In each case, pneumatic transport will involve a gas as a powder mover. This gas is blown on one side of the process and the mixture gas + powder flows in conveying pipes until the its final destination. In most of the cases, the gas used is air. There are however different ways to blow the air, introduce the powder in the conveying pipe. As a consequence different transport technologies have been developped over time.
A very common way to transport powder is to transport it in a dilute phase. The powder is diluted by the air which means that the ratio (kg product/kg air is low. The product is transported in the form of a cloud in the conveying pipes.
Instead of placing the Blower at the beginning of the conveying pipe, it can be placed at the end. In this case, the Blower is creating a VACUUM that will suck air in the conveying pipe. This air will drag the powder and transport it to the final receiver.
The other option, to transport powder, is to transport at a much higher concentration the product. A transport is called dense phase when the ratio (kg product/kg air is high. Again, such transport can be carried out in pressure or in vacuum.
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In pressure dense phase, air must be compressed at higher pressure than in lean phase. It can be done by installing a specific compressor, or simply by using air from the compressed air network of the factory. Pressures reached will range from around 1 bar g to several bag g. To introduce the powder in the transport pipe, some tanks, designed to withstand pressure, are often used. They are called pressure tank or pressure sender hoppers.
Vacuum dense phase will present a quite similar setup than the lean phase one. However, in order to accomodate higher solids loads, leading to higher pressure drop, a vacuum pump will be used instead of a simple roots Blower. Very high vacuum will then be generated.
Table 1 : Pros and Cons of the different conveying technologies
|Phase||Driving force||Solids load||Pressure||Achievable distance||Pros||Cons|
|Dilute||Pressure||5-15||Max 800 mbarg||150-200 m or mor||Simple
Instictive and easily understood by operators
|Requires star valve at product pick-up
Air may be hot after compression
|Dilute||Vacuum||5-15||Max -500 mbarg||100 m||Simple
Product pick-up is simplified (no need of star valve)
Air is sucked at the environment temperature
|Limited in distance compared to conveying in pressure
The operation can appear more delicate to production operators
|Dense||Pressure||25-30||From 1 bar g to several bag g||Can be 150+ m, but other factors may limit the distance||High distance can be reached
High product flow can be reached
|Compressed air consumption
Use of pressure tank
Difficult access to the inside of the pressure tank
|Dense||Vacuum||25-30||Up to -900 mbar||~100 m||No consumption of compressed air
Simple product pick-up, easy access to sender hopper
|Limited in distance and elevation|
When conveying in dense phase, suppliers may use different technologies inducing a different behaviour of the powder in the conveying pipe. In the simplest configuration, plugs of different length will naturally be created. This may cause some issues regarding the pressure drop. To decrease this phenomena, there are possibilities to force the creation of shorter plugs. It is done by pulsing a valve admitting air at the beginning of the conveying pipe or at intermediary sections. One drawback is that the air introduced in the conveying pipe will cause an acceleration of the product conveyed at the end of the line, which may be detrimental to the product quality (breakage).