Difference between Primary & Secondary Heat Transfer Surfaces of a Boiler
A boiler in every power plant is a type of heat exchanger. Heat from the combustion of fuel in the combustion chamber is transferred to the feed water circulating inside the boiler which ultimately evaporated to form steam which is utilized in a steam turbine attached to the boiler. So a heat transfer surface is essential for a boiler to perform its function. Heat transfer surfaces of a boiler are classified in to:
- Primary heat transfer surfaces - Eg: Evaporator, Super heater, Reheater
- Secondary heat transfer surfaces - Eg: Economiser, Air Preheater
Primary heat transfer surfaces
These are heat transfer surfaces which are the most essential for a boiler without which a boiler cannot perform its function. They utilize heat from the combustion of fuel and is transferred to the working fluid in the boiler directly. Evaporator and superheater are the primary heat transfer surfaces and if the boiler employs reheating, then reheater is also considered as a primary heat transfer surface. Super heaters and reheaters are primarily used for increasing the quality of steam or dryness fraction of steam at the exit of the steam turbine which results in fewer erosion losses in turbine and condenser. By deploying a superheater steam consumption of the plant also get reduced or in another way size of the plant can be reduced for same heating capacity if we deploy a superheater. Losses due to steam condensation in pipes and cylinders are also reduced because of super heating.
Secondary heat transfer surfaces
These are the heat transfer surfaces which utilizes the waste heat of the flue gases or they are surfaces for waste heat recovery which ultimately improves the efficiency of the boiler. Economizers and air preheaters are best examples of secondary heat transfer surfaces. Economizers are used for preheating the feed water before entering into the boiler. They are basically known as boiler accessories. Air preheater is a device which is used for preheating the air coming into the boiler. Economizer helps to reduce thermal stresses due to unequal expansion in the boiler and restricts the chilling of the boiler metal. It also enhances the evaporative capacity of the boiler. Whereas preheater helps to accelerate the combustion and facilitates the burning of fuel. However, both these devices improve the efficiency of the plant and reducing the size of the plant by reducing the specific fuel consumption. Air preheater is usually placed after economizer.
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can you please tell me in which model the thermic fluid heater work
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