The beginning of a movie or book sets the tone for the whole story. In a similar way, this also is true for continuous flow process dryers. Though instead of the first few pages or opening scenes, a process dryer starts with the introduction of the product to the dryer’s conveyor. This introduction is the most critical control point in the process. It affects capacity, efficiency and quality, and that, therefore, makes a good introduction essential to a successful drying story.
When introducing product to the dryer, the product must be presented at an appropriate volumetric feed and with a consistent application onto the dryer’s belt. The variance (in depth) of product placed on the belt should not be more than 10 percent of the specified depth. This means a dryer with a specified 5” bed depth should have no more than a 0.5” variation of product depth across the entire conveyor. This is a tall order and requires an operator’s attention. The consistency of depth, however, has a tremendous impact on product quality and the dryer’s efficiency and capacity.
Spreader Advantages and Disadvantages
|Spreader||Flowability||Advantages||Disadvantages||Product Depth Control|
|Hopper with adjustable gate||Free flowing||Simple, cost effective, no motorized mechanics||Clumping, clogging, pressure of product in hopper, must remain full to fill outside edges, can get some escaping exhaust gases||Gate height|
|Oscillating tube||Semi-flowing||Simple, cost effective, easy to airlock||Needs height to prevent flow restriction on direction changes||Belt speed|
|Vibrating pan||Semi-flowing||Low profile, fewer moving parts||Higher cost, hard to control with changing products with different characteristics, not as good for wider dryers, can break product shapes||Belt speed|
|Oscillating belt||Non-flowing||Applies the product without force, used for many products, controllable||Costly, requires significant dryer belt length for the conveyor arc||Belt speed|
|Wiper belt||Non-flowing||Applies minimal force to the product, used for many products, controllable||Pushes the product, which can cause damage||Belt speed|
TABLE 1. There are many types of spreaders and each is designed for different product characteristics.
The product characteristics will dictate how the material is distributed properly in the dryer. These characteristics include: product size (diameter, length and shape); product quality (moisture level, product stickiness and the amount of fines); and the flowability (free-, semi- or non-flowing). Knowing the product characteristics is critical to determining what type of spreader will work best.
If properly designed, a product spreader will handle even the most difficult product. (Typically, this is defined as the product with the least flowability due to shape, size and moisture). Each type of spreader — and there are many — is designed for different product characteristics. Common spreader types include a hopper with an adjustable gate, an oscillating tube or belt, and a belt wiper. Table 1 describes the advantages and disadvantages of specific spreader designs.
A consistent flow of volume of product must be delivered to the spreader for it to work properly. In some cases, the upstream process provides the volumetric feed. (Examples include extrusion and pelleting.) In other processes, the volumetric feed control must be implemented ahead of the dryer.
FIGURE 1. Even product distribution produces consistent airflow. Air follows the path of least resistance, so when product depth is inconsistent, drying will be inconsistent.
Volumetric Feed Boxes
With continuous flow dryers, volumetric feed boxes can be used as a volumetric control while also providing surge. This surge can prevent unwanted dryer shutdowns due to lack of product availability. A well-designed volumetric feeder box allows for a small amount of surge (roughly an hour or so of product storage). Depending upon the product, more storage volume can cause feeding problems.
The main purpose of the feeder box is to keep the dryer fed while providing precise volume control. As a result, it typically comprises a moving floor (auger or conveyor) and a distribution device (gate, rotary rake or choke) at the discharge. All functions should be controlled by a frequency drive to achieve the right volume.
The type of conveyor feeding the spreader is important. There needs to be a consistent flow without interruptions. Sometimes an auger or even a conveyor belt can cause issues if not operated at a proper speed (rpm). If the belt speed is too low, a conveyor or airlock can cause product surges. The result will be an inconsistent product depth on the dryer belt, and in many of these cases, the spreader cannot correct the volume irregularity due to surges.
A belt conveyor with chevron-type ribs is a preferred design to help prevent product surges. It offers consistent and repetitive cleats rather than distant cleats, which can create surges. Depending upon needed elevation of conveyance, however, the length required can be longer than other designs.
Entry of the product onto the spreader is critical to proper operation. In many cases, proper head room needs to be foreseen to allow the product to drop straight down and not at an angle. (Dropping at an angle can disrupt flow.) This is especially true with oscillating tube spreaders.
Keeping the dryer’s hopper full of product is important. The product must be able to fill to the outside edges of the hopper. Splitting the product stream with deflector plates, a rotating mixing bar or auger may be necessary to keep the hopper full. Also keep in mind that excess fines should be evenly distributed within the whole hopper.
The introduction of product to the dryer’s conveyor is a critical control point, affecting capacity, efficiency and quality.
When thinking about dryer design, understand that air takes the path of least resistance (figure 1). In the case of an uneven bed depth, the air will concentrate at the thin spots. This has a tremendous impact on the dryer’s performance. Inconsistent bed depth can affect:
- Capacity. A longer dwell time is needed to obtain required dryness, which can reduce capacity. In such cases, the dryer will never live up to its full potential.
- Efficiency. The dryer will need to run hotter to obtain needed dryness in thicker areas of product bed depth while thin spots will overdry. The results will be higher exhaust air temperature and wasted energy.
- Quality. Wet spots may clump and mold while dry spots can overheat.
- Safety. Thin spots can overheat due to the air channeling. This increases risk of a fire while also causing the increased discharge of fines from the dryer exhaust.
Like any good book, this is not where the story ends. Instead, it where the work starts.
A thorough dryer audit can uncover issues that are limiting a dryer’s performance. In many cases, however, the beginning is a great place to start: Examining the dryer and its product distribution mechanisms — and adjusting where needing — can provide a significant, positive impact on your dryer’s capacity, efficiency and quality.
Case in Point
Merell Brothers’ Biosolids Processing Facility
When the makers of FloridaGreen fertilizer were planning a new facility in Pasco County, Fla., the material handling to optimize the company’s biosolids drying process was top of mind. After evaluating a number of options, the company combined greenhouse drying with convection-heated belt drying to create its fertilizer via a treatment process known as PRFP, which requires pasteurizing the biosolids for 30 minutes at 158°F (70°C).
Ted Merrell, CEO of Merrell Bros., describes the critical necessity for even product distribution of the biosolids in the dryer. “Adjusting the depth of product in the day hopper which feeds our dryer has allowed the product to flow without disruptions. Prior to that adjustment, it was a constant battle to not get blow outs in the dryer due to surges out of the day hopper. This would clog our carbon filters and cause poor capacity and quality. It is important to balance the air with good product distribution.”