It is not always easy for a company to choose the best conveying system for their plant. We at STB Engineering have spent over forty years working in pneumatic conveying and we are always prepared to explain the full range of systems available before recommending one to a customer.
Frequently, the manufacturer of dense-phase systems will argue that their system would be the best, while the supplier of vacuum systems will counterclaim that a vacuum system is the ideal for the job. As the buyer, the plant engineer has little independent information on which to base his decision.
In reality, for any given application, one system will usually be preferable. Here we explain how to determine the system best suited to your requirements. This may simply be a question of costs, but there are certain technical factors that may point us in a different direction. It is worth explaining some of the technology, as there are three main types of pneumatic conveying systems in common use.
THE VACUUM SYSTEM
Also known as a negative pressure dilute phase system. Here all the material is airborne in the conveying pipe and is conveyed at relatively high velocities.
THE BLOWING SYSTEM
Also known as a ‘positive pressure dilute phase’ system or a ‘rotary valve’ system. This is similar to the vacuum system except that product is blown instead of sucked.
THE DENSE PHASE SYSTEM
Also known as a ‘pressure pot’ system. In this system, the conveying pipe is largely full of product, sliding along at much lower velocity but a higher pressure.
There are various derivatives of the above systems, such as low velocity dilute phase, medium phase, slug flow / dune flow and many others. Essentially they fit into two main categories Dilute Phase and Dense Phase and within these categories it can be divided into Vacuum and Pressure Systems.
COST-EFFECTIVENESS
The relative cost-effectiveness of each system is largely a function of two variables – throughput and conveying distance. For conveying low rates over reasonably short distances, a vacuum system will generally prove the cheapest option. On the other hand, if your requirement is for high rates and/or really long distances, then a dense-phase system would be most appropriate. The blowing system occupies the middle ground. The table below gives some examples.
| Conveying Distance | ||||
| Tonnage | 30m | 60m | 90m | 120m |
| 2 tph | Vacuum | Vacuum | Vacuum | Vacuum |
| 4 tph | Vacuum | Vacuum | Blowing | Vacuum or Blowing |
| 6 tph | Vacuum | Blowing | Vacuum or Blowing | Blowing |
| 8 tph | Blowing | Vacuum or Blowing | Blowing | Blowing or Dense Phase |
| 10 tph | Blowing | Blowing | Blowing or Dense Phase | Dense Phase |
| 20 tph | Blowing | Blowing or Dense phase | Dense phase | Dense phase |
THE RIGHT SYSTEM FOR YOU
It should be remembered that the differing systems have a considerable overlap in reality, and can operate outside of the above parameters where other circumstances dictate.
Complex systems
Vacuum systems are particularly well suited where multiple pick up points are required, and for complex multiple pick up and multiple delivery point system such a those used by polymer converters.
Hygiene
It is particularly desirable to avoid any leakage of product (if it is toxic or if hygiene is especially important) then a vacuum system has an advantage in that it tends to contain powder in the event of a leak.
Friable and Abrasive Products
If the material is particularly abrasive, then lower-velocity dense phase conveying will reduce wear on components. Similarly, if the product is particularly friable (easily damaged), the dense phase may prove attractive.
Limited Headroom
A mundane factor such as lack of headroom can sometimes be the most important. Regardless of any sophisticated technical arguments, there may only be room for a rotary valve or vacuum take-off pot.
Minimum Segregation
A dense phase system will also minimise segregation of a blended or variable product.
Cohesive Product
A dense phase system is often more suited to the conveying of cohesive powders
MAKING YOUR DECISION
Not all materials are happy in all conveying regimes, so if you are unsure it is wise to speak to an expert or someone who is using the same product.
Not all products are the same even if they are called the same. Varying particle sizes, bulk densities, moisture content, particle roughness and varying product formulations result in different handling characteristics for each variation of the same product. STB Engineering have a complete in house test facility which offers full pneumatic conveying trials and testing for all types of systems including Vacuum, Pressure and Dense Phase systems.
Consulting a professional with experience of all sorts of systems in many different applications is the best way to choose a system. We spend many hours advising on the appropriateness of systems for a particular product and situation. STB Engineering have four decades of experience in pneumatic conveying in the food, plastics, mineral and chemical industries and this has taught us that every situation is unique. As we are a company that produces all types of conveying systems, we can advise customers impartially on the best system for them.