OT: CO2 Instrumentation

[TheWaterboy]

I have to build a CO2 flow controller. We have a borrowed unit that uses 3-15 regulated CO2 through an I-P Controller to a Badger Meter valve.
This existing system comes right off the Cryogenic tank so it will see pretty cold gas when flow gets high.
I need to create this system with new parts and add a flowmeter.
Flows are calculated to be 0-100 SCFM but mostly around the bottom half of that. Pressure is under 100psi

Anyone done this and can steer me toward device choices? The existing unit works so I could just duplicate those parts but what gas flow meter would you recommend? And what about those combination flowmeter/ controllers like Brooks Mass/Flow ?

The cryogenic temp is what I puts me in unfamiliar territory. there are evaporators in front of this to flash it to a gas but stuff sometimes happens and I would not want liquid CO2 droplets to destroy an expensive flowmeter.

I appreciate any input you are willing to share.

 

[iant]

interesting problem, Have you contacted the GAS supplier as they may have the best idea of equipment supply. sorry I can't help

 

[TheWaterboy]

Currently this is all they provide, i.e. no flow metering, only proportional valve control. The tanks we use now are small 800 lb tanks and the company that supplies the small tanks like this don't know anything about the metering device. it was provided by a larger volume dept of the same company and apparently they don't talk to each other. Its wierd like that.
We rent all this and are designing a larger system with a big cryo tank. I suppose when they get a PO for the tank they might share some info but I dont want to wait that long.

 

[iant]

I am a SCUBA Diver with Qualifications to advanced (up to 100%)oxygen mix.
I know that different seals etc. are needed for 02 but have no idea for C02 - Maybe a differential (orifice) type flow device may suit the application

 

[leitmotif]

Make darn double sure your evaporator is suitably oversized.

Definitely put in a liquid gas separator. This could be as simple as a tank where liquid falls to bottom and gas is taken off the top. A float switch could be used to sound a high level alarm. Most common example is the receiver on an air compressor.

Since you are starting with cryogenic liquid and evaporating to gas you afe going to have very cold temperatures. A common problem in refrigeration mechanical tubing fittings is water collecting and icing thus damaging fitting. A good commercial refer outfit can help with this question and concern.

Last but not least is the flow rate measurement. The simplest is to use a rotameter with one side referanced to ambient pressure. Above (or below) ambient you will have to correct for pressure. I suspect you will also have to correct for gas temperature also.

Dan Bentler

 

[TheWaterboy]

Like the accumulator in an air conditioning system... I didn't think about it like that, good call.

I need PLC monitored flow and control so a simple rotometer will not suffice.

 

[danw]

Your 100 scfm flow rate is ~2850 slpm.

Thermal dispersion's killer app is clean dry gases, which is what you've got, and thermal dispersion is an inferred mass measurement providing an scfm measurement.

A look at their web site seem to show that the Brooks thermal dispersion meter/controllers max out at 1,000 slpm.

Other manufacturers have flow meter models to handle larger flow rates, either in-line or insertion: Thermal Instrument, Kurz, Sierra. All these are mass flow rate meters only, not controllers. You'd still need a controller, I/P or positioner, and a valve.

Coriolis will handle the flow rate measurement with the same caveat - flow rate measurement only; control components are additional.

You didn't say what measurement technology is in place now. Do you know?

 

[TheWaterboy]

First thank all of you for the help with this.

It's used for pH reduction and the only measurement available is the resultant pH change downstream. We simply apply more CO2 until we get what we want on the far end. No way to know how efficient it is with the current system except by measuring the volume of tank used... and that is not very precise. To be fair to the supplier this was an experimental system and was never meant to be here as long as it was. it just worked too well to stop using it.

We do have a Brooks combo unit on our nitrogen and it works really well but that flow is much smaller, on the order of .5 scfm.

I dont mind having two separate devices. We are 24x7x365 so whatever I use I will have to have a spare. A combo unit simply means one spare device instead of 3 and easier replacement.

What about the temperature/liquid issue. Is that a condition that must be avoided at all costs, i.e. huge evap and accumulator, or are there instruments that can handle it ?

 

[rootboy]

We use Thermal Instrument 9500's in a paint mix application, and they have performed flawlessly for five years now.

We chose them when we switched to non-conductive paint (goodbye wafer mags), and had to have something that wouldn't interfere with the paint (the paint will stick to protrusions and become gummy, not good for a paint job).)

Be sure to follow the manufacturer's specs when it comes to input and output (straight) pipe lengths. They are sensitive to non-laminar flow.