ENERGY_PERFORMANCE_ASSESSMENT_FOR_EQUIPMENT_AND_UTILITY_SYSTEMS_(Chapter-6: ENERGY PERFORMANCE ASSESSMENT OF FANS AND BLOWERS)

 

ENERGY_PERFORMANCE_ASSESSMENT_FOR_EQUIPMENT AND UTILITY SYSTEMS

(CHAPTER-6: ENERGY PERFORMANCE ASSESSMENT OF FANS AND BLOWERS)

Introduction

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 the Performance Test

The purposes of such a test are to determine, under actual operating conditions, the volume flow rate, the power input and the static pressure rise across the fan.

These test results will provide actual flow resistance of the air duct system, which can be compared with the design value of fan 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.

Scope

The procedure describes field testing of centrifugal fans and blowers for assessing performance and efficiency.

Reference Standards

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

Field Testing

1. Instruction for Site Testing

Before site tests are carried out, it should be ensured that:

a)Fan and its associated equipment are functioning properly, and at the rated speed

b)Operations are at stable conditions, i.e. 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 purposes of 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. 2Dc). For rectangular duct, equivalent diameter, DC is given by 2 LW/ (L+W) where L, W is the length and width of the duct. For circular ducts DC 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 ‘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 1.25 De.

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 0.25D,

from the fan inlet and no closer than 4D, from the fan outlet. The plane of pressure measurement

should be selected at least 4D, downstream of any bend, expander or obstruction which are likely to

cause separated flow or otherwise interfere with uniformity of pressure distribution.

Measurement of Air Velocity on Site

Velocity shall be measured by either pitot tube or a rotating vane anemometer. When in use, the tubes connecting the pitot tube to pressure measuring instrument should be air tight. The calibration of pressure measuring instrument should be up to date.

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 cancelled out and the manometer indicates only the velocity pressure.

Static pressure and velocity pressure measurements are shown in Figure 6.5. To ensure accurate velocity pressure readings, the Pitot tube nozzle tip must be pointed directly into and parallel with the air stream.

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.

Traverse readings: In practical situations, the velocity of the air stream 1s not uniform across the cross section of a duct. Friction slows the air moving close to the walls, so the velocity is greater in the center of the duct.

To obtain the average total velocity in ducts of 100 mm diameter or larger, a series of velocity pressure readings must be taken at points of equal area. A formal pattern of sensing points across the duct cross section is recommended. These are known as traverse readings. Figure 6.6 shows recommended Pitot tube locations for traversing round and rectangular ducts.

In round ducts, velocity pressure readings should be taken at centers of equal concentric areas. Numbers of points or traverse points depend upon the diameter of duct. For a circular duct of 1 m diameter, atleast 10 traverse points are recommended. In rectangular ducts, a minimum of 16 and a maximum of 64 readings are taken at centers of equal rectangular areas.

Actual velocities for each area are calculated from individual velocity pressure readings. This allows the readings and velocities to be inspected for errors or inconsistencies. The velocities are then averaged.

Example-Traverse point determination for round duct

Round duct: Let us calculate various traverse points for a duct of 1 m diameter. From Figure 6.4, for round duct of 1 m diameter (D). The radius, R is 0.5 m. The various points from the port holes are given below:

Example-Traverse point determination for rectangular duct

Rectangular duct: For 1.4 m x 0.8 m rectangular duct, let us calculate the traverse points. 16 points

are to be measured.

Dividing the area 1.4 x 0.8 = 1.12 m’ into 16 equal areas, each area is 0.07 m’. Taking dimensions of 0.35 m x 0.20 m per area, we can now mark the various points in the rectangular duct as follows:

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

The indicated velocity shall be measured at each traverse point in the cross section by holding the pitot tube stationary at each point. Velocity pressure reading shall be converted to velocity in m/s. The arithmetic mean of velocities at all traverse points gives the average velocity in the air duct.

Calculation of gas density corrected to normal temperature

Calculation of molecular weight of the gas (M) (dry basis), kg/kg mole

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 Static Pressure

The measurements of the static pressure on the inlet and outlet sides of the fan are taken relative to the atmosphere pressure using manometer in conjunction with the static pressure connection of a pitot tube or a U tube manometer.

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: If fan is not connected to motor directly, transmission efficiency should be suitable assumed depending upon the type.

Directly coupled                                                            1

Properly lubricated precision spur gears                      0.98 for each step

Flat belt drive                                                               0.97

V-belt drive                                                                  0.95

Other Prime Movers: When the fan is connected to non-electric prime mover instead of motor, 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

Solved Examples:

1) A cement kiln exhaust gas has the following composition on dry basis is CO2 - 24.7%, O2 — 5.1%,

CO-—0.1%, N2 — 70.1 %. The static pressure and temperature measured in the duct are -655 mmWC

and 316 °C. The velocity pressure measured with a pitot tube is 16.39 mmWC. The atmospheric

pressure at the site is 10334 mmWC. The pitot tube constant is 0.85. If the area of the duct is 8.3

m?’. Calculate the volumetric flow rate in m3/hr.

2) Given below is a set of curves for a centrifugal fan. At its Best Efficiency Point (BEP) determine

to the nearest approximation the following:

a) Static pressure in mmwce

b) Flow in m?/hr

c) Shaft power in kW

d) Work out the static efficiency of the fan by calculation

e) Power drawn by the motor if the motor operating efficiency is 90%

3) The following is a typical report on measurements taken and calculations made for a double inlet

fan in a palletizing plant.

A. Design Parameters:

Volume = 292 m3/sec.

Static Pressure = 610 mmwce

Measurements:

Instruments used

a) Suction pressure, outlet pressure =‘U’ tube manometer

b) For differential pressure = Inclined tube manometer

c) For temperature = Mercury in glass thermometer

d) Fan speed = Tachometer

e) Line current Tong tester

Ambient temperature = 32°C

Atmospheric pressure = 10334 mm WC

Area of duct (double entry) = 1.029 x 5.502 x 2

Fan

Damper position                                               = 80% open

Speed                                                                = 740 RPM

Average Static pressure (suction to fan)           = -20 mm WC

Average outlet pressure (discharge from fan)  = 470 mm WC

Average of velocities measured at various

Traverse points = 29 m/s

Velocity pressure or velocity head is measured at each traverse point and velocity is calculated.

Average velocity is arithmetic mean of the above calculated velocities.

Motor

Current = 220A

Voltage = 6.6kV

PF = 0.9

Motor power input = = 2263 (calculated)

Motor efficiency = 0.94

Transmission efficiency = 1 (directly coupled)

Power input to fan shaft =2127kW

C. Performance calculations:

a) Calculation of gas density

Fan is designed to operate at 78% efficiency at the design point. However, fan is operating at lower

flow than the design by partially closing the damper. Hence the fan is operating away from the best

efficiency point.

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Chapter 5

Chapter 7