Will the CFM be the same in straight and bent pipes?

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Discussion Overview

The discussion revolves around the comparison of airflow (CFM) in two pipes: one straight and one with 90-degree bends. Participants explore the implications of pipe geometry on flow rate, pressure, and turbulence, considering both theoretical and practical aspects of fluid dynamics.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant states that if the lengths and pressure drops across both pipes are the same, the straight pipe will have a higher flow rate due to the concept of "equivalent length" which accounts for bends and restrictions.
  • Another participant questions how the bent pipe can have the same flow rate despite the presence of turbulence caused by the bends, suggesting that turbulence should reduce flow rate.
  • A later reply agrees that turbulence can reduce flow rate but notes that velocity may increase, leading to a complex relationship between flow rate and velocity.
  • One participant draws an analogy between flow rate and electrical concepts, questioning whether an increase in velocity compensates for a decrease in flow rate, similar to volts and amps in electrical systems.

Areas of Agreement / Disagreement

Participants express differing views on whether the flow rates in the two pipes can be the same under the given conditions. While some argue that the straight pipe will always have a higher flow rate due to reduced restrictions, others challenge this by questioning the effects of turbulence and pressure dynamics.

Contextual Notes

Participants discuss the concept of equivalent length and its impact on flow rate, but there are unresolved assumptions regarding the specific conditions under which the flow rates are compared, such as the exact nature of the bends and the characteristics of the fluid being analyzed.

ISX
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So basically we have 2 pipes, one is straight, one has 90* bends in it and a concave surface to hit and stir things up even more. X shows that all the widths are the same everywhere, so it's not like the pipe is getting bigger or smaller in diameter. We have 20psi of pressure, will the CFM out of both pipes be the same?

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If the actual lengths of the two are the same, and the dP across them is the same, then the straight pipe will have a higher flow. There is something called an "equivalent length" for any pipe or tube with bends and other restrictions. It is this equivalent length that has to be the same in order for the two pipes to flow the same. Bends and other restrictions increase the equivalent length.
 
Alright so let's say they are the same length and everything and therefore have the same flow, why is it that they do have the same flow when the bent and screwed up one has to fight the 90* corners? Wouldn't the turbulence cause a reduction in flow rate?
 
ISX said:
Wouldn't the turbulence cause a reduction in flow rate?

Of course. There might be an increase in velocity, but the flow-rate itself, during restriction, is lower.
 
Hmmmm, so let's say I turn the pressure on to both pipes and hook it to a tank and 10 seconds goes by and I turn it off, would there be the same amount of air in both tanks?

The reason I ask is because you say flow rate goes down but velocity increases.. Are those 2 variables just like volts and amps? As in, if one goes down the other increases and you still have the same amount of watts. Or in this case, velocity goes up flow goes down but you still end up with the same volume..? Or is this wrong?
 
ISX said:
Alright so let's say they are the same length and everything and therefore have the same flow, why is it that they do have the same flow when the bent and screwed up one has to fight the 90* corners? Wouldn't the turbulence cause a reduction in flow rate?
I'm afraid you read me wrong. If they have the same length and the same pressure drop (ex: they are both connected to the same tank and discharge to atmosphere so the pressure drop is the same) then the STRAIGHT pipe flow rate will be HIGHER. So yes, the additional restrictions in the line will cause a reduction in flow rate.
 

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