For a given pressure, which pipe will have the fastest flow?
Discussion
For a given pressure, which pipe will have the fastest flow, 15mm or 10mm? (internal diameter)
Let's assume the liquid flowing is water, and both pipes are plastic. If pressure is relevant, then for arguments sake let's choose 1 bar.
Does the smaller diameter pipe result in a higher pressure, or does it simply offer more resistance to the pump (I'm think the latter as it must require more force to push the liquid through a smaller diameter pipe?).
If the length of the pipe matters, then assume 10m, open ended.
Let's assume the liquid flowing is water, and both pipes are plastic. If pressure is relevant, then for arguments sake let's choose 1 bar.
Does the smaller diameter pipe result in a higher pressure, or does it simply offer more resistance to the pump (I'm think the latter as it must require more force to push the liquid through a smaller diameter pipe?).
If the length of the pipe matters, then assume 10m, open ended.
My thinking is 1bar of pressure at the entry point will never change no matter how small you make the pipe.
Your reference to an attached pump would make a difference as the pump would increase pressure to its maximum permissible as the flow was reduced, but thats a different question.
If the 1 bar never changes, the only difference pipe diameter makes is the amount of water that flows.
If you squeeze a hose pipe together you are in fact increasing the pressure right at the exit point due to the release/squeeze effect, elsewhere in the pipe i believe the pressure remains the same.
Thats my uneducated view of it but probably completely wrong
and happy to know different as i like things like that 
Your reference to an attached pump would make a difference as the pump would increase pressure to its maximum permissible as the flow was reduced, but thats a different question.
If the 1 bar never changes, the only difference pipe diameter makes is the amount of water that flows.
If you squeeze a hose pipe together you are in fact increasing the pressure right at the exit point due to the release/squeeze effect, elsewhere in the pipe i believe the pressure remains the same.
Thats my uneducated view of it but probably completely wrong
and happy to know different as i like things like that Whatever123 said:
Yes, but the question was on ‘fastest’ flow, not volume 
flow rate of course is the correct way to measure how much stuff flows.Flow rate is in units of volume or mass per some unit of time.
The bigger diameter pipe will always flow more water, because it’s got more water in it.
At the sizes you’re talking about the backpressure won’t depend on pipe diameter.
Edited by PlywoodPascal on Sunday 15th September 14:10
Edited by PlywoodPascal on Sunday 15th September 14:11
Edited by PlywoodPascal on Sunday 15th September 14:13
I understand flow rate and the quantity contained there in, bigger pipe = higher flow rate as flow rate is a volumetric measure and the correct way to measure water ‘quantity’
Speed of flow is very different and there’ll be a calculation for pressure applied = x amount of flow and x amount of speed increase.
The op was worded such as its a bit of loaded question, nothing to do with flow rate (volume) but about speed of flow
Speed of flow is very different and there’ll be a calculation for pressure applied = x amount of flow and x amount of speed increase.
The op was worded such as its a bit of loaded question, nothing to do with flow rate (volume) but about speed of flow
Tye Green said:
'fastest' is a measure of speed.
'most' is a measure of volume.
the original question may need to be reworded....
I think it was intended, no? as hes interested in the speed of the water, thats how i took it 'most' is a measure of volume.
the original question may need to be reworded....
Its far more interesting anyway, especially in the scenario i explained re the pinching of a hose pipe where the pressure remains the same but the speed of the flow increases..
Which i suspect is where the op was going..
I'd suggest that regarding the SPEED of flow, given a uniform pressure where the water enters the pipe, the larger pipe would have the fastest flow - and I mean fastest, not move the most water, which it would do being a larger diameter, but that wasn't the original question.
The reason for thinking that the smaller pipe would have a slower flow is by considering the relative area of water in contact with the inside of the pipe. For a smaller pipe there will be relatively a higher proportion of the water in contact with the pipe walls, and as the pipe walls will cause a slowing of the water due to friction, the average speed of the water would be reduced.
Obviously, a key point in this is that the pressure remains the same (as specified by the OP).
The reason for thinking that the smaller pipe would have a slower flow is by considering the relative area of water in contact with the inside of the pipe. For a smaller pipe there will be relatively a higher proportion of the water in contact with the pipe walls, and as the pipe walls will cause a slowing of the water due to friction, the average speed of the water would be reduced.
Obviously, a key point in this is that the pressure remains the same (as specified by the OP).
Original question is a little confused. For a normal straight section of pipe, the bigger the cross sectional area, the lower is the ratio of the circumference of the pipe to that cross sectional area, and therefore the lower the resistance to flow. Thus the bigger pipe flows a larger volume of water per second at a higher velocity than a narrower pipe, assuming the pressure drop across the pipes are the same and their lengths are the same.
Thanks everyone.
It wasn't intended as a trick question.
The volume vs flow argument never entered my mind, however surely those go hand in hand, e.g. fastest flow = more volume? (or is that a rash assumption!).
According to the calculator posted by @PlywoodPascal, using my example it would seem that although there is some difference, it's minimal (if I interpret the output from the calculator at the bottom of the page correctly).
This question was prompted by two things; This morning my o/h and I were discussing our central heating and she idly wondered if 15mm copper pipe would be superior to the 10mm plastic which we currently have.
The other thing; Several years ago, I worked in an air & oil filter manufacturing plant, and my job as a QC operative was to test the effect of flow on different grades of filter paper.
We would heat engine oil and then pass the clean oil through an oil filter as a reference, and then contaminate the oil with various grades of 'dirt' to determine the effect on flow.
Which leads me on to the point I was going to make. My manager was a distinguished engineer (several letters after his name) and I remember him getting into a heated debate with a maintenance chap from a neighbouring factory about flow, volume and pressure etc. - the argument centred around the insertion of an orifice into the flow path and it's effect. All I remember was the argument revolved around pressure vs flow (rather than volume directly) - forgive me is this all sounds rather vague, it was some 40 years ago!
Anyway I digress...
Many thanks for the contributions, most interesting.
It wasn't intended as a trick question.
The volume vs flow argument never entered my mind, however surely those go hand in hand, e.g. fastest flow = more volume? (or is that a rash assumption!).
According to the calculator posted by @PlywoodPascal, using my example it would seem that although there is some difference, it's minimal (if I interpret the output from the calculator at the bottom of the page correctly).
This question was prompted by two things; This morning my o/h and I were discussing our central heating and she idly wondered if 15mm copper pipe would be superior to the 10mm plastic which we currently have.
The other thing; Several years ago, I worked in an air & oil filter manufacturing plant, and my job as a QC operative was to test the effect of flow on different grades of filter paper.
We would heat engine oil and then pass the clean oil through an oil filter as a reference, and then contaminate the oil with various grades of 'dirt' to determine the effect on flow.
Which leads me on to the point I was going to make. My manager was a distinguished engineer (several letters after his name) and I remember him getting into a heated debate with a maintenance chap from a neighbouring factory about flow, volume and pressure etc. - the argument centred around the insertion of an orifice into the flow path and it's effect. All I remember was the argument revolved around pressure vs flow (rather than volume directly) - forgive me is this all sounds rather vague, it was some 40 years ago!
Anyway I digress...
Many thanks for the contributions, most interesting.
Bugger, thought it was more interesting than that 
It did lead me to looking up my something for my own curiosity though and discovering why a pinched hose end increases velocity even if the origin or main hose pressure never changes, so I learnt anyway
It did lead me to looking up my something for my own curiosity though and discovering why a pinched hose end increases velocity even if the origin or main hose pressure never changes, so I learnt anyway
Edited by Whatever123 on Sunday 15th September 15:46
TonyRPH said:
Super Sonic said:
So was it flow rate or velocity you were asking about?
Flow rate would be what I was after.I guess bigger pipes would mean more heated water passing around the system, radiator temps might be more consistent, but slower to heat up?
And of course the critical thing with a radiator is that you want a decent temperature drop across the radiator, so colder water goes back to your boiler, you adjust that with a tap/valve at the radiator inlet. as long as your system can supply all of those radiators at sufficient flow rate there isn’t any benefit to using larger bore pipes that I can see.
PlywoodPascal said:
For your central heating there’d be other interesting effects too though.
I guess bigger pipes would mean more heated water passing around the system, radiator temps might be more consistent, but slower to heat up?
And of course the critical thing with a radiator is that you want a decent temperature drop across the radiator, so colder water goes back to your boiler, you adjust that with a tap/valve at the radiator inlet. as long as your system can supply all of those radiators at sufficient flow rate there isn’t any benefit to using larger bore pipes that I can see.
(my bold above)I guess bigger pipes would mean more heated water passing around the system, radiator temps might be more consistent, but slower to heat up?
And of course the critical thing with a radiator is that you want a decent temperature drop across the radiator, so colder water goes back to your boiler, you adjust that with a tap/valve at the radiator inlet. as long as your system can supply all of those radiators at sufficient flow rate there isn’t any benefit to using larger bore pipes that I can see.
Yes, this is pretty much what I had concluded looking at the calculator posted earlier. There's probably a (small) trade off somewhere, but it's doubtful as to whether that matters at all.
I was also pondering things like venturis in carburettors, and how those speed up the flow, and how this would apply to the water example I posted.
The venturi effect has always fascinated me as well. How there can be an increase in velocity, but yet there's always enough air going in on the input side - and how this would apply to water flow.
I just asked ChatGPT about that one and:
ChatGPT said:
Here’s how the Venturi effect works:
Constricted Area: As a fluid flows through a pipe that narrows (the constricted section), the velocity of the fluid increases because the same amount of fluid has to pass through a smaller area in the same amount of time.
Pressure Drop: As the velocity of the fluid increases in the constriction, the pressure decreases. This inverse relationship between velocity and pressure is a key aspect of the Venturi effect.
Return to Normal: Once the fluid exits the constriction and returns to the wider section of the pipe, the velocity decreases and the pressure increases again.
So I guess this is akin to squeezing the hosepipe which was mentioned earlier.Constricted Area: As a fluid flows through a pipe that narrows (the constricted section), the velocity of the fluid increases because the same amount of fluid has to pass through a smaller area in the same amount of time.
Pressure Drop: As the velocity of the fluid increases in the constriction, the pressure decreases. This inverse relationship between velocity and pressure is a key aspect of the Venturi effect.
Return to Normal: Once the fluid exits the constriction and returns to the wider section of the pipe, the velocity decreases and the pressure increases again.
It's certainly a fascinating subject.
EDIT
I just put my original question into ChatGPT (why didn't I think of that before asking here??!) and..
ChatGPT said:
Conclusion:
15 mm pipe will have the faster flow rate for the same pressure of 1 bar.
10 mm pipe will offer more resistance, reducing the flow rate and increasing pressure loss along the length.
The smaller pipe does not create higher pressure in the flow, but it does make it harder to achieve high flow rates due to increased friction and turbulence.
In practical terms, if you're looking for higher flow at the same pressure, you'd choose the larger pipe (15 mm).
So based on that answer, maybe 15mm bore would be better...15 mm pipe will have the faster flow rate for the same pressure of 1 bar.
10 mm pipe will offer more resistance, reducing the flow rate and increasing pressure loss along the length.
The smaller pipe does not create higher pressure in the flow, but it does make it harder to achieve high flow rates due to increased friction and turbulence.
In practical terms, if you're looking for higher flow at the same pressure, you'd choose the larger pipe (15 mm).
Edited by TonyRPH on Sunday 15th September 17:44
Whatever123 said:
Yes, but the question was on ‘fastest’ flow, not volume
volume will increase with a faster flow - as more will pass over a given time.Fluid dynamics is based around this fact.
a 10mm diameter pipe will pass 100 litres
a 20mm diameter pipe will pass 100 litres
100 litres being volume
so it follows for 10mm pipe it will need to pass twice the volume for a given time to compelte the task than what the 20mm pipe has.
Pressure loss in a pipe is proportional to the square of the velocity, and the absolute value is dependant on Reynolds number, as well as the characteristics of the pipe.
10mm id bore has about 1/2 the diameter of the 15mm bore, and therefore the pressure loss will be about x4 on the lower diameter pipe.
The pressure loss effects the flow, so the velocity of water within the pipe is effected both by the pressure loss, and the diameter of the pipe - to calculate the velocity of the water, one must first calculate the pressure loss in the smaller pipe, and thus the water flow, and then one can calculate the velocity. Since velocity is hugely influential on the pressure loss in the first place, this is going to be an iterative calculation.
It's a fairly straightforward 3rd year degree level calculation for fluid dynamics, which is normally studied in a mechanical engineering degree.
Ask me how I know.
10mm id bore has about 1/2 the diameter of the 15mm bore, and therefore the pressure loss will be about x4 on the lower diameter pipe.
The pressure loss effects the flow, so the velocity of water within the pipe is effected both by the pressure loss, and the diameter of the pipe - to calculate the velocity of the water, one must first calculate the pressure loss in the smaller pipe, and thus the water flow, and then one can calculate the velocity. Since velocity is hugely influential on the pressure loss in the first place, this is going to be an iterative calculation.
It's a fairly straightforward 3rd year degree level calculation for fluid dynamics, which is normally studied in a mechanical engineering degree.
Ask me how I know.
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