Can someone check my work, please? 4

[gt5pilot]

This is a bit out of my area:

For air @ 80F:

d = 1.5mm (dia lumen)
L = 15cm (lumen length)
mu = 2.07 Pa.s
rho = 0.9996 kg.m^3
V = 1 m/s

Re = (rho * V * d)/ mu

Re = 0.00072 - is that really right?

Q = (V * pi * d * d)/4

Q = 1.77E-06 cc/min

That would mean a pressure loss of:

delatP = (128 * mu * L * Q)/ Pi * d * d * d * d)

= 4.42E+6 Pa or 640 psi - something seems off...but, I did say this is not my comfort zone.

Thanks for any help you can give me.

Kirk


Kirk B Olson
Senior Principal Engineer
HEII, Inc.

 

[desertfox]

HI gt5pilot

Well if your formula are correct then your number crunching is also correct.

regards desertfox

 

[cah2oeng]

Uh, you may want to check your dynamic viscosity. I think you're about 5 orders of magnitude too large. Looks like that'll solve your "seems off" problem.

 

[gt5pilot]

That's what I get for pulling values from a table - I had input them into a spreadsheet had missed another needed conversion to get the numbers into Pa.s. Thank you for the "knudge"!

Kirk

Kirk B Olson
Senior Principal Engineer
HEII, Inc.

 

[gt5pilot]

BTW - 0.00001827 Pa.s...

Kirk

Kirk B Olson
Senior Principal Engineer
HEII, Inc.

 

[katmar]

As you have already seen, the viscosity number was wrong.

The units for your Q (flow rate) are wrong. The value of 1.77E-06 should have the units m3/s. If you want it in cc/min the value is 106. Assuming velocity of 1 m/s is correct?

The Reynolds number (if visc = 2.07E-5 Pa.s) is 72. This gives you laminar flow and fortunately you don't have to worry about the surface roughness to calculate the friction factor (which is implicit in the equation you used).

The pressure drop should be about 0.044 kPa (=44Pa =0.0064PSI =4.5mmH2O). This ignores entrance and exit effects, which will be very small at these low velocities.

If you plug the correct values into your formula you should get the correct answer (I have not checked your formula for consistency of units). At these low pressure drops you can safely ignore the complications of compressible flow - from your density and temperature I infer that you are working close to atmospheric pressure.

 

[MariusChE]

Hi g5pilot

You are using SI units. Therefor you'd need to get your lengths all into metres: 1mm = 0.001 m. the Pa.s is the correct unit of measurement for viscosity. The air viscosity is given by: AIR_VISC = -1.555E-14 * T ^ 3 + 0.000000000095782 * T ^ 2 + 0.000000037604 * T - 0.0000034484 (T in K). For that density, the air temperature is around 345K and the viscosity then should be around 2.00E-5 Pa.s. The Re = 75 i.e. laminar flow. The volumetric flow MUST be m^3/s = 1.76715E-06 m^3/s. Hence, the pressure drop is (using the formula that you gave) equal to 42.6 Pa.

remove.marius@mailbox.co.za

 

[MariusChE]

It would seem that Katmar and I posted at the exact same time. We got the same answer, so I guess that it has been "independently verified" ;>

remove.marius@mailbox.co.za

 

[katmar]

Hi Marius,

I hope the members of this forum don't get suspicious and think that we South Africans are conspiring together?!?

On a more serious note, I firmly believe that South African engineers have a huge advantage in that we are forced to work in SI units (mostly), but so many of our information sources are in US Customary or old metric units. This makes us so much more aware of the potential problems with units - although I have to admit I do still get tripped up now and then.

Harvey

 

[MariusChE]

Hi Katmar

Nice to see a familiar face...

Marius

remove.marius@mailbox.co.za

 

[IRstuff]

That's one of the reasons I've almost exclusively do that type of calculation in Mathcad, since the unit conversions are done automatically and in the background.

The other benefit of using such a program is that dimensional analysis also comes automatically; if your final units are not correct, you'll know that something got inputted incorrectly.



TTFN

 

[25362]

Checking tabulated air properties at atmospheric pressure I found that at 80 deg F the values are:

density: 1.1774 kg/m³
absolute viscosity: 1.983*10^-5 kg/(m.s) = 1.983*10^-5 Pa.s

The values given by gt5pilot seem to better suit a temperature of 170 deg F rather than 80 deg F. Any comment on the accuracy of my source ?

 

[MariusChE]

IRStuff

Aren't you the people who caused the first known traffic accident on Mars? ;>

M

remove.marius@mailbox.co.za

 

[IRstuff]

Nope, and it wouldn't have happened if they used Mathcad ;-)

Additionally, the failure of the interface was merely the tip of the iceberg. There were many occasions during that mission that the controllers were given indications that the probe was misbehaving and all the data was ignored until the probe was lost.

TTFN

 

[MariusChE]

Never ceases to amaze me: it was the scientists who landed the men on the moon but the engineers who caused Apollo 13 to "misbehave"; as you euphemistically put it.

Let me quickly see if I can get this forum activated: When are you Americans going to do do away with the dollars (or was it punds) per square thumb (or inches or something) system and get stuck into REAL units of measurement i.e metres and pascals and so on. I honestly do not know how any civilised nation can go on engineering in 12.34556 whatevers to 1.808748 whatever elses. Lesson 1 in SI: 1000 milimetres = 1.000 metre. EASY!!! see?

remove.marius@mailbox.co.za

 

[zdas04]

In another forum a member from the UK said his "... grass grew over an inch a week".

Metric countries - please tape up your glass houses prior to tossing stones.

David

 

[phex]

actually, UK is more or less the origin of those other non-SI units (i just remind you of "british thermal unit"). so, in this aspect, he wasnt tossing stones from INside the glass house, but actually from the outside. just my two cents.

 

[IRstuff]

Well, that's sort of the simplistic scenario, and if we were still stuck using slide rules, that might be an issue.

However, we now have computers and programs that can do the conversions, so the argument is quite moot.

Once "c" is entered as 2.99792458*10^8 m/s, it matters not whether you use it that way or use 1.80261750*10^12 furlong/fortnight (courtesy of v12, which includes both as built-in units).

So, the notion that we "must" use SI units, because it's somehow "easier" is moot, because the software has made that argument irrelevant and outdated.

Moreover, since real things are rarely that neat, 1 atm is 1.0132*10^5 Pa, so conversions are inevitable, no matter what unit system you use.

TTFN

 

[Cockroach]

Guys....Guys....Guys....let me remind you of the significant contributions America has made to our global civilization. Enough said!

On the subject of units, who cares if imperial or metric is the superior choice. I think this is a personal decision and is a reflection of an individual comfort zone. Ultimately, the answer should be EQUAL following system unit conversion. Just remember that mass is measured in "slugs" in the imperial, not poundal, pounds or otherwise.

Engineers should be equally comfortable working through the mathematical physics in either system. I can just hear Issac Newton groaning....

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[katmar]

The argument over which system of units to use is a very emotional issue, and there are probably only two factors that you claim with any degree of certainty.

Firstly, the system with which you are most familiar will always seem the "best"

Secondly, anyone who has seriously switched over to SI will never want to go back. If you want to go back, you have not made the switch completely.

Cockroach said "Just remember that mass is measured in slugs in the imperial system, not poundal, pounds or otherwise". I wonder how many people using the Imperial system on a daily basis know that this is why you have to include the gc factor in some equations?

It is great that computers can take care of the units for us in so many instances, but like anything else that the computer is doing for you it is best if you have the understanding to do it by hand if you had the time.