Problem With Your Dryer? Troubleshooting Plastic-Pellet Drying

Plastic Dryer Troubleshooting: Dew Point, Temperature, Airflow and Time

Drying is an important part of preparing hygroscopic plastics for moulding and processing. Materials such as PET, PA, PC, TPU and many engineering polymers can absorb moisture from the surrounding air.

If too much moisture remains in the resin during processing, it can cause hydrolysis, surface defects, reduced molecular weight and deterioration of mechanical properties.

When a dryer is not performing as expected, simply increasing the temperature may not solve the problem. Effective resin drying depends on four basic conditions working together:

• Temperature
• Low dew point
• Airflow
• Drying time

Checking these four factors systematically is usually more useful than assuming immediately that the dryer itself has failed.

The Four Fundamentals of Plastic Resin Drying

How a Desiccant Dryer Works

A typical desiccant dryer circulates dry heated air through a hopper containing the plastic resin. Moisture is released from the material and carried away by the return air.

The desiccant system then removes moisture from the air before it is reheated and returned to the hopper.

If any part of this cycle is not performing correctly, the pellets may receive air that is too wet, too cool or insufficient in volume.

1. Check the Drying Temperature

The temperature displayed on the dryer controller does not always represent the actual temperature experienced by the resin throughout the hopper.

Possible causes of poor temperature performance include:

• Heater problems
• Incorrect controller setting
• Temperature-sensor error
• Insufficient airflow
• Heat loss through ducts or hopper walls
• Incorrect hopper loading
• Air leakage

A temperature measurement at several positions can help determine whether the expected drying temperature is actually being achieved.

The temperature near the dryer outlet may be different from the temperature inside the resin bed. For troubleshooting, the measurement point should therefore be selected according to what needs to be verified.

2. Check the Dew Point of the Drying Air

Temperature alone does not tell you whether the air is sufficiently dry.

The dew point indicates the moisture content of the drying air. A lower dew point generally means the air contains less water vapour and therefore has a greater ability to remove moisture from hygroscopic material.

For many desiccant-dryer applications, low-dew-point air is required to obtain consistent drying performance. The actual requirement depends on the polymer, process and dryer design.

If the dew point becomes too high, possible causes include:

• Saturated or deteriorated desiccant
• Incomplete desiccant regeneration
• Valve leakage
• Air leakage into the drying circuit
• Restricted airflow
• Cooling or regeneration problems
• Incorrect desiccant-bed changeover

Where Should Dew Point Be Measured?

The correct measurement point depends on what you are trying to diagnose.

For permanent dryer monitoring, the supply-air or dryer-outlet point is often the most useful location because it indicates whether sufficiently dry air is being produced.

Watch What Happens During Desiccant Changeover

Twin-bed desiccant dryers normally alternate between drying and regeneration cycles.

During bed changeover, a temporary change in dew point may occur. The important question is whether the dew point recovers quickly and remains within the expected operating range.

A continuously installed dew-point transmitter can make this behaviour much easier to see than occasional manual checks because the complete drying and regeneration cycle can be trended over time.

3. Check the Airflow

Even very dry and sufficiently hot air cannot dry the material effectively if it does not flow properly through the resin bed.

Insufficient airflow may result from:

• Blocked filters
• Blower deterioration
• Restricted hoses or ducts
• Incorrect damper position
• Air leakage
• Incorrect hopper loading
• Material bridging
• Changes to the dryer or piping arrangement

Poor airflow can also create temperature differences inside the hopper, so airflow and temperature should often be investigated together.

4. Check the Drying Time

Drying is not instantaneous. Moisture inside the polymer must diffuse toward the pellet surface before it can be removed by the drying air.

If the throughput is too high or the residence time is too short, the material may leave the hopper before the drying process is complete.

Check:

• Material manufacturer's recommended drying time
• Hopper capacity
• Actual material throughput
• Hopper loading level
• Whether the material is being added continuously
• Whether wet and partially dried material are being mixed

Do Not Troubleshoot Dew Point Using Relative Humidity Alone

Relative humidity changes strongly with temperature. This makes it less convenient for evaluating very dry industrial air when process temperatures are changing.

Dew point is generally a more useful measurement for desiccant dryers because it directly describes the water-vapour content of the air independently of the normal changes caused simply by heating the air.

For dryer troubleshooting, it is therefore useful to consider:

• Drying-air dew point
• Drying-air temperature
• Material temperature
• Airflow
• Residence time

Permanent Dew Point Monitoring for Plastic Dryers

For dryers that need continuous performance monitoring, a permanently installed dew-point transmitter can provide an ongoing measurement instead of relying only on occasional checks.

The appropriate transmitter depends on the dryer size, required dew-point range, installation space, process temperature and required electrical output.

DMT143 and DMT143L for Compact Dryers and OEM Equipment

The Vaisala DMT143 and DMT143L Miniature Dew Point Transmitters are compact instruments suited to small industrial dryers and OEM equipment.

• Compact stainless-steel construction
• Suitable for low-dew-point industrial air
• Analog output options
• RS-485 with Modbus RTU
• Designed for integration into dryer and OEM equipment

The DMT143 family is particularly useful where installation space is limited and continuous dew-point monitoring needs to be built directly into the equipment.

View Vaisala DMT143 and DMT143L

High-Temperature Dryer and Oven Measurement

Some industrial drying processes require measurement directly inside hot process air rather than at a cooler dryer outlet.

For these applications, the probe must be selected according to the actual process temperature.

DMP5 for Processes up to 180 °C

The Vaisala DMP5 Dew Point and Temperature Probe is designed for direct in-line measurement in hot industrial drying processes.

• Direct process measurement up to +180 °C
• Long stainless-steel probe
• Adjustable installation depth
• Dew-point and temperature measurement
• Modbus RTU or connection to Vaisala Indigo transmitters

The DMP5 is the current direction for applications previously served by the Vaisala DMT345.

View Vaisala DMP5

DMP6 for Very Hot Processes up to 350 °C

For very-high-temperature industrial dryers and ovens, the Vaisala DMP6 Dew Point Probe allows direct measurement in processes operating up to +350 °C when correctly installed with its passive cooling arrangement.

• Designed for direct measurement from +100 to +350 °C
• Passive probe cooling
• No cooling water or powered cooling system required
• Modbus RTU or integration with compatible Indigo transmitters

The DMP6 is the current direction for very-high-temperature applications previously served by the Vaisala DMT346.

View Vaisala DMP6

Portable Dryer Troubleshooting with Indigo80 and DMP80

Not every dryer requires a permanent dew-point transmitter. A portable meter is particularly useful for maintenance personnel who need to check several dryers or compare different points in the drying system.

The Vaisala Indigo80 Portable Meter with DMP80 Dew Point Probe provides a current portable solution for dryer troubleshooting, spot checking and short-term measurement.

It can be used to:

• Check dryer-outlet dew point
• Compare several dryers
• Measure before and after maintenance
• Investigate drying-air problems
• Compare a permanent transmitter against a portable reference
• Log measurements during a dryer cycle

For existing sites using the older Vaisala DM70 portable dew-point meter, Indigo80 with the appropriate DMP80 probe should be considered for new portable measurement requirements.

View Vaisala Indigo80 and DMP80

Fixed Monitoring or Portable Checking?

A Practical Plastic Dryer Troubleshooting Sequence

When drying performance deteriorates, a systematic check can help narrow down the cause.

1. Confirm the material. Check the resin manufacturer's drying temperature and time recommendations.
2. Check actual drying temperature. Do not rely only on the controller setpoint.
3. Measure supply-air dew point. Confirm that the dryer is actually producing sufficiently dry air.
4. Observe the complete dryer cycle. Look for dew-point deterioration during regeneration or desiccant-bed changeover.
5. Check airflow. Inspect filters, blowers, hoses and restrictions.
6. Confirm residence time. Compare hopper capacity and actual production throughput.
7. Check for leakage. Inspect doors, seals, hoses, clamps and connections.
8. Compare before and after maintenance. Keep measurement records so that changes in dryer performance are visible.

Continuous Monitoring Can Reveal Problems Earlier

A dryer may appear to work normally during a single manual inspection but still suffer intermittent problems.

Continuous dew-point trending can reveal:

• Desiccant-bed changeover behaviour
• Gradual deterioration in dryer performance
• Moisture spikes
• Changes following maintenance
• Recurring operating problems
• Differences between production shifts

This can help maintenance personnel identify a developing problem before it begins affecting finished-product quality.

Dew Point Measurement Support for Plastic Dryers in Malaysia

Seacom Process Instruments supplies and supports dew-point, humidity and temperature measurement instruments for plastic drying, compressed air, industrial dryers, ovens and other low-moisture processes in Malaysia.

Our team can help determine whether the application requires a compact fixed transmitter, direct high-temperature probe or portable troubleshooting instrument.

When asking about a plastic-dryer application, it is useful to provide:

• Type of dryer
• Plastic or resin being dried
• Required drying temperature
• Expected drying-air dew point
• Location where the measurement will be installed
• Process pressure, if applicable
• Air temperature at the measurement point
• Permanent or portable measurement requirement
• Required output such as 4–20 mA or Modbus
• Existing dryer or dew-point instrument model

A good dryer diagnosis should not rely on one measurement alone. Temperature, dew point, airflow and drying time should be considered together to understand whether the material is receiving the drying conditions required by the process.

Ask Seacom about plastic dryer dew point measurement
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