http://www.authorstream.com PRESENTATION ON “ HEAT LOSSES IN BOILERS and EFFICIENCY OF BOILER ” IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR B.TECH. (MECH. ENGG. DEPARTMENT) MULLANA (AMBALA) SUBMITTED TO : SUBMITTED BY :- H.O.D VISHNU VASHIST 11071022 MECH.ENGG.DEPARTMENT
: BOILER Boiler is constructively combined device into one complex for produce steam or hot water by burning fuel. The heated or vaporized fluid exits the boiler for use in various processes or heating applications . The boiler design depends on type of used fuel, combustion method, steam production unit, pressure and temperature produced steam. Materials The pressure vessel in a boiler is usually made of steel (or alloy steel), or historically of wrought iron . Stainless steel is virtually prohibited (by the ASME Boiler Code) for use in wetted parts of modern boilers, but is used often in super heater sections that will not be exposed to liquid boiler water.
: Fuel The source of heat for a boiler is combustion of any of several fuels , such as wood , coal , oil , or natural gas . Nuclear fission is also used as a heat source for generating steam .
: TYPES OF BOILER
: 1.FIRE TUBE BOILER a) In fire tube boiler, hot gases pass through the tubes and boiler feed water in the shell side is converted into steam b) Fire tube boilers are generally used for relatively small steam capacities and low to medium steam pressures. c) Fire tube boilers are available for operation with oil, gas or solid fuels.
: 2.WATER TUBE BOILER In water tube boiler, boiler feed water flows through the tubes and enters the boiler drum. The circulated water is heated by the combustion gases and converted into steam at the vapour space in the drum. These boilers are selected when the steam demand as well as steam pressure requirements are high as in the case of process cum power boiler / power boilers.
: HEAT LOSSES IN BOILERS
Heat Losses in Boiler: Heat Losses in Boiler a) Heat Loss Due To Unburned Carbon When the fuel is burnt than small amounts of carbon will be left in the ash and this constitutes a loss of potential heat in the fuel. To reduce this heat losses:- Samples of ash must be analysed for carbon content. The quantity of ash produced per unit of fuel must also be known. This loss generally applies to solid fuel fired boilers. A small loss due to un-burnt carbon would also occur while burning fuel oils. b) Heat Loss due to Dry Flue Gas The major energy loss in a boiler occurs through flue gases, which escapes at a high temp. To reduce this heat losses:- Proper utilization of heat from flue gasses. Temperature of gas must be determined before leaving the chimney.
: d) Unburned carbon loss or loss on Ignition c) Heat Loss Due To Moisture in Fuel The moisture in the fuel absorbs heat and leaves the air heater at the gas temperature. The change in enthalpy of the water vapor due to moisture in the fuel constitutes a heat loss. It occur when not all of the available carbon comes in contact with combustion air. This loss can be caused by insufficient grinding of the fuel. If the fuel particles are too large than the surface area is reduced and not all of the particles complete burned. To reduce this heat losses:- Proper grinding of fuel should be done. e) Loss Due To Surface Radiation & Convection It represent the heat loss from the boiler skin to the surrounding environment. These losses are difficult to measure and can be reduced by proper maintenance of the boiler.
: e) Loss Due To sensible Heat in Bottom Ash There can be sensible heat loss from the hot ash which leaves the boiler. f) Heat Loss Due To Blow Down Some blow down is necessary to get rid of dissolved solids in the boiler water because these solids have absorbed heat, blow down is a heat loss.
: SOURCES OF HEAT LOSS
SOURCES OF HEAT LOSS : SOURCES OF HEAT LOSS a) Stack Gas The biggest energy loss in a conventional fossil fuel fired boiler goes 'up the chimney' that is, out the stack. The loss could
amount to as much as 30-35% of the fuel input in worst cases. This stack gas heat loss consists of three different ways: Heat loss due to dry flue gas. Heat loss due to moisture in the fuel. The size of heat loss depends on the temp and the volume of gas leaving the boiler. Therefore, reducing either one of these will reduce the heat loss. Stack gas heat loss is unavoidable, but to eliminate this loss altogether, the stack gas temp would have to be reduced to air temp around the boiler. Practically, this is impossible.
: 1) By keeping heat transfer surfaces clean on the water side will reduce the heat loss. 2) Ash deposits on water walls and on boiler tubes and scale deposits on the water side tube surfaces act as insulation, reducing heat absorption by the boiler water from the flue gas. The Three basic strategies for minimizing stack gas heat loss are:Minimizing excess air:- Excess air is defined as the air supplied to the furnace beyond the air required for complete combustion. By reduced excess air:- 1) Stack gas volume is also reduced. 2) Stack gas temp. is also reduced, because gas velocities are reduced, allowing the gas to spend more time inside the boiler where the heat can be absorbed. Keeping heat transfer surfaces clean:-
: Carbon in the bottom ash comes from unburned coal being dumped in the ash pit. This can be controlled by proper adjustment of the stoker. Adding flue gas heat recovery equipment:- By using flue gas heat recovery equipment heat loss can be reduced. b) Combustible Heat Losses This is the second largest source of heat loss in coal stokers and is caused by unburned fuel. It comes from three sources:- 1) Carbon in bottom ash:- 2) Carbon in fly ash:- Carbon in the fly ash comes from small coal particles that have not been burned. Fly ash re-injection from a mechanical dust collector will reduce this heat loss.
: c) Radiation Heat Losses A portion of heat from combustion escapes from the wall of the furnace without being absorbed by the boiler water. Heat loss from this occurrence is controlled through proper insulation techniques and the maintenance of insulation layers. d) Blow Down In boilers, the effect of water containing some dissolved solids and removal of pure steam results in concentration of solids in water. Above a certain level of concentration, these solids encourage faming, causing carryover of water into the steam, and could cause scaling inside the boiler leading to tube failure because of overheating. This concentration has to be reduced and therefore some boiler water is removed and make up water added. The blow down is at high temperature and represents a heat loss.
e) Combustion and Excess Air in Boiler :
e) Combustion and Excess Air in Boiler a) In boilers, heat is produced from combustion, and then heat is transferred to the water to produce steam. b) Oxygen is required to burn any fuel and this is obtained from atmospheric air. Carbon and oxygen are burnt with oxygen from the air, CO2→as shown in following reactions:Carbon (c) + Oxygen (O2) Water vapor →(carbon-dioxide) + Heat Hydrogen (2H2) + Oxygen (O2) (H2O) + Heat If insufficient oxygen is supplied, some of the carbon may pass through the combustion process, only partially burnt and may be seen in the form of solid deposits and or smoke. (2C) Carbon + Carbon Monoxide (2CO) + Reduced heat→Insufficient Oxygen (O2) Combustion:- Combustion is chemical reaction between fuel and oxygen which results is release of heat.
: Effect of Excess Air:- Excess air affects the process in two ways:- a) The higher the excess air quantity, the lower the flame temp and this results in a lower heat transfer. Maximum flame temperature required for rapid transfer of heat to the surrounding boiler metal is obtained only when no excess air is used. b) The more is the quantity of excess air, the greater will be the weight of flue gas per unit weight of fuel burnt. This increases the heat loss because air enters at ambient temperature and leaves the boiler at high temperature, taking a considerable amount of useful heat with it. Excess Air:- As we know that, more air than theoretical air is required for complete combustion. Less excess air means incomplete combustion and Too much excess air means large heat is dissipated to the chimney.
: Efficiency Of Boiler
: “The percentage of the total absorption heating value of outlet steam in the total supply heating value is” known as the efficiency of Boiler”. @What is the Efficiency Of Boiler? Boiler Efficiency may be indicated by:- a) Combustion Efficiency Indicates a burners ability to burn fuel measured by unburned fuel and excess air in the exhaust. b) Thermal Efficiency - Indicates the heat exchangers effectiveness to transfer heat from the combustion process to the water or steam in the boiler, exclusive radiation and convection losses. c) Fuel to Fluid Efficiency - Indicates the overall efficiency of the boiler inclusive thermal efficiency of the heat exchanger, radiation and convection losses - output divided by input.
@How to conduct Efficiency Test?: @How to conduct Efficiency Test? This test can be conducted by:- 1.Direct method This is i /p &o/p method because it requires only the useful o/p &the heat i /p (fuel) to evaluate the efficiency. Boiler efficiency (%)= heat o/p heat i /p 2.Indirect Method It
aims at calculating several heat losses associated with the boiler & subtracting heat loss % from 100. Dry flue gas losses Heat loss due to moisture in fuel Heat loss due to moisture from burning hydrogen Heat lost due to moisture in air
: Due to inaccuracy of direct method which denies heat losses, we adopt indirect method for our calculations.
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