Regular readers of Process Heating understand that each of the myriad dryer designs can effectively remove moisture from a range of materials. But, each type of dryer also comes with capabilities and characteristics that offer either advantages or disadvantages based on the materials to be processed and requirements of each application. Also, because each individual application has unique characteristics — even when processing the same materials — determining the “ideal drying system” can be a daunting task for anyone, including an experienced process engineer.
For purposes of this article, an “ideal drying system” refers not necessarily to the most or least expensive, or the most innovative or eco-friendly design. Instead, it is the most suitable drying system: the system that most effectively meets the required drying specifications with efficiency, consistency and reliability. Ideally, the dryer also operates at the lowest possible cost, considering both initial and ongoing operating costs.
From this perspective, it becomes clear that a process engineer exploring whether to invest in a new drying system — to most effectively, efficiently produce a new product, or to cut costs in producing an existing product — would be well served in talking to design engineers at a number of dryer manufacturers. For instance, designers and manufacturers of rotary dryers, belt conveyor dryers, vibrating fluid-bed dryers and indirect trough dryers — with long experience processing powders, granules, pellets and other materials — can offer insights into which dryers excel in which conditions.
To assist these engineers in helping you determine the most suitable drying system for the application, be sure to bring as much knowledge about your product, facility, budget and other areas as possible at the outset. Most dryer manufacturers are more than happy to sign a non-disclosure agreement, and the more information shared in early conversations, the more quickly and accurately guidance, recommendations and quotes can be provided.
To help streamline your dryer specifying process, be prepare to share these critical pieces of information:
- Knowledge of your product and facility.
- Familiarity with your electronics.
- Awareness of your maintenance capabilities.
- Knowledge of which regulations apply to your facility, process and product.
This article will take a closer look at each aspect.
Automatic weirs are shown on the side of this vibrating fluid-bed dryer. These gates go up and down in timed cycles under PLC control to allow adjustments to the retention time and promote plug flow.
Know Your Product
To devise a drying system that produces an end product at discharge that meets a desired set of characteristics, the engineer needs to know both:
- The characteristics of the desired end product.
- The nature of the upstream product as it flows into the drying system.
Simply describing the target as “similar to fish food” or “a lot like wheat flour,” or describing the material at infeed as a slurry, is a start, but it is not quite enough detail. It is essential to bring particle-size information to the table. Unfortunately, many companies run their processing lines without providing plant personnel with a particle-size analyzer. This is vital knowledge for the dryer design.
If the particle size is relatively large — say, more than an inch — then a vibrating fluid-bed dryer usually can be put on the back burner. Products like cookies, loaves of bread and larger agglomerates would require too much energy to fluidize economically. These larger products with high internal moisture require long residence times for proper evaporation. This often suggests a belt conveyor dryer as the solution. By one measurement of drying efficiency, a belt conveyor dryer’s typical heat transfer coefficient — 17 BTU/hr/F2/°F — might be considered a drawback for some applications. With products like cookies, breads and larger agglomerates, it can be an advantage, however. (By contrast, a vibrating fluid-bed dryer delivers 171 BTU/hr/F2/°F — several times greater heat transfer efficiency.)
Larger particles such as aggregates and mining materials may point to a rotary dryer as the initial solution. These heated, rotating drums handle the high capacities needed to process large volumes of sand, potash and other materials. At the same time, rotary dryers are inherently tough on the particles, however, due to the type of contact sustained as they fall during repeated rotation. This means the design may be more suitable for low value materials where there is minimal concern about product loss or changes in particle size.
As particles become smaller than approximately one inch in size, indirect trough and vibrating fluid-bed dryers become more viable. Both types can process pellets, granules and fine powders down to 10 microns.
Indirect trough dryers are simple to manufacture, offer ease of access for cleaning and can be sealed to prevent contamination risks. The heat transfer efficiency coefficient is 7 BTU/hr/F2/°F, which may make it impractical and costly to operate for some products. As already noted, the vibrating fluid-bed dryer offers notably higher heat transfer efficiency.
Yet, particle size is not the only factor. Particle shape, moisture content, type of moisture, flowability, stickiness, bulk density, specific heat and temperature ceiling are among the many factors involved. Each characteristic needs to be known at both infeed and discharge whenever possible.
In addition, given that designing the most suitable system demands weighing the advantages and disadvantages of each dryer configuration, it helps to know in advance which target characteristics are essential for meeting product quality requirements, and which targets may be stretched. For example, does a delicate product such as snack chips need to be discharged completely intact at a given size and shape, or is there an amount of breakage allowed? If allowed, by how much can that amount be stretched? If particle integrity is paramount, then a gentle process must be considered.
How the material arrives at the infeed also affects the specification process. Some drying systems — the belt conveyor and indirect trough dryers, for instance — require the material to enter the infeed in a single layer of uniform bed depth to allow the drying process to work at peak efficiency. This means auxiliary equipment for setting the bed depth must be included in the system design (at additional cost). This issue typically does not apply to rotary dryers because its rotation continually lifts, turns and drops the material during the process. Auxiliary infeed equipment also is not required for vibrating fluid-bed dryers because the design inherently disperses the product flow across the width of the bed as a fluid. As such, it forms a uniform bed depth every time without auxiliary equipment at the infeed.
This vibrating fluid-bed dryer sets an integral baghouse dust collector directly above the drying zone to capture fine particles entrained in the airstream and return them to the process as product. This enables fluid-bed drying to be used with particles as small as 10 microns without product loss.
Know Your Facility
In most cases, a new drying system must fit into an existing facility — and, often, into an existing processing line. Either way, the new drying system must fit within a specific footprint and height clearance. Because this can quickly rule out dryers that require long spans of floor space, bringing this information to prospective dryer manufacturers upfront can quickly narrow down the choices.
For example, suppose you want to dry 3,000 lb of a material with 30 percent moisture content using 300°F (149°C) air to 5 percent moisture content. A belt-style dryer would require approximately 323 ft2 of space, or an area 48’ long by 7’ wide. By contrast, a vibrating fluid-bed dryer would require 36 ft2 of drying area, or an area 12’ long by 3’ wide. Even if adding up to 100 ft2 to account for the heating air supply, the difference is significant.
Providing a floor plan and dimensions of the existing space is recommended. It is even more helpful if the electrical, gas and steam system locations are marked. Indirect dryers often generate heat using steam or hot oil. Belt dryers and fluid-bed dryers typically generate heat using a direct-fired gas furnace, which may require less equipment and infrastructure, depending on what is already in place.
In some cases, replacing less efficient systems with new drying systems that fit on tight footprints opens plant floor space while allowing auxiliary equipment — screw conveyors, valves, drums and ductwork, for example — to be removed. It helps to know where corners, columns and mezzanines may impact the design, and whether there is any flexibility available for reconfiguring existing machinery and equipment. When quoting, the design engineers can provide a projected layout of the floor plan to represent how the line may change if the project moves forward.
Know Your Electronics
Today, nearly every type of dryer comes with some level of electronic technology for improved process monitoring and control. These PLC-based systems can be integrated with central process PLCs to improve control over temperature and product flow. They also can provide a constant, real-time view of product temperature, exhaust air temperature, product moisture at infeed and discharge, and other process variables.
Be sure to bring an understanding of the type of technology currently in use in your facility to ensure the new drying system includes compatible technology. In plants with aging equipment, the new drying system may be used as the springboard for upgrading the level of technology up and down the processing line.
For smaller-particle products such as pellets, granules and fine particle powders, vibrating fluid-bed dryers can process materials down to 10 microns.
Know Your Maintenance
Belt conveyor dryers use a wire-mesh belt with a chain drive to convey product through an oven or dryer. This design requires a large number of moving parts and mechanical connections. Lubrication is needed periodically, and cleaning requires accessing many nooks and crannies.
Rotary dryers use less moving parts; however, inspecting the interior for wear and performing periodic cleaning and maintenance can be a hazardous job in a confined space.
Vibrating fluid-bed dryers use an external mechanical drive to create the vibratory action. This eliminates the need for gears, chains and other drive parts along with the gaps and cracks that can hide product in a belt. Some fluid-bed dryers use powered hoists and quick-release clamps to allow safe access to the interior for cleaning.
Because some drying systems demand more maintenance than others, it is important to consider the maintenance capabilities in the plant.
How many technicians on staff are trained to properly disassemble and completely clean a drying system to FDA requirements, then put it back together and restart the process — while keeping the product on-spec? If a part needs to be repaired or replaced, does anyone know how to diagnose and fix the problem? How easy is it to find people with these skills today? Given the high cost of downtime and the challenge of finding exceptional maintenance talent, choosing a dryer designed with few moving parts may offer a maintenance advantage.
Know Your Regulations
Most dryer manufacturers know the safety and environmental regulations involved at the federal level. They can offer guidance on designing a drying system that meets these requirements and minimizes the need for permitting and reporting. Regulations at the state and local levels, however, may conflict with federal regulations and with each other. Though many dryer manufacturers are well versed on relevant regulations at the state level, and in some localities based on experience, experienced engineers would recommend verifying the regulatory requirements involved at the plant’s location early in the process. Regulatory requirements can significantly affect the choice of drying system.
For example, the amount and type of process air allowed to be exhausted to the environment differs from place to place. An installation located where natural gas is inexpensive and the process parameters suggest exhausting 100 percent of the heated process air may appear to be a sensible approach, but this scenario may not be permitted in some locations. In this case, an experienced engineer may recommend capturing a portion of the process air, removing particulates in an integral baghouse dust collector, and recycling the clean, dry air back to the inlet to preheat ambient air in route to the drying zone. This type drying system can pay for itself in energy savings within two years and help keep the facility within its air permitting requirements.
In conclusion, with so many different approaches to drying and so many approaches suitable for the job, it makes sense to consult with a number of engineering professionals for guidance on the most suitable drying system for your application. As manufacturers, these engineers spend every day, year after year, considering how to design drying systems for every type of material on the planet, and their guidance and expertise represent significant value. In fact, the bulk of the time and energy involved in manufacturing a drying system happens upfront during the design- and quote-development phases.
To ease and speed this part of the process — and to help keep the investment as low as possible — begin the conversation armed with as much detailed information as is possible. Then, be sure to test the product on full-size machinery to simulate actual process conditions as closely as possible. One last word of caution: consider carefully any company that avoids including a guarantee.