Granular and powder ingredients are transferred within and between stages using pneumatic conveying (pressurized or moving air). There are four types of pneumatic conveying used, and which method is utilized depends on many factors.
- Lean phase pressure conveying (LPPC)
- Lean phase vacuum conveying (LPVC)
- Dense phase pressure conveying (DPPC)
- Dense phase vacuum conveying (DPVC)
Other terms used include dilute instead of lean, positive instead of pressure and negative instead of vacuum. Components used in pneumatic conveying systems include:
- Booster fans
- Butterfly valves
- Diverter valves
- Gravity diverter valves
- Rotary valves
- Vacuum pumps
Pneumatic conveying in the powder handling systems
Lean phase pressure conveying (LPPC)
Lean phase pressure conveying systems are the most common method used to transport powders and granules. This system, also known as dilute-phase pressure conveying, classically utilizes blowers supplying a large volume of pressurized air to a maximum of one Bar.
This system operates in a continuous manner, where the material is constantly supplied at the beginning and delivered to its destination without pause. Being easier to operate and requiring little head room, this system is possibly the most cost efficient process used in pneumatic conveying systems.
- A blower at the system’s beginning starts to supply a large volume of low-pressure air into the system
- The material is fed into the conveying line through a feeding device, such as a venturi or a rotary airlock valve
- The material is then entrained by the air stream and moves along in suspension (speed depends on the size and density of the material’s particles)
- A filter at the end of the line then separates the air from the material
- A wide range of convey rates and distances that can be tailored to the materials being delivered.
- Ideal for materials that are smooth and robust with a light bulk density, usually less than 92.27 kg/m (62 lb/ft)
- The ability to convey materials from single or multiple sites to multiple destinations
- Commonly used for systems which require materials to be dosed and weighed
- A simplistic and economical design, which makes it one of the most cost efficient conveying methods known
Lean phase vacuum conveying (LPVC)
Lean phase vacuum conveying systems use exhausters that provide up to a 50% vacuum to deliver the material through a conveying line. Fans may also be used as the base force if systems requires lower capacities. This system utilizes a method similar to the LPPC process where material may be transported on a continuous basis; however, by use of butterfly valves, material may be transported intermittently if required.
- Fans or exhausters are activated, forming a vacuum in the system
- Material is fed into the system; with a rotary valve airlock (or screw feeder) if metering or dosing is required
- Vacuum carries material and delivers to a receiving vessel
- Filters then extract the material from the air and the product is then transported to its destination
- A low to moderate conveying rate and distance, usually at a short distance with low capacity
- Suitability for conveying materials that tend to pack or compress under pressure, such as wood shavings or fibre material, as well as toxic materials that are hazardous in the workplace.
- Reduces dust hazard in the unlikely event of a system leak
- Allows material to be weighed and dosed if required
- Ability to convey materials from multiple sites to a single location.
Dense phase pressure conveying (DPPC)
Dense phase pressure conveying systems uses compressed air between one and eight Bar to push materials through a conveying line. This type of system operates on a batch basis. The pressure vessel is repeatedly filled with powder, pressurized and emptied. A panel of air valves and sensors may be used to control the air pressure in the pipe and the velocity of the material at intervals throughout the system to ensure smooth conveying of the material. This also allows the flow of the material to be stopped and started again with relative ease.
- Material is loaded into a pressure vessel through a valve (also known as a transporter or blow pot) until the vessel is full
- The material inlet valve, as well as the vent valve are then closed and compressed air is then metered into the vessel (The speed at which the compressed air flows depends on the material)
- The compressed air then pushes the material through the pipeline at a relatively slow speed, so as to minimize abrasion and degradation of the material and the system
- Once the material has been conveyed, the process begins anew
- A high conveying rate and distance, but sacrifices low convey velocity
- Can convey materials from a single point to multiple destinations
- Able to convey fragile, heavy or abrasive materials such as silica sand, carbon black, hazelnuts, cereal, sorbitol, dextrose and many more powders and granules
Dense phase vacuum conveying (DPVC)
Dense phase vacuum conveying systems utilize high capacity vacuum pumps to convey the material from a feed hopper or silo to another destination. The system operates similarly to the DPPC system. This system, much like the DPPC system, generally operates on a batch basis.
Valves and sensors are used throughout the system to control the power of the vacuum as well as the material’s fluidization settings and velocities of the system to ensure an efficient conveying process.
- Vacuum pump draws material into vacuum hopper (also known as the receiving vessel) until it is full
- Material is conveyed through the pipeline at a controlled low velocity, sometimes in a fluidized state to reduce friction and pressure drop
- Vacuum is then insulated and the conveyed material discharged into its destination
- Process is repeated again until the required material is conveyed
- Ideal for low to medium conveying rates and distances
- Better assurance of material quality throughout the process
- The ability to transport material from many locations to a single destination
- Reduction of dust hazards in the unlikely event of a leak
- Method utilized in systems that involve fragile materials requiring minimal degradation and those requiring low transportation velocity
- Ideal for gentle conveying of fragile, agglomerated and abrasive materials. This machinery is especially suitable for food, dairy and pharmaceutical systems involving powders