## effect of pipe size on efficiency

## effect of pipe size on efficiency

(OP)

I am a builder trying to understand one aspect of HVAC duct work design.

Non-engineering literature I have reviewed seems to not properly discuss the effect of pipe size on efficiency in delivering air through a HVAC system because it assumes the CFM is constant rather than the static pressure being the constant.

What I think I understand is that if two round pipes of different sizes carry gas (natural air) at the same static pressure the smaller pipe will deliver less CFM, but I believe the primary decrease in efficiency for the smaller pipe will come from the proportional increase in friction within the smaller pipe (e.g.: ratio of volume to wall surface area), is that correct?

If so, I would ideally like to find a formula or table that can compare the efficiency of BTUs delivered based upon pipe size ID, air temperature being carried, and distance.

A related question is whether if the distance is short (e.g.: laterals from the main trunk) does distance cease to be an important variable?

I am specifically interested in the efficiency of 2.5" ID smooth pipe versus 6", 7", and 8" smooth round duct, and if there was literature that reviewed efficiency as measured in the field that you could point me to then that would be a bonus!

Non-engineering literature I have reviewed seems to not properly discuss the effect of pipe size on efficiency in delivering air through a HVAC system because it assumes the CFM is constant rather than the static pressure being the constant.

What I think I understand is that if two round pipes of different sizes carry gas (natural air) at the same static pressure the smaller pipe will deliver less CFM, but I believe the primary decrease in efficiency for the smaller pipe will come from the proportional increase in friction within the smaller pipe (e.g.: ratio of volume to wall surface area), is that correct?

If so, I would ideally like to find a formula or table that can compare the efficiency of BTUs delivered based upon pipe size ID, air temperature being carried, and distance.

A related question is whether if the distance is short (e.g.: laterals from the main trunk) does distance cease to be an important variable?

I am specifically interested in the efficiency of 2.5" ID smooth pipe versus 6", 7", and 8" smooth round duct, and if there was literature that reviewed efficiency as measured in the field that you could point me to then that would be a bonus!

## RE: effect of pipe size on efficiency

Comparing flow rates with diameters basically looks like this,

https://external-content.duckduckgo.com/iu/?u=http...

Lesson #1 is use the largest diameters you can.

Lesson #2 flow rate vs pressure drop and distance. At low pressures, you need to keep pressure drop per foot to minimums, so you are running along the bottom of this chart.

A 4" pipe will give you 20x flow of a 1" over the same distance using the same pressure..

## RE: effect of pipe size on efficiency

By accident, you've posted in what is a rarely used forum designed for methane gas distribution, whereas next time try the HVAC engineering forum.

HVAC type distribution works at very low pressures (inches of water column). I assume this is what you mean by static pressure. HVAC also has certain velocities to avoid "whistling" or feeling a breeze.

The size of ducts and tubes is a balance between different factors including CAPEX, power, velocity, distance of pipe, available pressure.

For the same volume of air over the same length, the pressure drop varies by a ratio of somewhere around (D1/D2)^4

Friction is proportional to velocity squared. So especially in smaller pipe sizes, the difference in frictional losses between a 2" pipe and a 4" pipe is huge. Less so between say 16" and 20".

I'm confused though by your use of the term "efficiency"? Efficiency means the amount of one form of energy when it is converted.

I think you mean power consumed to transport a set volume of energy or air??

HVAC is a well worn path but each system is different so a simple table might not be what you need.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: effect of pipe size on efficiency

When I said "efficiency" I meant "energy loss pushing air through the pipe" as higher friction = higher energy loss = lower efficiency.

Thank you both!

## RE: effect of pipe size on efficiency

## RE: effect of pipe size on efficiency

## RE: effect of pipe size on efficiency

All I meant was that if you're looking for more info on HVAC, best to post it he HVAC forum - you'll get more answers.

Be aware that for hot air, all the friction losses from the air moving get transferred to heat so it's not "lost", but if you're moving cold air then it's not a great idea.

But HVAC is about lots of other things including low noise, low velocities at the exit and heat loss and gain from the outside.

Also a smaller amount of hotter or colder air may not mix well and lead to cold / warms spots in the room respectively. There are also practical limits on how hot or cold the air can be to not burn someone or start freezing the outside of the ducts or giving someone frost bite.

Like most things in engineering, it's all about getting the right balance between competing issues of cost, performance, noise and effectiveness.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: effect of pipe size on efficiency