Fan curves are published to enable users to calculate the amount of air the fan will move under different conditions. In its simplest form, the curve is based on vertical and horizontal scales worked in volume of air moved against resistance, m3/s and Pa. Sometimes m3/h or l/s are used to indicate volumes. In imperial days it would have been ft 3/min against inches waterguage. Conversion from SI to Imperial units has already been discussed. The curves that manufacturers publish in their catalogues, and computer selection programs should be to British Standards 848 parts 1 & 2, which deal with Performance Flow and Noise measurement respectively.

The layout of the curves will be similar, irrespective of the fan type, Axial, Mixed flow, Centrifugal, but the horizontal and vertical scales will probably vary, as indeed they will for differing impeller diameters of similar fans. The curves we will look at have another variation; the angle (pitch) of the impeller. Some form of cased axial flow fans, i.e., those built within a length of circular duct, have the option of variable angle blades. This ploy is to enable the actual fan performance to match the performance required more closely.

Figure 1 shows a curve for an axial fan. This is obtained by collecting a series of data points from the instruments on the test rig. The starting point is to measure the airflow at zero pressure, sometimes referred to as Free Inlet & Discharge or FID. The test rig is then adjusted to increase the pressure that the fan has to work against and to measure the airflow at each point. It is normal for only the “stable” part of the fan performance curve to be published. This curve is shown “smoothed”. The actual test points will not necessarily form a smooth curve, but these slight variations are omitted.

Figure 1. Axial Fan Performance Curve

Figure 2 shows a “family” of curves for an Axial flow variable pitch impeller fan. The volume of air moved decreases with the angle of the impeller. The speed of rotation remains theoretically constant.

Figure 2. Variation in Blade Pitch Angle

Figure 3 shows Fan Dynamic Pressure or Velocity Pressure. This is calculated as follows:

Dynamic Pressure = 0.5 × p × Velocity²

Where p is the density of the air or gas in Kg/m³ and Velocity is in m/sec.
This is obtained by Volume m³/sec
                               Area m³

This is important to understand because if the volume flows around or through an obstruction, such as a bend or damper, resistance will have to be overcome. A measure of this resistance is characterised by 'k' factors.

Figure 3. Fan Dynamic Pressure

Figure 4 shows Figure 1 reconstructed with the Static Pressure replaced by Total Pressure. The latter is the arithmetic addition of Static Pressure and Dynamic Pressure. Unfortunately one of the recommendations of the proposed replacement British Standard, is to publish performance data using the term Fan Pressure which is Total Pressure - as always, this is likely to cause confusion for years!

Figure 4. Total Pressure Curve

Figure 5 shows the efficiency contour lines. The efficiency is calculated using the test data by the following equation:

Fan Efficiency (Total)=

Total Pressure (Pa) × Volume Flow (m³/sec) Actual shaft power (Watts) × 1000

Figure 5. Efficiency Curve

The illustration shows that for a fan there is a ‘best’ efficiency point. In practice, there is only a small chance of operating at the duty point which meets this. The reason is that most manufacturers design impellers to operate over a wide range of flows and pressures, they are not designed for a specific duty point. However, there are manufacturers which do design impellers and fans for specific conditions including operating at high efficiency value - this is particularly important with fans absorbing large amounts of electrical energy.

This may seem relatively unimportant for small fans, but where multiple fans are required for ventilation systems in buildings, a minimum efficiency value is sometimes included in the Specification by the Specifier or Architect in order to keep electrical supply requirements to a minimum.

Figure 6 shows several features from earlier slides and is a practical example. The required duty point is plotted, volume of air against pressure, but is seen to be beyond the inherent fan speed characteristic curve. If the system curve is calculated as discussed earlier, and plotted, where the fan and system curves bisect each other is the actual duty point. It may be that an adjustable pitch Axial fan can be used with a slightly high blade pitch angle, which will give nearer the actual performance required.

Figure 6. Volumetric Flow Rate