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ventilation planning |
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Ventilation Planning
The relationship between ventilation pressure and quantity follows a simple square law, pressure is proportional to quantity squared, or for an airway pressure drop equals a constant, known as the resistance, times quantity squared.
By measuring pressure and quantity flowing in airways underground, values of resistance can be calculated for all individual lengths of roadway or for examples of roadway which are "typical" for a particular mine, seam or area. Alternatively, typical resistance values can be calculated for given airway types and sizes using standard "friction factors" affected by the roughness of the surface, etc.
Using the measured, calculated or assumed values, a ventilation model for the mine can be built-up as the combined resistance of roadways connected together in series or parallel, and the overall mine resistance can be calculated. Additional resistances have to be added to allow for shock losses at changes of direction, junctions, etc.
Once a model is constructed, the airflows which would result from a given fan can be calculated if the fan performance curves are known. Alternatively a fan duty can be calculated to provide desired flows around the mine based upon the determined mine resistance.
The most accurate models will be those which use actual measurements rather than typical or theoretical values.
"Solving" such a model for a large mine is a very arduous and time consuming process if done manually, especially as it can only be done by guessing a result then following an iterative process closer and closer to a correct solution.
Fortunately there are now computer programs available to carry out this process once the model has been constructed, Ventsim being the most widely used in Australia. Such modeling is done separately from mine planning models, but must be done in conjunction as both need to model the same mine layout and results from either may require modifications to the other.
Creating an initial model is a time consuming exercise, requiring ideally a large amount of underground measurement, reduction of results and construction of the model in the computer. For a number of reasons it is seldom that a model matches exactly the conditions in the mine and most models include some degree of "adjusting" often based on little more than intuition and the fact that it works (unless one is prepared to invest a large amount of time and money on very detailed measurements). To this extent, modeling has aspects of being an art rather than a science.
One difficulty in obtaining accuracy is that measurements normally have to be taken when the mine is working, at least to some extent. The measurements take a considerable time and most mines cannot afford to stand while such measurements are taken. In addition, a mine is continually changing – panels extend and retract, longwall faces may have chocks back or advanced to the face, face ends may be open or partly blocked with coal or brattice, roadways deform under stress, mobile machinery moves around, etc. Differences may be small, but it is unlikely a mine circuit is the same from one day to the next. Even the weather will have an effect on air density.
The degree of accuracy and detail has to be decided as a lot of time and money can be spent measuring and modeling detail which is not really necessary. It is often the case that projections of an order of magnitude rather than absolutely correct values are required, especially since future layout details more often than not differ from original plans.
At times complex sections of mine workings can be represented in a model as a single resistance roadway without including all the detail, provided there are no changes within that section. The pressure drop in such a section is easy to measure, but a representative flow may be difficult to define unless it is constant throughout.
Once a model has been constructed, it is important it be kept up to date, even if not being used. If this is done, it is ready for instant use if required. If not kept up to date, major work may be needed before it would be of use, a factor that could have serious consequences in the event that it was required to examine options in the event of a mine emergency.
Method of calculating ventilation requirements in tunnelling & dead ends – method of calculating auxiliary vent requirements – not free information, to be purchased.
IngentaConnect Modelling of auxiliary ventilation systems – not free information, to be purchased.
Paper on research programme to develop a real time vent monitoring & control system - does not result in an operational system, but progress towards one.