how can i convert the mas flow?

[shahr22]

hi
I have problem in converting the mass of gas to volume in a certain period of time which is transfered from a chamber of higher pressure to medium scale vacuume chamber.
can any boady help me or introduce software to solve my problem.
thanks
shahr22

 

[ChasBean1]

shahr - can you be more specific with your question?

 

[shahr22]

Thank you for your consideration chasbean1
in fact i want to calculate the mass of an ideal gas passing through a pipe with initial conditions of pressure and tempratur to a chamber with lower pressure (in certain period of time)ie. i have the mass, pressure,temprature known.

 

[Haf]

If you have a relatively large chamber and the period of time is relatively short, you can approximate the mass flow rate as constant over that time period. Using the ratio of the back pressure (Pb) to the tank pressure (P), you can determine the Mach number using isentropic relations. If the ratio is less than 0.52828, the flow is choked and you can use Fliegner's formula to calculate the mass flow rate. If not, you'll have to use the more complicated formula for mass flow rate (not difficult, just a bigger formula). You'll need pressure (P), temp. (T), area (A), and Mach number (M), along with gas properties (the gass constant (R) and the ratio of the specific heats (gamma)). The formulas are

Pb/P = (1+((gamma-1)/2)*M^2)^(-gamma/(gamma-1))
Mass flow rate = P*A*M*(gamma/(R*T))^0.5

Notice there are two roots when solving for Mach number. You want to use the positive subsonic root. Remember, if Pb/P < 0.52828, the flow is choked (M = 1). You cannot have supersonic flow, since you don't have a converging/diverging section.

If you're pipe is relatively long, you'll have to consider frictional losses. This can be done using Fanno flow analysis, which complicates things a bit.

If your chamber is not large and the time period is not short, I would suggest setting up a simple forward difference program that uses sufficiently short time steps to assume constant pressure over that time step. Since you know the mass in the tank, you can calculate the mass lost in each time step and use the new mass to calculate a new pressure (using ideal gas formula) which drives the next time step. Then you can still use the formulas above.

Good luck.

Haf

 

[DeltaCascade]

The old &quot;engineer in a box&quot; software request?

At first, your question seemed to deal with simply converting gas volume to mass units. However, it appears to be a fluid hydraulics analysis question.

As Haf indicated, it gets more complicated when friction losses come into play. Since you stated that the fluid flows through piping from one vessel (chamber?) to the other, frictional losses will likely be significant.

Hydraulic fluid analyses (pressure drops and flowrate calcs) are pretty standard for mechanical and chemical engineers. You may want to review fluid mechanics texts, or Perry's Handbook for Chemical Engineers, or perhaps Mark's handbook for Mechanical Engineers, or, Crane Technical Paper #410 (available for purchase from their web site). With an understanding of the problem (bournelli's equation, fanning friction factors, etc), software such as PIPENET from Sunrise Systems, or a host of others, can be purchased (Crane has a simple and inexpensive package), although you should likely just do it &quot;by hand&quot;.

If you provided all the details, you may even find a fellow tipster here that may be set up to crunch the numbers for you, as a demonstration. You would need to describe the gas composition, inlet and outlet pressure and temperature, piping configuration (ID, material, length, valves, # bends, elevation change, etc), and one could then calculate the flowrate. If it is not steady state, iterations could be done as Haf suggested, using the appropriate methodology.

Is this a lab bench-scale operation or an industrial-scale steady-state operation? Need more detail.

Good luck.