Economiser in a Steam boiler

In this post, we will discuss the economiser in a steam boiler. This post will help you to understand the concept of using economiser in a steam boiler.

An economiser in a steam boiler

Boiler Economiser ( Sometimes Called Economizer ) is the Energy improving device that helps to reduce the cost of operation by saving the fuel.

The economiser is used to economise the process of power generation. An economiser is a mechanical device which is used as a heat exchanger by preheating a fluid to reduce energy consumption. An economiser in a steam boiler is a heat exchanger device that heats up fluids or recovers residual heat from the combustion product i.e. flue gases in a thermal power plant before being released through the chimney. Flue gases are the combustion exhaust gases produced at power plants consist of mostly water vapor, carbon dioxide, nitrogen, soot carbon monoxide etc.

The recovered heat is in turn used to preheat the boiler feed water, that will eventually be converted to super-heated steam.

Boilers are designed to produce steam from water. Water is converted into steam by transferring both sensible and latent heat.

Sensible heat is the amount of heat required to increase the temperature of water at constant pressure without changing its liquid state

 Latent heat is the amount of heat required to change the state from Liquid to Vapor at constant temperature and pressure.

Working principle of economiser in a steam boiler

Working Principle of economiser in a steam boiler
Working Principle of economiser in a steam boiler

The Economiser in a steam Boiler works on the principle of Heat Transfer.

As shown in the figure above, the flue gases coming out of the steam boiler furnace carry a lot of heat. The function of economiser in a steam boiler is to recover some of the heat from the heat carried away in the flue gases pass to the chimney and utilize this heat for heating the feed water to the boiler. It is simply a heat exchanger with hot flue gas on the shell side and water on the tube side. Economisers in a steam boiler 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.

The process of Heat Transfer in Economizer, Evaporator, and Superheater

Heat transfer to water in steam generator takes place in 3 different regimes, as shown in the figure below.

Economiser in a steam boiler
Economiser in a steam boiler

Water is at first pre-heated sensibly in the economizer in the liquid phase at a certain pressure from state 4 to state 5 (refer to the diagram above) till it becomes a saturated liquid. It is then sent to the evaporator, where this saturated liquid is boiled associating a change of phase from 5 to 6 by absorbing the latent heat of vaporization, at that particular pressure. Now, this saturated vapour in state 6 is further heated in the super-heater, to bring it to state 1, that means, in gaseous or vapour form.

Out, of these 3 major heat ex-changer components, only the economizer operates with, zero fuel consumption, and thus it is one of the most vital and economical equipment in a thermal power plant.

Applications of Economizer

The economiser is used in all modern plants. The use of economizer results in

  • Increases boiler efficiency
  • Increases steaming rate
  • Saving fuel consumption.

In steam power plants it captures the waste heat from boiler stack gases (flue gases) and transfers it to the boiler feed water.

       Advantages and Benefits of Economizer

  • Due to the increase in the fuel prices, all power plants are facing pressure for increasing boiler efficiency. So by using economizer, boiler efficiency can be increased and hence less fuel will require to produce the same amount of steam.
  • It recovers more heat of flue gases which normal air pre-heater cannot do.
  • The efficiency of power plant reduced when steam air pre-heater required steam.

Power plants where it is not used, a large quantity of water is required to cool the flue gas before desulphurization which is minimized by using economizers.

      Types of Economizers Based on Boiler Efficiency

They are broadly classified into two types and have been described below.

Non – condensing Economiser

The most widely used one, in a thermal power plant, is the non-condensing economiser. These basically heat exchanger coils‚ that are finned around in the form of a spiral and are located inside the flue gas duct near the exit region of the boiler. These coils have the ability to reduce the fuel requirements of a boiler by transferring heat from the exit flue gas to the steam boiler feed water. It is used in the case of coal-fired boilers, where the lowest temperature to which flue gas can be cooled is about 250 degrees Farhenite (120 degrees Celcius).

non condensing economiser in a steam boiler
non condensing economiser in a steam boiler

cooling the flue gas below 250 degrees Farhenite and transferring that additional heat to the boiler feed water would have resulted in greater efficiency, but in a coal-fired power plant, this should not be done, since coal as a fuel contains sulphur in a very large extent as the impurity. And the flue gas thus formed by burning.

This coal results in the formation of sulphurous compounds as a by-product. Now if this flue gas is allowed to cool below 250 degrees Farhenite, condensation of the gaseous compounds result in the formation of sulphuric acid, which is considered extremely corrosive against the metal surface. This sulphuric acid may result in the scaling in the boiler tubes. Since the installation and maintenance cost of a power plant is huge, it is noteworthy that a non-condensing economizer is installed to limit the cooling capacity of the flue gas to about 250 degrees Farhenite.

      Condensing Economiser

The condensing economisers are mainly used in natural gas-fired thermal power plants, as the condensing economisers have the ability to improve the waste heat recovery by cooling the flue gas below its condensation temperature, which is about 80 degrees Farhenite (25 degrees Celcius).

condensing economiser in a steam boiler
condensing economiser in a steam boiler

These economisers result in greater efficiency of around 10 to 15% and more economical operation as it reclaims both sensible heats from the flue gas and latent heat by condensing water vapor present in the flue gas. This is contradictory to the conventional non-condensing economizers as they increase the efficiency to only about 5%.

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