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5 Reasons to Choose a Through-Air Dryer for Process Heating 

5 Reasons to Choose a Through-Air Dryer for Process Heating

Welcome to our series on the fundamentals of process heating dryers. This week we focus on through-air dryers. (If you missed last week’s installment and want to learn about impingement dryers, see “5 Reasons to Choose an Impingement Dryer.”)

What is a through-air dryer?

Through-air dryers, as the name suggests, use a heated airflow that passes through the material to remove moisture from permeable products, or to selectively melt bonding fibers. The heated air can be generated using any energy source available such as gas, steam, thermal oil, or electricity. The heated air is distributed uniformly above the product to be heated in a non-impingement fashion and then drawn thru the material using a controlled suction design.    

 This drying, or heating, method has exceptionally high heat transfer characteristics due to the air being uniformly distributed through the entire thickness of the product. It also produces high evaporation rates due to the combination of thermal drying (heat transfer and vaporization) and mechanical drying (airflow through the material removing moisture).

Through-air dryers generally will be seen in two forms: rotary and flatbed.

ROTARY THROUGH-AIR DRYERS

A rotary dryer is a rotating drum with an open-grated surface. The material to be dried covers the outside of this surface and moves with the drum.  An insulated enclosure and air distribution system is used to supply the conditioned hot air onto the product, and a suction fan pulling air from inside the roll draws the hot air through the web. Normally, rotary dryers operate at one set temperature and velocity and do not easily accommodate multiple heating zones.

FLATBED THROUGH-AIR DRYERS

A flatbed dryer holds the product on an open mesh conveyor belt and moves it through a long, horizontal suction chamber. Like rotary designs, flatbeds supply a stream of hot air onto the top of the web, with suction fans pulling the hot air through the material. Flatbeds, however, can be designed with multiple zones that vary the heat transfer by adjusting the temperature and airflow as the material moves through the dryer.

Common applications for through-air drying

Through-air drying is typically used for drying, curing, heat setting and thermal bonding of porous materials such as nonwovens, textiles, paper and fibers. In many thermal bonding applications such as auto carpeting, cotton shoddy and other recycled materials are mixed with low melt fibers such as Polyethylene.  When the mat of loose fibers passes through the through air dryer, the heat melts the fibers and they become bonding agent to produce a rugged product.

Dry laid or wet laid Fiberglass mat material is a good candidate for through-air drying, and manufacturers of the base material for roofing and siding shingles often use this drying method. Through-air drying is also used in the carpet industry for heat setting long rolls of carpet to prevent shrinkage.

5 reasons to choose a through-air dryer:

  1. Faster drying.  Through-air drying produces two to three times more heat transfer than other types of convection dryers. Intimate contact between the hot air stream and the product interior causes the web to heat and dry more rapidly.
  2. Uniform drying.  The high evaporation rates from thermal and mechanical drying, along with better heat penetration through the product, result in very uniform drying though the thickness of the material.
  3. Accurate temperature control.  The heated airstream can be precisely controlled in machine direction and across machine direction. Flatbed designs also provide the ability to create multiple heating zones to vary temperature and air velocity as the web moves through the dryer.
  4. Enhanced texture or “hand” of non-woven materials.  For certain types of nonwovens such as absorbent products used for baby diapers and sanitary napkins, through-air drying produces a desirable texture that can’t be accomplished with other types of dryers.
  5. Simple operation.  Through-air dryers are easier to operate than some other types of convection dryers. Threading is a simple matter of laying the material on the conveyor. 

Through-air dryer limitations:

  1. Works only with permeable webs.  Since the heated air stream needs to penetrate the web, non-permeable materials are not suitable for this type of dryer.
  2. Rotary designs can’t accommodate multiple zones.  When there are wet-to-dry permeability changes in the web, multiple zones are needed for efficient operation. If you need this capability, a flatbed design is recommended.
  3. Needs a powerful blower.  A higher blower pressure drop is needed for the higher air flow rates and to overcome product pressure drops. This requires a higher horsepower blower than other convection dryers, resulting in increased energy consumption. 
  4. Mechanical losses of fiber and coating material.  The high volume of air and the penetration through the material can result in loose fiber and coating material being carried through the air stream, resulting in frequent cleaning and higher maintenance.

Important design considerations for a through-air dryer

If your product seems suitable for a through-air dryer, there are a number of important design decisions to be made so that the dryer is optimized for your product and process requirements. When you work with an experienced dryer manufacturer, these design options can be precisely configured according to your needs:

  • Flatbed or rotary design
  • Temperature and air flow
  • Heating zones for flatbed dryers
  • Self-supported product or conveyor
  • Controls
  • Access for cleaning and maintenance