Mehran University of Engineering and Technology SZAB Campus Khairpur Mir’s

BOILERBOILER

THERMPDYNAMICSTHERMPDYNAMICS--IIII

AkashAkash Thahrani (MUET, SZAB KHAIRPUR)Thahrani (MUET, SZAB KHAIRPUR)

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Boiler, also called steam generator is theengineering device which generates steam atconstant pressure.

It is a closed vessel, generally made of steel inwhich vaporization of water takes place.

BoilerBoiler

which vaporization of water takes place.

Heat required for vaporization may beprovided by the combustion of fuel infurnace, electricity, nuclear reactor, hotexhaust gases, solar radiations etc.

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vertical boiler shell of cylindrical shape.

fire box of cylindrical type inside the shell.

Vertical passage of tubular type called uptake is provided over fire box for exhaust of flue gases.

Cross tubes are provided for improving

Simple vertical boiler

Cross tubes are provided for improving water circulation and increasing heating surface.

At the bottom of fire box a fire grate is provided for burning fuel.

Total heating surface area is about 7–10 times grate area.

Man hole and hand holes are provided in the shell for access to inside of shell.

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Hot gases raising from fire grate go upwardly andheat the water contained in shell and tubes.

Steam generated in shell can be tapped through asteam stop valve placed on the crown of shell.

Such boilers have steam generation capacity up to1000 kg per hour and maximum steam pressure1000 kg per hour and maximum steam pressureup to 10 bar.

Size of the boiler ranges from 0.6 m diameter to 2m diameter and height from 1.2 m to 4 m high.

Boiler efficiency is nearly 50%K-13ME-05

BOILER MOUNTINGS AND BOILER MOUNTINGS AND ACCESSORIESACCESSORIESACCESSORIESACCESSORIES

Boiler Mountings:

The necessary devices installed or mounted for the safetyof boiler and its control are called boiler mountings.

(i) Water level indicator(ii) Safety valves(ii) Safety valves(iii) High steam and low water safety valves(iv) Fusible plug(v) Pressure gauge(vi) Stop valve(vii) Feed check valve(viii) Blow off cock

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(I) WATER LEVEL INDICATORIt is used for knowing the level of water inboiler as water level inside boiler shouldnot go below a certain limit.

TWO TUBES: front glass metal tube.Water level is seen through glass tubeWater level is seen through glass tubewhich is made strong enough to withstandhigh steam pressure and temperature.

Two control cocks:For regulating steam and water passage

Blow off cock In case the glass tube breaks it closes the

opening by the two balls. K-13ME-05

(II) Safety valve: Its function is to prevent the steam pressure from exceeding a

limiting maximum pressure value.

Safety valve should operate automatically by releasing excesssteam and bring pressure down within safe limits.

These are of different types such as:

dead weight safety valve lever safety valve spring loaded safety valve etc.

Figure 11.25 gives the generaldescription of dead weight safetyvalve.

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It has a large vertical pipe on the top of which avalve seat is fixed. Valve rests upon this valveseat.

A weight carrier is hung on the top of valveupon which cast iron rings enclosed in cast ironcover are placed in weight carrier as deadcover are placed in weight carrier as deadweight.

When the pressure of steam exceeds the totalweight of valve, it is lifted and falls back assteam pressure gets reduced.

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3.FUSIBLE PLUG:

It is a safety device used for preventing the level of water from going down below a critical point and thus avoid overheating.

Fusible plug is mounted at crown plate of combustion chamber.

Gun metal body A copper plug

put with fusible metal at interface of copper plug put with fusible metal at interface of copper plugand gun metal body.

As water level goes down the heat available fromfurnace could not be completely utilized for steamformation and so the overheating may causemelting of fusible metal.

Fusible metal is a low melting point metal. Thusupon melting of lining the copper plug falls downand water falls from this opening onto furnace andthus quenches fire. K-13ME-05

4.PRESSURE GAUGE:4.PRESSURE GAUGE:

It is mounted at front top. Generally Bourdon type pressure gauge are being used for pressure measurement.

Pressure is continuously monitored so as to avoid occurrence of over shooting of boiler avoid occurrence of over shooting of boiler pressure.

Although safety devices protect boiler against pressure rising beyond a limit are provided but pressure gauges are also used for monitoring pressure.

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5.Stop 5.Stop valvevalve::

It regulates the flow of steam from the boiler. This is generally mounted on highest part of

boiler shell and performs function ofregulating the flow of steam from boiler.

6.Feed 6.Feed check valvecheck valve::

It is a non return valve at the end of delivery pipefrom feed water pump and is placed on boiler shellslightly below normal water level.

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7.Blow 7.Blow off cockoff cock:: It is used for periodical cleaning by discharging

the water and sediments from bottom of boiler.

8.Manhole 8.Manhole and mud boxand mud box::

Manhole provides opening for cleaning, inspection Manhole provides opening for cleaning, inspection and maintenance purpose.

Mud box is a collection chamber for collecting the mud.

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BOILER ACCESSORIESBOILER ACCESSORIES

Devices which are integral part of a boiler, Devices which are integral part of a boiler, but are not mounted on it

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These are devices which are used as integral parts of a

boiler, and help in running efficiently.

Some important accessories are

Feed pump

Superheater

Economiser

Air preheaterAir preheater

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Its purpose is to increase the temperature of

saturated steam without rising its pressure.

SUPERHEATER:SUPERHEATER:

ECONOMISERECONOMISER It’s a device used to heat feed water by utilizing

the heat in the exhaust flue gas before leavingthrough the chimneythrough the chimney

AIR PREHEATERAIR PREHEATER It is used to recover heat from exhaust flue gases.

The air required for the purpose of combustion is drawn

through the preheater where its temperature is raised.

It is installed between economizer and the chimney.

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Performance of Steam BoilerPerformance of Steam Boiler

EVAPORATIVE CAPACITYEVAPORATIVE CAPACITY

Performance of a steam boiler is measured in terms of evaporative evaporative capacity(Kg of water evaporated per hour).capacity(Kg of water evaporated per hour).

However evaporative capacities of two boilers cannot be compared unless both the boilers have same :

Feed water temperature Feed water temperature Working pressure Fuel

And final condition of steamIn actual practice feed water temperature and working pressure varies

considerably.

Which makes it difficult to compare two boilers.

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To overcome this problem, standard conditions are To overcome this problem, standard conditions are adopted,adopted,

Temperature of feed water = 100’CAtmospheric pressure

As per standard conditions 1 kg of water at 100 ‘C As per standard conditions 1 kg of water at 100 ‘C require 2257 kJ 2257 kJ to get converted to steam at 100 ‘C.

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Equivalent Equivalent Evaporation:Evaporation:

Equivalent Evaporation may be defined as the ratio of heat gained by the steam from Boiler at given condition to the amount of water evaporated from water at 100’C to dry and saturated steam at 100’C.

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BOILER EFFICIENCYBOILER EFFICIENCY

Boiler efficiency is the ratio of heat actually utilized in generation of steam to the heat supplied by the fuel in the same period.

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Example:02

The steam used by the turbine is 5.4 kg/kWh at a pressure of 50bar and a temperature of 350’C. The efficiency of the boiler is 82%with feed water at 150’C.

(i) How many kg of 28100kJ coal are required/kWh ?(ii) If the cost of Coal/Ton is RS. 500, what is fuel Cost/kWh?

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(i) Coal required , kg/kWh

Specific heat of water , C= 4.18 kJ/Kg

5050

hf1 = m C ∆T

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Fuel Cost/kWh:

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Example: 02

In a boiler test 1250kg of coal are consumed in 24hours. The mass of water evaporated is 13000 kg andthe mean effective pressure is 7 bar. The feed watertemperature was 40’C, heating value of coal is30000kJ/kg. The enthalpy of 1 kg of steam at 7 bar is30000kJ/kg. The enthalpy of 1 kg of steam at 7 bar is2570.7 kJ.

Determine:

(i) Equivalent evaporation per kg of coal:(ii) Efficiency of the Boiler

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hf1 = m C ∆T

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HEAT BALANCE ON BOILERHEAT BALANCE ON BOILER

Heat balance on boiler refers to the accounting for total heat released inside boiler and its distribution.

Total heat available inside boiler is due to burning

LEC NO: 03

Total heat available inside boiler is due to burning of fuel and can be quantified by the product of mass of fuel and heating value of fuel.

Heat distribution can be given comprising of the following, based on unit mass of fuel burnt.

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((ii) ) Heat Heat used for generation of steam: used for generation of steam:

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(b) Heat lost due to incomplete combustion:(b) Heat lost due to incomplete combustion:

Heat loss due to incomplete combustion can be quantifiedby knowing the additional heat that would be availablewith complete combustion

During complete combustion of carbon into During complete combustion of carbon into CO2, CO2, 3.38 3.38 ××10104 4 kJ kJ of heat is released of heat is released

While with While with incomplete combustion of carbon into CO, 1.012 incomplete combustion of carbon into CO, 1.012 ×× 10104 4 kJ kJ of heat is of heat is by by burning burning one one kg of kg of carbon.carbon.

Hence heat loss due to incomplete combustion of one kg Hence heat loss due to incomplete combustion of one kg of carbon of carbon shall beshall be

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(c) Heat loss to dry flue gases(c) Heat loss to dry flue gases::A large portion of heat getting lost goes along with flue gases. Flue gases leaving boiler comprises of dry flue gases and steam. Heat loss with dry flue gases can be given by

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(d) Heat loss to steam in flue gases:(d) Heat loss to steam in flue gases:

Steam is produced due to burning of hydrogen present in fuel into water vapour. Heat lost with steam in flue gases shall be

enthalpy enthalpy values of values of steam at steam at gas temperature gas temperature Tg and Tg and partial partial pressure of vapour in flue pressure of vapour in flue gas and enthalpy gas and enthalpy of water of water at mean at mean boiler boiler temperature respectivelytemperature respectively..

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(e) Heat lost in unburnt fuel(e) Heat lost in unburnt fuel::

Some portion of heat may get lost in unburnt fuel, which could be given by the product of mass of unburnt fuel per kg of fuel and its calorific value, as

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(f) (f) Heat loss due to convection, radiation and other Heat loss due to convection, radiation and other unaccountable losses:unaccountable losses:

In a boiler heat also gets lost due to convection, radiation from the boiler’s surface exposed to atmosphere.

Heat loss due to convection, radiation and other unaccounted losses losses

= Heat released by per kg fuel burnt– ∑ Heat loss components described from (a) to (e).

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