Wheat Flour Milling, Second Edition
Pages 365-395
DOI: https://doi.org/10.1094/1891127403.012
ISBN: 1-891127-40-3
Abstract
The miller in the modern mill regards the air-handling system as one of the more important portions of the milling operation. It is one of the controllable elements that millers can use to their advantage. Estimates indicate that, by weight, 10 times more air than wheat is moved in a modern flour mill during processing of a given quantity of wheat. Consequently, the miller's objectives are to understand and control this important part of the operation. Besides the mechanical aspects of air handling, which are important to its efficient usage, the miller is concerned about air quality and purity.
Ambient air in the facilities in which grain is handled or processed influences the process and the quality of the finished products. Air movement is used as a tool in segregating parts of the stock based on specific gravity differences and in separating foreign material from the wheat. It is also used in the creation of vacuum to aerate grain and collect dust and light material in machines and facilities. High-pressure air activates equipment, tools, and instrumentation in the ever-growing automation of the industry. Last, but not least, air is used to convey materials.
Air in the mill affects product quality. Contaminated air may contaminate the products it touches. In a well-designed mill, where a large part of the air used in the mill is washed and recirculated, it may be cleaner than the outside air. However, poorly designed air-handling systems are costly and can cause pollution in the immediate area of the flour mill. Excessive amounts of air in the process may cause too much moisture evaporation from intermediate materials, and as a result, flours may have poorer color and lower moisture content.
Materials considered for pneumatic conveying should have physical characteristics that allow them to be accelerated, moved, and separated from the air. Particles should have enough surface area to react to an airstream that will move or suspend them and should not have a cohesive or attracting force among them.
Because air is apparently “easy” to move, little thought may be given to the power consumed for its activation or to the efficiency of the system. As shown later, about the same energy level is needed to move air whether the mill is loaded with materials during grinding or running empty. The following sections include descriptions and principles characterizing air in the different forms used in the wheat flour milling industry.
