Saturday, June 3

Fluidized Bed Dryers and Finger Bags – Step by Step

We are going to discuss fluidized bed dryers, their construction, working, FBD finger bags, finger bag MOC, and their features and application in pharmaceutical manufacturing. Drying in pharmaceutical manufacturing involves removing water and other liquid solvents from granules or other materials using heat and a stream of air. A fluid bed dryer is used to dry granules in pharmaceuticals using FBD finger bags.

The principles of Fluidized Bed Dryer (FBD):

Now we look at The principles of Fluidized Bed Dryer (FBD):

  • In the fluidized bed dryer, hot high-pressure air is passed through a perforated bottom of the container containing the granules to be dried.
    FBD finger bag.
  • The granules are lifted from the bottom and suspended in the air stream, this condition is called fluidized state.
  • The hot air surrounds all the granules and dries them completely. Therefore, the granules are also dried.


The dryer is generally made of stainless steel. A removable container is attached to the base of the dryer that is used for loading and unloading material. The stainless steel bowl has a perforated wire mesh base that supports the material being dried. A fan is available on top of the circulating hot air. Fresh air enters; Filters and heat exchange are connected in series to heat the air to the required temperature.

Operation of the fluidized bed dryer

Various types of fluidized bed dryers are available. The simplest form is a stainless steel chamber with a removable perforated bottom or bowl. The granules to be dried are placed inside the container, suspended above the container on a wire mesh or within small finger bags that are suspended above the container. The air outside the chamber is filtered and heated before a fan blows the air up through the perforated container.

fluidized bed dryer image

Air velocity increases with time to a point where the frictional drag of the particles equals the gravitational force. The granules rise in the container due to the high speed of the air and then fall back in a random motion, like a boiling liquid. In this fluidized state, the granules are partially suspended in the air stream.

The hot air that swarms over the granules draws out any absorbed liquid from the particles, and since in the fluidized state all the particles are in the same gaseous environment, the process allows the granules to dry equally.

When the particles crawl into the bags, the granules tend to stick to the inner surface of the bag, which must be agitated when trying to remove all the dried granules.


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How to Work

Here is the step by step working process

  1. The wet granules to be dried are placed in a removable container. The bowl is pushed into the fluidized bed dryer.
  2. Fresh air passes through the pre-filter, which is subsequently heated by passing through a heat exchanger
  3. Hot air flows through the bottom of the cup while allowing the fan to rotate.
  4. Air velocity increases with time. When the air velocity is greater than the sedimentation velocity of the granules, the granule remains partially suspended in the air stream.
  5. After a while, a pressure point is reached where the frictional drag of the particles is equal to the force of gravity.
  6. The granules rise in the container due to the high speed of the air (1.5 to 7.5 meters per minute) and then fall back in a random boiling motion. This condition is said to be a fluidized state. The gas surrounds all the granules to dry them completely.
  7. Air exits the dryer through the bag filter. The entrained particles remain adhered to the inner surface of the finger bags. The bag is periodically shaken to remove adhering particles.

Finger Bag (FBD)

The filter bag of the fluidized bed dryer is made up of different kinds of superior quality fabrics, according to the requirement of the material involved in drying.

Finger Bags image
The different types of fabrics are:

  1. Antistatic
  2. Polypropylene
  3. Stain

Manufacture of fluidized bed dryer filter bags depends on the micron retention similar to the required factor, static change, and air permeability. Fluidized bed dryer filter bags are cost-effective and durable as they are manufactured to specification and regularly tested to meet standard quality. FBD finger bags have finger-like structures to increase their surface area and efficiency. The drying process in FBD depends on the MOC of the finger bags. The finger bags are made from a combination of antistatic polymer, nylon fabric, and cotton fabric.


  • Pharmaceutical industry
  • Chemical industry
  • Agricultural industry
  • Processing industry

Features of fluidized bed dryer finger bag:

  • Excellent resistance to wear and abrasion
  • Extremely durable construction
  • High flow rate combination with good particle retention
  • Increased surface area


Video of Fluidized-bed dryer | Vacuum dryer and working principle

Credit: Carewell pharma






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