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Pneumatic conveying of flours

Which pneumatic conveying systems to use for transporting flours ?

Flour pneumatic conveying : dilute phase and dense phase systems

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Section summary
1. Introduction : flours in industry ?
2. Pneumatic conveying of flour
3. Lean / Dilute phase
4. Dense phase

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1. Introduction : flours in industry ?

Flours are, with sugar, one of the most widespread food commodities in the world. Many cereals crops are milled to produce flours which is an ingredient of produce of basic necessity such as bread, pasta, baby cereals... Most popular flours are made of wheat, rice, rye, oats, soya... Around 700 million tons of wheat flour alone are produced worldwide every year ; most of this flour require industrial installations for production and also its usage. Those industry thus need to transport large quantities of flour in their plants, and one of the most efficient way to do so it to use pneumatic conveying.

2. Pneumatic conveying systems for flour

Pneumatic conveying presents many advantages that make it a good choice for transporting powders such as flours : it can reach high capacity, can convey over relatively long distances and with a flexible pipe layout, allows to contain well the dust as well as dust explosion risks.

In a pneumatic conveying system, air is blown on one side of the process and the mixture gas + powder flows in conveying pipes until the its final destination. There are however different ways to blow the air and introduce the powder in the conveying pipe. As a consequence different transport technologies have been developed over time : typically dilute phase and dense phase conveying.

Flours, particularly wheat flour, are relatively fine (< 200-300 microns) and can be suited for both types of pneumatic conveying, the decision on the technology being taken depending on the process requirements for an application.

3. Lean / Dilute phase

When can it be used for flours conveying ?

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 relatively low. The product is transported in the form of a cloud in the conveying pipes. The transport can be done by pressure or by vacuum (shorter distance).

This technology is very often used for flours, particularly for factories receiving flours as an ingredient for their products in bags or Big bags. The factory operators must then install the following equipment :

• Bag tipping station, or Big Bag tipping station
• Sieve and magnet to remove foreign bodies
• Product infeed to the pneumatic conveying line, for example with an airlock rotary valve for pressure conveying (the airlock rotary valve may be blow-through if the flour is not flowing well)
• The conveying system
• A receiver (a hopper / silo) equipped with a filter and explosion protection devices if the need is pointed out by a dust hazard analysis (DHA)

Figure 1 : typical flour conveying system

Flours are also very often transported in trucks, which must then be discharged to a silo. The conveying is then done thanks to a compressor on the truck and with the truck driver manipulating the valves to control the flow of flours. As the truck operator will try to reduce at maximum the duration of discharge of the truck, the conveying system operates at relatively high solids / air ratio which makes the conveying being in between dilute and dense phase (see below). The conveying line must be well sized (sufficient diameter) to make sure the conveying will not block and the filter of the silo must also be designed with sufficient area in order to accept the large quantities of air from the compressor, especially at the end of the conveying when the pressurized truck tank suddenly decompresses in the line.

Figure 2 : flour truck discharging to flour silo

4. Dense phase

When can it be used for flours conveying ?

Some flours, such as wheat flour, can be conveyed in dense phase which means that the transport is done at a much higher concentration the product.

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.

Dense phase conveying is often used to convey materials sensitive to breakage or sensitive to demixing, Flour is not in this case, if dense phase conveying is to be chosen it is more for its ability to convey high quantities of product with small volumes of air thus small energy requirement. This technology is thus more to consider for very high capacity lines (>10-15 t/h).

5. Flour dust explosion protection

Flour explosion have caused some of the largest disasters in powder handling industries. There are record of many flour silos exploding, leading to fatalities and the destruction of the whole plant. Dust explosion prevention and protection is thus to be taken as an utmost priority for all plant operators having to handle flours, in pneumatic conveying or with any other handling method. In general, the following must be respected :

• Have an updated dust explosion risk assessment, according to the regulation of the country where the factory is located
• APPLY the conclusions of the risk assessment, which are typically :
• Protection of silos against dust explosion
• Avoidance of ignition sources (electrostatic, mechanical...)
• Use of certified equipment
• A good housekeeping is fundamental when handling flour, any spillage or dust deposit must be immediately cleaned