Coating plays a pivotal role in producing roofing granules that not only provide the desired aesthetic appearance, but that can also withstand constant exposure to the elements. Uniformly and consistently coating granules to spec, however, can be a challenge.
One of the best ways to achieve a quality, consistent coating on granules is to optimize the coating drum design around the specific characteristics of the material and the product goals. Here’s why.
The Need for an Engineered Coating Drum
The coating applied to granules can serve many purposes. In most cases, the coating provides resistance to water and subsequent weathering, while also meeting homeowner expectations in terms of appearance and durability. In more advanced applications, coatings can also be used to prevent or deter algae growth, inhibit UV aging, and even create reflectivity that reduces energy costs for the building.
All of these objectives have allowed the roofing industry to provide consumers with an advanced solution that is durable and long-lasting, but doing so relies heavily on the ability to coat granules uniformly and to spec every time.
A coating, no matter how advanced in terms of formulation and performance, is only as good as its application. This is because the coating must be able to create an impenetrable barrier between the granule and the environment.
Any imperfection in the coating application on the granule negates its ability to protect the granule by creating an entry point for water, UV rays, bacteria, and more. For this reason, coating application is a critical aspect to get right in the roofing granule production process.
Optimizing Roofing Granule Quality Through Coating Drum Design
The coating drum is the roofing granule industry’s preferred type of equipment for coating, offering a high throughput, good mixing, and reliable technology.
Optimizing the coating drum around the specific product goals and the feedstock characteristics is essential to being able to reliably and consistently coat materials to spec.
FEECO Coating Drum
Several variables in the coating process can be adjusted in order to hone-in on the desired results. The two most influential variables are the spray system design and the material bed action.
Spray System Design
The spray system works in concert with the rotating drum to disperse the right amount of coating, in the right location(s), and at the right rate. For this reason, spray system design is a highly application-specific endeavor.
Spray rate, spray locations, nozzle type, and coating temperature are the primary aspects of the spray system.
Coating drum spray system
Spray Rate
Spray rate refers to the amount of coating delivered to the material bed. The spray rate for each individual spray location is determined by the total amount of coating required, in combination with the size of the drum, bed action, and number of spray locations.
Spray Locations
Spray location substantially influences coating uniformity. Oftentimes, introducing the entire amount of coating at once could cause buildup and nonuniform coating. Instead, it is often better to introduce the coating at different points along the length of the drum. The most optimal locations, along with spray rate at each location, are typically identified through testing.
Nozzle Type
While the spray rate establishes how much coating will be delivered, the nozzle type determines the way in which the coating will be introduced to the material.
Depending on the type of coating, various nozzle types may be available. Nozzles can be designed to deliver everything from a stream of fluid, to a completely atomized spray, and everything in between.
Coating (and/or Granule) Temperature
In many cases, the coating and/or the granules must be heated to achieve optimal application. This may require a preheater drum to bring granules up to temperature prior to entering the coating drum. It may also require a system for heating the coating itself prior to feeding the spray system.
Coating temperature can change how the coating behaves, and as such, must be carefully considered during spray system design.
Material Bed Action
The action occurring in the material bed is also hugely influential on coating application effectiveness and uniformity. This is because the action must facilitate a uniform distribution of the coating throughout the material bed, which often necessitates a tumbling and rolling action to promote sufficient granule-to-granule transfer and proper exposure to sprays.
The bed action is dependent on several factors, most notably the bed depth, drum speed, particle size and shape, and flighting.
Bed Depth
The bed depth is a function of percent fill, which is influenced by feed rate, drum slope, rotational speed, flighting, and drum length and diameter. Dams can also be incorporated into the unit to increase bed depth when necessary.
A bed that is too deep may cause the binder to unevenly distribute throughout the bed, or could inhibit the necessary bed action/mixing. A shallow bed may be too thin to promote good bed action/mixing and may also risk “overspray” of the coating onto the drum shell, or even the spray penetrating the bed and sticking to the drum.
There is no general rule for bed depth; it is dependent on the unique granule and coating characteristics.
Drum Speed
Drum speed is the main contributing factor in achieving proper bed action and is often very material specific.
As with bed depth, there is no definitive optimal drum speed. Rather, it depends on the material characteristics and how the speed influences their movement in the drum.
Flighting
Flighting also has a significant impact on bed action and is often incorporated to encourage bed turning and agitation (flights used for this purpose are often called tumbling flights). Advancing (spiral) flights can also be incorporated to move material away from the inlet and into the active processing zone.
Coating drum with tumbling flights and advancing flights (shown at right)
Other variables that influence the coating process include:
- Retention time
- Surface characteristics of the granules
Curing the Coating
It’s important to note that while the application aspect of coating is complete once granules exit the coating drum, curing plays an important role in ensuring that the applied coating is properly adhered to the granules for stability.
Curing is often conducted in a rotary kiln and serves to chemically bind the coating to the granules.
Process Development for Coating Roofing Granules in a Drum
As is evident, there are many aspects of the coating process and coating drum design that can affect the quality and uniformity of the finished product. This often necessitates testing in a facility such as the FEECO Innovation Center in order to identify the optimal drum design and process parameters.
Testing is especially crucial when working with a novel material as the starting granule, or a new type of coating material.
In the Innovation Center, roofing granule producers can confirm feasibility and test their intended product to establish key operating and equipment criteria for producing the desired results on a commercial scale.
The Innovation Center offers a number of process and equipment configurations to identify the exact combination of parameters. Producers can also create various samples for field testing.
A major advantage of testing with FEECO is that producers can test both granule coating and curing in the Innovation Center.
Conclusion
The essential role that proper coating application plays in bringing a quality roofing granule product to market cannot be overlooked. While the coating drum provides the industry with a high-capacity, reliable solution, the numerous equipment and process variables that can influence coating application effectiveness make developing a custom solution essential. The material bed action and spray system are the most influential factors in this effort, and testing is often a vital step in identifying and refining the process and equipment variables that will produce the best results at commercial scale.
FEECO is the roofing granule industry’s preferred provider for coating drums, as well as pre-heaters, rotary kilns, and rotary coolers. Our testing facility offers both the equipment and expertise to establish process and equipment parameters for commercial-scale operations. We also offer a comprehensive parts and service program for keeping rotary drums performing in optimal condition. For more information on our coating drums, testing services, or other roofing granule production equipment and services, contact us today!