Draught

Draught (Draft)

The difference in pressure of the combustion product with in a boiler furnace and cold air outside is known as draft.

Differential pressure between the air column (chimney height) outside and the hot flue gas column (chimney height) inside the chimney.

Classification

1.      Natural draught

2.      Artificial drought – I. Steam jet drought (Forced, Induced)

                                        II. Mechanical drought (Forced, Induced, Balanced)

Forced Draught: A blower or FD fan is installed upstream of boiler and air is forced to pass through the furnace, flues, economizer, air heaters and the chimney.

Induced Draught : A blower or FD fan is installed own stream of the air heater to suck out the air from the furnace and let it out via economizer and air heater to the atmosphere through the chimney.

Balanced draught: The FD fan will supply the combustion air for proper and complete combustion of fuel and will overcome the fuel bed resistance in case of stroke grate. The ID fan will remove the flue gas and excess air from the furnace, maintaining the pressure inside the furnace just below the atmospheric pressure.

	Natural Draught	Artificial Drought
	When the Draught is generated with the help of chimney only.	The Draught produced by steam jet or mechanical means like fans and blowers.
Adv.	Simple in design and construction.	No need for a large chimney.
	Low capital investment.	High operating efficiency (70%).
	Low maintenance costs.	Functionally independent of climatic conditions.
	Need no power input for operation.	Any grade of fuel can be burned.
	High dispersion of flue gases as chimney height is too high.	No need for high temperature of the exhaust flue gas.
	Long life.	Draught can be regulated as per the requirement of the furnace.
Dis Adv.	Draught depends upon climatic conditions (ambient air temperature) hw = 176.5 H/Tair	High investment.
	Low operating efficiency.	High operation and maintenance cost.
	Large chimney.
	About 20 % of heat released by fuel is lost due to high temperature of exhaust flue gas.

System RR

Resistance characteristic :

Interpolation of various static pressure developed within the furnace at varying air flows yields a certain curve is called system resistance characteristic.

 

Supply of air is controlled by:

1.      Damper control

2.      Speed control

Damper control	Speed control
For the fan operating at speed N1 rpm, point a marks the point of operation at full load whence the quantity of air supplied is Qa. Now if the load is to be reduced, the flow of air is to be reduced say to Qb. This is determined by increasing the system resistance by partially closing the damper. This brings about a new resistance curve (R2), which intersects the fan characteristics at point b. Now quantity of airflow Qb corresponding to this point is the desired value.	This is achieved by changing the fan rpm to bring about the desired change in fan characteristic. If the air flow is to be reduced from Qa to Qb, the speed of the fan is reduced from N1 to N2 rpm so that the new fan characteristic intersects the resistance curve R1 at point b corresponding to airflow Qb.
	Effect of speed in Fan performance Volume flow ~ N Pressure head ~ N2 Power input ~ N3
	Types of drive 1.      Variable speed steam turbine 2.      Hydraulic coupling 3.      Variable speed DC motor 4.      Variable speed AC motor 5.      Electronically adjustable motor driver.

 

Speed control is economical because the power (BHP) requirement in speed control is less than of damper control to bring about the desired change in air flow quantity.

 

Primary air: That part of the air supply to a combustion system which the fuel first encounters. It is uses to dry the coal in pulverizer and convey the powdered coal from pulverizer to furnace.

Secondary air: The second stage of admission of air to a combustion system, generally to complete combustion initiated by the primary air. It can be injected into the furnace of a boiler under relatively high pressure when firing solid fuels in order to create turbulence above the burning fuel to ensure good mixing with the gases produced in the combustion process and thereby complete combustion.

 Tertiary air:   A third stage of admission of air to a combustion system, the reactions of which have largely been completed by secondary air.  Tertiary air is rarely needed.

Theoretical air or Stoichiometric Air:   It is the minimum amount of air that would be theoretically needed for complete combustion of fuel, if mixing of fuel and air in burner is perfect. It is determined by the analysis of chemical constituents of fuel.

 

Theoretical air requirement for complete combustion of 1 Kg fuel

= 11.5 C + 34.5 (H-O/8) + 4.3S

Excess or Actual Air: it is generally defined as air introduced above the the theoretical air requirement to effect complete and efficient combustion of fuel. In actual practice, since mixing of fuel and air is never perfect, a certain amount of excess air is needed to complete combustion and ensure the release of entire heat contained in fuel.

Disadvantage of excess air:

1.      Increased stack loss

2.      Oxygen supplied in excess air will be affecting the tube and corrosion takes place.

3.      Higher volume of air may cause damage to boiler refectory wall, since higher pressure is developed.

The chimney Draught is a function of the temperature of the hot flue gas leaving the chimney. Higher the temperature of flue gas, greater will be the draught. So a sizable portion of heat generated in boiler furnace goes to heat up the flue gasses to the desired temperature to maintain the desired draught level. This heat could have otherwise been utilized for heating the BFP or preheating the combustion air to increase the efficiency of the boiler.

Wind pressure normally takes to design a chimney is 0.015 Kgf/cm2.

For maximum discharge to take place through a chimney, the chimney draught expressed in terms of hot flue gases should be equal to the height of chimney.

Cold Air Inversion: when the flue gas pressure inside the chimney is less than the air pressure outside the chimney. When several boiler working in partial load are connected to a common chimney.

Gas Recirculation Fan: It is used to recirculate gas from a point between economizer and APH, back to the furnace as a part of a steam temperature control system.

Prestart checks of ID, FD and PA Fan:

1.       Ascertain no body inside the boiler.

2.       All manhole doors, expansion flaps are closed.

3.       No mechanical work in fan and clearance from electrical department.

4.       Check that lubrication in DE/NDE bearing is normal.

5.       Check that coupling bolts, coupling, coupling guard, base frame, pedestal foundation bolts are healthy and tight.

6.       Check bearing temperature measuring RTD and motor all protection are healthy.

7.       Check that main/control supply of motor fan, dampers actuator is on.

8.       Check the IGV and discharge damper functioning normal.

9.       Now keep the IGV of fully shut position and discharge damper open.