6 replies to this topic

[ban]

I have a CO2 cylinder from which CO2 is taken through a valve. I know the inlet and outlet pressures of the value. Also the outlet pipe (from the valve) diameter is known. How may I calculate the gas flowrate?

[gvdlans]

I have a CO2 cylinder from which CO2 is taken through a valve. I know the inlet and outlet pressures of the value. Also the outlet pipe (from the valve) diameter is known. How may I calculate the gas flowrate?

You can not. You need to know the valve Cv value at the particular valve position. The diameter of the outlet piping is not relevant.

Why do you want to know?

[ban]

Hi gvdlans,

Thanks for the comment.

I need to buy a gas flow meter. However without having an idea about the flow rate, i cannot select one. Let me simplify the question.

"The gas (CO2) flows through a polyurethane tube (6mm ID) and the inlet pressure of the gas is 5 atm (absolute). The gas is just discharged to the atmosphere at the outlet. The length of the tube is around 2m. How may I calulate the flow rate"


Thank you in advance

[latexman]

Most gas cylinders have a shutoff valve immediately followed by a pressure regulator/gage set. Can we assume that 2m of 6mm ID polyurethane tube is connected directly downstream of the pressure regulator/gage set and there is 5 atm absolute (atma) pressure showing on the downstream gage which is at the upstream end of the tube?

Have you completed your graduate level fluid flow and thermodynamics courses? This will help on future posts.

You have not mentioned the temperature of the CO2, and this is critical when dealing with gas flow. If the CO2 source is a high pressure cylinder where liquid CO2 boils off to generate the gas, the gas can get quite cold which will affect the density of the gas which may affect your choice of flow meter. How you use the system may affect the temperatures you will get. If you use the gas for a short duration and then let the system sit idle for a long duration, the liquid will stay close to room temperature. If you use the gas for long durations and let the system sit idle for short durations, the liquid will stay very cold and you may have ice accumulate on the cylinder and tubing (definitely if any tubing is metallic). There will also be a further temperature drop as the high pressure gas goes through the pressure regulator, it's called the Joules-Thompson affect. Temperature may not be important to you to solve this problem if it's like homework, but if you are sizing a real flow meter, it may be.

Pressure drop through the tube is 80% (4atma/5atma) based on inlet pressure. Polyurethane has some insulating properties and the tube is not long, so no appreciable heat transfer will occur with the surroundings. This is an adiabatic, compressible, pipe (tubing) flow problem. I'd recommend you look in your/find a good Thermo and Fluid Flow text. It's not easy and it's not difficult to solve. I'd call it medium. It's a two martini problem!

[ban]

Hi Latexman,

Thanx for the reply. The temp of the tube is -10C and the process can be assumed as an isothermal process. I calculated the flow rate based on modified mechanical energy balance to accommodate compressible flow. According to my calculations, I should expect a flow rate ca.150 L/min. I am not sure whether this value is a realistic value under these conditions.

Thanx again

[latexman]

I am not sure whether this value is a realistic value or not either. I ran my handy dandy isothermal compressible flow spreadsheet and there are several problems. The first is you gave a volumetric flow rate and did not verify where (i.e. what pressure) that flow applied. I assume you meant at the end of the tubing. This can be averted by stating a mass flow rate. The second problem is there is choked flow. With Mach 1 at the exit, there is a shock wave with an oblique expansion wave. This means there is a pressure discontinuity. My program reports a 1.8 atma pressure upstream of the shock wave, and, as you know, a 1.0 atma pressure downstream of the shock wave. So, what pressure did you use for the volumetric flow rate? Now, are you looking for a flow rate of the system without a flow meter (i.e as is), or with the selected flow meter? Because, as soon as you add the flow meter, it will change the maximum throughput of the system. And, specifying a flow meter for sonic flow is something I have no experience in, but I suspect that is a 3 martini problem in itself!

Btw, I got 83 kg/hr for:

P1 = 5 atma
MW = 44
z = 1
D = 6 mm
L = 2 m
roughness = 0 mm
viscosity = 0.0014 cP
P2 = 1.8 atma
P3 = 1.0 atma

[ban]

I am not sure whether this value is a realistic value or not either. I ran my handy dandy isothermal compressible flow spreadsheet and there are several problems. The first is you gave a volumetric flow rate and did not verify where (i.e. what pressure) that flow applied. I assume you meant at the end of the tubing. This can be averted by stating a mass flow rate. The second problem is there is choked flow. With Mach 1 at the exit, there is a shock wave with an oblique expansion wave. This means there is a pressure discontinuity. My program reports a 1.8 atma pressure upstream of the shock wave, and, as you know, a 1.0 atma pressure downstream of the shock wave. So, what pressure did you use for the volumetric flow rate? Now, are you looking for a flow rate of the system without a flow meter (i.e as is), or with the selected flow meter? Because, as soon as you add the flow meter, it will change the maximum throughput of the system. And, specifying a flow meter for sonic flow is something I have no experience in, but I suspect that is a 3 martini problem in itself!

Btw, I got 83 kg/hr for:

P1 = 5 atma
MW = 44
z = 1
D = 6 mm
L = 2 m
roughness = 0 mm
viscosity = 0.0014 cP
P2 = 1.8 atma
P3 = 1.0 atma

Hi Latexman,

Thanx again for your reply. My calculations gave me around 51 kg/hr. Friction was taken into account. I feel itz reasonable since it is very close to what u got and wht is given by processcalculator.com.

The development of a shockwave is still a concern. I didnt install a flow meter yet. I want to select the best one. Due to constraints I gonna buy a VA flow meter. If I know the flow rates roughly I can choose the correct range. I may buy a flowmeter which can measure the range from 20-200 L/min (@ATP, air equivalent). It would hopefully accommodate my flow rates.

Meanwhile, I will reconfirm my calculations using an existing flow meter which can measure upto 20 L/min. It would not give 100% confidence as it coundt be employed to my requirments. However I can be confident on the calculation, at least.

Thanx again.
Ban