Pipe Flow Calcs

[Fastrotor]

Hi Guys, this one is for the mechanical engineers and techos out there.

I'd been struggling for a while to figure out in my head whether gas flow in a pipe would drop more pressure when hotter than cooler.

It seems hard to find formulas to work out pressure drop that include temperature in the calcs on the net..... but i found one site that i took the time to work out all the values to get a resonable result.

http://www.efunda.com/formulae/fluids/calc...iction.cfm#calc

here are my entered values :

Inputs

Pressure at A (absolute): 39 (psi)

Average fluid velocity in pipe, V: 110 (m/s)

Pipe diameter, D: 6.35 (cm) (2.5")

Pipe relative roughness, e/D: 0.000708661 (M/M) (stainless steel)

Pipe length from A to B, L: 6m (4m actual plus 2m for bends effective length, a guess)

Elevation gain from A to B, Dz: 0

Fluid density, r: .815 (160deg c)

Fluid viscosity (dynamic), m: 0.02464397 (cP)

Answers

Reynolds Number, R: 2.31 × 105

Friction Factor, f: 0.0196

Pressure at B: 37.7 psi

Pressure Drop: 1.33 psi

Volume Flowrate: 348 l/s

Mass Flowrate: 0.284 kg/s

When i change the temp of the air to 100 deg, (ie change Fluid density and Visosity numbers to .946 and 0.02210196) then i get the following result

Answers

Reynolds Number, R: 2.99 × 105

Friction Factor, f: 0.0193

Pressure at B: 37.5 psi

Pressure Drop: 1.51 psi

Volume Flowrate: 348 l/s

Mass Flowrate: 0.330 kg/s

NOW..... an explanation what this post is about..... i have an intercooler in an R33, using std piping route, then on to front mount.... so therefore one side is alot longer than the other side piping. My hot (compressor) side is the long side, going to drivers side endtank... cold side comes off P/S to plenum.

I wanted to prove mathematically what would be the best side to have each on... (ie hot = short or longest for LEAST pressure drop)

Now, going on thes calcs, it seems i have got it right, as for a given volume flowrate and pressure, a drop in temperature(the intercooler) increases mass flowrate and increases pressure drop for given pipe diameter and length.

I assume this is why some guys have 2.5" to cooler and then 3" to plenum ???????

Any techos want to comment, ? .... Have i got it right ?

Gary

[djr81]

You are probably thinking about things arse about.

The mass flow rate should be constant.

For a given pipe diameter & a given mass flow rate the hotter the gas the higher the pressure drop per unit length.

So to answer the question the hot side suffers more pressure drop.

But there are more factors to worry about (eg throttle response) than which side is hotter.

[Fastrotor]

You are probably thinking about things arse about.

The mass flow rate should be constant.

For a given pipe diameter & a given mass flow rate the hotter the gas the higher the pressure drop per unit length.

So to answer the question the hot side suffers more pressure drop.

But there are more factors to worry about (eg throttle response) than which side is hotter.

I think i got far to technical in the first post, so i rewrote it to ask what i want to know directly below

OK, heres the scenario.

You have a turbocharger pumping into piping , then into the cooler then on to the plenum.

Where is a better position for the cooler, closer to the turbo or plenum ? Assuming overall pipe length and size had to stay same. ie Long hot side, short cold OR vice versa.

For example

TURBO------#INTERCOOLER#-----------------------------PLENUM

OR

TURBO------------------------------#INTERCOOLER#-----PLENUM

Which configuration would effect the least OVERALL pressure drop ??

ps Before anyone says your constrained by physical location on vehicle anyway, my piping both come from the P/S, there for you have the option of short or long pipe run.... so i want to know if hot compressed gas drops more pressure than the cooler but similar pressure gas

So djr81, your saying in effect the the cooler closest to the turbo is best ??

Gary

Edited January 22, 2009 by Fastrotor

[djr81]

I think i got far to technical in the first post, so i rewrote it to ask what i want to know directly below

OK, heres the scenario.

You have a turbocharger pumping into piping , then into the cooler then on to the plenum.

Where is a better position for the cooler, closer to the turbo or plenum ? Assuming overall pipe length and size had to stay same. ie Long hot side, short cold OR vice versa.

For example

TURBO------#INTERCOOLER#-----------------------------PLENUM

OR

TURBO------------------------------#INTERCOOLER#-----PLENUM

Which configuration would effect the least OVERALL pressure drop ??

ps Before anyone says your constrained by physical location on vehicle anyway, my piping both come from the P/S, there for you have the option of short or long pipe run.... so i want to know if hot compressed gas drops more pressure than the cooler but similar pressure gas

So djr81, your saying in effect the the cooler closest to the turbo is best ??

Gary

The calcs you used were for incompressible fluids ie liquids, not gases & your assumptions were wrong. Apart from that the general principles are the same.

As I said for a given mass flow (constant in our case), a given pipe diameter (Constant) & the various other assumptions you can make about the pipework you can say the following:

The hotter pipe will suffer from more pressure drop.

So make the hot side shorter.

Better yet make both sides shorter.

Even better yet focus alot of effort on the inlet side of the turbo so it does not have to pump so much heat into the air in the first place.

[Fastrotor]

The calcs you used were for incompressible fluids ie liquids, not gases & your assumptions were wrong. Apart from that the general principles are the same.

As I said for a given mass flow (constant in our case), a given pipe diameter (Constant) & the various other assumptions you can make about the pipework you can say the following:

The hotter pipe will suffer from more pressure drop.

So make the hot side shorter.

Better yet make both sides shorter.

Even better yet focus alot of effort on the inlet side of the turbo so it does not have to pump so much heat into the air in the first place.

Thanks Djr81 ! Ok, i didn't notice they were for incompressible liquids, was just trying to work it out best i could given internet resources, and i am electroincs based

Appreciate your help.... so it will be worth while cutting and swapping the piping on my setup.

I guess another option would be to go to a downflow cooler.... reduce piping by about a meter !

Yes i know shorter is better , but i don't want to cut the D/S guard at this point.

The inlet to turbo is now full 4" with 4" * 8" long K&N pod. Have not made air box yet as i wish to prove i have eliminated power ceiling i am having trouble cracking. Dyno with bonnet up should negate most effect of excess temp into turbo. Once done i will fabricate a fully enclosed airbox, not just a half arsed "shield" as most seem to be.

Gary