The fundamental characteristics that determine how well a drying system will perform are:
- Moisture level in the air
- Temperature of the air
- Velocity of the air
- Number of nozzles
- Dwell time in the dryer
Some of the conventional Forced Hot Air Dryer Systems used in the industry today have been able to improve dryer performance by increasing either the number of nozzles, the web temperature, or the dwell time. This is because they have reached a plateau in terms of the improving the quality and/or the velocity of the air. The same applies to IR Dryers Systems except they tend to improve drying by focusing on temperature and dwell time.
Compressed Air Drying Technology
As with traditional forced hot air dryer systems, compressed air dryer systems can also improve drying by increasing the number of nozzles, the web temperature, and the dwell time. However, the cornerstone of compressed air technology is the ability to take advantage of characteristics inherent to compressed air that significantly impacts the level of moisture in the air and the velocity of the impinging air.
In terms of moisture level, traditional forced air dryer systems project heated ambient air at the web. Since this air is pulled from the surrounding environment, it has a certain level of humidity. In a sense it is like using a wet sponge to try to pick up water. As for compressed air, moisture is naturally removed from the air as it is being compressed. This is why the tanks on air compressors need to have the water drained from them every once in a while. So instead of using partially saturated air, compressed air technology uses a very dry air to remove moisture from the web.
Traditional forced air dryer systems use blowers to move air through the dryer system, and can reach nozzle velocities in the range of 5,000 to 10,000 feet per minute. Since compressed air dryer systems use pressurized air, much higher nozzle velocities in the range of 65,000 to 85,000 feet per minute can be achieved. This substantial difference in nozzle velocity significantly improves the drying process in two different ways.
First, the rate at which heat is transferred from the air to the web is proportional to the velocity of the air as it strikes the web. Since compressed air systems have substantially higher nozzle velocities, there is a considerable reduction in both dwell time and heat energy required to raise the temperature of the web. (Note: The cost savings in heat energy alone is enough to offset the cost of operating an air compressor. See Operating Cost Comparison Worksheet
Secondly, the high velocity air breaks down the boundary layer. The boundary layer is a very thin layer of air on the surface of the web that acts as a barrier between the web and the atmosphere. During the drying process, this barrier essentially traps and holds moisture at the surface of the web. By breaking through this barrier, saturated air that is trapped within the boundary layer is released and exhausted out of the dryer. Traditional forced air systems will disrupt the boundary layer, but they will not break it down with nearly the same intense scrubbing action as with the compressed air systems.
Once again, it is the combination of the very dry air and the truly high nozzle velocities that make this a superior drying technology over traditional systems.
FlexAir’s Air Bar Technology
FlexAir’s patent pending technology incorporates all of the benefits of Compressed Air Drying Technology into a single compact nozzle. Each nozzle contains its own heating element. When fully assembled the nozzle acts as an individual dryer because it receives, heats, and disperses the compressed air at the web. At FlexAir, we commonly refer to our nozzles as Air Bars.
Air Bars have a very compact 2” x 2” (50mm x 50mm) profile that are constructed out of a pair of custom aluminum extrusions. The inner extrusion houses the heating element as well as creating a series of air passages inside the Air Bar when assembled with the outer extrusion. The compressed air is ported into the top of the air bar, and is heated as it travels down through the air passages before being discharged out through the orifices at the web. In operation, the topside of the air bar that receives the compressed air will remain cool, keeping the heat towards the bottom where the air is being discharged.
Each heating element runs the entire length of the air bar, which is critical in maintaining an even temperature profile across the web, thus eliminating hot spots in the dryer. Each heating element is custom made to meet the voltage and wattage requirements of each application. The heating elements can be powered up and maintain operating temperature without consuming compressed air. In application, the compressed air is linked to the press run signal, where a solenoid valve is used to automatically turn the compressed air on and off when the press starts and stops.
The Air Bars have been designed to be a simple, yet robust device that are very efficient in delivering a great deal of drying, in a very compact package. As you can see by looking through our different dryer configurations, our Air Bar is the key component that allows us the tremendous flexibility in our dryer designs.