Figure 5.5 Real system pressure curve
As an approximation it may be said that when fans are connected together in series then, at any give volumetric flowrate, each fan adds its corresponding fan total pressure to the combined output with its corresponding power. In actual practice there is a slight loss in pressure in the connections between the stages.
In more exact work it should be noted that the total pressure of the combination is equal to the sum of the fan total pressures of the individual units minus the losses in the interconnecting duct. Thus the fan static pressure of the combination is equal to the total pressure of the first stage plus the static pressure of the second stage there being only one velocity pressure lost at the final outlet. With high pressures compression becomes important. The second stage will receive its air at a density increased by the pressure of the first. Due to this increased density its pressure development will be correspondingly greater, together with its absorbed power.
For normal commercial requirements, series operation is in use mainly for air supply to furnaces, which require a relatively high pressure at a small airflow. Two stages meet most needs, but a larger number of stages may be used for applications such as industrial vacuum cleaning, pneumatic conveying etc.
A test on a Sturtevant 2 stage STI type fan is shown in Figure
5.6 and the results are show in Table 5.1.
406 mm unshrouded impellers All tests at 3100 rpm 13.9°C/96.5 kPa
The transition point will vary from one system to another according to the amount of laminar flow present due to low velocities at filters etc. Only pneumatic conveying plant, dust exhaust and high velocity air conditioning are likely to have flows which are fully turbulent. These effects should be recognized especially when speed control is included. To repeat, fan efficiency will change and power absorbed will not vary as N3. Power savings are therefore likely to be somewhat less than claimed e. g. between N2 and N2 5. At very high turn down ratios, the savings will be even less.
It will be noted that the index for Q is continually varying and is not a fixed value. For small plants, the index appears to tend to smaller values — certainly below the 1.9 or thereabouts quoted by Loeffler et al.
It will however be concluded that a square law relationship assumed in applying tolerances to performance data as called for in AMCA211 and ISO 13348 (catalogue fans) is perfectly valid for small variations of 3% or even 5% of flowrate.
The curve assumes standard air, and if there is a variation in temperature and /or barometric pressure along the duct run then the curve becomes even more complex to calculate. Such cases are not unknown. Again, it should be emphasised that much lower indices are to be expected in grain drying, fuel beds, etc.
Figure 5.6 Example of test on Sturtevant 2 stage STI type fan
Table 5.1 Results of test on 2 stage fan
For a given system total pressure the volume delivered by the combination is the sum of the individual units at the same fan static pressure. This is only strictly true where the two fans are connected to a chamber.
If the fans blow directly into a common duct then neglecting losses, the volume delivered by the combination for a given total pressure is the sum of the volumes delivered by the individual fans at the same fan total pressure.
Multivane forward curved bladed fans are not usually suitable for parallel operation due to the shape of the fan curves. The stall of low volumetric flowrates means that there may be as many as three flowrates, where the fan pressure is the same.
Because of the pronounced peak in the pressure/volume curve, where there is any possibility of large and rapid fluctuation in system resistance, a forward curved fan selected at any pressure Q above the dotted line (see Figure 5.7) can be unstable. If, for any reason, the flow drops the point of operation can move from something normally around B to C where the fan head is slightly less. The change in volume may have been small and the system back pressure will have stayed almost unaltered. Thus the system pressure will be in excess of the fan pressure causing the flow to decrease rapidly back to A. Since the back pressure is still above the shut-off pressure a reversal of flow can occur.
Figure 5.7 Characteristic of forward curved fan showing instability
The system is then at a standstill and the system pressure (which we assume is x Q2) now drops below the shut-off pressure. Volume flow increases and the operating point moves up the curve past the equilibrium point. It then comes back and may tend to overshoot, thus repeating the cycle.
Such behaviour is accentuated at higher pressures, on long duct runs or when the fan discharges into a chamber of large dimensions. The instability is often not found during normal fan performance tests as these conditions do not then exist.
It will be seen that the practice of selecting over-large fans for a system to reduce the outlet velocity can be extremely dangerous. It may even lead to operating points to the left of the peak pressure B which should be avoided under all circumstances.
It is usually necessary to operate identical fans together to ensure that each does an equal share of the work.
Posted in Fans Ventilation A Practical Guide