Fan testing

In the UK performance testing is covered by BS 848: Part 1: 1980. The standard is comprehensive and internationally well accepted. It covers three types of installation:

A: Free inlet and free outlet.

B: Free inlet and ducted outlet.

C: Ducted inlet and free outlet.

D: Ducted inlet and ducted outlet.

The standard prescribes the parameters for the testing rigs that may be adopted and, in particular, ensures that adequate steps are taken to provide smooth airflow conditions at inlet and at outlet. An interpretation of a test arrangement for a type D installation, according to Keith Blackman (1980), is shown in Figure 15.23. A low loss entry piece (see sections 15.6 and 15.7) enables an accurate determination of airflow rate to be made by equating the measured static depression to velocity pressure. A measurement of the static pressure at fan inlet is comparatively easy in the large inlet chamber where smooth airflow conditions prevail. Establishing the velocity at fan outlet is more difficult because of the turbulent nature of the airflow. Static pressure is easier to measure and hence performance can be conveniently expressed as airflow rate against fan static pressure (see equation (15.22)).

The airflow rate is divided by the area across the flanged outlet from the fan (see Figure 15.22) to establish a notional mean air velocity at fan outlet. The corresponding velocity pressure is added to the measured fan static pressure to give the fan total pressure in accordance with equation (15.23).

Manometer

подпись: manometerEtoile airflow straightener that allows the static pressure to equalise radially as the. air flows through it

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Manometer

Eggcrate airflow straightener

Manometer

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Centrifugal fan under test

Fan shaft, bearings I and connexion to driving motor

Adjustable dampers to control airflow

Fan testing

 

Fixed resistance screens to equalise air velocity

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Low loss entry piece

 

Fan testing

Inlet

Chamber

 

Fan testing
Fan testing

Fan testing

Fig. 15.23 An interpretation of a testing arrangement for a type D installation (ducted inlet and ducted outlet) according to BS 848: Part 1: 1980. Based on a method adopted by Woods of Colchester Ltd (incorporating Keith Blackman) and used with their kind permission.

It is to be noted that the fan static pressure represents the sum of the total pressure losses before the fan inlet and the static pressure available to overcome the losses after the fan outlet, ignoring any benefit that may be obtained by converting the velocity pressure at fan outlet into static pressure. Hence, to take full advantage of the fan performance, every effort should be made to recover as much as possible of the velocity pressure at fan discharge, to augment the static pressure available.

It is further to be noted that, since the way the fan is installed on the site is never the same as the way in which its performance was tested according to BS 848 (1980), it is to be expected that there will be some shortfall in performance, unless this has been foreseen at the design stage and allowed for in the calculation of system total pressure loss. The discrepancy is sometimes spoken of as system effect. In this respect it is most important that smooth inlet and outlet duct connections are made to the fan.

Posted in Engineering Fifth Edition


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