Economizer
Function of economizer:
The economizer is a heat exchanger used to preheat the feed water using the hot gases leaving the boiler.
When the combustion gases leave the boiler after giving most of their heat to evaporator tubes, Superheater tubes and reheater tubes, they still possess lot of heat which if not recovered by means of some devices would go waste. The economizer is one of such neat recovering device in which the temperature of feed water is increases by means or outgoing flue gases before feed water is supplied to boiler drums.
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 economizer.
(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 reheater, having large amount of heat energy which 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 economizer is a heat exchanger used to preheat the Teed water using the hot gases leaving the boiler
Economizer consists of series of steel tubes through which the feed water flow as shown in Fig. below. The combustion gases pass over the tube and convective heat transfer taken place to feed water. Economizers may be parallel flow or counterflow, when the gas flow and water flow are in same direction the economizer are called parallel flow whereas in counter flow the gas flow and water flow in the opposite direction. A counter flow arrangement is nominally selected so that heating surface requirement is kept minimum for the same temperature drop in flue gas. Economizer tubes are made of steel either smooth or covered with fins to increase heat transfer surface area, Economizer tubes are generally 45 to 70 mm in outside diameter and coils are installed at a pitch of 45 to 50 mm spacings.
The temperature of feed water entering the economizer should be high enough so that moisture from the flue gases does not condense on the economizer tubes which may be absorb SO2 and CO2 from the flue gases and form acid to corrode the tubes. Also the feed water temperature at inlet of pump should be less than 85˚ C. Since there is a sight negative pressure on the suction side of pump, if temperature of feed water is greater than 85˚ C steam bubbles begin to from and boiler feed pump will not be able to pump mixture of steam and water and flow stops. Therefore, pump always placed before the economizer. The maximum temperature to which the feed water can be heated in the economizer is 25˚ C below saturation temperature corresponding the pressure.
Classification of economizers
1. Based on construction
i) Plain tube type economizers
ii) Gilled tube type economizers2. Based on part of steam generation
i) Steaming type economizers
ii) Non-steaming type economizers3. Based on location of economizers
i) Independent economizers
ii) Integral economizers
Plain tube type economizer:
In this economizer, the group of tubes are made of cast iron to resist corrosive action of flue gases. The ends of tubes are pressed into top and bottom headers. The feed water is distributed along the section by branched pipe coupled to the bottom headers. The economizer is located in the main flue between the boiler and the chimney. The waste flue gases flow outside the economizer tubes and heat is transferred to the feed water flowing inside the tubes.
The extremal 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. Every economizer is provided with a safety valve, a drain valve and release valve. These types of economizer are generally used in Lancashire boiler works under natural draught.
Gilled tube type economizer:
In this economizer, casting rectangular gills are provided on the bare tube walls. Hence, heat transfer surfaces are provided on the bare tube walls. Therefore, heat transfer surface are increased and size of economizer is reduced. Cast iron gilled tube economizers can be used up to 50 bar working pressure. At higher pressures (>50 bar) steel tubes are used instead of cast iron but cast iron gilled sleeves are shrunk to them.
In case of steaming economizer, some part of the water (about 5 to 7%) to DC converted into steam during its passage through the economizer. This types of economizer are used in large steam power plants. While in case of non steaming economizer feed water is heated within 75% of the saturation temperature of the boiler.
When economizer is installed outside the boiler house at any convenient place then it is known as independent economizer. While integral economizer is installed close to the boiler.
Q1. Why economizers are used in power plant?
Q2. Discuss different types of economizers.
Q3. Why economizers are located after feed pump? OR What is location of economizer in power plant?
Q4. With neat sketch, explain working of plain tube and gilled tube economizer.