HOT AIR DRYER DESIGN
DRYING is a very versatile and important unit process for the Process Industry. The process that results in reduction of moisture content in a given material is known as DRYING.
We will now discuss how to obtain a perfect hot air dryer design.
Obtain a Perfect Hot Air Dryer Design
The simplest and hence traditional method of drying a material, specially agricultural material in a country like India, has been to merely spread it under the sun and use the thermal energy available from the sun to gradually evaporate the water. Drying under the sun or the simple solar drying, though the easiest form of drying, has enough limitations since it requires long duration of drying, large area requirement for the material to be spread , vagaries of nature, etc. This is why, the Industry that is interested in a achieving and maintaining a steady output fromthe manufacturing facility , consistent quality in terms of its moisture content, best value for money, looks for a well designed mechanised hot air dryer design. Here again, relevant expertise is needed to identify the appropriate drying machine out of the several that are available now viz. tunnel dryer, rotary dryer, fluidised bed hot air dryer design, flash hot air dryer design, vacuum hot air dryer design, impingement dryer, etc.
Every material has its own drying characteristics and it requires thorough process engineering knowledge to understand the mutual dependence of the various operating parameters involved in the drying process.
It is the job of the Process Engineer to make hot air dryer for a given material which starts with studying the drying characteristics in the Lab. While it is one option to a hot air design from the first principles of arriving at the individual transfer coefficients with respect to heat and mass transfer that takes place all through the drying process, the other and most reliable method is to extrapolate the overall coefficient from study on a prototype dryer.
Achieving energy economy during drying is an important objective that the designer has to target. This involves conceptualising and detailing the method of recycling the hot air that exhausts from the dryer. One often argues asking the question
how can the exhaust air be recycled when it contains so much of moisture ?'Against such questions my typical reply is: ‘ leave it to the expert to calculate and decide much to recycle and how much of the exhaust air to be let out in the final exhaust’. Usually, the rate of moisture removal during the initial part of drying is high and then it gradually starts dropping as the material loses more and more moisture. Supply hot air and not recycling is meaningless since the energy bill becomes drastically high with such zero recycle. This is where the control dynamics becomes important. The process control introduced in dryers plays around with the percent recycle in the pilot plant design. In the initial phases of drying the recycle percentage is maintained low and as the drying progresses, the percentage recycle is increased.