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Resin dryers provide simple, fast and effective drying for molding and extrusion. A variety of types are available, providing quality drying performance for nearly any application.   Some of the most common types include: 

• Hot air dryers: No desiccant
• Compressed air dryers: No membrane or with low dew point membrane
• Desiccant bed/dehumidifying: Single- or multiple-bed type
• Desiccant wheel/dehumidifying: Rotary wheel
• Vacuum-type dryer: Heat and vacuum are used to remove material moisture

Dryers are available for molders and extruders processing as little as one pound per hour to over 6,000 pounds an hour.

Options range from small, energy-efficient portable units to large central systems. Many drying solutions feature a compact design with integrated conveying, ensuring dried material is delivered directly to the process machine.

Dryer types

Hot air dryer: These simple systems are the most economic resin dryers. Using a single-pass process, these dryers pull atmospheric air through a filter and through the heater, pushing the heated air through a drying hopper and exhausting it back into the atmosphere.

Ambient air is heated and introduced into the drying hopper. Surface moisture is removed from non-hygroscopic materials. Non-hygroscopic materials are typically dried at 180º F or less. For hygroscopic materials the air heats the pellet, and internal moisture is brought to the surface, allowing it to dry. The maximum hot air temperature is driven by the material’s softening (not melting) temperature. This type of dryer is not typically used for hygroscopic materials unless there are small quantities used from a sealed bag of resin. 

Compressed-air/dehumidifying/membrane dryer: Primarily used for low-throughput applications and small machine applications due to the amount of compressed air required. High pressure compressed air is expanded to produce higher volumes of low-pressure air. The expanded air is passed through a heater and then enters the drying hopper. Heated wet air is discharged from the drying hopper into the atmosphere. 

Non-membrane versions lower the plant dew point air (to process) by 40ºF to 50ºF. Compressed plant air typically provides approximately 0 to +15º F dew point air. In cases where the central compressed air system is equipped with a central compressed air dryer, this may reduce the compressed air dew point enough (0 to -15º F) to allow proper drying without the use of an additional membrane on the dryer. 

Membrane versions run compressed air through a membrane, which removes moisture from the air. Compressed air dryers which use a membrane can produce -40°F air or better.

Some applications may require a HEPA filter on the air outlet of the drying hopper to capture any exhaust dust particles and prevent them from entering the atmosphere.

Desiccant bed/dehumidifying dryer: This type of dryer uses a desiccant material (e.g., beads) to absorb moisture. As beads become saturated, they go through a regeneration process to remove moisture which will allow the desiccant to regenerate and allow moisture to be absorbed again into the beads (the process of adsorption).  Extremely hot air (400 to 800°F) is passed over the desiccant to release the moisture they’ve absorbed from the airstream. However, the beads must be cooled before returning to the drying process to avoid melting the resin with a temperature spike when the bed is put online for processing.

Multiple-bed desiccant technology can use one or two blowers. The blower(s) provide process, cooling and regeneration air, utilizing a switching valve to direct air through the desiccant beds.

Desiccant wheel rotor type (honeycomb) dryer: Desiccant wheel dryers provide a different type of desiccant drying. They are far more efficient, more compact, and require less maintenance. Traditional dehumidifying dryers use a large volume of molecular sieve (in pellet form) composed of at least 30% clay. This desiccant bead tends to degrade over time. The desiccant wheel design is completely different. It is constructed from a pure molecular sieve impregnated onto a durable media. This material is formed into a reinforced honeycomb structure.

 

Desiccant wheel dryers provide stable, extremely reliable dew point in nearly any ambient condition. Stable process temperatures mean no bed spikes. These dryers go from cold start to drying in minutes. They can achieve higher-than-average process air flows using minimal energy due to low pressure drop across the rotor. Large surface area to volume ratio of desiccant provides faster adsorption and desorption conditioning of the desiccant. The honeycomb has a thin layer of desiccant that uniformly absorbs moisture. At regeneration, the absorbed moisture is easily evaporated off the media, this allows the dryer to deliver effective continuous drying as the rotary wheel slowly rotates.

How a wheel dryer works.

A wheel dryer operates through three simultaneous processes: drying, regeneration and cooling:

• In the drying loop, humid air from the drying hopper passes through a filter, cooler and blower to the desiccant wheel, extracting moisture from the process air before returning to the drying hopper.
• The regeneration process removes moisture from the desiccant wheel using a separate filter, blower and heating chamber, expelling hot, moist air external to the dryer.
• Cooling follows regeneration, where a portion of low dew point, cooled air is directed to the desiccant wheel (before the heater) enhancing its moisture absorption for the next drying cycle.

Vacuum dryer: Vacuum dryers reduce the boiling point of water. For instance, at a vacuum of 90%, water boils at 122º F. Current designs feature an insulated vacuum vessel; insulated stainless steel hopper for drying temperatures up to 662º F; load cells that control material level and show material consumption; a dry air membrane for purging the vacuum vessel; and an insulated retention hopper for dried material. Pellets are heated to the required temperature. When the vacuum is applied, water vapor inside the resin boils.  Resin can be ready for processing in a short amount of time compared to traditional drying. Vacuum dryers have fewer moving parts and work well for some processing applications.

Controls

Controls monitor and maintain optimum performance in the drying process. Today’s control platforms offer many control and diagnostic features including air flow and temperature control. Some platforms are capable of adjusting dew point parameters and incorporate dryer maintenance routines in the typical operation and use.

Dryer controls typically feature colorful, easy-to-use touch screens with visual representation of drying parameters and processes. Intuitive layouts allow operators to quickly learn and initiate dryer functions.

 

Dryer controls are keeping up with the data-collection requirements of Industry 4.0 used in the plastics industry.  Advanced controls optimize moisture removal with capabilities like energy control algorithms and integrated conveying controls with purge takeoff capabilities.

Remote monitoring capabilities let operators keep tabs on operations form their phones, tablets or computers.

Process data collected by modern dryer controllers enables smarter facilities, with trend charts that show critical parameters.

Drying Hoppers

Drying hoppers are required for all types of drying applications. The drying hopper holds the correct amount of material in residence for the proper amount of time. This allows the dryer to remove the required amount of moisture from the resin.

The design of resin drying hoppers is another critical consideration for plastics drying systems. Flow design, sizing and insulation are the key factors to understand when choosing the ideal drying hopper.

Flow design — mass flow or funnel flow — affects material passage through the hopper.

In a mass flow design, all material flows through the hopper at the same rate, and material flow is counter current to the drying air. In a funnel flow system, material flows faster through the center of the hopper and slower along the hopper side and walls.  Mass flow designs are required to allow for proper residence time of the material before it enters the process feed throat.

Sizing your drying hopper can be done by weight or by volume. Small drying hoppers average from 0.1 cubic foot capacity to 30.0 cubic feet These are typically mounted on a floor stand or a portable cart and are generally located next to or near the process machine. For some lower-throughput applications, the drying hopper can be mounted directly to the machine throat. Larger drying hoppers average 30 to 425 cubic feet and are typically mounted on a floor stand or mezzanine due to the larger footprint requirements of the equipment.

To size your drying hopper by weight, it must equal the throughput rate of the machine (pounds per hour) multiplied by the residence time of the material in hours. For example, if your machine can move 100 pounds per hour and the material’s residence time is four hours, that would require a drying hopper with 400 pounds of capacity.

To size your drying hopper by volume, the calculation is machine throughput rate (pounds/hour) multiplied by material residence time (hours) divided by bulk density (pounds per cubic foot). For example, 100 pounds of material requiring four-hour residence time at a density of 40 pounds per cubic foot would require a drying hopper with capacity of 10 cubic feet.

Many engineering resins can be dried using 35 to 40 pounds per cubic foot as a reference for the virgin pellet bulk density (refer to material supplier data sheet).  Filled resins – for instance, ABS with 40% glass filler – weigh more, while regrind material tends to be lighter density. Particle size and shape of regrind also affects the material density. Examples include:

• PET virgin pellets: 52 to 53 pounds/cubic foot
• PET regrind from sheet: Typically is 17 to 23 pounds/cubic foot depending on sheet thickness
• PET preform regrind: Averages 26 to 38 pounds/cubic foot
• PET bottle flake: Typically 18 to 23 pounds/cubic foot
• PE virgin pellets: 32 to 35 pounds/cubic foot
• PE film: 7 to 11 pounds/cubic foot (depends on gauge)

Non-insulated drying hoppers are less desirable for a number of reasons:

• Waste energy
• Excess heat in process area
• Compromised operator safety
• Material adjacent to hopper walls will be at less-than-selected set-point temperature

Resin Moisture Measurement Devices

Understanding the initial moisture content of plastics resins is essential to determine how long to dry them. Drying time and temperature guidance from material suppliers often is based on an assumed initial moisture content, which can lead to resins being overdried or  under dried – creating process and part problems.

Resin moisture measurement devices are available in offline (laboratory) or inline formats. Lab devices can also provide complete chemical analysis, while inline devices are often less expensive and easier to use.

Offline devices include:

• Loss-in-weight analyzers: Measure total change in weight of the material when the resin sample is heated at a specific temperature.
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• Moisture-specific analyzers: Use a chemical process to detect moisture levels.

Inline moisture analyzers include:

• Capacitive sensor: Measure changes in capacitance caused by dielectric changes.
• Microwave: work on the principle that water has a significantly higher dielectric constant than most other materials. Due to a water molecule’s dipolar effect, a microwave resonator’s resonant frequency changes with variations in moisture content. These variations are detected by the sensor electronics, which are scaled by the calibration process to provide a precise readout of the moisture present.
• Near-infrared: Excite water molecules with light beams to measure light absorption in wavelength, which indicates resin moisture content.