Testing recommendations

Laboratory test stands

Tests for rating should be carried out on a duct system, with flow and pressure measurement and with instrumentation all meet­ing the requirements of ISO 5801.

It is not proposed to detail all the alternative set-ups, as there are a considerable number of these. The Standard totals 232 pages and has given the author many happy (?) hours of read­ing. Suffice it to say that the requirements are detailed and must be followed closely. However a typical duct arrangement is shown in Figure 4.22.

If a fan is provided with its own bearings it should be tested after a sufficiently extended “run-in” period. The inlet and outlet should be away from all walls. Free space should be sufficient to permit air to enter or leave the fan without setting up an un­measurable resistance. The laboratory should be of sufficient volume to ensure that it is free from any air currents that could affect the performance. If it is necessary to discharge the air into another room, then make-up air will be needed.

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Testing recommendations

Inlet side common part Outlet side common part

Figure 4.22 Typical example of a standardised test airway

Field tests

The use of standardised laboratory test stands in the field is usually impossible. Long lengths of straight ducting to “calm” the flow are rarely feasible whilst permanently installed flow measuring devices such as orifice plates, venturis etc., will have too high a pressure loss. All these lead to higherthan nec­essary absorbed power.

Whenever the real installation differs from the idealized (and recommended) laboratory arrangement there will be a loss of fan performance due to the effects of swirl and/or distorted un­developed velocity profiles. This is especially true where there are duct bends directly on the fan inlet and/or outlets. It is rec­ommended to read AMCA201 or The Fan and Ductwork Instal­lation Guide, published by FMA (Fan Manufacturers Associa­tion). Both of these give information on how to calculate the magnitude of likely performance reduction.

Measuring flowrate

Fan flowrate can be expressed as either the volumetric flowrate in m3/s or the mass flowrate in kg/s. If a laboratory test is to comply with ISO 5801 it is essential that readings are taken at the prescribed measuring planes and are downstream of any flow straightening device and at a sufficient distance to ensure flow calming. Many flow measuring devices are permissible within the Code e. g. orifice plates, inlet cones, venturi meters, multi-nozzles etc. All are valid provided the correct coefficients of discharge are used.

Pitot static tube traverses are permitted, but these are perhaps more dependent on operator skill. They are however often the only method possible on site. All types of pitot head are permit­ted, but the writer would recommend the NPL modified ellipsoi­dal type, which is less susceptible to pitch or yaw errors.

Measuring fan pressure

Fan pressure is defined as the stagnation pressure at outlet mi­nus the stagnation pressure at inlet. Up to about 2.0 kPa this is virtually the same as Fan Total Pressure. Care should be taken to ensure that the appropriate value is specified i. e. “total” or “static”. This may depend on the data used for calculating the system pressure and therefore whether “velocity” pressure is included.

Measuring air density

Fan performance is a function of the air (or gas) density han­dled by the fan. It is therefore necessary to take such measure­ments of wet and dry bulb temperature, barometric pressure and even perhaps chemical composition so that the density may be calculated. It should be noted that standard air density is assumed to be 1.2 kg/m3. This equates to dry air at 20°C and 101.325 kPa or to moist air at 16°C and 100 kPa and 50% RH, but these properties are not part of the definition.

Measuring fan speed

Rotational speed can be measured by various types of tachom­eter. A good accuracy is essential as fan performance is very sensitive to even small variations in speed. The fan laws (see Section 4.6) show that flowrate varies directly as the speed, pressure as the square of the speed and absorbed power as the cube of the speed.

Measuring absorbed power

Various prime movers can be used to drive a fan, but more than 99% are electric motor driven. To obtain good figures for ab­sorbed power, it is necessary to at least use a calibrated motor where input volts and amperes can determine the output power. The so-called two-wattmeter method may also be used. For the highest accuracy, however, it is essential to use a dynamome­ter or torque meter.

Calibration and uncertainties

Instruments used for a fan test should be calibrated frequently and this calibration should be traceable back to National / Inter­national Standards. There will be uncertainties associated with any calibration correction and the measured quantities may have a random error, which may be superimposed on a system­atic error. If measurements are repeated over a sufficient pe­riod of time then it may be possible to obtain the magnitude of the systematic error.

Test results

The results of a fan test should be expressed in terms of volu­metric flowrate against fan pressure at a constant rotational speed. Fan absorbed power and fan efficiency may also be given. Inlet air or gas density is also essential.

A fan characteristic curve may be plotted for either the duty range or the full curve from SND (fully closed) to FIO (fully open).

Posted in Fans Ventilation A Practical Guide


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