Determining the performance of fans in-situ

The need to revise existing national methods of measuring the aerodynamic performance of fans under site conditions has been felt for some time. Hence, early in the life of ISO Technical Committee TC117, work commenced on a “stand-alone” docu­ment. Again the time for preparation has been extremely long, but compromises have been reached which enabled Standard ISO 5802 to be published. This is largely an amalgam of the French AFNOR X10-201 for siting of the velocity-area measur­ing points and BS 848:Part 1:1980 Section 3 relating to pres­sure, calculation, instrument calibration and uncertainties. Thus all the commonly encountered airway cross-sections are addressed together with relevant velocity-area methods.

Performance ratings

Catalogue rating tables and performance curves are produced from tests carried out according to the procedures specified for standardised airway conditions. In actual systems, however, it is rare for fans to be installed exactly reproducing those speci­fied in the laboratory Standard. It will be remembered that ISO 5801 specifies “common parts” both upstream and down­stream of the fan. These ensure a fully developed, swirl free and symmetrical velocity profile presented to the inlet.

The fan is enabled to develop its full potential and also to re­cover the excess velocity (dynamic) pressure at the fan dis­charge and convert it into useful static (potential) pressure. At the same time any useless residual swirl is removed. For these reasons, it is likely that the site performance will be degraded when compared with a laboratory test in a standardised airway.

The magnitude of the difference may be considered as an indication of the quality of the system design.

Measuring stations

A major problem of testing in the field is the difficulty of finding suitable locations for making accurate measurements of flowrate and pressure. Wherever possible, the system de­signer should consider the provision of a suitable measurement station before manufacture. If this is not possible then tempo­rary or permanent alterations to the ducting may be necessary to improve the accuracy of the test.

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Figure 4.14 Location of pressure measurement planes for site testing

Most field tests will need to be carried out by some kind of veloc­ity-area method using either pitot-static tubes or anemometers. A traverse plane suitable for the measurements necessary to determine flowrate (Figure 4.14), would have the following attributes:

A) the velocity distribution should be uniform throughout the traverse plane

B) the flow streams should be at right angles to the traverse plane

C) the cross-sectional shape of the airway in which the tra­verse plane is located should be regular

D) the cross-sectional shape and area of the airway should be constant for some distance both upstream and down­stream of the traverse plane

E) the traverse plane should be located to minimize the ef­fects of leaks between the traverse plane and the fan.

A location at least five equivalent diameters downstream of the fan in a long straight uniform cross-section duct would provide ideal conditions for a pitot traverse assuming a vane axial or centrifugal unit. For a tube axial a location upstream would be preferable to obviate the errors resulting from swirl. In all cases where the traverse plane has to be close to the fan, an up­stream location is preferred. This will give a more acceptable velocity profile from symmetry, fullness and swirl-free points of view. It will also minimize the effects of leakage. In some instal­lations with parallel flow paths it may be necessary to use more than one traverse plane and add their results.

Flowrate measurements

The Standard includes recommendations for the number and distribution of measurement points in the traverse plane when a velocity-area method is used. For circular ducts the measuring points are spread over a minimum of three diameters with at least three points per radius. The positioning may be to either log-Tchebycheff or log-linear rules (Figure 4.15).

Similar information is given for annular, rectangular (Figure 4.16) and other common regular shapes. Rules are also in­cluded for duct cross-sections, which do not correspond closely to any of the standard shapes.

Since the flow at a traverse plane is never absolutely steady, the velocity pressure measurements indicated by a pitot­tube/manometer combination will fluctuate. Each measure­ment will, therefore, need to be averaged on a time-weighted

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Figure 4.18 Tapping connections to obtain to obtain average static pressure in circular airway (Shown interconnected to single manometer)

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figure 4.18 tapping connections to obtain to obtain average static pressure in circular airway (shown interconnected to single manometer)

Figure 4.15 Siting of measuring points in a circular section with four diameters and three measuring points per radius

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figure 4.15 siting of measuring points in a circular section with four diameters and three measuring points per radius

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Power measurements

The drive shaft power may be determined either directly through a torque meter or deduced from the electrical power in­put to the motor terminals and using the summation of losses method.

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the drive shaft power may be determined either directly through a torque meter or deduced from the electrical power input to the motor terminals and using the summation of losses method.
Figure 4.16 Rectangular section with six cross-lines and five measuring points per cross-line

Basis. The four designs of pitot-tube permitted in ISO 5801 are all considered primary instruments and may be used without calibration provided they are in good condition. They do not all have the same insensitivity to pitch and yaw. ISO 3966 and ISO 7194 indicate likely errors for each type under non-normal flow. The modified ellipsoidal head of the NPL design is preferred as it is the least sensitive to misalignment.

Determining the performance of fans in-situ

Figure 4.19 Typical performance curves for a forward curved centrifugal fan to different installation categories

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подпись: inlet volume flow m3/sPressure measurements

Care must be taken to ensure that static pressure measure­ments on both the inlet and outlet of the fan are taken relative to atmospheric pressure or to that existing within a common test enclosure. Under reasonably uniform flow, free from swirl and separation, four interconnected wall tappings may be used (Figures4.17and4.18). As with ISO 5801, Fan Pressure is de­fined as the difference in stagnation pressures at fan outlet and inlet. At pressures less than about 2.0 kPa, this is virtually the same as the previously defined Fan Total Pressure.

Determining the performance of fans in-situ

Figure 4.17 Construction of wall pressure tappings Note: a to be not less than 1.5 mm nor greater than 10 mm and not greater than 0.1 D

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