How to dertermine and describe fan performance

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IN this, the first of a “Back to Basics” series, we discuss the all-important issue of how to determine and describe fan performance. Accurate fan performance is essential information for anyone designing or specifying an air movement device for a ventilation or air conditioning system. This and the next issue of Tech Talk will explain: - How a fan’s airflow and pressure performance is established. - How the performance is presented to the design engineer in a concise, easily understood format.

Background
Formerly, and in some countries still, fan performance was presented in tabular form - thought to be useful for people who did not understand performance curves. However, in recent decades performance curves for products have become so common they are almost universally understood. Curves have the advantage of disseminating much information in a small space, giving the user an understanding of the complete fan performance and what safety margin is available. Fantech uses performance curves exclusively to describe the air-flow pressure performance characteristics of their fans. But how is a fan curve, or any performance curve, initially created?

A curve
A curve is the result of connecting a series of points to make a continuous line, the points being derived from some sort of test. An example would be to consider water flowing from a tap into a pail. With the tap fully open, measure the amount of water that goes into the pail in one minute. Next, close the tap a quarter turn and measure the amount of water that goes into the pail in one minute. Repeat the exercise of closing the tap a quarter of a turn and measuring the flow of water in one minute. If the tap takes two full turns from fully open to fully closed, seven readings will be obtained. As the tap is closed with each quarter turn, the flow rate of water will reduce until, eventually, the tap is closed and no water comes through. We can now plot these points on a graph, with the horizontal axis being the flow rate of the water/minute and the vertical axis being the tap position. Fig. 1 shows the sort of points that could be obtained from such a test. The results obtained are not always in an even line. This is normal and comes from minor errors in the reading of instruments, etc. Connecting these points on a point-to-point basis, as in Fig. 2, does not look very good so a ‘curve of best fit’ is usually adopted, as in Fig.3. The level of inaccuracy introduced by such a measure is miniscule and can be ignored.

Fig. 1 - plot of test results

Fig. 2 - point-to-point connection

Fig. 3 - curve of best fit

Not simple
Performance testing of fans is not as simple as the above example. The design of a fan test rig is defined in Test Standard ISO5801:1997, with its construction having to conform to very strict standards. As well, the measuring instruments have to be calibrated on a regular basis to ensure they are providing correct readings. If this is done it is expected the performance curve of a fan tested on such a rig will be reliable. Testing a fan is difficult, as the fan is immersed in the medium being measured, that is, air. An analogy would be testing the performance of a pump under water, which is also difficult. However, although you cannot see air, you can feel it because it is moving. It is this particular feature that makes it possible to determine how much air is being handled by a fan.

Fig. 4 - Minivent Roof Unit Performance Curves

Velocity Pressure
Moving air develops a pressure, or head, called Velocity Pressure and, depending where it is measured in the rig, provides both the Fan Static Pressure and Fan Velocity Pressure values. With these two values we can determine the Fan Total Pressure. Static, Velocity and Total Pressures are explained in Tech Talk No 43, July 2002. A copy can be provided on request. As with the tap example, there is a facility within the fan test rig that enables flow of air to be restricted, i.e. imposed pressure loss, with the value of the loss to be measured as well as the reduced flow rate. In a fan test, the measurement of the restriction value will be of pressure rather than turns of a tap, but the outcome will be the same. A series of coordinates, flow rates and pressures, will be obtained which can be plotted on a graph so that a curve can be drawn.

Fig. 5 - Axial Flow Fan Performance Curves

Specific speed
Many products, such as centrifugal, mixedflow or fixed pitch axial flow fans, have a single curve for a fan of a particular size running at a specific speed. Fig. 4 shows the curves for a range of axial roof units. The performance of each diameter is shown running at one speed. Adjustable pitch axial flow fans are different because, for any one size and speed, the angle of the fan blade can be adjusted from approximately 10° to 40° in one degree increments. However, showing the performance data for each pitch angle could result in a very crowded set of curves. This is overcome by showing the curves in five degree increments (Fig. 5). This set of curves has the volume/pressure performances in the upper graph and the power absorbed data in the lower graph. Where a required performance is between two curves, the angle required is interpolated.

The Standards
The International Standard for airflow testing of fans is ISO5801:1997. Prior to 1997 many countries had their own standards, dominated by Britain (BS) and America (AMCA). Their methodology was fairly close. France and Germany also had standards, with the French version being close to BS and AMCA, and the German method somewhat different. How this test data is transformed into a concise, understandable format will be covered in our next edition.

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