Raking In Fluid Bed Dryer

Fluidization is the main principle by which FBD or fluid bed dryer works. The fluid bed dryer is frequently used in the drying process due to its reduced drying time & increased capacity means it can dry more volume of product in a short period as compared to the tray dryer.

Raking Process

Raking is the process that is routinely performed by all the operators in FBD during the drying process but we don’t use this technical term in a routine so most of you are not familiar with this word Raking.

Raking is the manual process in which we remove the product trolley & manually mix the granules with gloved hands or with the help of stainless steel spatula & break large lumps formed in the FBD product trolley to ensure that all the material in the product trolley will be lifted uniformly & result in uniform drying 

Why We Need Raking

One of the main contributory factors of FBD for uniform drying of products is that it should lift all the product volume equally from all areas of the FBD product trolley but sometimes the uniform lifting is not achieved & the drying process is not uniform.

So to ensure uniform drying we use the process of Raking.

Factors Resulting Non Uniform Drying In FBD / Product Lifting Issue

Following are some factors that may result in non-uniform drying due to lack of uniform product lifting in the product trolley of FBD & require Raking after specified time intervals.

  • Fan Motor Speed
  • Product Load
  • Filter Blocking
  • Dutch Woven Sieve Blocking

Fan Motor Speed

The speed of the Blower Fan is responsible for increasing or decreasing the air velocity which is required for fluidization.If the speed of the blower fan is not proper it will not lift the product properly so drying will not be uniform & large lumps will be formed which will further hinder the drying process.

So removing the product trolley & performing the Raking process means breaking the large lumps & mixing all the granules in the product trolley manually & increasing the speed of the blower motor for proper fluidization will result in proper lifting and uniform drying.

Product Load

If the product trolley of FBD is overloaded than its recommended capacity, it will result in non-uniform drying due to product lifting issues.

Remove excessive load, perform Raking & dry the product in lots if the product load is higher than the recommended capacity.

FBD Filter Blocking

If the filters of FBD are choked or blocked, the required suction will not be produced & air velocity will be low so the product will not be lifted uniformly.

It is practically observed that on the filter choking product from one side is lifted & other remains unlifted & contains lumps. 

Remove the product trolley, perform Raking & replace or wash the filter to ensure proper suction for uniform lifting & drying.

Dutch Woven Sieve Blocking

The screen or mesh of the FBD product trolley is known as Dutch Woven Screen & if this screen is blocked the product is not lifted uniformly & lumps are produced.

Remove the product trolley, perform the Raking activity & clear or clean the dutch woven screen for uniform product lifting & drying.


Drying in FBD is not always smooth & uniform. Sometimes product lifting issues are observed which results in non-uniform drying so to avoid the issue of non-uniform drying in FBD or fluid bed dryer we use the manual technique which is known as Raking in which we manually mix & break the lumps.

Also Read

  1. Working of fluid bed dryer.
  2. Size Of FBD Mesh
  3. Critical Process Parameters Of FBD
  4. Kevlar Rope For FBD
  5. Application of tray dryers in pharmaceuticals.