RESIN MOISTURE & DRYING

You have lots of latitude when choosing a drying system. This section reviews those choices with an eye towards what kind of installation choices you have to make. Your decisions will influence the initial cost and the efficiency and performance of your drying system for years to come.

Machine vs. Central Installation Configurations

In broad terms, you have two basic choices about where to place your dryers and drying hoppers.

• On or near processing machines
• Remote, central location away from the production floor

Your choice will be largely determined by the type of processing you do.

• Extrusion, blow molding and long-run injection molders may benefit by locating drying systems on or near processing machines
• Injection molding operations using multiple materials and frequently changing materials and molds, benefit from remote placement of drying equipment

Four Steps for Energy-Saving Installation

You can take some simple steps to ensure minimal heat loss during the drying process, which will pay nice dividends by reducing energy costs. 

1. Hopper Insulation

Manufacturers do not always follow the same rules. It is up to you to ensure that the hoppers you buy have at least 2" thick insulation sandwiched between the walls of:

• Hopper sides
• Hopper access doors
• Hopper cone

It is not necessary to have insulation above the normal material level in the hopper because that is where cooler material enters the hopper and air is returning to the dryer.

2. Dryer Insulation

Proper application of insulation to the following dryer components can also reduce heat loss.

• High temperature flex hose carrying hot air to hopper cone
• Hard metal ducting carrying hot air to dryer cone
• High temperature flex hose carrying hot air within the dryer cabinet (depending on space and practicality)

Note: Do not insulate the hose returning air from the top of the hopper to the dryer.

3. Proper Process Heater Placement

• This depends largely on the size of the system and the distance between the dryer and hopper
• Small systems include the process heater within the dryer cabinet
• Larger systems (where distance between dryer and hopper are greater) should have the process heater mounted on the dryer to minimize heat loss

4. Tight Hose Connections

Process Integration/Layout of Drying Systems

The integration of a drying system into a process workflow for plastic processing can make a big difference in the success of your drying task, the ease of service, and the need for additional equipment.

An Overview of Resin Drying Equipment

Drying systems are typically made up of two major components - the dryer and the drying hopper. The dryer typically provides heated, dry air to the hopper and is connected to utilities which depends on the dryer type. These may include: 

• Electrical supply
• Gas supply
• Compressed air
• Water

The drying hopper is the vessel that holds your plastic material and it is connected to the dryer with hoses or tubing for the transfer of heated, dry air to the hopper for drying. 

The hopper is typically equipped with:

• A vacuum loader or vacuum receiver to fill the hopper
• A slide gate at the bottom to allow the material to be drawn out of the hopper to the process machine
• A port, to allow materials to be drained from the hopper for material changeover
• Insulation to maintain resin temperature

Some dryers combine these components into a single cabinet or onto a common stand or cart for easy installation and/or portability.

Resin Drying Equipment Location

There are several choices when it comes to dryer location.

• Machine-mounted dryer advantages include:
  • No chance of material loss, contamination or exposure to ambient air that might cause moisture pickup
  • Ideal for systems dedicated to a single material
  • Drying hoppers mounted directly on the machine throat may allow gravity flow  to the machine throat, eliminating the need for additional receivers or loaders (this does not work with batch-drying equipment)
• Off-machine dryer location advantages include:
  • No need to climb on ladder for hopper cleanout
  • Easy access to loaders and dryer for maintenanc
• Portable Drying Systems
  • These compact systems usually integrate a dryer, hopper and loader on single cart with casters for easy transfer from one process machine to another. The cart size usually gets impractical with throughputs over 200 lb./hr.
• Mezzanine Mounting:
  • Dryers may be mounted on a mezzanine directly above the process machines they supply
  • The mezzanine may be used as a location for the drying equipment that saves floor space and allows any dryer to supply dry material to a number of processing machines, below
    • Ideal to minimize equipment on processing floor and for clean rooms
    • Reduces initial cost of system and installation
• Central Drying System: Dries multiple materials with a single central dryer

 

Layout Considerations

Moving fresh material to the drying hopper (and in many cases, moving dried material away from the hopper) is a large factor when considering layout.  Any equipment needs to be occasionally serviced, and loading equipment high atop a large drying hopper is no exception, so it should be taken into consideration.  When transferring dried material, considerations should be made as to distance and how often the material will be moved and how long it will be at rest away from the drying hopper before it is consumed by the processing machine.  As a result, purging the take-away material line of all dried material after each load should be considered.

For this, a purge valve at the base of the hopper is employed which allows conveying air to sweep the transfer line clean after the purge valve closes off material flow from the hopper.  In many cases, using a separate or ‘borrowed’ supply of dry air for conveying is recommended, so that even during conveying, the material is in contact with only dry, not ambient air.

In some cases, typically for larger drying systems, additional equipment for filtering closed loop dry air or coolers for the drying air stream are required and their location close to the drying process can drive the location of the drying systems dryer and hopper.

In summary, the flow of fresh material from storage, to the drying system and finally to the processing machine(s) should be fully examined, along with the need for all additional pieces of support equipment before a layout decision is made for a drying system. In addition, service of all associated devices should be considered.