Cereals processing technology

34 Cereals processing technology
3.4.2 Machine construction
Significant changes have occurred in the way machines are built. Steel has
replaced timber as the material of choice for the construction of plansifters.
Roller mills were constructed from cast iron and wood until recently but many
roller mill manufacturers now use prefabricated steel sheets for the main frame
of the roller mill, with the grinding forces being contained in what are known as
‘roll packs’. 5 This has made the roller mill much lighter than its cast iron
predecessors and this has had implications for machine cost and building design.
The advent of the roll pack has also dramatically reduced roll replacement times.
This has become increasingly important where flour mills are expected to run for
extended periods without shutting down.
Developments in roller mill technology are not just limited to the method of
fabrication and the way the roll chills 6 are fitted. Significant strides have been
made in minimising the amount of noise generated by roller mills in operation.
This includes the replacement of chain and gear drives with timing belt drives on
the differential mechanism. Application of exhaust air to the grinding zone of
rolls has minimised fluidisation and roll ‘bounce’ 7 is less of a problem.
Sophisticated electronic control systems provide reliable roll engagement and
disengagement and instances of rolls running without feeds have been
eliminated. This is a considerable advance in terms of increasing roll life
expectancy and safety. This is because rolls running in contact with each other
wear rapidly and pose a significant fire risk.
3.4.3 New machines
The most notable process developments in flour milling have been the double
grinding of intermediate streams before sieving and the debranning of wheat
before main processing. New machines have been developed to exploit these
techniques. These machines and their application are well documented in trade
literature and are the subject of numerous patent applications (Forder 1996,
Posner and Hibbs 1997). The function of these new machines in the flow sheet
has been to reduce the number of machines used.
The double-grinding roller mill has been employed successfully in flour mills
only in the last decade, even though it was first examined during the original
development of the gradual reduction system (Storck and Teague 1952). A
variation, the six-roller mill, has been in use in the malting industry for many
years (Buhler 1981). The configuration of the double-grinding roller mill
enables mill stocks to be ground twice without intermediate sifting or grinding.
5 This refers to the self-contained bearing housings that support the roller mill grinding forces.
6 Roll chill is the term used to describe the cast metal rollers that form the working components of
roller mills. The phrase is derived from the method of production of the steel used in their
manufacture, namely chilled steel.
7 Roll bounce is the phenomena experienced in grinding using roller mills when fluidisation of the
material being ground occurs at the point of contact between the rolls. The rolls literally bounce as
they encounter inconsistent feeds to the grinding zone.