Traditional forced hot air dryers use blowers to pass ambient air through a burner where the air is heated and ultimately discharged through a nozzle at the web.

The air movement in these dryers systems addresses an important element of the evaporation process. There is a very thin layer of air on the surface of the web called the boundary layer 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. The air discharging from the nozzle at the web is enough to disrupt this barrier so that moisture can be released.

The limitation of forced hot air dryer systems is the inability to significantly improve the quality or velocity of the air. Since the air is pulled from the surrounding environment, it has a certain level of humidity. So these systems are relying on partially saturated air to efficiently pull moisture from the web. The velocity of the air striking the web directly affects the rate at which the web can accept heat and how well the boundary layer is broken down. The use of blowers to move the air limits the velocity of the air discharging from the nozzle.

Advantages:

  • Moderate Drying Capacities – These systems address some of the key elements of the drying process, but will not be considered high performance drying systems until the moisture content and the velocity of the air are significantly improved.
  • Familiar Technology – Converters are comfortable with this technology and know what to expect when using it.
  • Can be used in Hazardous Applications – The heat source is located outside of the hazard environment, and blowers are configured to purge the ductwork of explosive fumes before the burner is ignited.

Disadvantages:

  • Limited Evaporation Rates – Inability to significantly impact the moisture level or the velocity of the air will continue to limit performance.
  • Large Ductwork & Blowers – Dryer manufacturers are moving higher volumes of air in an attempt to increase the velocity of the air at the nozzle. This change alone accounts for many of the disadvantages and inefficiencies of the system.
      • Very Noisy – Larger blowers are very noisy.
      • Large Infrastructure – The larger blowers and ductwork take up a lot of space around the press and make the gear side of the press difficult to access.
      • High Operating Costs – More energy is required to heat larger volumes of air, resulting in even higher energy costs.
      • High Heat Loads to the Plant Environment – a tremendous amount of heat radiates from the ductwork and the dryer housings.
  • High Web Temperatures – Dryer manufacturers will try to improve drying by increasing the temperature of the dryer. This not only further raises the energy costs, but also makes it more challenging to run temperature sensitive webs.