Optimizing Grain Flow in Vibro Thermal Disinfector (VTD) Using Iron Mesh

Introduction:

The Vibro Thermal Disinfector (VTD) is a machine developed at Vigyan Ashram for disinfecting grains like wheat and rice without using any chemicals. It works on two simple principles — heat and vibration. You can read about the original VTD design here.

Grains are poured from a hopper at the top and travel through a series of vibrating trays inside a heated chamber. By the time grains exit from the bottom, they are disinfected by the heat they were exposed to during their journey.

Basic Design:

The machine consists of:

A hopper at the top where grains are fed
6 vibrating trays arranged in a zigzag pattern inside a heated chamber
A vibrator motor with an eccentric weight that creates vibration
A heating element that maintains temperature between 55-70°C
An outlet at the bottom from where disinfected grains are collected

The vibration is created by an eccentric weight attached to the motor shaft. Since the weight is unbalanced, it creates a rotating force when the motor runs which causes the entire frame and trays to vibrate. This vibration combined with the slight slope of the trays makes grains travel forward and downward through all 6 trays.

The Problem:

For proper disinfection, grains need to spend at least 8 minutes inside the heated chamber. However currently grains were passing through in just 75 seconds — which is far too fast for effective disinfection.

What We Tried:

To increase the residence time of grains inside the machine, we decided to add iron mesh on the trays. The idea was that mesh would create a rougher surface than plain MS sheet, slowing down grain movement and increasing the time grains spend inside the heated chamber.

We started by adding mesh on the first and second trays to observe the effect- are as follows.

Observations:

During the trials we made several interesting observations:

When the machine was started, grains were accumulating on the right side of the trays due to vibration direction
Grains were also sliding below the mesh instead of travelling on top of it
To fix both these issues we taped the mesh to the trays to hold it firmly in place

After adding mesh on two trays the residence time increased from 75 seconds to around 3 minutes 40 seconds — a significant improvement but still short of the 8 minute target.

Mesh Size Experiment:

We also experimented with different mesh sizes. The current mesh has a diagonal of approximately 2.20mm which gives a hole size of around 1.55mm — smaller than the wheat grain width of 3-4mm so grains cannot fall through.

When we tried a smaller mesh size, we observed that wheat grains were slipping rather than percolating through the mesh. This is because smaller mesh creates a smoother surface, reducing friction. The 2.20mm mesh proved better as it creates a rougher surface giving more resistance to grain movement.

Interesting Finding:

One interesting observation was that on plain MS trays grains slide smoothly, but on the mesh surface grains bounce while travelling. This bouncing is actually beneficial as grains get exposed to heat more evenly from all sides during their journey!

Grains falling from mesh and tray