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?
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
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
"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
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
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
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
"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
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
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
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
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
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
TTFN
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RE: Calculating pressure change in a vacuum pipe system
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