|1. Introduction :
flours in industry ?
|2. Pneumatic conveying of flour
|3. Lean / Dilute phase|
|4. 4. Dense phase
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.
Pneumatic conveying present many advantages that make it a good choice for transporting powders such as flours : it can reach high capacity, can convey over relatively long distance 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, 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.
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 :
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.
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.
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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.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).