Calculating Orifice Size

Try this Google search there may be something to help you.

Thank you for that simple direction. Not being a math whiz or an engineer, I was afraid I was dealing with something that required a magic formula. I will try to work through the formulas and see what I come up with.

Bill

What is the temperature?

Is the flow at actual or standard conditions? If standard, which one?

What is the design pressure drop across the orifice?

Is the orifice in a pipe (dia. ?) or a vessel wall?

You need all above to make a start.

Cheers.............Codey

Hi Codey,

As you probably can tell, this is not my area of expertise. I don't know about standard conditions, but I believe you are referring to temperature and pressures. Let me give you a little more detail.

I have a pressurized cylinder containing a small amount of liquid CO2. The cylinder is connected to a valve. The output of the valve is connected to a 5/16" aluminum discharge tube (ID ~ .2425 in), about 24" long. The other end of the tube is open to the atmosphere. The restrictor/orifice will be pressed or screwed into the end of the discharge tube where it connects to the valve and when the valve is opened the gas will discharge from the cylinder, through the valve and restrictor into the 5/16" tube, and into the open air.

The cylinder contains 16 grams by weight of CO2. My best math :) tells me that that's about .308 cubic feet of gas at 1 atmosphere. My understanding is that the pressure in the cylinder will be between 850-900 psi at room temperature (~70-80 deg F).

The intent is to be able to open the valve and discharge the entire 16g of CO2 in about 3 minutes, or at a rate of ~ .1 cubic foot of gas per minute. The rate is not critical - between 2 and 4 mminutes is fine. Is this enough info to determine the correct size of orifice?

Bill

You have a slight problem using an orifice. For a slow release,

1/ It will have to be very small at the pressure indicated.

2/ As it discharges the pressure will fall due to cooling of the CO2.

3/ There will be a tendency for a small orifice to ice up restricting the flow. Also as more of the liquid is used the colder the remaining amount will get due to thermal transfer restriction.

4/This will give an indeterminate discharge rate depending on temperature equalisation.

5/ You will probably have to experiment due to the large amount of variables it would be difficult to calculate as the calculation would have to have all these variables inputted into the calculation.These are derived by experimentation, so find a supply of small orifice plates and do just that.

Hi Bill

One or 2 things I'm not clear about.

You say 16g of CO₂ in the cylinder. In a cylinder most of the CO₂ is liquid as it is below critical temperature. Critical temperature = 31°C, critical pressure ~ 74bar. Your pressure 60bar corresponds to ~ 22°C so is about right. If the temp is maintained as gas is drawn off, the pressure stays at 60bar till all the liquid has evaporated, then decreases to atmospheric. Similar situation to a propane cylinder, but at higher pressure.

But as the gas evaporates heat is absorbed and the remaing liquid cools. Does the cylinder hold only 16gm? If so the cooling is likely to be substantial and pressure falls, as does flow through a fixed orifice. But if it's an ordinary-sized ciylinder and you draw off 16gm at a time, the remaining liquid only cools a little and the cylinder can warm back up to ambient ready for next time (if there is a next time).

Also the gas coming out cools. I believe dry ice (powder CO₂) is made by releasing gas from a cylinder, the gas solidifies and the solid is caught in a bag (liquid CO₂ does not exist at atmospheric pressure). It doesn't affect the orifice flow, but if I'm right, is this OK for what you're doing?

Assuming above is OK, I've calculated orifice to pass 16gm in say 3 minutes. Flow is critical until upstream pressure falls to about 2 bara.

If u/s pressure stays at 60bar, I make orifice dia. about 0.1mm. (mass flux ~ 10.5 gm/mm²/sec, discharge coefficient taken as 0.62). If u/s pressure is average 30bar, orifice dia. about 0.15mm.

But I'd be inclined to use a small needle valve instead of an orifice. Then you can adjust it to suit and trim if necessary during the 3 minutes. Valve Cv something like 0.002 wide open, turning down to 0.0003 to give design flow at 60bar.

Cheers........Codey

For 200 cc of gas at 60 atm pressure and a fairly long discharge time the oriifice will be tiny. A better option would be a capillary tube.

Bioramani

Good idea, the capillary tube [a long one ] could be sandwiched between finned heat sinks. This would tend to stabilize the temperature of the liquid giving a more constant discharge.

Talk to the folks at O'Keefe.

Thanks for the suggestion.

I did contact them by email and gave them the basic info of what I am trying to do. They responded by suggesting a particular line of devices, which is helpful, but unfortunately I need to figure out the size of the orifice I need and they didn't step up to the plate on that one.

I know it probably isn't too difficult for someone who works in this area, but for me it looks like a mountain. I don't know anyone I can call on for help in this area so I thought I would ask on this forum. Maybe it is more difficult than I realized.

Bill

Did you look at the charts O'Keefe provides?

You'd better ask Al Gore if he will allow the release of CO2 in such a manner!

JL Mealer
http://mealercompanies.com
America's Next Major Automaker

Yeah, I didn't think of that! Do you think the Carbon Police will get me???

Bill

hi dear,

u hav not provided the complete data for orifice calculations.if u would provide the following data, iw ould calculate it for u...

max gas flow in sft3/m.

Normal gas flow in sft3/m

flow temperature in F

inlet pressure in psig

Differential range in in h2O

cp/Cv ratio for the gas

viscosity @FTP in cp

Pipe inside diameter in inches (this is most important)

density @FTP in lb/ft3

all above listed data is required when u design or size an orifice.

when u will provide the required information,i will send u the size of the orifice.

cheers,

M.Naeem

Dear Mr. Naeem,

Can you please provide me orifice plate size based on the following information?

Maxmum gas flow 100,000 scfh

Normal gas flow 100,000 scfh

Flow tempeature 60 deg F

Inlet pressure 125 psig

Diff. pressure 30" water

Diff pressure normal 12.25 inches

Cp/Cv ratio 1.0002

Viscocity @ FTP 0.0189 cP

Pipe size dia 6.6.065 inches

Specific gravity 0.967

Material Nitrogen

M.W 28

Thanks,

Vas P. Kenyen

Exhausting a pressurized CO2 tank to empty is a transient compressible gas analysis. As the gas exhausts, the tank pressure drops which reduces the gas density and the exhaust flow rate. Depending on what your actual need is, there are a few approaches. The cheapest is an orifice sized to meet an exhaust time requirement, but which would yield a continually changing flow rate from maximum at the start to near zero at the end. The next is a pressure regulated exhaust that would approximately keep the flow rate constant while meeting the time requirement. The last is a flow regulator that maintains a near constant flow rate during the exhaust time.

For the first cheapest option, an off-the-shelf cheap ball valve (~$10-$20 USD) would work. Then do a couple of trials to find a a good setting which would meet the desired time requirement. But the flow rate would vary considerably while emptying the tank.

For an almost constant flow rate, an adjustable air pressure-regulator (~$20-$50) would work. Again, a couple of trials to get the desired exhaust time.

If an exactly constant flow rate is needed, then that is flow regulator territory, and gets relatively expensive (easily several hundred dollars or more). Such devices need to actively monitor conditions and adjust for pressure and temperature changes, thus the high cost.

If needed, an actual analysis can be done, but better definition of the actual requirements is needed. Required inputs are: tank volume, initial tank pressure, initial tank temperature, ambient pressure, and target exhaust time. Also whether a simple orifice (i.e. ball valve or orifice plate), pressure regulator, or flow regulator is to be used. If using a pressure / flow regulator, an additional input need is the target flow rate (volume per time, or mass per time). From that, a simple Excel sheet could be setup to do a pseudo-transient analysis of emptying a CO2 tank.

The problem is complicated by the fact that the pressure in the tank will initially decrease only as the temperature of the liquid CO2 changes due to evaporative cooling and will be the vapor pressure of the CO2 at the liquid temperature. This will also depend on the mass and specific heat of the tank via heat transfer. Once the liquid is all evaporated, the pressure will more rapidly drop and be a function of the volume of CO2 remaining in the and its temperature. The solution depends on how exact you need to be and if you desire the flow to be somewhat constant.

Heh, my bad. Missed the comment about the original state being liquid. That does complicate analysis considerably since phase changes, and associated cooling, need to be handled.

The idea of a precision needle valve (post15 - Codey) is probably the simplest approach. You can then play with various adjustments to get the flow and time you want.

The idea of an orifice is Ok if you can make one. It is no good being able to calculate the exact size if you do not have a suitable drill. Why not try a couple of standard gas jets made for cookers/blow lamps etc.

But you might need a pressure regulator (a proper spring/diaphragm type) in line to the orifice (upstream). This will hold the pressure fairly constant at the orifice and this will give a fairly steady flow.

good luck.

Google "Spraying Systems" for a nozzle sizing vendor. I have bought CO2 nozzle from them before...