic designs and features.
To give optimal versatility, the dryer is normally linear and divided into distinct and separated zones. Each zone has its own heat source, which can be direct gas, indirect gas, thermal oil, dry saturated steam or electrical. Coupled with this are zonal air velocity, air direction and humidity. To further optimize the operating efficiency of these units, please consider the following practices.
Basic dryer housekeeping and maintenance includes ensuring that the airflow circuit is not restricted.
1. Basic Housekeeping and Process
No apology is made for making this the number one point on optimization. Most issues reported to dryer OEMs concerning product quality, capacity, consistency, operating costs or a combination of these relate to the basics. Yet the basics often are overlooked, and the issue is presumed to be due to more complex reasons.
Under the presumption that burners, heat exchangers, thermal oil or steam boilers are maintained and operating effectively, there are some basic areas to explore: airflow circuit and product depth.
The Airflow Circuit. Ensure that the perforations on the conveyor plates are not blocking and restricting airflow. If this is the case, there are various conveyor cleaning devices that can be employed, either “on the fly” or periodically such as after each shift. Also, make sure to check the inside of air exhaust stacks. Volatiles, fats and oils can condense within exhaust stacks, leading to a restriction and resistance that the exhaust fan cannot overcome. This issue is particularly prevalent in the ground nut roasting industry.
Product Bed Depth. It is of critical importance to ensure a consistent and steady flow of product to the dryer. Uneven and inconsistent flow will vary the bed depth. Air will take the path of least resistance. Any areas of thinner bed depth will be overdried while thicker depths will remain wet.
This also applies to any feeding devices. These are normally provided by the OEM because this factor also is critical. Ensure that they are providing complete and even product bed depth across the full width of the conveyor. Ultrasonic/laser sensors across the feed end can be employed to ensure an even feed is being presented.
After clean down and maintenance, routinely perform the following checks:
- Check the rotational direction of fans after major maintenance.
- Check the calibration of probes and sensors, and check for broken wires or common issues such as bent temperature sensor tips.
- Regularly check that there is no air leakage from the feed and delivery ends (for example, if the dryer is operating at slightly negative pressure). Also, check that any zonal curtains and partitions are set correctly, minimizing any air exchange between zones.
- Check that any air distribution plates or veins are set at their correct positions after cleaning and that any zonal partitions such as curtains are present and set properly.
Every dryer should be designed for a specific product or range of products. The overall configuration is dependent on the drying curve for the product. Each product has unique drying characteristics even if the material seems closely related. For instance, the drying curves for three different rubber crumb products are shown.
Distribution plates and zonal partitions are critical in ensuring an even airflow across the width of the dryer. In the case of distribution plates, these often are sectioned for ease of removal. The sections may have different percentages of open areas. If replaced incorrectly, this can lead to varying airflow and temperatures from one side of the bed to the other.
In addition to the points already mentioned, also perform these checks:
- Check the integrity of any Teflon or similar sealing strip between conveyor and dryer.
- Check any silicone or metal-to-metal door, roof and floor seals for wear and air leakages. Areas where seals appear to be intact but material has become brittle lead to a poor seal, air escape and energy loss. Replace brittle seals to restore proper air sealing.
This list is by no means exhaustive, but it covers the most commonly experienced issues. All OEMs supply operating manuals, which normally contain a maintenance and troubleshooting section. So, one last to-do for the basics: Do familiarize yourself with the recommendations in the OEM manual.
Most OEM dryer manufacturers can assist with laboratory-scale testing at their facility. In such tests, all product and process parameters can be optimized without compromising production time or risking rejected product.
2. Product Type, Capacity and Operating Conditions
While this is a broad subject that requires an in-depth knowledge of drying technology, if a few rudimentary principles are kept in mind, in-depth knowledge is not essential.
Capacity. Every dryer is designed for a specific product or range of products. The product will dictate the zonal configuration in terms of length ratio and energy load required (air temperature), as well as humidity and air velocity and direction. The overall configuration is dependent on the drying curve for the product. Each product has unique drying characteristics even if the material seems closely related.
As a result, issues in energy load, product consistency and quality can be seen under certain circumstances:
- Product changes.
- Capacity changes.
First, the product may have changed. For example, its evaporative load may be greater or less. Or, if the particle size has changed, it may have other, less apparent impacts such as product agglomeration, which will affect evaporative rate and consistency (as will any variability in particle size). Increased particle size will increase drying time or reduce capacity.
Second, the capacity requirement may have increased or decreased. It is not atypical to find dryers operating beyond their design capacity. As this capacity increases over the limit, efficiency starts to decrease in terms of energy consumption. A point is reached where temperature setpoints, capacity or product quality are no longer achievable or acceptable.
Running under capacity also has issues. Normally, product mass loading is decreased, reducing bed depth. This, in turn, requires a lower airflow in each zone, which then requires the exhaust fan volume to be reduced because the drying rate and humidity within each zone will change. The residence time also will need to be adjusted.
It is worth remembering that if one factor in the operation is changed, it will influence all of the others. In cases of reduced capacity, it is a far better policy to run the dryer under its design conditions but achieve the required capacity in fewer operating hours.
Overcapacity has two possible solutions: Extend the square footage of drying capacity, or optimize the existing dryer’s performance. The latter may involve changing the process parameters.
Many facilities will not be able increase the size of the dryer. However, optimizing the existing dryer’s performance is achievable. First, ensure all of the elements of the dryer are maintained and cleaned. Second, optimize the bed depth. The aim is to increase bed depth to a maximum level while maintaining finished product requirements. To do so, slowly decrease the product conveyor speed incrementally while monitoring the finished product moisture and other characteristics. Running deeper and slower will increase evaporation until the resistance of the increased product bed reduces the airflow to the point that humidity of the leaving air begins to inhibit the process (wet-bulb depression). If these steps do not yield sufficient additional capacity, then consider adding more air-circulation fans.
In the above process, the monitoring of finished product moisture should show the requirement to adjust the exhaust volume. If the above process is deemed to be too risky or time consuming, most OEM dryer manufacturers can assist with laboratory-scale testing at their facility. In such tests, all product and process parameters can be optimized without lost production time or the risk of rejected product.
The importance of the PLC system cannot be overemphasized in optimizing performance. PLCs offer the ability to reduce human error and automatically take into account variables in climatic conditions. In some cases, the PLC can even account for product mass flow and volatiles variations within the climatic conditions.
The importance of the PLC system cannot be overemphasized in optimizing performance. PLCs offer the ability to reduce human error. Also, they can automatically take into account variables in climatic conditions and, in some cases, product mass flow and volatiles variations. Once regarded as a costly option but now deemed essential by most industries, PLCs assist with some key functions. They include zonal automatic humidity control, sequential operation, feed forward/feed-back product flow and product bed pressure monitoring.
Zonal Automatic Humidity Control. This feature helps ensure that during seasonal or day/night climatic humidity and evaporative load changes, the percent humidity setpoint is maintained consistently. This function is best suited to lower temperature drying (212°F [100°C] or below).
Sequential Operation. PLCs can optimize operation during sequential zonal shutdown, dryer idle mode and run in, run out mode. This capability minimizes product waste at the start and finish of production, and it helps to optimize energy usage during these stages or during production stoppages.
Feed-Forward/Feed-Back System. This refers to mass flow of product into the dryer and percentage of volatiles by weight, allowing the dryer to make slight adjustments to compensate if standard deviation is exceeded. In some processes, it also is possible to monitor finished product volatiles and allow the dryer, within certain parameters, to compensate.
Product Bed Pressure Monitoring Per Zone. This ensures that there is no major disruption or compaction of the bed.
There are, of course, other benefits of a PLC: datalogging and traceability, security (multi-level access), and menu-driven programming, enabling the changing of all process parameters with a single command.