10 most frequently asked questions about dryers.

What kind of drying equipment do you need? How do you use it correctly to get the best results? If you have questions like these, rest assured that you are not the only one scratching your head.

1 Do I need a desiccant dryer?

Whether you need a desiccant (dehumidifying) dryer rather than a hot-air type depends on the resin being processed. Hot-air dryers are mostly used for non-hygroscopic materials, such as polypropylene, polyethylene, and polystyrene. For these plastics, drying is only necessary to remove moisture condensation adhering to the surface of the pellets or granules. Hot-air dryers simply blow heated ambient air over the surface of the pellets or granules to evaporate the moisture and carry it out of the hopper. If your process or the end product can tolerate some moisture, you may not need to dry non-hygroscopic materials at all.

You can't avoid drying hygroscopic plastics like ABS, PET, PBT, nylon, and most engineering resins. Those materials absorb moisture within the pellets or granules. Heating those resins to processing temperatures with the moisture still there can destroy the polymer - or lead to bubbles and splay in the molded part. Such materials are most effectively dried with a desiccant dryer, which removes moisture from the material using dehumidified hot air.

Some processors use desiccant dryers with non-hygroscopic resins. The dehumidified air acts as a sponge to remove moisture more quickly than hot ambient air. However, a desiccant dryer is likely to use more energy than a hot-air dryer and to cost much more to purchase for the same air-flow capacity.

2 Which dryer configuration is right for me?

You have three basic dryer configurations from which to choose: machine-mounted, portable cart-mounted, and central drying systems. Each has its proper place.

Machine-mounted dryers come in two configurations. In one, the dryer and the drying hopper are mounted directly on the machine. In the other, the drying hopper is mounted on the machine and the dryer generally stands on the floor next to it. For large process machinery, it is not uncommon for the dryer to be located above the machine, such as on a mezzanine. For any given machine size, there is a maximum dryer size that makes sense for machine mounting because of the instability of the dryer and drying hopper caused by rapid cycling and vibration of the molding press.

Safety is a consideration with machine-mounted dryers because workers must climb up on the machine to set up and service the dryer or drying hopper. Remember that your process machines may experience some additional downtime while waiting for set-up, service, and cleanout of machine-mounted drying hoppers. (Some manufacturers provide quick-release clamps so that dryers can be removed from the machine for servicing.)

Cart-mounted or portable dryers, which put the dryer and drying hopper on a wheeled cart, can be moved from one location to another to serve multiple process machines. These have become the most popular dryer configuration because of the flexibility and mobility they offer. For example, material can be predried on the cart away from the machine and then delivered to a machine-mounted vacuum receiver. Service and cleanup can be accomplished off-line to reduce downtime for process machinery during material changeovers.

Central drying systems can be more economical to install and operate than others, particularly where large quantifies of similar materials are processed in multiple machines. When floor space is limited, these systems can be installed away from the central production area.

One version consists of a centrally located dryer and drying hopper that convey material to several processing machines. This type is ideal for long process runs when all machines are running a single material.

A second central drying configuration is appropriate for processing several types of materials. It has a centrally located, high-capacity dryer that provides dehumidified air to multiple machines with individual machine-mounted drying hoppers.

A third configuration consists of a central drying station that delivers dehumidified air to individual drying hoppers mounted on a common stand in a central location. This arrangement allows several materials to be predried simultaneously and then conveyed to multiple process machines.

3 Should I convey with dry air?

Whether to convey with dry air is a frequent subject of debate. A thorough system survey can be used to determine whether the additional expense of dry-air conveying is justified. In most cases, the benefits are more a matter of perception than reality.

Some dryer manufacturers will recommend dry-air conveying whenever a material-handling system is installed. However, our view at AEC/Whitlock is that whether to convey with dry air depends on your process and is only necessary in limited situations. The only justifiable reason is to prevent material from absorbing moisture from the conveying air. However, just as it takes hours to dry materials, it can take an hour (and typically more) for material to reabsorb moisture from the air. Since material moves at approximately 3000 ft/min in the conveying lines, it only takes a few seconds to transfer material from the remote drying hopper to a machine's vacuum receiver.

You may hear two reasons cited for conveying with dry air: One, that it prevents outside contamination. Two, that it provides drying while conveying. Neither is correct. For one thing, the conveying system is closed, so there is no air movement after the conveying cycle is complete. In addition, the dried material and the conveying air are typically hotter than 150 F. Air will not effectively release its moisture at temperatures greater than 150 F. Therefore, it doesn't matter whether air that hot has been dried or not - it won't give its moisture up to the material.

There are alternatives to dry-air conveying that ensure material stays dry after conveying. Try drying the material on the processing machine after it has been conveyed, keeping minimal inventory of material on the processing machine, keeping the material hot after conveying, and purging material from the conveying line with dry or ambient air to reduce the amount of material sitting in conveying lines between loading cycles.

4 What size dryer do I need?

The optimum dryer size is a function of the machine's production rate, or how many pounds of material are used in one hour. The two most important parameters to consider in sizing a dryer are air flow (cubic ft/min or cfm) and process-air temperature. But cost, anticipated future requirements, and type of material being processed are also key considerations for dryer size.

The rule of thumb for air flow is to use 1 cfm of air per pound of material being processed in one hour. (Although some sources use a range of values from 0.5 to 1 cfm/lb/hr, we always recommend the maximum.) Once you have determined the required cfm, then specify the next largest standard size closest to that amount.

Keep in mind that dryer model-number designations do not always indicate their true cfm rating. Make sure you ask the dryer supplier for a true indication of air flow in order to avoid undersizing the dryer.

Required drying temperatures are typically specified by the materials supplier. Most dryer manufacturers offer standard and high-temperature options on their equipment. Before you choose, make certain you consider your present and possible future requirements. Also ask if there is an add-on cost for the high-temperature option (in some cases it is included in the price of the dryer). Generally, standard temperature ranges are 150-300 F while high-temperature models offer 150-400 F.

5 What size drying hopper do I need?

The drying-hopper size is also a function of the machine's production rate, material residence time, and material bulk density.

Recommended residence time at drying temperature is found on the material supplier's data sheets. Most non-hygroscopic materials have a residence time of 1.5 hr. Some hygroscopic materials take longer than others to release moisture and thus require more time in the drying hopper.

Material bulk density, measured in weight per cubic foot, is the other key parameter when sizing a drying hopper. A critical consideration is whether regrind will be dried together with virgin material. If regrind will be used, an appropriate bulk-density value must be considered when determining the hopper size. The bulk density of virgin materials is generally higher than for regrind of that same material. So you may need a larger hopper if regrind is to be used.

Dryer suppliers use typical bulk density values to determine drying-hopper size for regrind and for virgin materials. These values are used along with the specified regrind percentage to determine the overall bulk density.

Most equipment suppliers state drying-hopper capacity specifications in cubic feet. The hopper size can be determined with this formula:

Production rate (lb/hr) [divided by] bulk density (lb/cu ft) x residence time = drying-hopper size (cu ft) required.

Specify the next larger standard size closest to this amount.

6 What's the right drying temperature and do I need an aftercooler or precooler?

Resin suppliers provide the recommended temperatures for drying their materials. Because there can be heat losses somewhere in your drying system, you may need to use higher settings than the resin maker specifies in order to ensure that the material is being processed at the recommended temperature. A temperature sensor in the drying hopper will give you confidence that the material is being dried at the desired temperature. An insulated process-air hose is a good idea to minimize heat loss between a floor-mounted dryer and a machine-mounted drying hopper.

Aftercoolers and precoolers are needed for applications that specify extremely high or low temperature settings. An aftercooler, which is mounted in the return line between the drying hopper and the dryer, is recommended when drying at temperatures of greater than 250 F and is a necessity for drying temperatures greater than 300 F. Drying continuously at high temperatures without the use of an aftercooler may reduce the efficiency of the desiccant beds in dehumidifying dryers.

A precooler, which is mounted in the air delivery line between the drying hopper and the dryer, is recommended to achieve drying temperatures below 180 F and is a must for drying at less than 160 F. When using a precooler, you will also need to place a thermocouple at the inlet to the drying hopper.

7 How do I know my material is dry enough?

Some dryer suppliers offer monitoring devices that work in conjunction with the drying process to estimate when materials should be dry. We at AEC/Whitlock think that taking a material sample and testing it with a moisture-content analyzer is the only true and accurate way to know that your material is dry. Any other method is an estimate, not a true measure, of material dryness.

8 What dryer features ensure easy servicing?

Compactness (small footprint) is important to most processors when choosing a dryer. However, other criteria also merit consideration, including the location and ease of access to serviceable components such as filters, heaters, and desiccants. Ask the supplier for a list of routine maintenance items and their required frequency of service. Find out how simple or complex it is to assemble and disassemble the dryer. How easy is it to clean out the hopper for material changeovers?

Also, consider the overall complexity of the drying system. For example, consider the number of desiccant beds that may need to be serviced or replaced and the ease with which this can be accomplished. Every dryer maker has a different design philosophy. Some designs are more complex than others.

Next, consider the type of process controls on the drying system. Proprietary controls, for example, may be more difficult to repair or replace than an industry-standard brand of rugged, dependable, off-the-shelf controls.

9 How can I tell if a dryer is energy efficient?

There are a number of high-efficiency and energy-saving features available for you to consider, depending on your process requirements. These include control features such as dewpoint and temperature monitors and auto-shutdown and standby modes. For example, a switch-on dewpoint feature can turn fan blower motors on as required, ensuring minimum regeneration to maintain setpoint.

Closed-loop desiccant-regeneration air circuits speed up the regeneration cycle and reduce energy consumption. High-efficiency blower motors are also available. Use of insulated process hose and hopper-mounted heaters instead of dryer-mounted heaters can further reduce energy costs. For certain areas of the country where electricity costs are extremely high, cost-saving gas-fired dryer options are now available from several suppliers.

10 What options might I need?

Here is a brief description of popular dryer options:

* Aflercooler. Reduces return-air temperature to enhance dryer performance. Recommended when drying above 250 F. Required for drying above 300 F.

* Precooler: Extends the low-end temperature range of drying equipment. Recommended when drying at or below 180 F. Required for drying below 160 F.

* Insulated hose: Insulated process-air delivery hose reduces heat loss between dryer and hopper. Never insulate return-air lines between the drying hopper and the dryer.

* Redundant high-temperature safety: A back-up temperature-control system that shuts down delivery of process air to the hopper if the temperature rises uncontrolled. Minimizes risk of melting plastic in the hopper. Commonly used for processing temperature-sensitive materials.

* Self-cleaning filter: Compressed-air pulse jets in the filter housing extend intervals between cleaning.

* Closed-loop regeneration: Saves energy by accelerating the regeneration process. Also maximizes the desiccant bed's moisture-holding capacity, which extends the time range that the process bed can stay on line.

* 13X Desiccant: High-efficiency desiccant with smaller pore openings and increased surface area for premium drying applications. Required for drying PET, which releases volatiles (acetaldehyde) during drying that will saturate a standard desiccant and degrade it relatively quickly.

* Heated drying hoppers: These come with just a heater, or with a heater and independent temperature control, or with a heater, control, and blower. They all cut costs by reducing heat loss between the dryer and drying hopper.

* Seven-day timer: Automatically starts the dryer to ensure proper residence time for material.

* Heater burnout indicator: Confirms proper heater operation.

* Audio/visual alarm annunciator: Provides a loud audible signal and visual strobe to indicate an alarm condition in the drying process.

* External communications: Provides a "gateway" to the temperature controls on the dryer. Allows for higher-level distributed control and process monitoring.

Example of Sizing a Dryer & Hopper

* Application: virgin molding-grade ABS.

* Bulk density: 42 lb/cu ft.

* Residence time: 2 hr.

* Production rate: 50 lb/hr.

* Drying temperature: 200 F

* Dryer size required: 1 cfm (rule of thumb) x 50 lb/hr (production rate) = 50 cfm.

* Required temperature range: 200 F.

* Hopper size required: 50 lb/hr (production rate) [divided by] 42 lb/cu ft (bulk density) x 2 hr (residence time) = 2.38 ft. The next largest standard size from AEC/Whitlock has a 3-cu-ft capacity. This provides some additional capacity to allow for virgin and regrind mixtures.