Role of Dryer Nozzles
Air-bar nozzles of various sizes, shapes, and configurations are commonly used in the drying of continuous-web products, such as coated paper, films and foils, as well as various types of printed materials. The type of nozzle and its configuration depend on several factors, including whether the material is coated/printed on one or both sides of the substrate, whether the materials to be dried have any heat and mass-transfer limitations, and the amount of space available for the drying system.
The dryer designer has numerous nozzles and systems from which to choose based on the specific needs of the coating process. The choice comes down to application experience, knowledge of the drying process, and the testing of a given set of materials. All are necessary to make the right drying process set up for a given application.
Basic Nozzle Systems and Designs
The most common types of nozzles can be classified into two categories: conventional impingement nozzles and flotation nozzles. Impingement nozzles are defined as hole bars or slot nozzles and utilize impinging jets to impart heat and mass transfer. They are typically used in combination with a means of supporting the web material, such as with rolls, belts, or other contact devices.
Flotation nozzles are defined as air bars or air foils. They are specialized nozzles designed with dual purpose – to provide high heat and mass transfer, and to provide non-contact support for the web material, which eliminates the need for rolls, belts or other contact devices. The non-contact flotation is accomplished by using nozzles on one side or both sides of the web. The web support is provided by a cushion of air created by a positive pressure between the face of the air bar or air foil and the web. When staggered above and below the web, they support the web in a sinusoidal fashion. With airfoils, both a positive support pressure and a negative support pressure are created using a single slot, which in turn provides support for the web using nozzles on one side of the web only.
Special Nozzle Systems and Designs
For some applications, special nozzle configurations and hybrid nozzle designs are needed to meet the processing requirements. One way to accomplish this is to combine flotation and/or impingement nozzles into special arrays that can provide enhanced web-handling support for some products; gentle drying with solvent dilution for products where the evaporation rate must be limited due to product quality, but additional air is required for safe operation; or by providing nozzle arrays that combine flotation and impingement nozzles to create a high-heat and mass-transfer system.
An example of such a system is the DualDry® nozzle configuration from Babcock & Wilcox MEGTEC. This system combines flotation nozzles in conjunction with hole bars.
An alternative drying system combines convection nozzles with other heating technologies such as infrared. By combining flotation or impingement nozzles with infrared emitters the dryer designer is able to create a hybrid design that offers the benefits of combined heat and mass-transfer characteristics that convection nozzles alone cannot provide. An example of this hybrid design is the B&W MEGTEC FloatIR® nozzle system, which combines the benefits of a flotation nozzle with those of electric infrared, creating a nozzle that provides enhanced heat and mass transfer, as well as stable flotation.
More info: Todd Rueckl, regional sales mgr., at firstname.lastname@example.org; 920-337-1533, www.babcock.com/megtec