effect of pipe size on efficiency 2

[Superchief]

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!

 

[1503-44]

You are correct, a smaller d pipe will have a lot more friction for any given flow. Friction is much dependent on velocity, which is proportional to pipe area, which is proportional to d. Half the pipe area doubles velocity and friction increases by the square of velocity. I guess you see where that goes. It is very important to keep velocity low. Use large d ducts when you can. Total friction is a direct multiple of distance, not squared, so not quite as important as velocity, but at low pressure, it is important as you don't have much pressure to play with. Using shortest distances will help there, as will large diameters. Larger diameters help the most.

Comparing flow rates with diameters basically looks like this,

https://external-content.duckduckgo...id=OIP.mm0VzxrOpptyqa3XYMhLwgHaC7&pid=Api&f=1

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..

 

[LittleInch]

Superchief,

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.

 

[Superchief]

Apologies for posting in the wrong forum - so the critical insight I gained is that if we desire to use smaller ducts then we need to keep the velocity low which can be done by delivering the target CF over longer time or by delivering warmer/colder air to decrease the CF demand, or both strategies.

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

Thank you both!

 

[1503-44]

That is called "pressure loss per unit of length", psi/ft, or bars/meter. Not efficiency.

 

[Superchief]

understood and thank you for helping me understand the terminology ... I was looking at it from a "how much electricity to run the fan" and if there is a higher pressure loss it means more electricity is needed to run the system, correct? That is what I was referring to as efficiency but I understand your point.

 

[LittleInch]

superchief

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.

 

[1503-44]

Yes it would generally require more energy to run at a higher pressure loss per foot. Or you might be able to almost do the same using a higher efficiency motor. That is what efficiency means. Getting more or better performance, useful work (per unit), with the same or lesser power.