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# Calculate the diameter of the dryer

If the drying time can be determined in some way, (In an existing drying unit or in a laboratory under conditions that are present in the dry condition) this time is used for mechanical design of the dryer, which is used for a small amount of drying when the material stays there longer than usual. The important thing here is the equality of the conditions in the dryer in a laboratory and the main dryer; this means that the drying time should be determined from an industrial scale dryer and compare with a semi-industrial design. Calculating the drying length is a factor that is associated with both mass transfer and heat transfer. The calculation method is such that the total amount of moisture content in the dryer is divided into a suitable number. These divisions include the preheating stages of the feed and the drying stage with a constant intensity and a drying step with descending intensity. If calculations are done using a computer, you can increase the number of divisions and increase the accuracy of computations. The designer of a dryer should consider each design with a single and distinct pattern; the designs of others can only be a guide.
The temperature of the feed and the air and the wet bubble is known, so The length of the preheating portion of the feed and the delivery of the feed to the bubble temperature and the constant drying portion are calculated by using the heat transfer coefficients (It is assumed that drying does not occur in the preheating portion of the feed) For calculating the length of the dryer, in which drying is carried out with a descending intensity, The mass transfer coefficient in length is required in terms of the function of moisture content. We can assume that constant heat transfer coefficient is same throughout the dryer. For calculating the total time Perhaps it’s better to first estimate the drying time, and then the length of the dryer.
Rotary dryer design
To design a rotary dryer, the following should be calculated:

• Length and diameter of the dryer
• Dryers Tilt
• The amount of air necessary for drying
• Required amount of the heat
• Flow direction
• The Number of cylinders per unit
In order to obtain the above, we must have a series of information that includes:
• Humidity and available air temperature
• Moisture and temperature of the air exits from the heater
• Humidity and temperature of the air exits from the dryer
• Moisture input material
• Product amount per unit
• Product moisture content