An important in part of creating ventilation models in Ventsim™ is to check or audit the model before final release or use. A ventilation model may consist of thousands of airways each with dozens of different attributes. If an attribute such as a friction factor or airway size is incorrect or inconsistent with similar airways, this can create a string of problems in the model resulting in inaccurate airflow and pressures.
Often, these problems are inadvertently included in models without the full knowledge of the user, particularly when the model or parts of the model may have been done by someone else. A system to inspect for common inaccuracies is therefore an essential part of any design process.
A simple system is to create a list in Excel of common items you need to check to assist with model validity. This creates a formalised way of auditing model parameters that may have been incorrectly inputted or forgotten about.
Create a list of important attributes that influence airway resistance. Airway resistance is the single biggest cause of model inaccuracies. Common attributes include (if not using directly measured resistances)
- Friction Factors
- Airway Size and Area
- Shock Factors
If measured resistances are being used, Linear Resistance is a good check.
The easiest way to check attributes is to use the attribute COLOURS to display variations on screen. Airways that are inexplicably different in colour than other airways are easy to spot, and the cause of the changed attribute can be investigated. The example below shows changes in friction factor that require further investigation.
Incorrect Fan Curves are another source of inaccurate models. They should always be checked for accuracy and authenticity.
- Check the fan curve against the manufacturers fan sheet.
- Ensure the correct pressure type is specified for the curve (Fan Total or Static Pressure) and that the simulation matches the fan curve.
- Ensure the blade setting for the fan curve in Ventsim™ matches the blade setting of the fan actually being used in the mine.
- Ensure the fan curve density matches the specification in the manufactures fan sheet, and the model air density is set correctly in the Settings.
- Fixed Flows are not a good idea in a final model. Only use fix flows to represent variable regulators if possible – check these with the “Restrict Only” option to allow the simulation to report problems.
Heat and Moisture
Thermodynamic simulation can be a complex modelling process, and for good accuracy requires many checks. Some of the more important ones are: Simulation > Environment. Make sure the following global items are checked:
- Surface temperatures
- Rock parameters and geothermal gradients.
- Wetness Fractions (specify this for each airway if the mine wetness is variable)
In the underground environment, the following factors should also be checked for each airway. Again, Colouring by Heat Attribute is a good way to examine for variations.
- Exposure Age – this is critical for accurate heat flow from rock strata – newer exposed airways are more critical to get right than ones that have been exposed for many years.
- Equipment Heat – Ensure the heat specified for machines is the average utilised heat output, not the maximum output. A common mistake is to include maximum engine power from diesel machine – this must be the average output and can be better estimated with the fuel consumption calculator in Ventsim™.
Require the accurate estimation of power costs, mining costs and airway useful life. If these factors are not correctly input, size optimisation recommendations in Ventsim™ will not be correct. Use the Mining Cost Calculator in the TOOLS menu for more accurate mining cost factors if you are using the QUICK or GLOBAL optimisation methods.
Finally, the Simulation Parameters should be checked for the type of simulation required. The RUN > SUMMARY sheet provides an overview of some of these.
- Airflow Compressibility (normally turn on, unless validating against a Ventsim™ Classic or VnetPC model)
- Airflow Natural Ventiltion (normally turn off, unless an accurate heat simulation model has been done)
- Simulation Accuracy – normally set to BALANCED for general use, or HIGH if simulation for final reports. This will force the simulation to achieve a higher level of convergence (at the expense of greater simulation time)