Boiler is perhaps the most important component of a power plant. In a boiler, the water is heated to high temperature and high pressure steam, which is then used to drive a turbine to generate power. In other words, it is that device which transfers the chemical energy of the fuel into pressure and heat energy that is directly used in producing electricity.
But, the process is not as easy as it seems. It is a challenging task to ensure that a boiler works safely and efficiently. There are many practical issues which an engineer has to deal with, to ensure the same. A lot of boiler accessories and mountings are used to ensure the same.
What is a Boiler Accessory and a Boiler Mounting?
In simple words, a device that enhances the performance of a boiler and boosts its efficiency is called a boiler accessory. It may or may not physically be a part of the boiler itself. We may or may not use an accessory.
On the other hand, devices that are used to ensure that the boiler works properly and safely, are called mountings. Generally, mountings are physically a part of the boiler itself. Most of the mountings are essential to be used with the boiler.
We shall discuss about boiler mountings in another article. This article is going to be dedicated to boiler accessories.
Most Important Boiler Accessories
Though there are many Boiler Accessories that are used in a real power plant, we shall discuss about the 3 most important boiler accessories in this article. If you are a mechanical engineer, you should have guessed them by now. They are Economiser, Feed Water Heater and Superheater.
A. Economiser
Just as the name suggests, an economiser enhances the boiler economy. It is one of those boiler accessories which helps in getting more quantity of steam output at higher temperature by combusting the same amount of fuel (generally coal). It basically heats the water before entering into the boiler by extracting heat from the waste flue gases of the boiler itself. It ensures economy of the fuel and hence the name.
It is placed between the boiler and the chimney, so that the flue gases of the boiler may flow through it before exiting from the chimney. It is practically see that an economiser also increases the life of the boiler.
B. Feed Water Heater
Feed water heater is also one of those boiler accessories in which the feed water is heated before entering the boiler. It’s function is similar to that of economiser but the only difference is that the heat used in this device is taken from the exhaust steam coming from the steam turbines instead of using flue gases, unlike in economiser.
The advantages of using Economiser and Feed Water Heater are similar and in a practical thermal power plant, both are employed in the same order, one after the other. Using feed water heater means that some quantity of steam is tapped from turbine after partial expansion and is used to heat the feed water. This resembles the process of regeneration and ensures increase in thermal efficiency of the system.
C. Superheater
Just as the name suggests, the function of a superheater is to superheat the steam. Once water is completely vaporized to steam at a particular temperature and pressure, it is called dry saturated steam. When this steam is further heated at the same pressure beyond its saturation temperature (boiling point corresponding to that pressure), it is called superheated stem.
Superheated steam has more energy content (enthalpy) than ordinary dry saturated steam and hence it is preferred to have superheated steam to drive the turbine. Superheating increases the mean temperature of heat addition and also increases the area of the Temperature-Entropy property diagram. Thus, it ensures that both, net work output and thermal efficiency of the cycle is greatly boosted. The overall fuel consumption is also reduced by 10-15% by using a superheater. It is evident how important a superheater is for the profitable running of a thermal power plant.
A superheater generally consists of multiple tubes in a grid like structure through the dry saturated steam from the boiler outlet passes and over these tubes, the hot flue gases are made to pass so as to ensure efficient heat transfer. Owing to such an arrangement, a considerable amount of heat is absorbed by the steam from the flue gases and it starts to get superheated.