Image Credit: Glenro Inc. | Despatch Industries | MEGTEC Systems

Web dryers are devices which dry webs, rolls, and other continuous sheets of material, typically incorporated in a process line for web coating applications. Selecting web dryers requires an analysis of design type, heating source, and secondary capabilities.

Types of Web Dryers

Web dryers can be classified and selected based on design type or heating method.

Design

Web dryers differ primarily based on design type.

• Belt conveyors use perforated belts to transfer feed materials through the dryer. Heated air is either passed under and through, or over and through the belt and product bed before being reheated and re-circulated.
• Cylinder dryers or roll dryers usually consist of a steam-heated drum with a smooth outer surface around which a moving web or sheet is passed for drying.
• Festoon dryers or loop dryers are used to heat webs or continuous sheets with minimal contact. The web is held on bars or threaded onto movable idler rolls that are driven by a conveyor. Festoon dryers are used to dry or cure coated abrasive webs, flocked wall paper or coated fabrics.
• Flotation dryers use a series of air bars or louvers to support and gently dry webs of fragile materials. Air bars produce dispersed drying air.
• Impingement dryers use blasts of hot air to convectively dry moving webs, boards, or other large bulk materials.
• Predryers or preheaters provide initial heating or drying of a material before the unit is passed into a larger dryer. Often, radiant heating dryers are used to pre-dry webs before the material is fed into a hot-air dryer.
• Suction drum dryers consist of a series of perforated cylinders or drums with an internal vacuum. Web or sheet materials are held against the drum by the vacuum or suction generated as a fan draws air from the interior of the drum. This suction holds the material to the surface of the drum, permitting air to pass through the material being dried. A portion of the drum at the transfer has no suction and permits the material to transfer to the next drum without interference.

Heating Method

Web dryers may also be grouped according to how they heat and dry the web material.

• Direct heating involves hot air or combusted (or heated) gas being directly circulated through the material being dried to convectively heat the web or evaporate moisture from its surface.
• Indirect heating involves heating walls, tubes, jackets, or discs by steam, gas, thermal oil, or hot air. These heated elements transfer their heat to materials that come in contact with them via conduction. Indirect dryers may be useful when direct contact with combustible gas or hot air is not desirable due to product or process requirements.
• Radiant heating involves radiant heat generated by electric or gas-fired infrared heaters. Radiant heat dryers are useful for drying surfaces, flat products or web materials where a clear line of sight can be provided. Straight pass infrared dryers are used to pre-dry web materials before these materials are fed into a conventional, direct, hot dryer.

Secondary Capabilities

Web dryers may also be distinguished based on other capabilities which may be incorporated into the equipment.

• Coating - Dryers provide the application of a coating on the web material before, during, or after drying.
• Cooling and freezing - Web materials are cooled or frozen in the dryer during or after the drying process.
• Moisturizing and humidifying - Dryers can restore a level of moisture to an over-dried and possibly brittle product.
• Preheating - Dryers have an integral preheater or preheating capability, which reduces the heating demands on the drying unit and increases line speed.
• Washing and rinsing - Dryers have an integral washing or rinsing capability to remove dirt, particles, or coatings from the web material.