Central drying: is it right for you?

Central drying systems are not a new concept for plastics processing, but their popularity has increased significantly in recent years. This renewed interest is not surprising because central systems can satisfy the operational needs of many, if not most, processors.

Up to now, an individual drying system dedicated to each process machine has been the simplest and most generally accepted approach to material drying. But there are drawbacks to this traditional route. For one thing, individual drying systems often consume valuable floor space around the processing machines. Additional floor space is sacrificed to the raw-material gaylords that are frequently used to feed individual dryers. Also, putting gaylords and dryers at the processing machines typically occasions considerable forklift traffic. Moving gaylords around the plant, and the inevitable material spills that go along with such activity, are not only unsightly but are potential safety hazards.

If you could move all the drying and accompanying material handling off the main process floor, you would both reduce traffic in the work area and realize improvements in plant appearance, manpower efficiency, and personnel safety.

WHY GO CENTRAL?

Central drying systems typically are located away from the main processing area. They can be designed to provide the same drying capabilities as several individual drying systems but require less equipment. That's a bonus, given the "downsizing" trend throughout industry, which results in many companies having less personnel to maintain dryers and other equipment. Not only are central systems easier and less costly to maintain, but by reducing equipment they can reduce total energy usage as well.

A common requirement among custom processors is frequent materials changes, often on short notice. A more subtle requirement is to maintain an attractive appearance for their facility. A shop that is clean, neat, and uncluttered makes a positive impression on prospective customers.

Are central drying systems the right choice for everyone? Probably not, but not necessarily for any technical reason. Personal choices, likes, and dislikes affect many buying decisions. Small-budget operations with an existing set-up of several individual dryers, for example, may not want to consider making such a move for economic reasons. On the other hand, converting to a central drying system by using a beside-the-press dryer to feed banks of multiple hoppers can cost up to 25% less than purchasing an individual dryer for each material hopper.

An often-heard objection to central drying systems is the fear of "putting all your eggs in one basket." The concern is that if a central dryer malfunctions, widespread production delays could result. An obvious solution, although one that may be prohibitively costly, is to have a standby dryer that can instantly be brought on line. Fortunately, central systems can be made quite reliable without going to such expense. Most dehumidifying dryers, particularly those of twin-bed design, are relatively simple machines and not prone to catastrophic failure. When opting for a central system, the best approach is to institute an effective preventive-maintenance program and to maintain an inventory of critical spare parts. This way any malfunctions can be quickly corrected with little or no lost production time.

THREE CONFIGURATIONS

There are three common central drying arrangements. The first and most common [ILLUSTRATION FOR FIGURE 1 OMITTED] incorporates a single dehumidifying dryer that provides dry air to multiple drying hoppers through common trunk lines. This set-up is best suited for running multiple machines and a variety of materials. Each hopper is equipped with a process heater and manual airflow-balancing valves provide the range of drying conditions required when drying a different resin in each hopper. Temperature control can be provided by a central PLC mounted on the dryer or by individual devices on each hopper.

Generally, a vacuum-loading system is used to transfer the dried resin to the individual process machines. However, for processors that run small injection presses and rely on hand loading, multiple-hopper central systems can also be loaded and unloaded manually. One or more of the hoppers may be used as "swing" hoppers to predry a new material off line, with the ability to be brought on line quickly for fast material changes.

In the second configuration [ILLUSTRATION FOR FIGURE 2 OMITTED], which is the simplest and least flexible, the dryer and hopper are sized to provide a single material to multiple processing machines. This arrangement is best suited to running one type of material on several machines. Here, again, a vacuum-loading system is typically used to distribute dried resin to the multiple use points.

The third central-drying arrangement [ILLUSTRATION FOR FIGURE 3 OMITTED] incorporates a single dehumidffying dryer to supply dry air to multiple machine-mounted drying hoppers via common trunk lines. Each hopper is equipped with a process heater/blower/temperature-control package to maintain desired temperature and air flow for the material being processed. This design is well suited for processing materials that require the dryness and temperature to be within very strict limits as they enter the processing machine (e.g., injection molding PET bottle preforms).

VACUUM LOADING IS KEY

The design of the vacuum-loading system is the key to a successful installation of either of the first two central-drying configurations. Once the resin is dried it must not regain excessive moisture in the time it takes for it to actually enter the machine's feed throat. Several features can be incorporated into the vacuum-loading system to prevent this from occurring. For example, a minimal-inventory receiver can be mounted at the feed throat of each machine. These incorporate a transparent holding chamber and an adjustable level sensor. Operators can see the material and the adjustable sensor allows them to vary the amount of material in the chamber to accommodate changes in throughput at each machine.

Additionally, dry air can be used for conveying and each line can be "purged" after each conveying cycle. Such steps provide further insurance against potentially regaining harmful moisture.

Cross-contamination of materials, or sending the wrong material to the wrong machine, is a common concern voiced about the multiple-hopper central-drying arrangement. By purging each line after each conveying cycle, you will eliminate the potential for cross-contamination. And by incorporating line "proofing" into the system, you will eliminate the potential for sending the wrong material to the wrong machine. Line proofing uses electronic interlocks to ensure that the right conveying tube is connected to the right material hopper.

Some equipment suppliers offer vacuum-loading system controls that do this very reliably without use of separate proofing switches. These are distributed-type control architectures, which are best suited to centralized drying systems. They distribute some of the control "smarts" to each loading station. Total wiring is reduced, and very little additional wiring is needed to add new stations. While they cost more than other controls, distributed systems are much less costly to install.