Flow rates and psychrometry for gas mixtures in coal mine gas management systems

[Roy Moreby]

Hi

I am using ventsim to model various gas management systems in coal mines in which there will be variable mixtures of air and gas (CH4, CO2 and N2) at various pressures (down to -50kPa guage) and temperatures.

Due to the wide range in densities involved, I prefer to model on a mass flow basis.

Please clarify if the various Ventsim algorithms account for various gas mixtures when specifying mass flow rates and also if calculating moisture content from wet and dry bulb temperatures.

The temperature/moisture content aspect my sound pedantic but it is a significant issue in coal mine monitoring systems when calculating actual CO2-e emissions from sampling of "wet" or "dry" gas streams. At one mine this made a difference of about AU$1M per year!!!

Thanks

Roy

 

 

 

 

 

 

 

[Martin Griffith]

Hi Roy, you can make Ventsim calculate air densities from the gas mixture by turning on the option in Settings | Simulation | Gas | Use Gas Density for Air Simulation. With this on, the density in an airway will be calculated from the gas mixture present after a Gas Simulation. This density will be used in any subsequent Air Simulation, Heat Simulation and all of the psychrometric calculations for moisture content. I think this is what you are after. I'd be happy to take a look at this with you.

We've been trying to push people to use Pumpsim for this sort of application, but this wasn't possible since Pumpsim 3.1 couldn't do what the Ventsim Gas Simulation does. This has changed with Pumpsim DESIGN, which allows similar Gas Simulations to Ventsim.

Another comment on using Ventsim for this: the gas mixtures in the Gas Simulation are calculated for dry air. There is another setting, Setting | Simulation | Gas | Dry Air Gas Balance, with default setting of On. With this default setting, water vapour is ignored in the Gas balance (although it of course is integral to the psychrometric calculations). If you select No, then the software will calculate the amount of moisture in the air and then squash this into the gas mixtures; you will start to see a water vapour content in the Gas Simulation. This water vapour is not transported around the model in the same way as the other gas components, but is rather a straight function of the dry and wet bulb temperatures and pressure at the given location. 

 

[Roy Moreby]

Thanks Martin - sounds perfect

[Roy Moreby]

Hi Martin

Found a bug (I think)

When I run a model with air (i.e. no gas specified) and fix the flow rate on a mass flow basis it works fine

When I set the inlet to some gas mixture then fixing the mass flow produces a wrong answer e.g. I fix at 1kg/s but Ventsim changes that to 0.81kg/s

The mass flow rate of dry gas should stay the same if setting the mass flow as a fixed number i.e. if I have 1kg/s of CH4 then the quantity will change with pressure but the mass flow rate should not.

I also see that if I run vent first to balance the network, then gas then vent then repeat a few times again it keeps changing the mass flow rate

It also takes three complete iterations to get the mass flow right with the gas turned off

Thanks Roy