From an economic and manufacturing point of view, choosing the right pneumatic conveying system is not easy for everyone. Let’s start saying that there is no standard that determines with absolute certainty whether a pneumatic system operates in dense or dilute phase. Just because a system has a rotary valve does not necessarily mean that it is operating in dilute phase, just because it has a propeller (the transporter) does not necessarily mean that it is operating in dense phase. To try to make this distinction, however, a few pointers may be useful:
When deciding whether to adopt a dense or dilute phase pneumatic conveying system, the first thing to consider is the nature of the material to be moved. Main characteristics in this respect are: particle size and shape, apparent density, moisture content, abrasiveness and friability level. Secondly, cohesion, hygroscopicity (the ability to absorb water molecules), static charge, explosiveness, toxicity and melting point.
Then, it is necessary to consider how and where the system has to be installed. How will pipes and systems have to fit into your warehouse or company premises? Are there volumetric constraints, such as other machinery or low ceilings? The changes to be made to the system in order to adapt it to these constraints must be examined, as well as ensuring the right spaces for any future maintenance.
The last factor to take into account is the amount of material you want to transfer per hour.
In the preliminary phase, technicians will assess the feasibility of the system. If the material to be moved is not extremely sticky, does not have a very high bulk density and is made up of compliant elements, it can be moved. Then, in order to understand which type of system to adopt, we start with a cost analysis. The initial investment to implement a dilute phase system is lower than for a dense phase system. However, also considering the long-term running costs, it requires larger pipes and filters, is more prone to deterioration and the star valve requires recurrent maintenance.
We then move on to consider the operation mode. Whether the dilute phase system should operate under suction or pressure depends essentially on two factors: air losses and the material behaviour during transport. If air losses into and out of the system will have negative effects, such as with toxic or explosive materials, a suction system will be the best choice. A suction pneumatic conveying system is also the best choice when the material tends to compact tightly during handling, as wood chips do. Suction is also the choice if the substance is extremely cohesive and prone to accumulate on surfaces, such as titanium dioxide, or if it is very fibrous, such as buckwheat hulls. This is why suction tends to keep the material particles separated during transport, whereas pressurised systems compact them together and on the pipe walls.
On the other side let’s see when the dense phase may be the best solution. Dense phase systems rely on high flow velocities to keep the material entrained by the air stream. On the contrary, high velocities can damage friable materials, resulting in increased friction and product waste. Another undesirable consequence can be the separation of mixed materials during transport. This occurs especially when the mix is made from substances with very different particle sizes and varying apparent densities. The third problem with the dilute phase is that combining abrasive materials with high speeds, the conveying lines and other components wear out more quickly. In all three cases, the smoother transfer provided by the dense phase, which relies on high pressure differentials rather than high speeds, is often a better option.
How piping influences
A further factor in the choice is the length of the transport lines required by the system. The dense phase generally needs smaller diameter piping than a dilute phase system of comparable size. This means that dense phase systems not only require a smaller volume of air, but also require less heavy piping, making them easier to install and replace.