Two fans, twice the air flow?
Original article date: April 2000
Ian McLeod of Papst, explains why doubling the number of fans doesn’t always produce the desired results
The idea of mounting two fans in series or parallel often arises when one fan isn’t up to the task of cooling and a larger one just won’t fit. The only time two fans of equal size can provide double the airflow, however, is when they are operating in free air, ie with no back pressure to restrict the airflow. Usually, engineers are working with an enclosure, and need to know how to optimise the performance of multiple fans.
Fans produce turbulent air currents which, when forced through equipment enclosures, remove heat from internal components. Obstructions to this airflow not only provide a reverse pressure, which the fan must overcome, but can also mask components from the cooling air stream. To design the layout of the box, the designer must consider the cooling paths. Densely packed enclosures exhibit airflow resistance, manifested as pressure loss in the direction of airflow. It’s analogous to an electrical generator forcing current through a resistor.
In theory, weighting factors can be applied to determine the flow/pressure characteristics of systems. In practice, the variety of designs used in enclosures and the presence of internal components that interfere with airflow, mean that it is impossible to calculate weighting factors using general formulae. Designers must rely on measurements or rough approximations. For practical purposes, the pressure loss ‘p’ of an enclosure is approximated by the formula:
p = Rv x Q/2 x V2
where Rv is a weighting factor for pressure loss, Q is the density of the displacement medium and V is the velocity of air flow through the system. It can be seen that pressure loss increases as the square of flow rate.
In the characteristic curve based on this formula where pressure loss is plotted as a function of flow rate, we see the air flow characteristics of a given enclosure. When fitted within an enclosure, the fan has to overcome the inherent airflow resistance. To achieve this, the fan needs to produce a pressure increase which will in turn decrease the flow rate. A characteristic fan curve expresses the relationship between flow rate and pressure.
The operating point of the fan is determined by the point at which the characteristic enclosure curve and characteristic fan curve intersect. At this point, the pressure loss of the enclosure is just compensated by the pressure increase of the fan and this point determines the flow rate available within that enclosure.
With parallel mounting, the flow rate is multiplied by the number of fans – but the results must be plotted over the entire characteristic fan curve. If fans are placed too close together, other interference effects come into play and reduce the overall flow. This is largely because the flow of air into a fan is usually laminar and smooth, while the exhausted air is much more turbulent. Even in an ideal environment, where interference effects could be ignored, a pressure increase of four times would be needed to produce a doubling of air flow, as pressure loss increases with the square of flow rate. Here, the airflow only increases by approximately 20 to 25% over that achieved with a single fan.
When fans are mounted in series, the pressure increase, in theory, is doubled. But if the fans are close together, results will again fall short of the theoretical performance due to the angular component of airflow introduced in the exhaust of the rear fan. This limits the suction effectiveness of the front fan. One solution is to direct the angular component back into the main air current using guide vanes, but this eats space. A better approach is to use one fan on the intake and one on the exhaust side of the enclosure. The presence of internal components and the large cross-sectional area between the individual fans will mean that airflow is essentially unidirectional. This provides effective airflow and relatively low noise levels.
The choice of series or parallel fan combinations depends on the individual application; some applications may require a combination of both. They key point is that two fans never mean twice the air flow.
- Papst
April 2000