Desiccant Maintenance

There are three basic types of desiccant dryers used by plastics processors:

• Dual Bed (Twin Tower) Desiccant Dryers
• Carousel Dryers
• Desiccant Wheel (Rotor) Dryers

The first two types use desiccant beads and the third type uses a pure crystalline molecular sieve desiccant that is grown onto a substrate.

The purpose of the desiccant, in all case, is to remove moisture from the process air stream. If the desiccant is doing its job and the heaters and blowers are working as designed, the dew point of the process air is reduced to -40 degrees. This is required for many of the engineering grade materials in use today.

It is important to avoid contamination of the desiccant:

• Follow instructions for filter maintenance to minimize the chance of desiccant contamination from dust
• Follow instructions for regeneration blower maintenance to make sure that desiccant is being properly drier after removing moisture from the air stream
• Utilize a plasticizer filter to avoid plasticizer contamination

Desiccant beads break down faster because:

• They are composed of 30% clay binder
• The constant heating and cooling breaks down the bond between the clay and the desiccant
• Dust forms and the desiccant become less effective in removing moisture
• The dust can cause contamination of the resin in the drying hopper
• Replacement of desiccant beads is recommended every two years

Desiccant wheels work longer and more efficiently:

• 100% pure molecular sieve desiccant
• Desiccant wheel warranties range up to 5 years
• Actual useable life can be up to 15 years

It's time to replace desiccant beads when:

• The dew point of the process air won’t attain -40˚
• Cleaning/replacing filters does not help and air flow from regeneration blower is good
• You can crush the desiccant beads between your fingers
   

Replacement of desiccant beads can be a labor-intensive job:

• Turn dryer off and allow to cool down
• Tops must be removed from both towers
• All desiccant must be scooped or shoveled out of the bed
• Vacuum dust from inside the towers
• Check the manufacturers recommendation for desiccant type
• Carefully pack new desiccant beads tightly into each tower being careful not to create any air pockets

NOTE: Smaller twin tower dryers (up to 50 lb./hr. throughput) offer ready-made desiccant cartridges, making replacement far easier.

Multi-bed Carousel Dryers use less desiccant than dual bed dryers but:

• All desiccant and dust must be removed from each tank
• Beads must be packed into the tanks extra tightly (best to use tank vibration)
• It may be best to purchase tanks from manufacturer
• Check/replace seals to avoid air leaks

Desiccant wheel replacement should be considered when:

• The dew point of the process air won’t attain -40˚
• Cleaning/replacing filters does not help and air flow from regeneration blower is good

Desiccant wheel replacement:

• Some manufacturers require that a complete rotor wheel assembly be ordered for replacement
• Other manufacturers sell just the desiccant wheel replacement, which is less expensive
• In either case, the disassembly and installation is fairly straightforward, following the manufacturers instructions

Wheel/Rotor Dryers provide the longest lasting desiccant medium on the market today and therefore require the lowest frequency of replacement than any other type of desiccant dryer. The wheel technology does not employ desiccant beads, but pure molecular sieve desiccant that is permanently bonded to the air-passing corrugations of the wheel element. These corrugations are less likely to trap dust and the desiccant cannot break down over time, so desiccant life is greatly enhanced. If/when the desiccant stops performing its moisture absorbing task, the wheel assembly itself, complete with desiccant medium and its cylindrical enclosure, and on some models, its end caps and drive system, is replaced as a complete unit. This complete replacement assures the user of a 100% restoration of drying capability, with no concerns over quantity of desiccant or even the rotation of the assembly.