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How Diameter Affects Pressure Loss

I've been doing a lab report and have come across the fact that the pressure Loss experienced in a 4mm diameter pipe is much greater than in a 7mm diameter pipe, even though they are the same length. I can see why this is in terms of the Darcy-Weisbach equation, but that doesn't give me the complete understanding. If anyone could help that would be great, cheers
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Original post by podrodow1
I've been doing a lab report and have come across the fact that the pressure Loss experienced in a 4mm diameter pipe is much greater than in a 7mm diameter pipe, even though they are the same length. I can see why this is in terms of the Darcy-Weisbach equation, but that doesn't give me the complete understanding. If anyone could help that would be great, cheers


might help or not but:

pressure = force/area

From this it's clear that the smaller the area the higher the pressure will be. Think of a water hose....if you open the tap fully the water will flow out at some rate but if you put your finger over the hose and block some of the opening with your finger the water comes out a lot faster i.e. more pressure (the area has been decreased).
Original post by podrodow1
I've been doing a lab report and have come across the fact that the pressure Loss experienced in a 4mm diameter pipe is much greater than in a 7mm diameter pipe, even though they are the same length. I can see why this is in terms of the Darcy-Weisbach equation, but that doesn't give me the complete understanding. If anyone could help that would be great, cheers


I'd imagine the friction factor would be much higher on a lower bore pipe. My hunch would be because the ratio of the surface area of the pipe wall to the volume of fluid passing through is much higher.
Original post by podrodow1
I've been doing a lab report and have come across the fact that the pressure Loss experienced in a 4mm diameter pipe is much greater than in a 7mm diameter pipe, even though they are the same length. I can see why this is in terms of the Darcy-Weisbach equation, but that doesn't give me the complete understanding. If anyone could help that would be great, cheers


If you look at the engineering Bernoulli equation, we express the mechanical energy of the fluid in terms of pressure, potential, and kinetic energies, plus a loss term (due to friction). The loss term increases with the square of velocity.

So, with a smaller cross-sectional area, the velocity needs to be higher to maintain the flowrate. Therefore the loss term is greater and this manifests itself as a pressure drop.

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