Chapter Two steam power plant

2.04 - draught

fundamentals• Definition

draught is the small pressure difference which causes a flow of gas to take place.

• Functionin case of a boiler, draught is to :

1. Force air to the fire.2. Carry away the gaseous products of combustion.• In a boiler furnace, proper combustion takes place only when

sufficient quantity of air is supplied to the burning fuel.• Tools

To obtain draught as defined above and to function as stated above, the tool required is called “chimney”

Flue gas stack (chimney)A chimney is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the stack, or chimney, effect. The space inside a chimney is called a flue. Chimneys may be found in buildings, steam locomotives and ships. The term funnel is generally used for ship’s chimneys and sometimes to refer to locomotive chimneys..Chimneys are tall to increase their draw of air for combustion and to disperse pollutants in the flue gases over a greater area so as to reduce the pollutant concentrations in compliance with regulatory or other limits.

A flue gas stack is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air. Flue gases are produced when coal, oil, natural gas, wood or any other fuel is combusted in an industrial furnace, a power plant's steam-generating boiler, or other large combustion device. Flue gas is usually composed of carbon dioxide (CO2) and water vapor as well as nitrogen and excess oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as particulate mattr, carbon monoxide, nitrogen oxides and sulfur oxides. The flue gas stacks are often quite tall, up to 400 meters (1300 feet) or more, so as to disperse the exhaust pollutants over a greater area and thereby reduce the concentration of the pollutants to the levels required by governmental environmental policy and environmental regulation. When the flue gases are exhausted from stoves, ovens, fireplaces, or other small sources within residential abodes, restaurants, hotels, or other public buildings and small commercial enterprises, their flue gas stacks are referred to as chimneys.An important note must be taken, that :• draught is the process of induction as well as exhausting gases• chimney is the tool through which and by which a draught is created.

Draught classification

I – natural draughtNatural draught is obtained by the use of chimney, and the draught thus Produced is due to the density difference between the column of hot gas inside the Chimney and the cold air outside.

The above figure shows a diagrammatic arrangement of a chimney of height 'H' above the grate

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• GrateIt is a platform in the combustion chamber, upon which the solid fuel (wood or coal) is burnt. It generally consists of cast iron bars which are space apart so that air (required for combustion) can pass through them. The surface area of the grate, over which the fire takes place, is called grate surface.

Static draught

the difference of pressure causing the flow of gases is known as "static draught"

Chimney effect

• The stack effect in chimneys:

• the gauges represent absolute air pressure

• the airflow is indicated with light grey arrows.

• The gauge dials move clockwise with increasing pressure.

or stack effect• Stack effect is the movement of air

into and out of buildings, chimneys, flue gas stacks, or other containers, and is driven by a specific force.

• Such force occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences.

• The result is either a positive or negative force.

• The greater the thermal difference and the height of the structure, the greater the force, and thus the stack effect.

• The stack effect is also referred to as the "chimney effect", and it helps drive natural ventilation and infiltration.

chimney effect calculation The combustion flue gases inside the flue gas stacks are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called "natural draft (or draught)", "natural ventilation“ , "chimney effect", or "stack effect". The taller the stack, the more draft (or draught) is created.The equation below provides an approximation of the pressure difference, ΔP, (between the bottom and the top of the flue gas stack) that is created by the draft:

Where: ΔP = available pressure difference, in (Pa)C = 0.0342a = atmospheric pressure, in Pah = height of the flue gas stack, in mTo = absolute outside air temperature, in (degree K)Ti = absolute average temperature of the flue gas inside the stack, in K

Chimney Height

• Assuming that under same (T&P) conditions :volume of combustion products = volume of air supplied

• And let :

And also

Sub values of (ρa & ρg ) in above static draught eq.[

Assuming draught pressure (∆P) is equivalent to (H1)m height of burnt gases.

Equating the two expressions for (∆P)

Let (hw) be the height of water column in (mm) shown by U-tube manometer Which will produce (∆P)

Chimney diameter• Assuming no losses, the velocity of the gases passing through the chimney

is expressed as:• C = √2gH1• If there is losses, then the pressure loss in the chimney is equivalent to a

hot gas column of (h‾), then• C = √2g(H1 – h‾)

= 4.43 √(H1 – h‾) = 4.43 √(1 – h‾ / H1) [ √H1 ]= K [ √H1 ]

• Where • K = 0.825 for brick chimney

= 1.1 for steel chimney

The mass of the gases flowing through any cross section is given by:ṁg = ρg . A . C kg/s

and sinceA = π/4 D²

Then chimney diameter can be evaluated in terms of other variables, where (C ) and (ρg) evaluation as derived above.The final form of chimney diameter evaluation can be presented as

The flue gas flow rate induced by the draftAs a "first guess" approximation, the following equation can be used to estimate the flue gas flow rate induced by the draft of a flue gas stack. The equation assumes that the molar mass of the flue gas and the outside air are equal and that the frictional resistance and heat losses are negligible :

where: Q = flue gas flow rate, m³/sA = cross-sectional area of chimney, m² (assuming it has a constant cross-section)C = discharge coefficient (usually taken to be from 0.65 to 0.70)g = gravitational acceleration at sea level, 9.807 m/s²h = height of chimney, mTi = absolute average temperature of the flue gas in the stack, KTo = absolute outside air temperature, K

Max draught

• The chimney draught is most effective when the max weight of hot gases is discharged in a given time.

• This will occur when :Tg = f (Ta)

The gas velocity when the losses to be negligible is

and since

Sub (H1) in velocity equation

The density of the hot gases

The mass of the gas discharged per second

Inserting values of C & gas density to get

II - Artificial draught

II.A – Mechanical draught

• The common methods applied for mechanical draught in steam power plants are :

1. Forced draught2. Induced draught3. Balanced draught

1 - Forced draught

2 – induced draught

3 – balance draught

II.B - Steam jet draught

Designing chimneys and stacks to provide the correct amount of natural draft involves a great many factors such as :

1. The height and diameter of the stack .2. The desired amount of excess combustion air needed to assure

complete combustion .3. The temperature of the flue gases leaving the combustion zone .4. The composition of the combustion flue gas, which determines the flue

gas density.5. The frictional resistance to the flow of the flue gases through the

chimney or stack, which will vary with the materials used to construct the chimney or stack .

6. The heat loss from the flue gases as they flow through the chimney or stack .

7. The local atmospheric pressure of the ambient air, which is determined by the local elevation above sea level .

The calculation of many of the above design factors requires trial-and-error iterative methods.

tutorial• All chimneys produce a suction at their base due to the difference in density of the

hot flue gases rising in the chimney.Suction Effect = Weight of air column at Tamb - weight of column of hot flue gases.• Or;• ∆p = H x g ( ρ air - ρ flue gas )• where;• ∆p = suction pressure or draught (N/m2) obtained from boiler catalogue.• H = height of column (m) or chimney height• g = acceleration due to gravity (9.81 m/s2)• ρ = density (kg/m2)• Chimney or Flue height can be determined from the following;• H = ∆p / [ g ( ρ air - ρ flue gas ) ]

Example 1

• Calculation the minimum flue height for an oil fired boiler and determine a suitable termination height for the flue given the following information;

• DATA• Building height at eaves level = 6

metres.• Height of roof ridge = 8 metres.• Height of highest openable window =

3.2 metres.• Outside air density = 1.2 kg/m3.• Flue gas density = 1.0 kg/m3.• Minimum flue gas suction pressure at

boiler = 6 Pa (N/m2).• Pressure loss in straight flue and flue

fittings = 4 Pa.

• Answer•• Flue Gas Suction Pressure

required = minimum boiler suction pressure + Flue pressure losses.

• Flue gas Pressure required at boiler = 6 + 4 = 10 Pa

•• Minimum flue height:•• H = ∆p / [ g ( ρ air - ρ flue gas ) ]•• H = 10 / [ 9.81 ( 1.2 – 1.0) ]•• H = 10 / 1.962 = 5.097 metres.•• A suitable termination height would be 6

metres since this is; more than the calculated minimum height, above openable windows, lower than the ridge height and at eaves level.