Using the ASHRAE tables I'm able to get an equivalent length for copper fittings if it's a short radius or long radius fitting. In some of our pipework we also bend to 2xDiameter bend radius and 1.5xDiameter bend radius. How can I find or calculate the equivalent lengths for these?
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Equivalent length of a 90° elbow with different bend radius? 1
- Thread starter MechEng92
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LittleInch
Petroleum
Not going to be much different.
5D maybe, but not 1.5 to 2D
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5D maybe, but not 1.5 to 2D
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
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- #5
- Thread starter
- #7
I believe the pressure drop of the custom fitting bends can only be measured through testing, not calculations. In practice, I think you can just use the worst case because it's likely not worth your time to differentiate between the 1.5 and 2.0, especially if you're using a safety factor.
EnergyProfessional
Mechanical
I think you are over-thinking this. You will see that different publications show different K-values and other data and in real-life many fittings get made in a shop and vary from the text-book fitting. Especially the more complex ones. So there is quite some variety. Imagine a Y, or bullhorn etc. Also note, that the tests are made with long duct up and down-stream. In real life you have multiple fittings close to each other and they influence each other. For design, just use the higher pressure drop to be conservative.
Also allow for some field-changes, like routing around a structural element. And your plans may show a 90°, and the contractor will use an assembly of 45° or some other deviation since the sprinkler guy decided to install his pipes before the ducts.
Also allow for some field-changes, like routing around a structural element. And your plans may show a 90°, and the contractor will use an assembly of 45° or some other deviation since the sprinkler guy decided to install his pipes before the ducts.
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- #10
Cameron Hydraulic Data provides a table of "Resistance coefficient K" to calculate the equivalent length of pipe for 90 degree bends of different r/d (bend radius / pipe diameter).
Unfortunately for you, the r/d values are integers, and there is a big difference between 1 and 2.
The K value then gets plugged into a formula that includes the "Friction Factor" from the Moody diagram, which in turn is dependent on pipe roughness and the flow Reynolds number.
How many of those inputs do you know with any certainty? (Hint: Probably none)
Unless pipe bends are a dominant feature it's really not worth the effort of going through the work when all the inputs are guesses.
Unfortunately for you, the r/d values are integers, and there is a big difference between 1 and 2.
The K value then gets plugged into a formula that includes the "Friction Factor" from the Moody diagram, which in turn is dependent on pipe roughness and the flow Reynolds number.
How many of those inputs do you know with any certainty? (Hint: Probably none)
Unless pipe bends are a dominant feature it's really not worth the effort of going through the work when all the inputs are guesses.
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- #11
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