Generally, the chamber design depends on the atomizer used and on the air-fluid contact system selected.
The height H of the drying chamber as a function of droplet diameter d and temperature differences between the drying air and the particle.
I need the formula to calculate the diameter of the evaporator (Air-Droplet Contact System_Cocurrent contact).
The primary property that determines the tower diameter is the gas velocity in the tower - specifically a gas velocity that is low enough to not entrain the droplets. This depends on the droplet properties (size, density, and shape) and on the gas properties (viscosity and density). A first approximation to calculate the droplet terminal velocity and determine a tower diameter than generates a gas velocity that is about 1/2 of the terminal velocity or a droplet velocity that provides the residence time you need. However, the droplets generally drop a little faster than their terminal velocities for various reasons and the gas velocity profile is not flat. Still, the above formula is a good starting place.
I thought about your question since it is an interesting one.
Let me formulate this way:
- the diameter should be such that no liquid droplet will reach the wall.
that means droplet diameter should be zero before the droplet comes to the wall.
In the conditions created in the dryer the fluid will evaporate with a rate "k" per surface unit in contact with drying fluid (be it air or any other kind). The droplet left the spay with an angle (alpha) with respect to the drum axis and with a velocity Wo. The drying fluid has a ascending velocity Wa. You have several equations to write for the droplet:
- variation of its liquid volume due to evaporation losses
- balance of forces acting upon the droplet which are the weight and the drag due to the relative movement in the gas flow.
If Wo is in an angle leads to a complex movement as well axial as radial. The distance on the radial will be proportional to the time the droplet will vaporize completely. The drum radius should be > than the T*Wo*sin (alpha) where T is the complete vaporization time.
Let us put Ro the droplet initial radius and assume that the volume loss is )proportional to a constant K * S where S= area of droplet in contact with drying gas. You can write your first equation as dV/dt= K*S if you consider the droplet spherical you can deduct the evolution law for the radius and the time T.
If you write the other balance of forces you can compute as well the height you will need.
The drag is as I wrote depends on the Reynolds criterium and because of the very small dimensions of droplet you will find yourself in laminar domain.
Let me know how you finalized it.
Larry L. Baxter that's great.
but, this approximation to calculate the diameter.
Using a special rotary atomizer outside a reactor condition, the spray diameter is 3 m. I need to know what effect this spraying diameter has on the reactor conditions when the hot gas enters the rotational speed. when hot gas enters to the evaporation chamber, The mass of droplets aligns with the flow of motion. therefore the spray diameter decrease.
Ijaz Durrani
Thanks for the explanations and resources.
Best regards.
Nick Hamburger
That's great.
I'll definitely let you know.
Best regards.
I thought it is a vertical dryer. For a centrifugal one only the basics as logic can stay valid. In stead of "g" you have an other field.
Nick Hamburger Yes, it is a vertical dryer.
As in the following figure:
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