Calculating Residence Time

Question

I am conveying and processing a specific bulk material that needs to stay in the conveyor for a certain amount of time for a curing process. We are injecting a gas into the conveyor to mix with the solids and need a specific time in the conveyor for the reaction to take place. How do I know how long my bulk material will be in the screw conveyor?

Answer

The length time a bulk material spends inside the screw conveyor is known as residence time. A few inputs are needed to determine the residence time of a screw conveyor. Residence time (RT) is a function of screw pitch (P), conveyor speed (S), and the centerline distance from the inlet to the discharge (L). With these values a simple formula can be used to determine residence time:

• RT: Minutes
• L: Inches
• S: Revolutions per minute (RPM)
• P: Inches

It is also possible to calculate the required centerline distance between inlet and discharge if residence time is given. The same factors are used to determine this value:

L=RT × S × P

Any of the above factors can be manipulated to yield a desired residence time to meet the needs of your application. Knowing the residence time is especially critical when designing conveyors for pyrolysis, gasification, and other thermal processing applications.

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