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Understanding Fan Curves in Ventsim

Ventsim is a powerful program to model mine ventilation, factors and conditions. Mine ventilation can have a huge number of factors and knowing how to use Ventsim will help mines get the most from their ventilation system. Fan curves are graphs and data sets that show how the pressure, efficiency and power of a fan will change given different system resistance.  The operating point of a fan is essentially the intersection of the mine system resistance to the fan, and the fan pressure and flow curve. There are several parameters that can be set to model fan curves.

This article will outline key terms and values within Ventsim fan curves, as well as the steps to add a fan curve to a fan in a mine’s Ventsim model.

VENTSIM FAN CURVES IN DATA POINT NUMBERS

  • Ventsim supports a maximum of 10 points per fan curve. Users can add multiple fan curves to a fan.
  • A minimum of 5 points per fan curve is recommended for modelling. The fewer points entered, the less likelihood of accuracy. There are multiple methods to estimate a curve based on available data.
  • In the Fan Database (Tools > Fans), up to 1000 fans and their associated curves can be entered, edited, and viewed.

PICKING WHICH CURVE FUNCTION TO USE

Users can pick whether the fan curve uses the linear,cubic spline or polynomial method to estimate the curve based on the points inputted. Which method to choose will depend on the available fan curve data available.

Figuge 1: Choosing the fan curve calculation method is available under ‘curve Type Estimation’ in the fan curve menu. Steps to access fan curve settings are available further along in this article.

  • The linear method will model a straight path between each point, which is faster during modelling, and may provide sufficient accuracy when many fan curve points are provided. Only having a few available fan curve points however could result in significantly inaccurate values.
  • The cubic spline method requires at least 3 available points and is useful when there are fewer fan curve points available, however, this method does not work as well if there are sudden changes in direction between points.

The polynomial method requires at least three points and is often the most accurate method when few points are available and spread apart.  However where more than 3 points are used and are even in nature, it may approximate the closet curve path between the points.

Users can switch between curve functions to see each method easily modelled.

Note: Ventsim can also predict fan reversal performance, which may be used in emergency situations. The reversal values are defaulted at half of the maximum pressure and half of the maximum airflow. However, manufacturers do not normally provide values for fan reversal output, so accurate modelling may require testing and experimentation of fan reversal outputs within the mine.

PRESSURE, EFFICIENCY AND POWER CURVES

Fan Pressure Curve

To model a fan pressure curve, at a minimum Ventsim requires values for airflow in m3/s, as well as for either fan static pressure or fan total pressure in pascals (Pa).

  • FAN TOTAL PRESSURE METHOD: Using the fan total pressure method will allow the full consideration of a fan performance for accurate results. This method considers the contributions of both fan static pressure and velocity pressure to make its calculations. When airflow exits a fan, velocity pressure is often partially recovered and converted to static pressure, assisting in mine ventilation. Users need to consider pressure losses such as shock and exit losses and make sure exit flow influences such as diffusers, silencers or tunnel geometry are properly incorporated into the model. If factors that decrease pressure aren’t properly modelled, the model may overpredict available pressure.
  • FAN STATIC PRESSURE METHOD: Using the fan static pressure is a more traditional approach that may result in more conservative simulation results. This method assumes that fan velocity pressure is wasted and does not contribute to overall model pressures and airflows. However, surface intake fans, or underground booster fans may contribute some useful velocity pressure and ignoring this by using only fan static pressure curves can decrease the accuracy of the method. Ventsim will ony uses this method if a Fan Total Pressure curve is unavailable or if the method is chosen in the simulation settings. This method is technically incorrect as there is rarely a situation where fan velocity pressure is completely wasted, however the method is still useful as it allows users to disregard velocity pressure loss consideration in the model, while still obtaining conservative results.

Figure 2: The Fan Total Pressure curve is green, the Fan Static Pressure curve in yellow. Fan Static Pressure is always lower and if used in simulation, results in  more conservative results unless additional losses are included in the modelling to reduce the influence of velocity pressure.

Figure 3: Fan Static Pressure values can be used in lieu of Fan Total Pressure. Fan Static Pressure  values exclude velocity pressure and may lead to more conservative, or lower performing results than realty..

The fan pressure curve is the most important curve in a ventilation flow simulation. Ventsim can use default values to calculate power and efficiency if required, however a pressure curve is essential if airflow from a model is to be estimated.

Fan Efficiency Curve

The Fan Efficiency Curve value is used to calculate the efficiency of the fan at a particular airflow volume. This curve can also be used by the simulation to estimate power consumed, however if a Fan Power Curve is available, Ventsim will use that curve instead. If neither the power or efficiency curve is available, the model will estimate power using the fan pressure and airflow, and a default fan efficiency.

Figure 4: This figure shows the values for total efficiency curve (the blue curve). On this fan, maximum efficiency (peak pressure) occurs at an airflow volume of 32 m3/s, before steeply dropping off.

Fan Power Curve

When available, the Fan Power Curve is used to calculate fan shaft power, which represents how much power the fan impeller draws or is absorbed from the fan motor. This value is generally more accurate than power estimated from only the Fan Efficiency Curve.  Where possible the fan power curve should be entered. If no efficiency curve is entered, Ventsim will estimate the efficiency in the simulation by the fan power curve.

Figure 5: The four (4) type of fan curve information shown as different coloured lines.  In most cases only the FTP (fan total pressure) and the PWR (fan power) need to be entered, although all four (4) curves can be entered in desired..

ENTERING A FAN CURVE INTO VENTSIM

  1. Fan curves are available within the Fans menu. To access this, in the toolbar select Tools then Fans. A form will show like this:

Figure 6: This image demonstrates the fan curve menu with the fan menu. Circled are certain value that these instructions go through.

  1. The fans menu will be on the left of the window. Users are able to select the individual fan dropdown to view added fans. To add a new fan type, click the ‘Add Fan’ button (circled in blue above).
  2. Users are able to name individual variations of the fan in the ‘Fan Variation’ section, as well as select the Curve Type Estimation (linear, cubic spline or polynomial as explained earlier).
  3. The green circle on the image above shows the different data option available for each curve. Simply toggle through each of the four fan curve data types by clicking them. In most cases, only the Fan Total Pressure, and the Fan Power need to be entered. The other option can remain blank.
  4. Fan model variations are typically variation of the fan model representing other curves derived from different blades angles, vane angles or other settings.
  5. The is normally no need to enter different fan curves for different fan speeds. The fan speed can be modified directly in the simulation without the need for entering a different fan curve.

Note: Manufacturers will typically use a default of sea level air density (1.2kg/m3) for their fan curve calculations. If a different fan curve density has been provided by the manufacturer, this is changeable in the fan settings. The modelling simulation will automatically adjust the fan curve based on the air density in the simulated model. There is no need to provide a fan curve at the actual mine location if the adjusted density has been included in the Ventsim model.

Fan Losses

The installation of fans in a mine or tunnel system will result in pressure losses which need to be deducted from the fan performance. The losses can be entered into the ‘Installation Resistance’ option in the main fan and may represent shock losses (loss of velocity pressure due to turbulence or obstruction), and other resistance losses such as silencers or installation restrictions. If no installation losses are entered, the fan is assumed to perform at full capacity into the system, and simulations may overpredict actual performance.

Figure 7 -Optional Entry for Installed Resistance Losses for Fans

The Fan Digitiser

To assist in copying the fan manufacturers curve, the fan digitiser is available in the fan toolbar. This tool allows users to directly ‘trace’ the fan curve image graphically, rather than entering the individual values in the spreadsheet. A PDF file or image can be imported into the fan digitiser, and traced over with the mouse.  The fan curve points are estimated by the user identifying the region of the form with the curve, and specifying the ‘Graph Region Minimum and Maximum Values’ of the pressure, flow and power on the image.

 


Nick Mclean | 13/12/2021 15:28:34 | 0 comments