With the changeover from solvent-based to water-based coatings, in accordance with the VOC Directive, drying times in hot air circulation ovens have increased, reducing throughput. One way to ameliorate this situation, according to tests performed by Heraeus Noblelight GmbH (www.heraeus-noblelight.com/en/infrared-heat), is the introduction of infra red dryers just before the main drying oven. By raising the temperature sufficiently to achieve water evaporation before entry to the drying oven, infrared ‘boosters’ make it possible to attain time savings of up to 50% compared to conventional drying ovens.
The tests carried out by Heraeus Noblelight also demonstrated that the choice of infra red source is significant. The tests showed in particular that carbon medium wave infrared emitters out-performed conventional short wave ones and consumed up to 30% less energy.
This drying performance can be improved still further by the addition of an air knife.
Using an air knife supplied by Air Control Industries, it was demonstrated that by exhausting the water-saturated air efficiently from the drying zone drying times are reduced significantly. This is because there is not a build up of moisture laden air for the drying system to overcome. Consequently, energy is used more efficiently, again helping to reduce costs.
Characteristics of air knives that make them particularly suitable for this drying application are their ability to deliver accurately controlled air flows as regards volume and velocity, air is delivered as a continuous even curtain across the full width of product, and they can be produced in lengths to match the application.
These features are said to complement infrared’s ability to dry coatings from the inside out. This characteristic eliminates skinning and blistering to ensure excellent surface qualities are achieved.