We often get asked by customers why the mine resistance of their models keeps changing in their Ventsim™ Summary when they change their model airflow. Mine Resistance is the combined total of the resistance of all air moving through the mine, and is a useful figure to know, because it can be combined with Atkinsons turbulent flow pressure equation P=RQ2 to assist in calculating how much additional (or reduced) pressure may be require to achieve a certain volume flow of air through a mine.
Because resistance is simply a function of airways size, length, friction factor and shock loss, you would think therefore mine resistance would be combined sum of all the parallel and series airways, and would be independent of airflow changes through the mine. However, airflow distribution which is altered by fans unevenly in different parts of the mine WILL AFFECT Mine Resistance in the same way as using regulators (ie additional resistance) to alter airflow distribution.
Consider two separate cases. A mine with a single entry and exit is adjusted for flow. If the airflow is doubled through the mine, and as expected the overall pressure will be expected to increase by a factor of four (4).
Now consider a case with a single entry, but two (2) exhausts. If we were to double the overall airflow in this mine by only mounting additional fans on one of the exhaust routes, we would require a much greater additional pressure than allowing the air to flow evenly through both exhausts. Effectively we have increased our mine resistance because we have changed the relative portions of airflow that can travel through each airway! The same net effect could be achieved by adding regulators to parts of the mine which would similarly change mine resistance.
In summary, Mine Resistance will only remain the same if the changed airflow proportions remain consistent through the mine. If we increase or decrease airflow through one part of the mine and not the other (with either fans or regulators), we also change the overall mine resistance.