The Parts and Performance of WEB CONVECTION DRYERS
- Published: September 01, 2001, By Matthew Tielkemeier, Dri-Tec Inc.
If any component of your drying system is subpar, it can affect the outcome of this crucial step in your production process.
There are many methods for drying or curing the coatings on converted web products. Ultraviolet lamps can cure certain types of 100% solids coatings, such as silicones. The concentrated heat generated by infrared lamps commonly is used to crosslink some specific heat-activated solventless coatings. Warm melt pressure-sensitive adhesives sometimes are finished with electron beam curing.
Of all these curing methods, air convection drying is still the primary means of drying for coating and laminating applications. At this time, most of the coatings available for converting require either water or a solvent to be evacuated from the solids contained in the coating solution. Convection drying in one form or another is the best, most time-proven technique for accomplishing this goal. In the future, because of advances made in the chemistry of the coatings, rising fuel costs, and legal limitations for carbon dioxide emissions, the popularity of convection drying may diminish.
One Goal, Two Types
Air convection dryers in their basic form are simply an engineered air distribution system; they are a mechanism for drying a printed or coated web. Essentially, convection dryers deliver heated air to the surface of the web and remove solvent- or water-laden air, drying the web in the process.
There are two types of convection dryers commonly used for converting: the roll support dryer and the floatation dryer. Each design has its own characteristics and advantages.
In the roll support design (Figure 1), typically a roller is mounted under each air nozzle to support the web. Frictional drag from the rollers must be considered based on the characteristics of the webs to be processed. Tendency or direct drive methods commonly are employed to the rollers to overcome frictional buildup when necessary. Roll support dryers generally are more economical to manufacture. They keep the web tracking straight through the oven, because the web is supported mechanically and the tension is always under positive control through the drying chamber.
Floatation dryers (Figure 2) use opposing air streams to support the web as it transports through the oven. The design is more expensive but very efficient, and it may be necessary to dry the web when coatings are applied simultaneously to both sides of the web.
Bringing in the Heat
The major energy sources commonly used for drying the printed or coated web are natural or LP gas, steam or oil (via a heat exchanger), and electricity.
Natural gas is the most common, as it is readily available and has a relatively low operating cost. Gas heating can provide high temperatures with fast heatup, and it has a good, responsive turn-down capability.
On the other hand, the combustion output may contain impurities that are unacceptable in some applications. One must also take great care because of the risk of fire or explosion, especially when solvent-based coatings are being processed. Gas-fired heat systems also require a scheduled preventative maintenance program to retain optimal performance.
Both steam and oil require a heat exchanger in conjunction with the combustion source. This provides clean, heated air that will not contaminate the product.
Steam units are cheaper to manufacture than gas. The incentive to chose steam often is based on initial cost and existing steam capacity in the plant. If steam is not currently available on site, and a clean, isolated air source is needed, oil is usually the choice. Heat exchanger systems are slow to respond when temperature changes are required and can't supply the higher operating temperatures of gas and electricity. Like gas units, they also require a good preventive maintenance program for peak performance.
While no heat supply system is completely safe, heat exchanger systems are somewhat less prone to accidents when flammable solvents are used as part of the process. This is especially true if recirculation is utilized as part of the drying scheme for energy conservation.
Electric heat sources have a number of advantages. They are capable of high temperatures and have a better response time both for heat-up and cool-down conditions. Electricity supplies very clean air, which may require a filter but does not need a heat exchanger for most clean room applications.
Design flexibility is also an attribute of the electric heater. Since no open flames are needed, the possibility of explosion is somewhat reduced when solvents are used. Electricity is almost always available, even when all other fuel sources are hard to come by. Its main disadvantage is its high long-term operating cost, and that factor alone keeps most production facilities from using electricity.
Supply Fans Begin The Process
The supply fan is the first component of the air-handling system. Its function is to deliver pressurized air to the nozzles by way of the ductwork. There are two fan designs commonly used for convection oven applications: “paddle wheel” and Acoustafoil.
The paddle wheel fan was the first and only fan to be used in the early days of web drying. The design provides stable air output over the entire pressure range. Its main advantages are that it can supply high static pressures that result in higher air velocities and better heat transfer at the web. More horsepower is required by the paddle wheel fan in order to produce these benefits. The drawbacks are significantly higher operating costs and increased noise levels. The initial cost for a paddle wheel fan also is higher.
Acoustafoil fans can be an excellent alternative when higher static pressures are not required. Because they supply higher air volumes with reduced horsepower, their long-term operating cost is lower. The design also provides quieter operating levels, and because of the manufacturing techniques, the initial cost is relatively low.
You must consider carefully the fan design best suited for each dryer application. Factors that contribute to the final specification include web substrate characteristics, desired line speed, solvent type, and percentage of solids in the coating. The stability of the web and the laydown thickness and viscosity of the coating often will be the determining factors. Variable frequency fan controls help the drying system adapt to a wide variety of operating parameters.
Air Distribution is Critical
The air distribution system includes the ducting straights, elbows, tapers and “Ts,” filters, dampers, and the dryer housing.
Well-engineered and -installed ductwork is critical for a successful application. It is the aspect of the drying system that is most frequently overlooked and taken for granted. Ducting diameters must be specified carefully to accommodate the air volume and the velocities required at the web for proper drying.
The length of the ductwork from the supply fan also will be an important factor for determining the ducting diameter. For best efficiency, duct runs and elbows should be minimized wherever possible (Figure 3). Seams and joints should be sealed with a high-temperature-resistant caulking to avoid air blowout. Insulation on the dryer housing and the ductwork reduces heat loss and will help lower the sound levels. Dampers also are a common area for heat and pressure loss. Superior quality, sealed dampers should be provided or retrofitted into your system. Obviously, all the filters should be properly installed, then checked frequently and replaced or cleaned when necessary.
Controlling the Temperature
There are three basic types of temperature controllers: on/off; proportional; and Fuzzy Logic.
On/off controllers are the oldest and most basic type. They operate the same way that the thermostat in your home works for the furnace or air conditioner. After the desired temperature has been set, the controller monitors the temperature and turns the heat source on when the set point is not satisfied. The controller turns the heat source off when the desired temperature has been achieved. This type of control results in uneven sinusoidal temperature variances. The system is in a constant state of “hunting and chasing.”
Proportional controllers were the next advance in temperature control and are the most commonly used today. As the controller senses the temperature, a variable signal — usually analog — is introduced to the heat source. So, as the actual temperature approaches the target set point, the controller starts to feather back the output of the heat source. The closer the actual temperature comes to the set point, the less heat is supplied. This control scheme provides a more even air temperature to the web.
Fuzzy Logic controllers that employ Artificial Intelligence are beginning to take precedence all over the world. Although the technique was developed in the US, Japan has been quick to take advantage of its superior performance. Artificial Intelligence controls actually “learn” through conditions-monitoring and computer memory how best to achieve the programmed results.
Fuzzy Logic eliminates the standard binary code commonly used by digital and computer controllers and replaces it with a way to handle “shades of gray” or judgment decisions by using percentage calculations. While Fuzzy Logic is not a cure-all for every programming application, it has proven to be very effective when used for controls of all types, and its use is likely to become common in the future.
Two Types of Air Nozzles
Although there are many variations, two basic air nozzle configurations are common for impingement type dryers (Figure 4). The best choice for a given application will be determined by the design criteria. Some of the design factors to consider are the required air volume and velocity, air turbulence, and the web's stability. Even distribution delivered through internal baffling is critical.
The slot design air nozzle (a) has become more popular in recent years. It is more costly to manufacture but provides very even air distribution over the surface of the coating. This is especially important for heavier laydowns when the coating is susceptible to being disturbed before it has set up. Dual slot designs greatly increase drying efficiency by creating more turbulence, but they are more costly to manufacture.
The punched hole air nozzle (b) has been around for a long time. Although it can't be used for every application, it is often more effective because it produces a great deal of heat transfer through its wide impingement area and highly turbulent air output. The punched hole design is easier to manufacture and can be provided at a lower cost.
The ability to remove and change the nozzle tube faces quickly and easily (Figure 5) affords some important advantages. When a web break occurs at the coating station, the usual result is coating being “slopped” onto the face of the air nozzle. If the coating is not cleaned off immediately, dryer performance will diminish, and the coating will be harder to clean off later after it has dried and cured on the faces. When an operator can replace the dirty tube faces with ready spares quickly, production can continue at optimal performance while the dirty nozzles can be stored in a solvent bath and cleaned at the operator's convenience.
The removable tube face option also allows for inexpensive dryer upgrades when the operating parameters change or new and more efficient nozzle designs are developed.
Exhaust System Must Be Balanced
The function of the exhaust system is to remove the water- or solvent-laden air from the drying chamber. It is important to have enough capacity to draw a negative pressure within the system. The exhaust must be balanced so the solvents are drawn into the capture system where they can be handled safely.
Usually about 10% more exhaust than supply is required for a good balance. If the exhaust rate is too high, it will cause the web to cool, which will decrease efficiency and raise the operating cost by wasting fuel. The exhaust also can create web-handling problems in the oven if it is so high it lifts the web off the support rollers and sucks the web against the nozzle faces. Proper balance is one of the keys to optimal dryer performance.
Don't Forget Recirculation
Recirculation does not aid in drying the web, so why should it be considered?
With the introduction of more water-based coatings and inks, drying has become more challenging. Water-based emulsions typically evaporate four to five times more slowly than solvent-based solutions. They require more heat and velocity to complete the drying process. Water-based coatings also tend to cool the web more rapidly than solvent-based coatings, demanding more energy consumption. Recirculation has been a viable response to these challenges. The technique is utilized to maximize efficiency and conserve energy.
The relatively drier, exhausted air from the last zone of the oven is used most commonly for recirculation. It can be processed back through the first-zone burner or recovered via a heat exchanger.
Another method for utilizing recirculated air is to apply the exhaust from the last zone to the underside of the web in the first zone, where the impinged air is more saturated. The residual heat helps keep the web from cooling off too much and impeding drying performance.
These heat-recovery techniques result in less overall energy consumption. Of course, you have to exercise great care when solvent-based coating or inks are used. In this case, a professionally installed and maintained LFL (lower flammability level) monitoring system is required.
Experience has shown recirculation to work best on paper or board webs and when higher temperatures are required. When 50% of the exhaust air is recirculated, we can typically expect to profit by a 20% reduction in energy consumption. An important added benefit will be a 50% reduction of the air volume induced in the pollution control system.
Think of your dryer as a system. Each component in the system has a vitally important function. In order for your drying system to operate at optimal levels, every aspect of the equipment must be specified properly and operating at peak performance. In this case the old adage is certainly true: “The chain is only as strong as its weakest link.”
Matthew Tielkemeier is the VP of Dri-Tec Inc., Milwaukee, WI. He has been in the converting industry for 15 years and has published and presented numerous technical papers and case studies. He can be reached at 414/354-3540.
The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to author(s).
