Fan testing & performance – Back to the Basics, Part 2

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IN this second part of our “Back to Basics” series, we discuss an International Standard for airflow testing of fans, ISO 5801:1997.

The ISO (International Standards Organisation) standard, introduced after much deliberation, means that fans manufactured and tested in different countries can be fairly compared to each other regarding aerodynamic performance. ISO 5801:1997 has close agreement with the British (BS 848.1:1980) Standard, upon which the ISO document is largely based, and the American (AMCA) and French standards. However, it has significant differences to the German DIN Standard.

The main difference between the newer Standards and those they replaced is the installation types A, B, C, and D (Fig 1.0), such that if a fan is nominated for use in any one of these arrangements the variation in performance should be established and shown. Previously there was no performance testing of fans with regard to the effect of the intake and/or discharge configuration, and a single performance curve was used for all.

Fig 1.0 - Axial Flow Fans Installation Types

This depicts a fan with free inlet and free outlet, meaning there is no connecting ductwork involved. A typical application for this arrangement would be fans on air-cooled condensers.

Of the four arrangements shown, this is the least efficient. However, to minimise the loss, the sketch depicts the fan has an inlet cone fitted, which improves the flow of air into the fan, resulting in a lower pressure loss than if it was simply a straight edge entry. Also, as the flow of air is smoother, the air enters the impeller close to an ideal manner, resulting in a lower noise level and, importantly, maximising the energy efficiency of the fan.

On the outlet side the air is simply discharged to atmosphere. This means the value of the velocity pressure (pdF) is lost, a key result being that some of the energy is therefore waisted. A discharge evase (referred to by some as discharge cone) can be used to help regain some of the pressure, and hence the otherwise lost energy. (You can refer to the Fans by Fantech catalogue Do’s and Don’ts section for more information on discharge evases.)

This depicts a fan with free inlet but ducted outlet. A typical application for this arrangement would be supply air fans at the beginning of the system. The inlet side is the same as for the Type A installation arrangement. However, the outlet side is ducted which means the pressure loss at this point is less than with the Type A arrangement so this arrangement is slightly more efficient.

This depicts a fan with ducted inlet and free outlet. A typical application for this would be an exhaust fan at the end of the system. As the inlet side is ducted there is no change to the air velocity so there is no inlet loss. On the other hand, on the outlet side the air is being exhausted to atmosphere so the value of the velocity pressure, pdF, is lost. This arrangement is between Types A & B as far as efficiency is concerned.

This depicts a fan with ducted inlet and outlet. This installation type would be the most common arrangement we meet and is typical of most ventilation systems occurring in the HVAC market.

As the inlet and outlet sides are ducted there are no losses at these points, as a result, this installation arrangement is the most efficient.

The above installation arrangements cover only airflow and pressure. The noise testing of fans requires a totally different test facility to the one for testing airflow, effectively doubling the cost of testing. There are plans being worked on by the ISO to come up with a test facility that will test both airflow and noise performance on the same rig at the same time.

How are the configurations used in practice?

Fig 2.0

As is the industry norm the performance curves shown on the graph in Fig 2.0 are based on a Type D (fully ducted) test installation. The pressure loss corrections for the Type A, B and C installations are shown by the horizontal continuums below the graph. The “selection pressure” is determined by extending a line downwards at the air volume point along the graph and adding the “Relative pressure loss” at the point where the air volume crosses the relevant continuum to the input pressure.

The sad thing about fan testing is that fan manufacturers around the world spend very substantial sums of money testing their products and then produce nice catalogues and/or CDs showing the results of the tests. They then sell the product which has been selected on the basis of a performance derived from this very exacting test rig and stuck into a duct system that bears no resemblance to it.

Because the space available for such an item as a fan in buildings, or on equipment, is frequently restricted, the idealised layout provided by a test rig is rarely experienced, so it is amazing so few fans actually have a problem when installed.

However, it would be impossible to test fans for every variation of duct layout but what is being done appears to work. It should be remembered that most fans are a repeat of many other identical fans made before them whilst every duct system is unique.

What this means is that, if there is a problem, unless it is obviously a mechanical or electrical problem with the fan, the chances are the problem will lie somewhere within the design of the duct system or how the fan was installed. Particular points to look for are bad airflow entry or discharge conditions to the fan as well as poorly designed duct transformations and contractions. To know what you should look for onsite, become familiar with the Do’s and Don’ts as shown in Section “O” of the Fans by Fantech catalogue, or as shown on our website.

As fan testing is a fairly big ticket expense it is possible that companies who have tight budgets will be tempted to persist with their existing performance data which has been tested using an obsolete code. Fan specifiers need to beware that the data they are using has been generated using an appropriate standard, preferably ISO 5801:1997.

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