Pipe & Profile Extruders – Compounders – Wood/plastics Composites – Blow Molders

Though the conveying of pellets and powders share some similarities – the differences are numerous. Pellets are typically free-flowing materials and relatively simple components can be used to vacuum convey them from source to destination. Powders are a different story…they may be compacted or damp and not flow well or they may be aerated or have characteristics that make them flow too easily. Dust wants to escape from the system at every opportunity. There are times when the only way to define a powder’s characteristics is to test it in our lab. Sometimes a simple vacuum system works fine but there are some limitations.

Vacuum Powder System Limitations:

• Limited throughput capacity
• Limited long distance conveying
• Intermediate stations required over 750’
• Receiver/separator required at each station

Typically, powders must be conveyed in a pressure system and the design of these systems must be carefully thought out. There are multiple styles and designs of virtually every system component and they must be properly matched – not only for the material and the process – but to ensure that the components themselves are suited to work together efficiently.

Pressure systems are more costly that vacuum systems, use more sophisticated components and generally require more maintenance but their advantages are listed below.

Some Advantages of Pressure Systems:

• Typical 5,000-30,000 lb./hr. throughputs
• Conveys to 1000’ equivalent distance
• Ideal for high throughput processes
• Can meter out of a pressure convey line into multiple stations with bypass valve

Many powder conveying systems can utilize vacuum instead of pressure but there are some limitations and the system has to be engineered correctly – using components designed for the specific powder, throughput and distance your process requires.

For instance, PVC and similar free-flowing powders may work fine with standard pickup devices and powder receivers but powders with more than 20% of things like Ti02 , may require eccentric hoppers and rotary valve pickup devices to get the powders into the airstream.

Here are some rules of thumb for successful use of vacuum powder systems:

• Keep conveying distance below 750 Ft. Equivalent Distance
• Do not try to exceed 15,000 lb./hr. throughput
• Staging of material is required for longer distances

• Low initial cost
• Low maintenance
• Vacuum keeps powder in system
• Simple components

Data Sheet : VRP Powder Receivers

Data Sheet : VLP Powder Loaders

Data Sheet : Vacuum Takeoff Boxes

Pressure systems pick up where vacuum leaves of when it comes to conveying powders.

Railcar and Truck

Complete systems to unload plastic powders from railcar and bulk trucks and convey to silo storage. Custom engineered pumps, receivers, airlocks and peripherals ensure successful startup and efficient operation.

Designed specifically for powder storage with 60° slope on the cone section to improve material flow. Top or bottom loading bin vent filters are a requirement. Air cannons, bin sweeps and other bin activation accessories are included if necessary to enhance material flow.

These quality blowers range in size from 10-150 Hp to provide a full CFM range for any application. Pressure systems operate on a high volume of air at a low pressure. The blower is chosen based on:

• CFM
• Conveying distance
• Pressure required

Typical features include:

• Silencers
• Inlet filters
• Pressure relief valve
• Pressure switch
• Check valve
• Sound enclosures

These carefully designed receivers must meet OSHA requirements for air emissions. They may be concentric or eccentric - for powders with poor flow characteristics.

The filter media may be top loading or front loading.

Design Considerations Include:

• Can velocity
• Hopper design
• Discharge valve
• Material activation
• Need 3:1 - 5:1 air/cloth ratio

Terminal Pressure System

From a single source to a single destination…to bin, dryer, extruder or blender.

From a single source to multiple destinations with bypass material returning to silo. Typically for weigh scale applications.

There are probably hundreds of air lock designs and thousands of models. Basic operation is simple. The air lock is sized to meter a given volume of material into the air stream provided by the pressure blower. Air trapped between the blades is returned to the material source or vented through a filter.

• Housing
• Cast iron
• Chrome plated, other
• Rotor
• 8 blades (10 optional)
• Adjustable, relieved tips
• Clearance
• Bearings & Seals
• Outboard bearings
• Air purge seals
• Pressure Relief
• Conveying With Cooled Air
• Air from the blower typically gains about 14°F/ 1psi. That means that a 10 psi system can add as much as 140°F to the temperature of the conveying air. A cooler placed in the air line is an important addition to protect temperature sensitive powders. It can also protect the blade tips within the airlock from unnecessary wear.

Bulk bags are a popular way to receive powder shipments. Unfortunately, when the bulk bags are received, the contents are often greatly compressed. This often requires some special methods of improving the material flow-ability to an acceptable level. A bulk bag massager is one such method. Two paddles (located on opposite sides of the bag) move in unison to loosen compacted material and allow it to exit the bag. Dust can be filtered with a plant-wide dust collection system or by an individual unit next to the bulk bag unloader.

Bin Vent Filters

Bin vent filters are required on powder silos to filter exiting air that results from bypass return air or filling the silo. They also keep the silo from being pressurized. Filters may be loaded from the bottom but top loading units have gained popularity. Blowback is used to prolong filter media life and a simple pressure differential device monitors whether the filter media is becoming clogged.

Design Considerations: 

• Air to cloth ratio (3-4 ft³ per 1 sq. ft of filter area)
• Impact of external devices
• Breathing for material discharge

Air cannons are typically used on silos or bins. They inject blasts of air into the vessel to keep materials with difficult flow characteristics flowing through the vessel to the takeoff device.

Bin sweeps provide a simple but effective means of keeping materials moving that may have a tendency to “cake” in the silo cone. Wood flour is a good example of a material with these characteristics but powders that tend to “cake” – maybe because of dampness – are also candidates.

A motor drives a rotating screw around the inside of the silo cone to sweep the material down to the pickup device.

Here a Gyrator causes the cone of an in-plant hopper to vibrate. The cone is allowed to move because it is connected to the body of the bin with a flexible sleeve that is clamped into place.

For powders with particularly difficult flow characteristics, a variety of flow aids can be combined. Here you see an eccentric hopper with a very steep cone angle. In addition a vibrator is being used to agitate the material and an air pad, which utilizes a blower or compressed air, forces air into the cone to aerate the material.

These valves are typically used when filling weigh bins mounted on load cells. When the destination is filled to a pre-determined weight, the Fill/Pass Valve causes all air and material to pass onto the next weigh bin requiring material or it returns any remaining material in the line to be returned to the source – acting to purge the material line.

Once again there are multiple designs and one is shown here. A typical use would place a delumper beneath a bulk bag unloading station where, as the material exits the bag, the delumper breaks up any clumps and a screw is used to meter the material into the next destination.

Powders can become compacted within a plant for a variety of reasons including basic material characteristics, moisture gain or even low turnover which means the material sets in a bin or hopper for an extended time. In these cases, material agitation is necessary to get the material flowing. An agitator with a series of rotating tines is an excellent way to accomplish the task. One style is shown here. Note the rotating screws beneath the tines. They meter the material out of the bin or hopper.

Gravimetric

Gain in Weight:

• Feeder uses variable frequency drive screw and meters into weigh hopper with load cells.

Loss in Weight:

• Feeder uses variable frequency drive screw or vibratory tray and meters into weigh hopper with load cells.

Volumetric

• For free-flowing powders – very dry with low moisture
• Feeder uses variable frequency drive screw or vibratory tray operating on time to fill bin.

Accuracy Depends on:

• Proper Agitation
• De-lumping
• Aerating
• Particle distribution