Claiming that a dryer can dry or dehydrate almost any solid may seem like a bold and sweeping statement. Yet, it is true. Continuous drying systems will dehydrate or dry a range of products: fruits, vegetables, extruded starch snack and cereal products, natural breakfast cereals, granolas, nuts and seeds, minerals, chemicals such as metal oxides, filter cakes, charcoal briquettes, alginates and gelatins, super-absorbent polymers, fibers and tobacco products, to name a few.
Often, continuous-drying systems are overlooked in favor of rotary or fluid-bed technology. At first glance, rotary and fluid-bed systems may initially appear to process some materials at faster rates and at a greater consistency. Given the profusion of continuous industrial dryers and dehydrators available however, such is not the case. Continuous dryers are offered with options that can optimize thermal transfer, product handling, maintenance, capital cost and overall operating costs. This article will explore the pros and cons of each type of drying system.
With the use of linear, conveyorized, forced-convection dryers and ovens, all the aforementioned products can — and are — produced. Depending on the specific application and design, these forced-convection units typically have a temperature range of between ambient and 392°F (200°C) maximum, dry-bulb temperature. The air velocities normally are fully adjustable, ranging to 500 ft/min or, at times, beyond. Energy transfer rates up to 100 W/m2K are possible, subject to material and process conditions.
Not all products are equal in the treatment required, however. For this reason, logical range limits are incorporated within the control system. These limits can be applied to air temperature, velocity and process air humidity, according to specific requirements.
Conveyors can be perforated, hinged plates, or woven or mesh conveyors, depending on the product loading and air mass flow required.
Each product carries its own handling requirement. For instance, consider how to process rubber compounds. Typically, the conveyors themselves can be perforated, hinged plates, or woven or mesh conveyors, depending on the product loading and air mass flow required. Perforated plates are favored. This type of conveyor always has a reinforcing underside plate called a girt. Some conveyors use a double girt.
The correct selection of conveyor-plate gauge and girt ensures deflection does not occur even under severe loading by dense materials such as minerals or rubber compounds. Any deflection can — and usually does — result in uneven drying as well as the inevitable premature failure of the conveyor or its drive system.
These heavy-duty conveyors typically are driven by roller-side chain-and-sprocket systems. Attached to each plate is a side-traveling guard. The guard retains the product on the conveyor and eliminates air bypass around the sides. This drive arrangement adds to the ability to process heavy loads and helps increase system longevity. Product loadings in excess of 1200 lb/ft2 are not uncommon.
The conveyor plates can be treated to form a nonstick surface, either by coating them with materials such as PTFE or by electroless polishing, which gives a mirror-like finish. The plates themselves typically are 304 or 316 grades stainless, depending on application.
The conveyor plates can be treated to form a nonstick surface, either by coating them with materials such as PTFE or by electroless polishing, which gives a mirror-like finish.
Handling Floor-Space Limitations
It is true that most fluid-bed dryers will perform the same drying function approximately one-third faster than forced-convection units — providing that fluid-bed technology is suitable in the first place. It should come as no surprise, however, to learn that a fluid-bed drying system’s drying area is also proportionally reduced.
That said, installations with limited floor space are not precluded from using forced-convection dryers. Several options minimize the floor-space requirements for forced-convection dryers.
In process applications, product drying and treatment times can vary from just a few minutes to as much as 15 hours, depending on the product. In cases of long drying times, multiple horizontal overlapping conveyors can be used within one dryer body to minimize floor-space requirements. Each conveyor is designed with independent frequency-controlled drives, allowing for an increase in product-bed height at each critical stage. In this way, the overall thermal treatment area is minimized. Such a design also allows for good product release from the conveyor. On certain materials, failing to take product release into account could cause issues with agglomeration and adhesion.
If space is at a premium on the factory floor, then the conveyor configuration can be vertical rather than horizontal.
Most fluid-bed dryers will perform the same drying function approximately one-third faster than forced-convection units. It then follows that the drying area is also reduced proportionally.
There are, of course, limitations to conveyor dryers in terms of how fine a material can be while being transported. This criterion very much depends on the material’s physical characteristics, conveyor open area and type of conveyor used.
As a rule of thumb, the limitation on free-flowing materials is 0.04”. Because conveyor dryers are designed to transport product without product agitation — as a function of its conveying action or airflow — it is possible to process materials that contain a particle size lower than 0.04” without sieving out the smaller fractions. In small particles do fall out of the product bulk during processing, various fines-recovery systems can be employed within the dryer. Such systems optimize yield, minimize the need for frequent cleaning, and help avoid hygiene issues.
In fluid-bed systems, the air velocity will carry over less-dense particles in the airstream. These must be removed with cyclones or solid-media filtration prior to airflow recirculation or exhaust.
This is not the case with forced-convection units, where velocities through the product are typically within the 180 to 200 ft/min range. Indeed, the object is to not disturb the bed load and maintain an even differential pressure throughout the process.
As a result, the footprint of a forced-convection drying system does not need to contain external cyclones or other filters, or any onboard filtration. Recall that, as previously noted, the process speeds and square-footage drying requirements of fluid-bed drying systems are typically one-third less than forced convection dryers. Eliminating the need for external filtration brings the overall footprint of both units much closer. In cases of vertically stacked or overlapping conveyors, the overall footprint of the forced-convection unit often will be less than that of a typical fluid-bed dryer.
Extruders can be used to produce extruded noodle strands from materials such as red/yellow oxides, titanium dioxide, clays, laponites and filter cake media.
Operating costs do not just relate to the total energy consumption per pound of finished product, of course. Instead, they also include downtime for cleaning, preventive maintenance and other necessary activities.
For the purpose of this simple comparison, however, if only energy (electrical horsepower and BTU energy) is considered, then a few general statements apply when comparing most forced-convection units against typical fluidizing dryers.
Given the same product, capacity, evaporative load, energy source and climatic conditions, the typical fluid-bed unit will consume approximately 15 to 18 percent more overall energy than a typical forced-convection unit. The reasons behind this are numerous but mostly relate to the following:
- Forced-convection dryers rely on psychrometry to recirculate and replenish air only as required, according to the absolute humidity within the linear zones. They must have negative pressure at both ends (feed and delivery) to ensure air does not leak into the plant, which reduces efficiency slightly.
- Fluidizing units rely on high velocity air to maintain a fluid-bed condition. This does not in itself mean a large mass flow. However, to balance the airflow without air leakage, these units normally require a high exhaust volume and fresh-air intake. These required air additions must be brought up to the operating temperature.
This is a simplified explanation, and it does not take into account any energy-recovery systems that each type of dryer may contain. It reflects and is verified by what I have found in the field. Processors should carefully evaluate energy consumption assumptions among drying systems before selecting the style most suitable for the process.
Continuous Dryer Processing 101
Understanding the basics about operating a continuous-bed dryer will help optimize operations and minimize drying time.
Product-Bed Loading. An even loading of product — in terms of bed depth and density — is critical. This helps ensure the heated air mass flow is evenly distributed through the product bed and helps accomplish energy transfer and volatiles evaporation.
Several different feeding devices can be used to ensure proper bed depth and density. The feeder used depends on the product’s physical characteristics and the required capacity. One feeder option is a rolling extruder, which can produce extruded noodle-like strands from materials such as red/yellow oxides, titanium dioxide, clays, laponites and filter-cake media. Rolling extruders are well suited for thixotropic materials because the shear forces through them are minimal. Also, they are designed to extrude over the usable width of the dryer.
A feeder option for free-flowing materials is a simple hopper with adjustable-discharge gates. Alternatively, hydraulic or electric oscillating spreaders can be used. These spreaders are capable of handling a range of products with varying characteristics.
The product-loading stage is so critical to the drying process that most dryer manufacturers will insist on supplying the feeding device. Uneven loading in terms of depth across the conveyor width, or blank spots on the conveyor, will inevitably lead to patchy drying, with some parts being overdried and some underdried. In many cases, ultrasonic or laser level sensors are placed over the feed-end to monitor the product depth on the left, right and middle of the conveyor.
Even Product Treatment. Having loaded the product as evenly as possible on the conveyor, the next objective is to ensure that air velocity is even from side to side and top to bottom of the product bed. This can be a challenge, with some effective conveyor widths being up to 14.5’ and product-bed depths to 3’.
Continuous dryers are offered with options that can optimize thermal transfer, product handling, maintenance, capital cost and overall operating costs.
The differential pressure of the product bed and conveyor have a significant influence on even airflow. Depending on the specific dryer design, either a horizontal or vertical air-distribution plate is used prior to air entering the treatment chamber. These plates typically consist of various open areas from one side to the other. The plates create the required pressure across the width of the dryer treatment area as well as ensuring adequate airflow onto the product.
The creation of independent zones along the dryer length allows for changes in air direction. These may alternate up and down, or combine those with other changes in direction, depending on the product’s requirements. The construction of these units allows for ambient or forced cooling to be incorporated within the same body. This ensures no overprocessing. Where product is to be stored or packed immediately after drying, this also eliminates the possibility of condensation forming in the pack or storage vessel. The cooling section typically is sized to give a finished product temperature within 60°F (33°C) of the ambient air temperature.
In conclusion, with the ever-changing demands of the markets, product versatility and future needs should be key considerations when selecting an industrial dryer. Can one unit produce your current or anticipated future products with ease of changeover? Will your capacity requirements escalate in the short, medium or long term?
No one has a crystal ball, and these questions are sometimes impossible to answer with any degree of accuracy. Long-term risk is increased on an investment that has a lifespan of many decades and a typical write-off period of 5 to 10 years.
Most forced-convection dryers are of a modular construction. Standard modules of an appropriate width are combined and divided into independent zones as required for the process. This allows for several benefits. The first is ease of separation of zones at an appropriate stage. Adding extra modules within a zone or adding a supplementary zone allows an existing drying system to accommodate product-mix changes. The second benefit is ease of reconfiguration of the zones along the length of the dryer. Changes such these can allow completely different product to be processed. Such retrofits and redesigning of the drying chamber are far more difficult — and, in some cases, impossible — with many fluid-bed dryers.