The OptiDry Coat high intensity air dryer is a compact air dryer using gas as a heat source. The high intensity air dryer combines high drying capacity, reduced space requirement and excellent runnability. OptiDry Coat is an especially attractive replacement for energy-intensive infrared dryers on coated lines.
In modern coating lines, coating drying is done with economical and ecological air dryers. The OptiDry Coat high intensity air dryer has been specially designed for coaters where one-sided drying is the most advantageous alternative for the space requirement.
Coating drying is a very energy demanding process and with air dryers it is possible to significantly cut the energy costs of the drying process. The energy efficiency of air dryers ranges from 60 to 80% as compared to the energy efficiency of infrared dryers which is only 25 to 35%.
The energy savings when converting from IR to air drying with OptiDry Coat high intensity air dryers is in the neighborhood of 50 %. (In our calculations the drying energy efficiency is defined as the energy transferred to the web divided by the energy used for dryer heating.)
OptiDry Coat is a cost-efficient rebuild solution with a short shutdown time, typically one week. There is no need to extend the machine length or change steam production.
Electric and gas IR dryer poor efficiency is due to large losses to cooling air (electric IR), significant losses to exhaust air (gas IR), transmission losses and reflection losses. For example, 30-50% of gas energy converges to infrared radiation. The infrared radiation is partially absorbed in the paper web, partially transmitted and partially reflected. About 25-35% of the gas energy is absorbed in the paper web as thermal radiation plus about 2-3% as convective heat transfer. In electric IR dryers the quartz glass becomes dirty which also reduces energy transmission.
Comparatively, OptiDry Coat's high energy efficiency keeps the heat load into the machine room to a minimum and energy costs low. The high energy efficiency is provided by the air circulation system, effective nozzle technology and no unnecessary heating of the paper web.
With OptiAir Recovery, further optimization of exhaust air heat recovery can save an additional 10% in fuel costs and corresponding CO2 emission reduction. The hot and humid exhaust air is used for preheating combustion air and makeup air in the air-to-air economizers. Energy savings come from preheating the burner air from 35 to 200 ºC. Payback time for OptiAir Recover is typically less than one year.
OptiDry Coat needs little maintenance and does not require regular replacement of emitters or lamps. The gas burner and circulation air fan are located outside the machine, such as on the mezzanine or in a fan room.
OptiDry Coat has no costly wear parts such as emitters. (Typically IR emitters must be replaced every two to four years.) There is less service time required due to less cleaning and maintenance shutdown time. OptiDry Coat presents no fire hazards and provides better working conditions due to lower ambient temperature. It produces low heat and humidity loads to the machine hall due to the drying air recirculation.
The heat transfer capacity of the blowing nozzles from the OptiDry Coat high intensity air dryer exceeds the capacity of conventional nozzles by about 40%, which guarantees plenty of drying capacity in a small space. At 450 °C drying air, the air dryer is about 20% more effective for a given machine length than the conventional IR dryer. The OptiDry Coat high intensity air dryer’s circulation air system offers energy efficient and effective drying. Evaporation starts immediately after the coating station with no unnecessary heating of the web.
The OptiDry Coat high intensity air dryer is a non-contact air dryer that provides excellent web stability by means of an air pad generated between the nozzle and the web. The carrier surface with overpressure ensures non-contact drying. The distance to the web is controlled automatically for different paper grades and the web tension stays constant in the dryer area. Mechanical seals minimize leakage. The exhaust slot circulates the drying air and keeps an optimal blowing distance. The dryer surface is slightly curved for a more stable web run.
The high intensity air dryer located after the coating station offers quick solidification of coating and optimal coating quality. Fiber roughening of the paper is eliminated by immediately evaporating water from the coating through the surface. The binder concentration develops positively and its uniformity improves. This improves surface strength and decreases print mottle.
Only a small part of the drying energy penetrates into the base sheet, which provides an excellent drying method for board grades. The OptiDry Coat high intensity air dryer evaporates only the water from the coating and does not needlessly heat the base sheet. OptiDry Coat is also better for the environment because with 50% less fuel burned, there is 50% less CO2 formed.
In general, paper quality with OptiDry Coat compared to infrared drying will be at least as good on all grades, due to lower web temperature and less water penetration. This, combined with the greatly decreased energy consumption, reduced maintenance costs and higher drying capacity make it a preferred upgrade for existing coating lines with infrared dryers.
At a coated board machine in Sweden, the mill desired to get more drying capacity and energy savings. Their electric infrared dryer using 2.4 MW operating power was replaced with a high intensity OptiDry Coat air drying on the top coating station #3. The new air dryer was located immediately after the coating station. The air dryer exhaust preheats combustion air in the heat exchanger.
The results were increased evaporation capacity and reduced running costs – and the paper quality was even better than before.
According to the board machine production manager, "The new Valmet air dryer works great! All coated board quality properties stayed unchanged or somewhat improved after the rebuild, as expected. All the rebuild targets were met: more drying capacity and high energy efficiency. Maintenance costs are very low, only yearly service of the burner. There are not any fire problems and no need for heavy cleaning. The cooperation with the Valmet people was great during the project."
For more information on the benefits of replacing existing infrared dryers with the OptiDry Coat high intensity air dryer, contact your Valmet representative.