Because size reduction depends totally on energy used-more energy means more breakage— the link between the two merits special mention (see Table 1.3). Progress in size-reduction machines has always depended on available sources of energy. First, it was necessary to supplement human muscle power as an energy source and then to replace it entirely. Tools that used animals—horses and oxen—to drive grinding devices were developed. Next, water wheels and windmills were invented, followed by motors powered by steam. Finally steam-driven turbines—now a source of energy around the world—were developed to generate electricity. This largely eliminated the use of human energy to drive size-reduction tools and machinery (except in some developing countries, where grain is still ground by hand).
Today, grinding mills use about 2% of the electricity generated in the world, and dry-grinding processes consume about 75% of this energy. (Note that some of the major size-reduction processes that consume a great deal of energy, such as the grinding of trees to produce wood pulp cellulose, are beyond the scope of this book.)
Size-reduction machinery is generally described as mechanical breakage mechanisms capable of applying available power to break down material from a specified feed size to a specified product size. The following key points summarize the types of energy used for size reduction as they developed over the years:
■ The fuel for muscle power is the food eaten by humans and animals. Muscle — powered machines work at about 0.5 kW (0.66 hp).
■ The fuel for waterpower is from the flow of water that drives the water wheel. Water-powered machines work at about 5 kW (6.6 hp).
■ The fuel for steam power is the wood, coal, or petroleum that, when combusted, provides the heat to convert water to steam. Steam-powered machines work at 50 kW (66 hp).
■ Electrical energy is produced in electrical generators driven by water, steam, or internal combustion engines. Effectively, there is now no limit to the power that can be delivered by electric motors.