RESIN MOISTURE & DRYING

You must keep drying systems in good working order to pre-condition your material satisfactorily for processing. Here’s an overview of the critical, and often overlooked elements of the drying process that can help you get the most out of your drying equipment and material investment:

• Process and Regeneration Heaters
• Air Flow Components
• Filters and Blowers
• Moisture Absorbing Desiccant

Heater Maintenance in Plastics Processing

Maintaining Process Heaters

The process heater elements in a dryer are a critical part of the dehumidification process because they elevate the material temperature to purge moisture from the resin. 

Monitor process heaters closely to ensure they are:

• Reaching the desired temperature promptly
• Properly switching on and off in response to the temperature controller’s signals

Slowness, or inability to attain the temperature set on the controller, is a sign of a burned out or failing heater element. 

When your process heater slows down: 

• The dryer should be taken out of service
•  The heaters should be replaced 

It is best to replace all heaters within a multiple-element heater box at the same time, since the expected life of each element is approximately the same.

Maintaining Regeneration Heaters

There is a second set of heaters in an electric dual bed, desiccant wheel or carousel drying system.  It is for the drying or regenerating of the desiccant. The manufacturer usually sets the temperature on a hidden temperature controller. The regeneration heater plays an important role in the operation of the dryer, keeping the desiccant dried out, so it can efficiently remove the moisture from the process air stream.

How to Monitor Regeneration Heaters:

• Observe controller lights thermometers or amp meters if supplied by manufacturer
• Observe heater contactors
• Measure heat output
• Compare temperature reading to values specified in dryer instruction manual for that dryer model that type of desiccant

Blower Maintenance

Every dryer is designed with an airflow volume to match its throughput capacity along with the heaters, desiccant, etc.  If the airflow rate is compromised, the dryer efficiency is reduced.

There are two blowers in desiccant dryer systems:

• A process blower, which may be located within the dryer cabinet or on the drying hopper
• A regeneration blower, inside the cabinet, forces hot air through the desiccant to remove the absorbed moisture and make it ready to remove more moisture from the process air stream

Ensure maintenance of maximum airflow in your resin drying blower by:

• Following the instructions for filter maintenance
• Checking hose connections for tightness
• Repairing/replacing hoses that are crimped or crushed
• Repairing holes in hoses or replacing the hose
• Making sure hose linings have not collapsed – blocking airflow
• Ensuring that hoses are the shortest possible length

NOTE:
Any blower that changes pitch over time or begins to make abnormal noises should be examined and replaced as soon as possible to assure full air flow is available for the dehumidifying and regeneration processes.

Filter Maintenance

Air cleansing filters are used in drying systems to:

• Prevent dust and fines carried over from the material in the drying hopper from contaminating the desiccant’s ability to remove moisture
• Clean ambient air used in the regeneration of the desiccant
• Prevent dust from contacting the dryer’s heaters, creating a fire hazard

Keep all filters in the drying system clean to allow the critical airflows of the dryer to do their job. 

Optimal Resin Dryer Filter Maintenance Routine

• Turn the dryer heaters off and allow heaters to cool off before the blowers are stopped
• Carefully remove the filter elements
• Clean or replace the filter elements 
• Air blow or vacuum the filters to remove accumulated dust and fines
• Do not puncture or distort the filters
• Replace filter elements if they are damaged, distorted or can’t be cleaned
• Carefully re-install filter elements so no leaks are allowed around the base of the filter or the filter housing or lid.

Desiccant Maintenance for Optimal Resin Drying

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.

Desiccant Beads are 30% clay.

To avoid contamination of resin drying 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
   

Desiccant Wheels use 100% pure crystalline desiccant

When should desiccant wheels be replaced?

• 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

Considerations for 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.