FORCED DRAFT FAN

BY JUSTICE. A. OFUGARA

INTRODUCTION

• For combustion to take place, we require the “3 T” and one “A” which refers to Time, Temperature, Turbulence and Air.

• It takes time to establish combustion, it requires the right temperature which is the source of spark or ignition, the mixing (turbulence) of the fuel and oxygen, and then air which provides the oxygen.

THE NEED FOR COMPLETE COMBUSTION

• In accordance with existing Environmental laws, we are mandated to ensure we always carry out efficient combustion and to control the flue gas emissions.

• When the quantity of air supplied is insufficient (i.e. the air-fuel ratio is below that required for complete combustion), we would have an increase in fuel wastage, a decrease in efficiency and increase in pollution as we would have un-burnt carbon particulates, carbon monoxides and potentially explosive gases in our exhaust system as by-products of combustion.

• On the other hand, excess air tends to reduce the heating effect of combustion. Hence we need to control the quantity of air supplied so that we have the right air-fuel ratio.

• For complete combustion of natural gas, the Air/Fuel ratio is approximately 10:1 by volume.

• Any mixture with Air/Fuel ratio less than 9.5:1 is known as a rich mixture (higher quantity of fuel).

• Any mixture greater than 10:1 is known as a lean mixture (lesser quantity of fuel).

THE DRAFT SYSTEMS• The flow of air through the boiler is known as Draft. Draft

can be achieved either Naturally or Mechanically through the use of fans and blowers.

• Natural Draft System is the system in which air or flue gases flow naturally due to differences in density between the hotter (less dense) air and the cooler (denser) surrounding air. In most Electric Power Generation Plants, Natural Draft cannot supply the adequate volume of air for complete combustion due to the high pressure of the fuel from the nozzles and also the resistance of the fuel chamber and stack; Hence the usage of fans.

• Mechanical Draft is divided into two which are: 1. Induced Draft (ID) and 2. Forced Draft (FD) systems. • In the FD fan system, air is blown or forced into the combustion

chamber; While in the ID fan system, air is pulled or drawn through the combustion chamber.

• The FD fan creates a positive pressure in the combustion chamber; While the ID fan creates a negative pressure.

• When both systems are used, it is known as a Balanced-Draft System.

• Figures 1 to 4 below show the diagrammatic representation of the Draft systems.

Diagrammatic Representation of Natural Draft

Diagrammatic Representation of Forced Draft

Diagrammatic Representation of Induced Draft

Diagrammatic Representation of Balanced Draft

• Most boilers today use mechanical draft rather than natural draft as more definite control is possible with this method.

• Also a high chimney is not necessary to produce mechanical draft and mechanical draft does not vary with wind and weather conditions but natural draft does.

• Like the natural draft, both the forced draft and the induced draft systems also use a chimney. Although the chimney does not produce the draft in both the FD and ID systems, it is necessary to discharge the combustion gases high enough above ground level to reduce air pollution.

• Balanced Draft is used only in very large boilers.

COMPONENTS OF THE FD FAN SYSTEM

• The forced draft fan system in a thermal station supplies the air required for combustion and also air required for the purging of the boiler.

• Purging is the process in which fans force air through the boiler with all the vents open so as to expel gases that might have accumulated after the previous combustion.

• Purging reduces the risk of explosion.

• The FD fan system includes the following for each unit.

• (a) Two-50% forced draft fans.• (b) One-100% steam Coil air heater.• (c) One-100% regenerative or main air heater• (d) Two-100% Ignition and Cooling air fans.• (e) Two-50% air flow measuring Venturis• (f) One-Boiler stack.

• It also includes associated air and flue gas ducting and dampers.

• The combustion air is delivered by the forced draft fans to the air heaters via a common cold air duct.

• The hot air leaving the main air heater divides into two, passes through air Venturis before getting to each burner via individual burner wind boxes.

COMPONENTS OF THE FD FAN• The FD fan system consists of the following components:1. The Duct: This is the channel/piping through which ambient air enters and

passes through to the wind-box of the boiler.2. Inlet Screen/Mesh: This is the wire mesh at the inlet which prevents bird

and particles from entering into the fan. It filters and cleans the air. 3. Bellows: These are piping expansion joints which absorb vibration. They

are often made of thick treated leather. They pull air in when they expand, and push it out when they contrast thus supplying the required air pressure.

4. Dampers: These are manually or automatically operated panels used to restrict the inlet or outlet area of air flow thus providing control of the air or gas flow rate.

5. Inlet Guide Vanes: These direct the air flow towards the eye of the impeller

6. Outlet Guide Vanes: These direct the air flow towards the combustion chamber.

7. Electric Motor: This provides power for the fan8. Lube Oil Pump: This pumps oil for lubrication of fan bearings and

dampers9. Impeller: This is the part of the fan which directly pressurises the

air10. Shaft and Coupling: This transfers the power from the motor to

the fan.11. Base: This provides support for the FD Fan System. 12. Silencers: Modern FD fans such as those in Egbin Power Plc are

fitted with Silencers to reduce the noise and vibration.

Left And Front View Of The FD Fan

Right Side View OF FD FAN With Impeller

Typical FD Fan Specifics

• The FD fan has a large capacity induction motor (3-phase) and has an impeller carrying moving vanes.

• It has dampers that control the air-flow in and linkages which link the dampers and the vanes to the outside.

• The vane is controlled by a pneumatic actuator. • The linkage is a shaft that aids transmission of power for

control purposes of the vanes. • The mechanism inside the actuator pushes the linkages

which in turn moves the vanes.• Moving the handle thus moves all the glasses.

• The forced draft fans are of the double inlet type and are located at basement floor level at the rear of the boiler with the suction end located on the 4th floor.

• The fans are controlled by variable Vanes in the inlet boxes and can be isolated with a power operated shut off damper in the discharge duct.

• Each fan is driven by constant speed, air cooled electric motor through a flexible spacer type coupling.

• The FD fan has Radial and Thrust bearings which are ring lubricated.

• In addition a forced lubrication system is provided which is designed to be put into service prior to the fan start but may be taken out of service later.

• The lubricating oil system is cooled by water taken from the general service water system.

CHARACTERISTICS OF FD FAN

• The inlet of the fan is placed in the Plant house and not directly outside as the Plant house is warmer and this raises the temperature of the air.

• The FD fan inlet has a wire mesh to prevent birds and particles from entering into the system.

• In cold regions, some FD fan inlets are fitted with heaters to raise the air temperature and prevent ice/snow from entering thus increasing the thermal efficiency of the plant.

• If this isn’t done, more fuel will be used up in heating the air during combustion.

• The FD fan used in a typical Steam Plant is a centrifugal fan and it can be driven by a motor or a turbine.

• The assembly of the hub, disk and blades is known as the fan wheel or impeller.

• The fan uses the centrifugal force of rotation of the wheel to impart movement to the air and thus increase its pressure.

• Although FD Fans are used to control the flow of air using damper adjustments, flow control by the use of variable speed is smoother and more efficient.

• Variable speed can be achieved through the use of an adjustable frequency AC controller, a DC motor and drive, a steam turbine driver, or a hydraulic variable speed drive unit (“fluid drive”).

FD FAN PROBLEMS

• CASE 1: DAMAGED VANES• Vanes naturally get stiff with time and thus gets

damaged when they are forced to open. Hence, vanes must be lubricated efficiently.

• Linkages must be greased regularly.• Check bearing oil regularly. • Also check cooling water valves to ensure cooling

water is flowing as water cools the oil that cools the bearings.

• CASE 2: INCOMPLETE COMBUSTION• A typical scenario of FD Fan problem is that of incomplete

combustion in the boiler. • It was noticed that the fan wasn’t supplying enough air for

combustion. • Troubleshooting the fan revealed that one of the damper

pins sheared off so the linkage connection was separated so that damper would move without it moving the vane.

• This led to insufficient air coming in.• A new pin was installed and the damper was properly

positioned. This solved the problem.

• CASE 3A: NOISE AND VIBRATION• The boiler was provided with a high pressure FD fan to meet the combustion air

requirement of a 30 TPH (tonnes per hour) FBC Boiler. Client reported issue of noise and vibration from the FD fan especially at partially loaded condition. The sound could be heard at the factory entrance which was nearly half a km away from the boiler house.

• FAN vendor could not resolve the problem• On a close inspection of the Inlet Guide Vane (IGV), it was observed that the IGV

would induce a swirl motion of the entering air in a direction opposite that of the Fan rotation.

• The linkages were removed and the IGV was totally modified so that the rotation of swirl was in the same direction to that of the Fan.

• Incidentally the Fan vendor had tried just reversing the IGV. This would not change the swirl direction. The problem was solved after the turbulence created by the IGV was removed.

• CASE 3B: NOISE AND VIBRATION • Excessive noise and vibration was noticed during

operations. • Troubleshooting revealed misalignment between the fan

and motor. • Re-alignment of the shaft and motor, after ensuring that

the coupling is still in proper condition, solved the problem.

• Noise and vibration leads to a decrease in efficiency as energy is lost in the form of the kinetic energy of vibration.

• A list of possible causes of noise and vibration includes:1. Worn bearings2. Unstable foundation3. Foreign material in the fan causing an imbalance4. Misalignment of bearings, couplings, wheel or v-belt drives5. Damaged wheel or motor6. Bent shaft7. Worn coupling8. Loose dampers or variable inlet vanes9. Speed too high or incorrect fan rotation10. Vibration to fan transmitted from other source11. Uneven blade wear12. Loose or broken bolts or set screws

• CASE 4: OVERHEATING OF BEARINGS• Inadequate lubrication leads to overheating of bearings. Poor

lubrication may arise due to the use of substandard lube oil. It may also arise from the use of lube oil pumps which are not of the rated capacity.

• A list of possible cause of bearing overheating includes:1) Improper lubrication2) Poor alignment3) Damaged wheel or driver4) Bent shaft5) Abnormal end thrust6) Dirt in bearings7) Improper belt tension

• CASE 5: OVERLOAD ON DRIVER• Overload on driver occurs when:• Speed is too high• Direction of rotation is incorrect• Bent shaft• Poor alignment• Improper lubrication• Wheel wedging or binging on fan housing

• CASE 6: FAN CAPACITY BELOW RATING• This can occur due to:I. Dampers or variable inlet vanes are not

adjusted properlyII. Fan inlet or outlet conditions are impairedIII. Multiple air leaks within the systemIV. Damage sustained to the fan wheelV. Direction of rotation is incorrect.

FD FAN OPERATIONS

• 4.1) CONDITIONS FOR FD FAN TO START• For us to start the FD fan we will first start the MAH.• Check that the 3.3kv breaker is racked in and ready

to start• Check that the lube oil is running and the pressure is

up to 1.2kg/cm2

• The damper must be closed• The Vane must be closed• MAH in service

• 4.2) ISOLATION OF THE FD FAN • Rack out the 3.3kv breaker and tag it• Close the Vane and it will show that you

closed it• Rack out the lube oil pump of the FD fan and

tag it• The damper closes on its own when you close

the Vane

THE END