Calculating the Suction Generated by a Shop-Vac

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Discussion Overview

The discussion revolves around calculating the suction generated by a shop-vac vacuum for a design project involving the collection of kevlar fibers. Participants explore the implications of using different attachment sizes and the effects on airflow and pressure, with a focus on both theoretical calculations and practical considerations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes using a 0.25" diameter attachment with a shop-vac and seeks a mathematical proof of its effectiveness compared to the original 1.25" attachment.
  • Another participant suggests conducting a full-scale test instead of calculations, emphasizing the need to understand minimum vacuum and airflow requirements for handling fibers.
  • Concerns are raised about significant airflow restrictions caused by the smaller nozzle, which could affect the shop-vac motor's cooling and overall performance.
  • A later reply clarifies that reducing the nozzle area does not proportionally decrease airflow, as the air velocity increases through the smaller nozzle, though the exact flow rate reduction depends on various factors.
  • One participant requests more details about the attachment and the method of fiber collection, indicating that understanding the setup is crucial for developing a mathematical model.
  • It is noted that obstructing airflow can lead to increased pressure drop and potential issues with motor cooling, highlighting the complexities involved in vacuum design.

Areas of Agreement / Disagreement

Participants express differing views on the best approach to validate the effectiveness of the smaller attachment, with some advocating for empirical testing while others focus on theoretical calculations. Concerns about airflow restrictions and their implications for motor performance are also debated, indicating unresolved issues in the discussion.

Contextual Notes

Participants mention the need for specific details about the attachment and the fiber handling process, which are currently lacking. The discussion also highlights the dependence on various factors such as nozzle shape and fan characteristics, which are not fully explored.

mbeard9
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TL;DR
I am looking to determine the suction power generated by a shop-vac given differing attachment areas.
I am working on a design project where a shop-vac vacuum will be used to gather kevlar fibers in place for a cut to be made. Currently, the vacuum has a 1.25" diameter attachment. I have created a 0.25" diameter attachment that completes the task perfectly. However, I cannot figure out how to prove this mathematically. I have attached the vacuum specs below.

https://www.lowes.com/pd/Shop-Vac-6-Gallon-3-5-HP-Shop-Vacuum/1000351357

Air Flow (CFM) 145
Sealed Pressure (Inches) 58
 
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There are calculations to scale from a small experiment up to production size. The size of your numbers, and your background, indicates that you would be best off to run a full scale test.

If you are determined to calculate, you need to know exactly the minimum vacuum and/or air flow velocity and/or air flow rate to do what you want to do with the fibers. Then you need to understand if scaling factors apply. Search scaling factors pneumatic conveying for some idea of what scaling factors are and whether they apply to your situation.

I have over 20 years experience in handling materials ranging from tissue paper and paper towel stock to medical gauze using vacuum. We did a lot of testing.
 
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Be cautious about a major air flow restriction. You are restricting the hose cross section area by a factor of 25, less than 4% of design airflow. The Shop-Vac motor needs good airflow for cooling. Been there, done that. :cry:
 
Tom.G said:
Be cautious about a major air flow restriction. You are restricting the hose cross section area by a factor of 25, less than 4% of design airflow. The Shop-Vac motor needs good airflow for cooling. Been there, done that. :cry:

Shrinking the nozzle area by a factor of 25 does not decrease airflow by that same factor. The air velocity through the smaller nozzle will be much higher than through the large one. It will reduce flow rate somewhat as well, but the exact amount that it reduces it by will depend both on the shape of the nozzle and the characteristics of the fan and motor.
 
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You have given no information about the attachment that requires the mathematical model.
Start by describing the attachment.
How are you gathering the fibres by using an airflow?
How are the fibres arranged?
How long are the fibres?
What do you cut the fibres with?

You specify the vacuum cleaner you are using. A vacuum cleaner is designed with a centrifugal pump to generate a high volume airflow. If you obstruct the airflow, a greater pressure is dropped across the obstruction. The pump usually runs faster when obstructed, with lower air flow and a lower pump input pressure. Motor cooling can then becomes a problem.