Pneumatic Conveying Systems 101

If you’re not that familiar with pneumatic conveying systems, but have been asked to scope one anyway, you may be feeling a little overwhelmed and apprehensive.

That’s understandable. These systems can be complex, winding and task-specific. And if one part of the system can’t do its unique job, it won’t just be the system that breaks down – it will be the entire process. That’s bad for business (and for you).

Fortunately, a little knowledge goes a long way. That’s why we’ve put together some answers to a few of the most common pneumatic conveyance questions for you right here. They’ll help you make more informed decisions up front and avoid mistakes and consequences down the line.

What is pneumatic conveying?

Pneumatic conveying refers to a type of system that uses compressed air or another gas to transfer bulk materials like powders and granules from one process area to another.

A pneumatic system works by moving the material through an enclosed conveying line using a combination of pressure differential and the flow of air (or another gas) from a blower or fan. By controlling the positive or negative pressure and the airflow inside the conveying line, the system can move materials very successfully with very little damage or loss.

What are the ideal materials for pneumatic conveying?

A pneumatic conveying process is ideal for fine, dry powders. We’ve designed and installed systems for a variety of food and industrial products, including:

• Soy flour
• Wheat flour
• Starch
• Sugar
• Cement powder
• Carbon black
• Coal fines
• Sands
• Metal powders
• Granular materials
• Pelletized materials

Pneumatic systems also work very well with batch ingredients when you need to weigh out a certain amount of material or when you need a product to cool a bit during conveyance.

What are the advantages of pneumatic conveying?

Every system has its pros and cons. Pneumatic conveying equipment is no different. But the list of advantages is pretty long, which is why this type of system is very popular for certain processes.

• Ease of Routing – One of the biggest advantages is the ability it gives you to route the system just about any way you like. Pneumatic systems can run horizontally or vertically, can cover long distances, and are easy to design around existing equipment, obstructions or anything that doesn’t present a straight path. That gives you a fair amount of flexibility, which is the second advantage.
• Adaptability – Pneumatic systems are easy to adapt to existing equipment, so you don’t have to start from scratch. They also don’t take up a lot of floor space, so they’re easier to locate. You have the ability to set up for multiple pick-up points and discharges, too, so you can create a system that’s surprisingly flexible and efficient.
• Construction Flexibility – Because pneumatic conveying lines are relatively light weight, they don’t require a lot of structural support. That saves plant space and money, and removes a number of potential construction challenges.
• Safety – Being enclosed is another plus for these systems.
• Ease of Maintenance – Unlike other options, pneumatic systems only have a few main parts (air compressor or pump, in-feed device, tubing, and a separator), and each of those components can be repaired independent of the rest of the system. That means maintenance is quicker and downtime is limited.

What are pneumatic conveying’s disadvantages?

A system like this isn’t always the best option. Here are some of the key disadvantages with pneumatic conveying:

• Lower Product Volume – Pneumatic systems are typically used for smaller volumes of material, with most pipe sizing ranging from 3-12”. That’s not to say they can’t work for higher volumes. They just present greater challenges.
• Scalability — There is a limit to the blower size and large systems require significant horsepower. ?If you need more than 300 horsepower from your blower, this may not be the system for you. Each application is unique.?This is where you need to ask yourself if there is a more economical way to convey product.
• Line Plugging/Product Damage – Pneumatic systems aren’t the best for wet and/or fragile products. Wet products (Pressed corn germ is a good example) can plug the line from time to time and kill productivity. Fragile products can also fall victim to the speed and creative routing these systems allow. The higher velocity and sharp turns can damage more sensitive materials and impact the end product.
• Environmental - Open circuit systems usually discharge conveying air to the atmosphere through a dust collector.?This may require a permit as an emissions point.
• Noise - Blowers can produce a considerable amount of noise.?It is common to use a sound enclosure with large blowers.

How do I choose between pneumatic conveying system types?

Now that you know what these systems are (and are not) capable of, how do you decide which type of system will work best for your process? Well, there are two kinds of systems to consider.

Dilute Phase Pneumatic Conveying

The first and most common is the dilute phase system, which can be either pressure- or vacuum-fed. These systems use high velocities to fluidize the product in the air/gas stream and use a higher ratio of air to product.?These systems have a number of applications including conveying long distances, batch-weigh systems, or multiple pick-up and discharge points.?The higher velocities usually mean some product breakage during transfer, however in many applications, this is not a concern.Consistently metering the product into the blow line is very important with dilute phase systems.?A properly installed rotary airlock or other metering device with appropriate venting will ensure that the system is reliable day-in, day-out.

The higher velocity means a dilute phase system is not ideal for fragile or sheer-sensitive products, which can break,?coat up and potentially plug the line.

Dense Phase Pneumatic Conveying

For conveying fragile or sheer sensitive products, a dense phase system is the better choice. These systems are designed to operate at low velocities to minimize product breakage and sheering.?Dense phase uses a pulse of product pushed by a pulse of air, then another pulse of product followed by a pulse of air.?This requires the feeding systems to be precisely calculated, timed and designed. Particle size, density and length or run all factor into the ratio of product to air.?There are equipment manufacturers that specialize in designing, building and installing dense phase systems.

What are some of the biggest challenges in system design and scoping?

Over the years we’ve consulted on dozens of pneumatic conveying systems and seen project leaders make every possible mistake. Here are some of the most common, along with a few specific suggestions for how to avoid them on your project.

1. Pressure Drop Miscalculation – One of biggest challenges with these systems is properly calculating the pressure drop over the length of the line. There are lots of variables here, including pipe size, line layout, product density and particle size, to name a few. There’s software available to calculate pressure drop, and it’s a wise investment, because pressure drop problems can have a domino effect on the entire system.?
2. Low Pick-up Velocity - Remember that there is a pressure gradient across the conveying system.??the air velocity at one end may be 4000 ft/min and 9000 ft/min at the other.?If you don’t account for the pressure drop through the system, often the pick-up velocity will not be high enough to fluidize the product and the pick-up will plug.
3. Underestimating Flow Rate – A common mistake in feeding systems is designing them for an average flow rate. Production rates can vary, and you want your system to function at peak flow. Designing for average flow will put you at a disadvantage when you need the system to perform at the high end, and you may end up plugging up the blow line. So be sure to design for peak capacity.
4. Poor Piping Design – Poor piping design can lead to a host of problems. If particle size is important to your process or breakage is a concern, incorporate wide-sweep elbows in your system design. They’re gentler on the product and create fewer trouble spots.?Elbows that are too close together can be a plugging concern. If the pipe is sized too small, it will incur a high pressure drop.
5. Absence of Venting – If you put in a dilute phase pressure system with no venting, there’s a good chance your system will airlock and you won’t be able to get your product to fall out of the bin. As the rotary valve turns and each pocket transfers product into the blowline, the pockets fill with pressurized as they return to pick up more product.?That air has to go somewhere.?Be sure you spec and install the proper venting on the feed end to prevent this problem.
6. Incorrect Air Flow Diversion – In a system with multiple pick-up points and multiple destinations, make sure you’re diverting air flow correctly by using air tight diverters.?The airflow is only designed to pick-up from one point and deliver to one point.?Air tight diverters are necessary to isolate each conveying pathway from multiple pick-ups points to multiple destinations.
7. Improper Filtering – Some plants make the mistake of relying on the vent filter on their silo.?Vent filters are designed for the air displacement inside the bin, not conveying air. ??That can pose a problem with the air flow in the new system. It’s important to have a baghouse on the other end that’s big enough to handle the conveying air flow in addition to the small vent filter on the silo.
8. Environmental Discharge Planning – If you’re not designing a closed-loop system, keep in mind that there could be an environmental discharge point down the line. If that’s the case, be sure to have the facility environmental coordinator check your state and local compliance requirements. This could be modifying an existing permit or applying for a new permit.
9. Contact Part Corrosion – On vacuum and closed-loop systems, any product that’s corrosive (e.g., dry salt) or acidic (e.g., biomass) can damage carbon steel, so make sure your contact parts are stainless.
10. Underestimating Ancillary Components – Finally, keep in mind that you’re not just pricing a blower here. There are lots of ancillary components and equipment involved (material storage, loss-and-weigh system, feed system, piping, venting, dust collector, diverter valves being some of the bigger items). Make sure each is called out and accounted for in your scope so your system is complete and your pricing is true.

Where do I go from here?

Scoping, designing and installing pneumatic conveying systems can be complex. There are lots of variables, as we’ve shown here, and mistakes can be costly both at the outset and down the line.

Don’t let inexperience or lingering questions keep you from getting it right. Take your time. Read more about systems design and installation. Talk to an engineer. That way you’ll wind up with a system that doesn’t just work for your process, but for your budget and your boss as well.

AMG has designed dozens of pneumatic conveying systems. AMG has the tools and experience to help you design yours.?We know the right questions to ask. We know what works.