CENTRIFUGAL FAN PERFORMANCE TESTING BS848

FAN TESTING

This section describes the method of testing a fan installed on site in order to determine the
performance of the fan in conjunction with the system to which it is connected.

PURPOSE OF FAN TESTING :

The purposes of such a test are to determine, under actual operating conditions, the volume flow
rate, the power input and the total pressure rise across the fan.These test results will provide actual value for the flow resistance of the air duct system, which can be compared with the value specified by supplier.

Performance Terms and Definitions:

Static Pressure: 

The absolute pressure at a point minus the reference atmospheric pressure.

Dynamic Pressure: 

The rise in static pressure which occurs when air moving with specified velocity at a point is bought to rest without loss of mechanical energy. It is also known as velocity pressure.

Total Pressure: 

The sum of static pressures and dynamic pressures at a point. Fan Shaft Power: The mechanical power supplied to the fan shaft Motor Input Power: The electrical power supplied to the terminals of an electric motor drive. 

 

Reference Standards:

British Standard, BS 848 - Fans for general purposes Part 1, Methods of testing performance.

Field Testing 

Instruction for Site Testing
Before site tests are carried out, it should be ensured that:
• Fan and its associated equipment are functioning properly, and at the rated speed
• Operations are at stable conditions, e.g. steady temperatures, densities, system resistance
etc.

Location of Measurement Planes

General: 

The flow measurement plane shall be located in any suitable straight length, (preferably on the inlet side of the fan) where the airflow conditions are substantially axial, symmetrical and free from turbulence. Leakage of air from or into the air duct shall be negligible between the flow measuring plane and the fan. Bends and obstructions in an air duct can disturb the airflow for a considerable distance downstream, and should be avoided for the purposesof the test.

Test length: 

That part of the duct in which the flow measurement plane is located, is termed the 'test length' and shall be straight, of uniform cross section and free from any obstructions which may modify the airflow. It shall have a length equal to not less than twice the equivalent diameter of the air duct (i.e. 2De). For rectangular duct, equivalent diameter, De is given by 2 LW/(L + W) where L, Wis the length and width of the duct. For circular ducts De is the same as diameter of the duct.

Inlet side of the fan: 

Where the 'test length' is on the inlet side of the fan, its downstream end shall be at a distance from the fan inlet equal to atleast 0.75De. See figure 6.1. In the case of a fan having an inlet box , the downstream end of the test length shall be at a distance from the nearest part of the inlet cone of the fan equal to at least 0.75De.

Outlet side of the fan: 

Where the 'test length' is on the outlet side of the fan, the upstream end of the 'test length' shall be at a distance from the fan outlet of at least 3De. See figure 6.2. For this purpose, the fan outlet shall be considered as being the outlet of any expander on the outlet side of the fan.

Location of the Flow Measurement Plane within the 'Test Length':  

The flow measurement plane shall be located within the 'test length' at a distance from the downstream end of the 'test length' equal to at least 1.25De.  

Location of Pressure Measurement Plane: 

For the purpose of determining the pressure rise produced by the fan, the static pressure shall be measured at planes on the inlet and/or the outlet side of the fan sufficiently close to it to ensure that the pressure losses between the measuring planes and the fan are calculable in accordance with available friction factor data without adding excessively to the uncertainty of fan pressure determination. If conveniently close to the fan, the 'test length' selected for air flow measurement should also be used to pressure measurement. Other planes used for pressure measurement should be no closer than 0.25De from the fan inlet and no closer than 4De from the fan outlet. The plane
of pressure measurement should be selected at least 4De downstream of any bend, expander orobstruction which are likely to cause separated flow or otherwise interfere with uniformity of
pressure distribution

Measurement of Air Velocity In Site:

Velocity shall be measured by either pitot tube or a rotating vane anemometer. When in use, the
pitot tube shall be connected by means of airtight tubes to a pressure measuring instrument. The
anemometer shall be calibrated before the test.

Pitot Tube: 

In Figure 6.4, note that separate static connections (A) and total pressure connections
(B) can be connected simultaneously across a manometer (C). Since the static pressure
is applied to both sides of the manometer, its effect is canceled out and the manometer indicates
only the velocity pressure.
In practice this type of measurement is usually made with a Pitot tube which incorporates both
static and total pressure sensors in a single unit. Essentially, a Pitot tube consists of an impact tube (which receives total pressure input) fastened concentrically inside a second tube of slightly larger diameter which receives static pressure input from radial sensing holes around the tip. The air space between inner and outer tubes permits transfer of pressure from the sensing holes to the static pressure connection at the opposite end of the Pitot and then, through connecting tubing, to the low or negative pressure side of a manometer. When the total pressure tube is connected to the high pressure side of the manometer, velocity pressure is indicated directly. See Figure 6.5.
To ensure accurate velocity pressure readings, the Pitot tube tip must be pointed directly into
(parallel with) the air stream. As the Pitot tube tip is parallel with the static pressure outlet tube,
the latter can be used as a pointer to align the tip properly. When the Pitot tube is correctly
aligned, the pressure indication will be maximum. 

Figure 6.4 Types of Pressure Measurementtion

    

Calculation of Velocity: 

After taking velocity pressures readings, at various traverse points, the velocity corresponding to each point is calculated using the following expression.

 Anemometer: 

The indicated velocity shall be measured at each traverse point in the cross section by holding the anemometer stationary at each point for a period of time of not less than 1 minute. Each reading shall be converted to velocity in m/s and individually corrected in accordance with the anemometer calibration. The arithmetic mean of the corrected point velocities gives the average velocity in the air duct and the volume flow rate is obtained by multiplying the area of the air duct by the average velocity.

Determination of Flow

Once the cross-sectional area of the duct is measured, the flow can be calculated as follows:
Flow, (m3/s) = Area (m2) x Velocity (m/s)

 

Determination of Fan Pressure:

General: 

Precautions shall be taken so that the measurements of the static pressure on the
inlet and outlet sides of the fan are taken relative to the atmosphere pressure.

Measurement of Static Pressure: 

 This shall be done by using a manometer in conjunction with the static pressure connection of a pitot tube or a U tube manometer. When using a pitot tube it is necessary to carry out a traverse in the pressure measurement plane taking individual point pressure readings in a manner similar to that for determining flow rate. In general, a smaller number of readings will be found adequate where individual readings do not vary by more than 2% from each other. The average of all the individual readings shall be taken as the static pressure of that section.

Determination of Power Input:

Power Measurement:  

The power measurements can be done using a suitable clamp- on power meter. Alternatively by measuring the amps, voltage and assuming a power factor of 0.9 the power can be calculated as below:

 

Transmission Systems: The interposition of a transmission system may be unavoidable introducing additional uncertainties. The following values shall be used as a basis for transmission efficiency in the case of drives rated at 20 kW and above unless other reliable information is available:

Properly lubricated precision spur gears 98% for each step
Flat belt drive 97%
V-belt drive 95%

Other Prime Movers:  

When the fan forms one unit with a non-electric prime mover it is recommended that the fuel consumption (oil, steam, compressed air etc.) should be specified and determined in place of the overall power

 

Factors that Could Affect Performance

• Leakage, re-circulation or other defects in the system;
• Inaccurate estimation of flow resistance;
• Erroneous application of the standardized test data;
• Excessive loss in a system component located too close to the fan outlet;
• Disturbance of the fan performance due to a bend or other system component located too
close to the fan inlet;
• Error in site measurement