An economiser is a mechanical device which is used as a heat exchanger by preheating a fluid to reduce energy consumption. In a steam boiler, it is a heat ex-changer device that heats up fluids or recovers residual heat from the combustion product i.e. flue gases in thermal power plant before being released through the chimney. Flue gases are the combustion exhaust gases produced at power plants consist of mostly nitrogen, carbon dioxide, water vapor, soot carbon monoxide etc. Hence, the economiser in thermal power plants, is used to economise the process of electrical power generation, as the name of the device is suggestive of. The recovered heat is in turn used to preheat the boiler feed water, that will eventually be converted to super-heated steam. Thus, saving on fuel consumption and economising the process to a large extent, as we are essentially gathering the waste heat and applying it to, where it is required. Nowadays however, in addition to that, the heat available in the exhaust flue gases can be economically recovered using air pre-heater which are essential in all pulverized coal fired boiler.
(ii) Integral economizers: economizer is installed close to the boiler
4.Types of Economiser Based on boiler Efficiency
(i) Non condensing Economiser
The most common type of economiser used in thermal power plant is the non condensing thermal power plant. They are Heat exchanger tubes with fins around them in the form of spiral and they are loacated inside the duct at the exit region of the boiler.In this economiser used for reducing the flue requirement of boiler by transfering the heat from the flue gas to the feed water inside the tubes. These are used in coal fired thermal power plant where the temperature of the flue gas can be reduced to 120 deg c and not beyond it.This type of Economiser increases the efficiency by 3-6%
(ii)Condensing Type Economiser: They are used in nautral gas fired thermal powerplant. They reduced the fluegas temperature below condensation temperature as low as 25 deg c. They increase the boiler efficency by 10-15%. while using the condensor the fuel should not contain sulphur.
Working Principle of Economizer
As shown in the figure above, the flue gases coming out of the steam boiler furnace carry a lot of heat. Function of economiser in thermal power plant is to recover some of the heat from the heat carried away in the flue gases up the chimney and utilize for heating the feed water to the boiler. It is simply a heat ex-changer with hot flue gas on shell side and water on tube side with extended heating surface like Fins or Gills. Economisers in thermal power plant must be sized for the volume and temperature of flue gas, the maximum pressure drop passed the stack, what kind of fuel is used in the boiler and how much energy needs to be recovered. When the water is boiled in steam boiler, the steam is produced which is then super-heated after which it is passed to the turbines. Then the exhausted steam from turbine blades, is passed through steam condenser of turbine in which the steam is condensed and this condensed water then is pre warmed first in feed water heater then in it before re-feeding in boiler. It is placed in the passage of flue gases in between the exit from the boiler and the entry to the chimney. In this a large number of small diameter thin walled tubes are placed between two headers. The flue gases flow outside the tubes usually in counter flow.
Classification of economizers:
1. Based on construction
(i) Plain tube type economizers: The external surface of tubes is kept clean
and free from soot by soot scrapers moving up and down the economizer
tubes. Otherwise heat transfer resistance is increase and efficiency of
economizer decreased.
(ii) Gilled tube type economizers:
Rectangular gills are provided on the bare tube walls to increases the
heat transfer surfaces .
2. Based on part of steam generation
(i) Steaming type economizers: some part of the water (about 5 to 7%) to
be converted into steam during its passage through the economizer.
(ii) Non-steaming type economizers: While in case of non steaming
economizer feed water is heated within 75% of the saturation
temperature of the boiler.
3. Based on location of economizers
(i) Independent economizers: economizer is installed
outside the boiler house at any convenient place
(ii) Integral economizers: economizer is installed close to the boiler
4.Types of Economiser Based on boiler Efficiency
(i) Non condensing Economiser
The most common type of economiser used in thermal power plant is the non condensing thermal power plant. They are Heat exchanger tubes with fins around them in the form of spiral and they are loacated inside the duct at the exit region of the boiler.In this economiser used for reducing the flue requirement of boiler by transfering the heat from the flue gas to the feed water inside the tubes. These are used in coal fired thermal power plant where the temperature of the flue gas can be reduced to 120 deg c and not beyond it.This type of Economiser increases the efficiency by 3-6%
(ii)Condensing Type Economiser: They are used in nautral gas fired thermal powerplant. They reduced the fluegas temperature below condensation temperature as low as 25 deg c. They increase the boiler efficency by 10-15%. while using the condensor the fuel should not contain sulphur.
Advantages of economizers:
(1) It improves the boiler efficiency. It has been found that about 1%
efficiency of boiler is increased by increasing temperature of feed water
by 6 °C with help of economiser.
(2) It reduces the losses of heat with the flue gases, The temperature of flue
gases is about 370°C to 540°C at exit of last superheater or rehea ter,
having large amount of heat energy which otherwise otherwise would have been
wasted.
(3) It reduces the consumption of fuel. It has been estimated that about 1% of
fuel costs can be saved for every 6 °C rise in temperature of the boiler
feed water.
(4) It reduces thermal stresses in the boiler due to reduced temperature
differential in the boiler.
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