How to get the right dryer for your process.
A processor wants to make quality products and drying the resin is often a necessary evil to accomplish that. Drying a material correctly, requires an understanding by the dryer manufacturer of the processors requirements as well as an understanding by the processor of the dryer types and the drying system features that are offered. We will cover both sets of requirements so that the reader can get the best fit for their process and the most bang for the buck.
This article takes an in-depth look at:
* Material types
* Resin drying requirements
* Available dryer choices with drying principles
* Energy usage
* Hopper requirements
FACTORS THAT MUST CONSIDERED FOR PROPER DRYING
(1) Hygroscopic vs. Non-hygroscopic Materials
Polymers attract moisture from the surrounding environment. The amount of moisture that they attract is a function of the moisture in the surrounding environment. Some materials only collect surface moisture while others actually absorb water vapor inside the pellets. Resins do not all attract moisture at the same rate.
* Hygroscopic Resins--Materials like PET, PC, Nylons (and many others) attract moisture throughout the pellet as the moisture is attracted to the polymer molecules. These resins require some type of dryer system to remove the water within the pellets, otherwise the end product will have either structural or appearance defects.
* Non-Hygroscopic Resins--In these resins (things like PP, PE, PVC and some others) the moisture does not penetrate the pellets outside surface and is only present on the surface of the pellet. If a processor is producing good parts without a high percentage of rejects, drying is not necessary. But, if the structural and dimensional requirements are stringent, either a hot air dryer or even a dehumidifying dryer may be well worth the investment. It should be noted that dryers that utilize de-humidified air remove surface moisture more thoroughly and faster than hot air dryers.
* New Type of Non-Hygroscopic Resins--Increasingly, we are seeing non-hygroscopic resins with additives like talc or carbon black added. These additives allow resins like PE and PP to be used for more demanding applications but they have higher moisture absorption and can lead to moisture being encapsulated in the resin pellet. The amount and type additive need to be understood and they must typically be dried with de-humidified air.
(2) Material Rate in lb./hr. or kg/hr.
It is important to specify the upper and lower limits for throughput rate. The entire range needs to be considered so that the system is designed to dry the resin at the highest rate but not over-dry and possibly degrade the polymer at the minimum rate. A level control system can be incorporated in the hopper that allows the residence time to be varied. A system is also available to automatically control drying temperature or air flow rate to match the material rate. Having automatic control of at least one of these can insure proper drying without the danger of polymer degradation.
(3) Material Type
The type of resin can influence the parameters for sizing the dryer.
* Amorphous Resins--Resins like PET, PLA and some co-polyesters are the primary resins that need special handling. In many cases, if the quantity of amorphous material is over 15-20% of the total blend, it's necessary to crystallize these in an agitated hopper (known as a Crystallizer) instead of in the main drying hopper. Crystallizing the amorphous resin separately allows drying in a standard hopper without having sticky material cling to the sides of the drying hopper. Crystallizing a material may take 1 hour and drying it in a separate system may take another 4-5 hours.
* Nylons and other Moisture-Sensitive Resins--Other resins, such as nylons, are very moisture sensitive. Poor part formation may result if the resin is under-dried but over-drying can produce brittle parts. In these cases, automatic control of the drying temperature, in combination with specialized moisture-control features can insure the performance characteristics of the resin are maintained without operator intervention. Managing the drying process is very important in these situations.
* Polymers such as PBT can be damaged by overexposure to heat. Some grades will significantly lose their properties to be molded or extruded after 8-10 hours exposure to heat.
(4) Special Considerations
There are a number of factors that can change what would be the standard recommendation for a dryer system. These special factors can significantly change the vendor's recommendations:
* Excess Surface Moisture--Often, there is a spin dryer to remove the majority of surface moisture from pelletized or compounded material but the moisture can exceed the specified levels if the throughput rate is too high for the spin dryer or it is not properly maintained. In these cases, the temperature and the amount of process air needs to be higher than the standard sizing would recommend.
* Regrind--Often the regrind has a lower bulk density, a higher surface area per pound and can attract higher moisture levels than the base pellets. The amount of regrind and the shape of it may change the drying system recommendation.
* Improper Storage--Usually, materials, like nylon 66, come from the manufacturer with a specified range for ppm moisture but this changes quickly after the bag or bulk box is opened. After only a few hours of exposure to ambient air moisture, the moisture level in the material can be several times what was in the unopened bag and, if not sealed properly, can be 10-25 times as high in a month or so.
(5) Air Flow Requirement
The quantity of air required for a particular drying applications is a function of the resin and the heating temperature. Non-hygroscopic resins will require less air flow than hygroscopic resins and both, very high and very low temperature drying applications, can require more air flow rate.
(6) Drying Temperature -Pre-cooler/After-cooler
Almost all dryers will provide drying temperatures up to 350[degrees]F but when drying at temperatures at the higher or lower end of the range, a cooling coil is required for desiccant type dryers.
When drying resins at temperatures in excess of 220[degrees] F, a cooling coil should precede the desiccant (pre-cooler) to ensure that the temperature of the hopper return-air to the desiccant does not exceed 140-150[degrees] F because the desiccant's performance declines considerably above this range.
In cases where the drying temperature is below 160[degrees] F, an external cooling coil, after the desiccant, is a required option for most desiccant type dryers. This is because there is a temperature rise across the process blower (heat of compression) and also a temperature rise across the desiccant (heat of adsorption). This coil, with tower water, will lower the dryers exit temperature so that the process heater can control the drying temperature properly. Another option would be to use a membrane dryer, like the NovaDrier, that does not have the blowers or desiccant and can typically control temperatures as low as 120[degrees] F very well. (Maximum throughput about 200 lb./hr.)
FACTORS TO CONSIDER WHEN CHOOSING A DRYER
* Throughput range
* Consistent dew point and heating
* Energy usag--What is the operating power at the material rate (kw/kg or kw/lb.)
* Is an economical gas-fired heater available/advisable?
* VFD (variable frequency drive) control of regen heater/blower available to reduce energy usage?
* Ease of interfacing with control
* Level of comfort with the drying technology
* Maintenance requirements & maintenance monitoring
* Whether the dryer meets any special requirements you have
* Pre-cooler/after-cooler availability
* Plasticizer filter/collection included
* Continued fast availability of parts
* Availability of well-designed hopper
* Can your dryer supplier provide lab testing if you are processing a new material?
CONSIDER THE FULL RANGE OF DRYER TYPES BEFORE DECIDING
This is important because not all suppliers offer all types of dryers. If your supplier only manufactures one or two types of dryers, they will naturally try to pigeonhole you into one of those.
(1) Desiccant Wheel Dryers (Rotary Wheel) (Throughputs from <25-5,000 lb./hr.)
Desiccant wheel dryers have become the most popular dryer for many applications because of their more consistent temperature and dew point to the drying hopper. They use a "honeycomb" wheel, impregnated with a pure crystalline desiccant and there is less variation throughout the drying cycle because there is internal cooling after desiccant regeneration and bed-changeover is eliminated. This continuous process typically provides a compact unit with few moving parts and a constant source of -40[degrees]dew point drying air.
Wheel Dryer Advantages:
--More consistent drying for improved quality
--Reduced maintenance-more "up-time"
--Reduced energy usage
--May have advanced controls for ease-of-use
(2) Twin Tower Desiccant Dryers (Throughputs from < 5-3,800 lb./hr.)
Twin Tower (also called Dual Bed) dryers used to be the most commonly purchased type of resin dryer and there are thousands still in use today. While one bed of desiccant supplies dry process air to flow through the drying hopper, the other bed--with saturated desiccant--is regenerated, by forcing hot air through it. When the regeneration is complete, that bed becomes the one supplying the dry process air and the first bed goes into the regeneration mode. Dual Bed dryers were typically used because they attained a -40[degrees] dew point and generally do a good job of drying most resins.
Twin Tower Dryers have been around a very long time but they can exhibit inconsistencies in drying temperature and dew point as the towers switch. It uses molecular sieves in ball form with about 30% clay binder.
Dual Bed dryers have some disadvantages:
* High energy usage
* Large footprint
* Relatively high maintenance due to number of moving parts and desiccant replacement
* Spikes and deviations in temperature and dew point during bed changeover
(3) Central Drying Systems (Throughputs from < 200 through 5,000 lb./hr.)
Central Drying Systems incorporate one or more dryers to serve as a central source of -40[degrees]dew point dry air which is piped to a number of hoppers.
(See Central Drying article on page 20)
(4) Membrane vs. Compressed Air Dryers (Throughputs from < 5-200 lb./hr.)
Membrane dryers, much like desiccant dryers, produce -40[degrees] low dew point air for drying--year round. The low dew point is achieved by drying high dew point compressed air with a hollow fiber membrane that separates the moisture from the airstream. These systems can provide similar economies to a desiccant dryer if equipped with the necessary flow. The membrane has an extremely long life. They have no moving parts and are very well-fitted for smaller injection molding applications.
Ordinary compressed air dryers do not employ a membrane. They depend on a simple expansion of standard compressed air to reduce the dew point. Typically the dew point is lowered by 40-50 degrees so they never attain the -40[degrees]dew point air required for resin drying and during hot/humid conditions, they often have to be taken offline.
(5) Vacuum Dryers (Throughputs from < 30 - 1,000 lb./hr.)
Vacuum Dryers have come into the mainstream recently because of their speed in drying and low cost of operation. These batch dryers incorporate a heating position, a vacuum position and a material discharge. They use low vacuum to cause the moisture to "boil" off. The 30 lb./hr. models are also well positioned for lab applications.
(6) Hot Air Dryers (15--1000 cfm)
Hot air dryers were the first dryers used for drying plastics but they dry with heated ambient air so they are used only for drying the surface moisture off of non-hygroscopic materials.
Vacuum drying is a 3-stage batch process.
Heats the resin.
Vacuum is applied to remove moisture.
Material held for processing.
(7) Infrared Dryers (Throughputs from <500-4,000 lb./hr.)
Infrared Dryers are generally used in drying PET for sheet and some fiber applications. These dryers use infrared heat to crystallize and/or dry PET. They have a higher cost than a typical desiccant dryer but can be very competitive when doing both crystallizing and primary drying. The infrared heat penetrates well to the inside of the resin flake or pellet and drives the moisture out while crystallizing the resin at the same time. These dryers can shorten the drying crystallizing process start up from 6-10 hours to one hour.
OTHER DRYING SYSTEM FACTORS TO CONSIDER
Dryer systems should have intuitive, easy-to-use controls. The dryer should need no more than a "quick start card" for normal operation. Complicated codes and look-ups should not be required. (See Controls Article page 26)
(2) Energy Usage
The energy usage of various dryers varies widely, as you can see by this chart. You can minimize the ongoing cost of ownership by considering energy costs before purchasing a dryer. Depending on the dryer throughput, the operational hours/year and your cost/kwh, the additional operating cost may amount to several thousand dollars/year/dryer.
Typical Energy Costs/100 lb. of Throughput NovaWheel[TM] with Adaptive Control $0.27 NovaWheel[TM] w/o Adaptive Control $0.30 Vacuum Dryer $0.44 NovaDrier[TM] Membrane $0.50 Dual Bed $0.68 Compressed Air Dryer w/ membrane option $1.30
(3) "Right-Sizing" the dryer minimizes costs
One of the best ways to minimize the capital and utility costs in the plant is to just use a correctly sized dryer. In trying to be safe, most people end up putting a safety factor on top of a safety factor and the power used by the dryer is far above what is necessary. Consider adding a VFD ( variable frequency drive) to your dryer if you are faced with this scenario. The VFD will, generally, have a payback of only months. Just be sure to discuss this with the dryer's manufacturer to insure that the flow isn't reduced so far as to cause any overheating in motors. Reducing air flow as much as 50% will be available to you without any issues. With this, you can have the dryer operate at the flow rate that's correct for the application. Some modern dryers have this "Adaptive Control" system and are able to automatically adjust for changes in material or moisture loads.
(4) Lab Capabilities
If you have a new resin that may have unusual properties, think about getting some testing done by the dryer manufacturer. Often this is done with no cost if the application has specialized requirements.
(1) Basic Design--Mass Flow
The hopper is sometimes overlooked but this is the point where all the drying happens. It's a critical point in the entire process. A well-designed hopper ensures that the air flows up through the material uniformly and the resin moves down to the hopper exit uniformly.
(2) Bulk Density/Residence Time
The volume of the hopper is primarily based on the bulk density of the resin and the residence time (drying time) for a particular resin. In some cases different materials will be dried in the same hopper and this may include regrind. When the range of materials and throughput rates is taken into account, the hopper should be suited for having the volume to run at the maximum rate but not to damage the resin when running at the lower rates. Ignoring these factors can lead to material property degradation or changes in color.
(3) Moisture Level of the Resin
The moisture level of the resin can lead to a very significant change in the residence time (drying time) required. Some nylons that can be dried (out of the bag) in 3-4 hours can take 24 hours to dry after they have reached their saturation level.
(4) Ease of Cleaning--cross contamination and material incompatibility
There should be an access door large enough that a complete air cleaning and/or wipe down is possible. Also, unnecessary seams can result in places than cannot be adequately cleaned.
(5) Designed for Drying Temperature
The hopper should be designed such that all seals, gaskets and materials are adequate for the widest range of its operation. This includes any vacuum receivers and take off assemblies. High temperature seals and flex hose should be used, when necessary.
(6) Insulation--Heat Loss
In general, the hopper should be designed such that any energy introduced to the hopper stays in the hopper. No exterior surface should ever exceed 120 F.
(6) Cone/Discharge requirements Flow/Bridging/Regrind
There are several types of cone/ distribution systems that provide good air and material flow. Drying cannot be done in something similar to a silo or surge bin. Below is an example of an adjustable -height cone that provide good distribution.
(7) Level Control
Some resins are very temperature sensitive and over exposure to heat and/or dry air can cause either resin degradation or a color shift. An elongated sight glass can be used to allow a capacitance sensor to vary the level of the resin.
* Don't assume that all dryers are alike-they are NOT!
* Be very specific about your drying requirements-now and in the future
* Give details of any current drying problems you are experiencing
* Ask lot of questions and don't hesitate to call the manufacturer's drying expert.