Given the diversity of the 50,000 or so materials that are dried commercially at various scales for a variety of reasons, the wide range of drying times required, the diverse physical forms of the materials dried and the multitude of quality constraints required to be met for the dried products, it is not surprising that literally hundreds of dryer types are in use today.
Drying is a highly energy-intensive operation that also determines the quality of most products that are dried. It combines transport phenomena with material science. Since fossil fuels are commonly used as the energy source for dryers, drying also has important environmental implications. It is estimated that from 9-25% of the national industrial energy consumption is attributed to thermal dehydration in developed countries.
This wide range is a result of the fact that different industrial sectors have widely varying demands as far as drying is concerned, ranging from a high of 35% in papermaking to just over 3% in the chemical process industries. Also, the thermal efficiencies of dryers in use today range from a low of around 20% to a high of 80%; the latter is difficult to achieve and is most common for indirect dryers which have so far limited industrial applications today.
Research and Development
It is interesting to note that about one ton equivalent of oil is consumed for, on average, six tons of water removed by thermal dehydration. Since it has already been shown that there is a positive correlation between the amount of water removed in industrial processing and the GDP (or standard of living) of the country it is clear that efforts must be made to improve the drying processes as the large developing economies of the world will soon make accelerating demands on fossil fuels to provide the energy for drying in various industrial sectors.
Drying is crucial to food preservation and agricultural processing. Thus, it is also at the heart of the nexus of food, water and energy.. This will encourage R&D required to enhance dryer performance and discourage non-optimal operation of existing dryers. Carbon footprints of dryers per unit of water removed should be placed on boiler-plate so that the buyers are aware of the performance of the units they wish to install.