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Calculating pressure change in a vacuum pipe system

Calculating pressure change in a vacuum pipe system

Calculating pressure change in a vacuum pipe system

(OP)
I have always used published charts for sizing vacuum piping (typically used for medical vacuum), but have never dug into the engineering behind it.

Most charts stop at 0.75" and says a 0.75" tube would have a pressure "drop" of 0.53 in Hg per 100 ft for 3 scfm. But i want to calculate the pressure change in a short piece of 0.5" L copper tube.

The starting pressure would be 14 in Hg with an allowable maximum pressure "drop" of 5 in Hg.

Could I use Darcy's equation for compressible fluids (assuming air as the fluid) or would i use something else?

RE: Calculating pressure change in a vacuum pipe system

First off, get rid of the "drop" terminology. IF you work in psia then flow is always from higher pressure to lower pressure and you are not trying to wrap your head around numbers increasing as a parameter is physically dropping. The traditional way to solve this is the Spitzglass Formula.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Calculating pressure change in a vacuum pipe system

What's wrong with "drop"?

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Calculating pressure change in a vacuum pipe system

(OP)
I can work with Spitzglass formula.

So, basically the way to envision it is, flow is from outside the system into the system. So initial pressure is atmospheric which will be approximately 29.9 in Hg, accounting for altitude.

Pressure inside the system is set at 14 in Hg, but since this is a vacuum (gauge) pressure, it has to be translated to absolute, which would be approximately 15 in Hg (absolute).

So I have my pressures, and my pipe equivalent length (with all the fittings and accessories added in). So I just plug into Spitzglass and it gives me a flow for the system that meets the requirements.

If it is greater than 3 scfm, i am ok. If it is less, then I exceed my pressure drop limits.

Does that logic seem correct?

RE: Calculating pressure change in a vacuum pipe system

I don't see it that way. You're not being at all clear.
29.92 in HG is standard atmospheric pressure at sea level and 69°F. That is an absolute pressure.

If your 14 in Hg is gauge pressure, then the absolute pressure inside the system is 29.92 + 14 = 43.9 in Hg absolute.
If your 14 in Hg is absolute pressure inside the system and (if) flow is from atmosphere to inside, then your pressure drop is 29.92-14 = 15.92 in Hg

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Calculating pressure change in a vacuum pipe system

Let's look at it from 0 psia instead of 0 psig. Atmospheric pressure is a function of altitude, lets say you are in Denver, CO and atmospheric pressure is 12.4 psia. That means that the high pressure in your system is 12.4 psia. If you have 14 inHg (6.876 psi) of vacuum, then the low pressure is 12.4 psia - 6.876 psia = 5.524 psia. So the numbers you put in your equation are P1=12.4, P2=5.524 psia. That really is a pressure drop (not quotes needed) and works with Engineering arithmetic.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Calculating pressure change in a vacuum pipe system

I understand that perfectly. The problems are that Ped didn't say what altitude he is at, or how he gets 15 absolute. If 14 Hg is gauge pressure, that makes no sense, unless maybe he's at 70,000 ft altitude.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Calculating pressure change in a vacuum pipe system

(OP)
The 14 in Hg is a gauge pressure read from a vacuum gauge so it would measure from 0 in Hg downward to ~-29 in Hg which would be a perfect vacuum. So the absolute pressure would be atmospheric + gauge, but gauge would be negative. So absolute pressure within the piping would be ~15 in Hg. I can make allowances for altitude to determine absolute pressure.

From zdas04's response, i think my approach is correct - I use atmospheric as P1 and the absolute pressure in the piping as P2 and go through Spitzglass to find if 3 scfm works.

RE: Calculating pressure change in a vacuum pipe system

No, you've got it backwards. A gauge pressure is referenced to atomospheric, not 0. Absolute pressures are referenced from 0.
Good thing you're in the 14-15 range in Hg, becuase at those values you get the wrong answer, but the right value.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Calculating pressure change in a vacuum pipe system

(OP)
But on a gauge, whether it is vacuum or not, isn't atmospheric pressure the reference point, or 0 psig. If the gauge reads 14 psig, then we add 14 psig to what atmospheric pressure is to get absolute pressure (psia).

With a vacuum gauge, zero is atmospheric and you subtract (or add a negative pressure) the gauge reading to get the absolute pressure.

I have never actually seen a real vacuum gauge, only specified them, but this link (http://www.anver.com/document/pdfs/components/gaug...) indicates to me the starting point is 0 and the needle moves counter clockwise to 30" which would be perfect vacuum.

Please let me know if this is correct.

RE: Calculating pressure change in a vacuum pipe system

The reading is zero; the reference is 29.92
29.92-14 = 15.92 absolute.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Calculating pressure change in a vacuum pipe system

BigInch,
I don't think so. Like PED says, any gauge at rest is at local atmospheric pressure (or zero dP for a dP gauge) That is why people report increasing vacuum with larger numbers instead of smaller numbers. Someone decided that negative psi units were too course and came up with inH20, inHg, mmHg, etc to indicate small changes in a capped number. At 2000 m elevation a perfect vacuum (i.e., all mass evacuated from a container) is 24.4 inHg lower pressure than their local atmospheric pressure.

To further confuse this already murky issue, the weather people pretend that atmospheric pressure is 29.4 everywhere (i.e., they calibrate zero psig as 29.92 inHg) then they add or subtract the effects of high and low pressure fronts. Doing it properly would have "normal" pressure in Vale, CO, lower than the the pressure in the eye of a hurricane which is not what people have come to expect.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Calculating pressure change in a vacuum pipe system

Vacuum pressure is almost always in absolute units. 1E-6 Torr, which corresponds to 1E-6 mmHg, does not refer to an absolute pressure of 759.999999 mmHg; it is 1E-6 mmHg absolute pressure. The alternative just doesn't make sense to measure in that fashion.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Calculating pressure change in a vacuum pipe system

Not 29.4, it's 29.92 A perfect vacuum in Vale (if it is at 2000 m elevation) is 23.4749 in Hg (on a day with a Standard Atmospheric composition above).

29.92-14 = 15.92 in Hg absolute (positive) is correct for the OP's interior pipe pressure, if the atmosphere above him is the Standard Atmosphere and he's at 0 msl.

NASA's standard atmosphere model.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.g...
see page 52, mb are converted to in Hg courtesy of Uconner.

0 ft 29.92126 in Hg
6,561 ft (2000 m) 23.4749 in Hg
11,000 ft 6.683245 in Hg
20,000 ft 1.616734 in Hg
32,000 ft 0.2563258 in Hg

"Doing it properly would have "normal" pressure in Vale, CO, lower than the the pressure in the eye of a hurricane which is not what people have come to expect. " People should expect the unexpected, because normal pressure in Vale is certainly lower than the eye of a hurricane.

http://www.ncdc.noaa.gov/special-reports/wilma.htm...
Hurricane Wilma on 19 Oct 2005 reached a pressure of 882 mb = 26.0455 in Hg

It's all relative. Above 18,000 feet, FL180, aircraft set their altimeter readings for 0 msl to 29.92 in Hg, no matter what the pressure, or temperature is outside, or on the ground below.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

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