Knowing the differences between dense phase conveying and dilute phase conveying (also called lean phase conveying), particularly in terms of fluid mechanics, will allow you to precisely design and calibrate your pneumatic conveying system.
If you’re a plant manager or plant engineer, you’ll know the importance of carefully calibrating the speed and pressure of air inside a pneumatic conveying system. The calibration will largely depend on the type of bulk material you’re conveying, and this is where the dense phase and dilute phase come in.
Dense phase conveying is still a relatively new concept in the industry, and so system designs are still held more close than with dilute phase conveying, which has been around much longer. (This also means it’s important to have the proper training and knowledge when you’re working on dense phase systems.)
As the name implies, dense phase conveying involves tightly packed bulk material particles, such as dog kibble, frozen veg, powders, and plastic pellets. In the dense phase, the product isn’t suspended in the air because it’s either too heavy or very abrasive.
The velocity of the air current is also kept high (100kPa or higher). This means the product travels in “waves”, “plugs” or “strands”, and creates less abrasion, so dense phase conveying is better for friable or delicate products that could break up during conveyance.
Dilute phase conveying involves bulk material particles that are largely spaced out. These particles are lighter and are more abrasive. This means that the material can be conveyed at relatively faster velocities and higher pressures than in a dense phase. For instance, talcum powder is lighter and much less abrasive than something like plastic pellets, so it can be conveyed at higher velocities and air pressures because it causes less damage.
In dilute phase conveying, you use a blower to move product through the system by the air stream. The air stream needs to be just right in order to keep the product in the flow and prevent them from dragging on the bottom of the pipe or tube.
You’ll also hear dilute phase conveying referred to as lean phase conveying. You’ll typically find the term lean phase conveying being used when you’re discussing a conveying system being turned up or down. There’s also something called ‘dilute medium phase conveying’, which sits between lean phase and dense phase conveying and happens when you turn your system down.
There are several differences between dense phase pneumatic conveying and lean phase pneumatic conveying. Some differences are inevitable because they’re inherent properties of the bulk material itself - for example, dilute phase conveying often handles lighter particles. Other differences are necessary measures that ensure the proper handling of materials, and prevent things like abrasion or powderization of particles.
Here are some major differences between dense phase conveying and the dilute phase conveying: