Gentle and energy-efficient

Sustainability requirements and stricter VOC regulations are making solvent-based paints less popular. However, the switch to more environmentally friendly systems also requires a new approach to paint drying. Depending on the time and energy requirements, there are various systems to choose from.

"In general, modern water-based lacquers are much more user-friendly than they are said to be," says Reinhard Huber, a trained carpenter and application engineer at Adler-Werk Lackfabrik Johann Berghofer GmbH & Co KG in Schwaz, Austria. With many water-based paints, drying is somewhat slower than with solvent-based paints. "The right ambient conditions are important here. An air-conditioned drying room with regulated humidity in which the air is in motion is ideal," says Huber. In the industry, an oven has often replaced this drying room in the past, whereby air circulation was also important here.

Paint drying requires a lot of energy

However, strong fluctuations in energy prices and occasional fears of a gas shortage made other drying options appear interesting. This is because a lot of energy is needed to dry water-based paints: around 2350 to 2500 kilojoules are required to evaporate one liter of water.

Reason enough to take a close look at the drying process and search for efficiency potential. Harter GmbH in Stiefenhofen has done just that and offers heat pump-based condensation drying systems. They operate at temperatures between 20 and 90 degrees Celsius, depending on the application - which suits a sensitive substrate such as wood.

In addition to the heat pump, the dehumidification technology also contributes to energy efficiency: The humid and heated air from the drying room is fed into a pre-cooler and then into an air cooler. There, the moisture condenses on the fins of the air cooler and runs out of the drying system. Condensation also releases the evaporation energy, which can be used to reheat the air in the preheater - without any additional energy input. The air heater in the next stage then heats the air to the required process temperature before it is returned to the drying chamber.

Another advantage of this process is its speed: according to Harter, condensation drying can save up to 50 percent of the time compared to conventional processes such as hot air drying or simple blower technology.

Hellmann-Hygrex GmbH in Kaltenkirchen also offers its own drying process, which consumes significantly less energy than conventional dryers - up to 80 percent. The advantages are based on the reduction in humidity using an industrial dryer. The free water vapor in the moist air is removed by the machines through freezing or condensation and the air is then reheated via the cooling circuit. The warm air in the drying room can then absorb more moisture than cold air.

Thanks to the heat pump principle, the process is very energy-efficient and additional heating coils are not required. The evaporation energy - up to 2500 kilojoules per kilogram of water - is released again during the condensation of water vapor in the dry air generator. It is transferred to the dry air heating system and fed back into the drying process, creating a closed circuit. A dry air generator from the company generates around three to four kilowatts of cooling power (condensation of water) and four to five kilowatts of heating power (heating of the dry air) with one kilowatt of drive power, just like a heat pump.

Gentle drying well suited for wood substrate

The process is well suited to the sensitive substrate wood: At a dry air temperature of 20 to 35 degrees Celsius, the water-based paints are dried without thermal stress. The risk of skin formation is significantly lower than with thermal drying or infrared radiation. This is because, even with a thick top coat, the paint film only forms when the water has evenly diffused from the inside to the outside and evaporated. This means that even thick layers can be dried with optimum surface quality.

Harter projects show what can be achieved in practice. In one case, a manufacturer of window frames wanted to use low-temperature drying for gentle dehumidification, a safe process and to save energy. Harter converted an existing drying chamber with a chain conveyor to its process. The chamber was equipped with an air circulation system and a heat pump module was connected. At a temperature of around 40 degrees Celsius, the window frames dry within around ten minutes, with the system requiring a nominal output of 9.5 kilowatts.

In addition to the energy savings from the heat pump dryer and exhaust air-free drying, the process enables a reproducible process without bubbles and boilers because drying takes place from the inside out. In a second case, a manufacturer of beer bench sets wanted to improve the quality of the coating and achieve block resistance, ensure the downstream process in production, shorten the cycle time, increase production capacity, save energy and also integrate a new system into extremely tight spaces. To achieve this, Harter installed a drying cabinet in a niche and installed the heat pump module on the second floor.

Both parts of the system are connected via insulated piping. At a temperature of around 40 degrees Celsius, the beer benches now dry within 20 minutes - previously it took two to three days. A system with a nominal output of 8.3 kilowatts is sufficient for this. The manufacturer has also achieved block strength and a reproducible process. The shorter cycle time also enables a higher output. Added to this is the energy saving

Infrared drying is also evolving

However, other drying processes are also continuing to develop. Excelitas Noblelight GmbH in Hanau, for example, has launched carbon infrared emitters onto the market. While conventional medium-wave emitters are equipped with a heating coil made of alloyed resistance wire made of iron, chrome and aluminum, the new emitters use a carbon strip (see also pages 36-38).

Noblelight uses an example to explain how infrared emitters can be used effectively in practice: in the manufacture of furniture, panels made of untreated wood are stained in color and then coated with a UV varnish. The stain must be completely dry before the UV coating can be applied. Water-based color stains can cause problems here, as water dries relatively slowly.

Fast drying has priority depending on the application

A Scottish company compared various methods in the search for an efficient drying process. They initially considered gas convection ovens, but rejected this option: they would have required a footprint of around 20 meters and a dwell time of around 30 minutes would have been required for complete drying.

In the end, the decision was made to use two medium-wave infrared modules from Noblelight. They are around two meters long and, with an output of 12 kilowatts each, dried the panels completely, even at a line speed of 10 meters per minute. The infrared modules reach the required temperature in less than two minutes so that they can be switched off at lunchtime and during longer breaks. Gas ovens, on the other hand, have to be kept at the right temperature throughout the working day. The carbon emitter infrared technology therefore helps to make paint drying more energy-efficient.

UV radiation hardens the paints even faster. However, this only works with special UV paints. Strictly speaking, these paints do not dry either: the transition from liquid paint to solid paint layer takes place via a chemical reaction, the paint molecules cross-link with each other.

For this reason, "UV drying" is only comparable with other processes to a very limited extent. Its advantage is that the paint hardens within seconds with little energy input. The surface is dry, abrasion-resistant and can be processed immediately.

Adler-Werk Lackfabrik Johann Berghofer GmbH & Co KG
www.adler-lacke.com

Excelitas Noblelight GmbH
www.noblelight.com

Harter GmbH
www.harter-gmbh.de

Hellmann-Hygrex GmbH
www.hygrex.de