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How to Calculate for a Thin Film Agitated Evaporator

A thin film agitated evaporator is used to separate liquids from unwanted chemical components. The liquid is heated to its boiling point, at which time the pure desired chemical vapors are fed through a vent in the top of the unit for further refining. The unwanted elements are heavier and vaporize at higher temperatures and fall back into the base of the unit as froth. This froth is strained away through a collection grid at the base of the unit. Heat transfer calculations must be made to determine the correct temperature for the heating element to vaporize the wanted materials without contaminating them with other elements.

Things You'll Need

Desired liquid boiling point
Thin film agitator

Instructions

- 1
  Determine the basic equation for the transfer of heat by means of convection and conduction between two media separated by a wall. The equation for a thin film agitator is Q = UA (Delta)Tl where Q is the heat duty, U is the heating coefficient, A is the area of conduction and (Delta)TI is the logarithmic mean temperature difference.
- 2
  Determine the value of the heating coefficient (U) using the following formula: 1/U = 1/Ho + t/k + 1/Hi where t is the thickness of the wall, k is thermal conductivity of the wall and Hi and Ho are the inside and outside heating film coefficients. These values can all be found in the material data sheets that come with the agitator.
- 3
  Calculate the value of Hi and Ho with a scaling factor. This means that whatever size your machine is in comparison to the model unit is the scalable factor that should be used to determine both Hi and Ho. If your machine is 12 times as large as the model referenced in the instructional material, your values for both Ho and Hi should be multiplied by a factor of 12.
- 4
  Solve the original equation for Q. This will be the constant temperature that the internal coil needs to be kept at for the vaporization of the liquid to occur and be maintained until it has reached the condenser at the top of the agitator.

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