What is Economiser in Thermal Power Plant?

Economiser Definition

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.

Working Principle

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.

Types of Economise

CI Gilled Tube Economizer

The gilled tube economizers are made up of cast iron which are fabricated using graded cast iron fins, have following features,

• High optimum efficiency due to proper contact of gills with tubes.

• Commonly used in plants where intoxicated flue gas is generated due to the quality of fuel burnt.

Round Gilled Tube Economizer

This is made by mild steel fabricated with square and round fins, welded on carbon steel seamless tubes, have the feature,

Proper contact between the tubes and fins are ensured for optimum efficiency.

Coiled Tube Type Economizer

These are used mostly in thermal power plants and large processing units. These coiled tube type Economizers are fabricated out of carbon steel seamless, have following features,

• These are very efficient in recovering the heat from gases.

• Occupy very little space.

Horizontal Finned Tube Economizer

In this is carbon steel seamless tube sealed – welded with horizontal fins to make a complete assembly of economizer for heat transfer, have following features,

• Proper care is taken for making the contact of fins with tubes for perfect heat transfer.

• These are used mainly used by Thermal Power Plants.

Non-Condensing vs. Condensing

Non-condensing economisers are used in coal-fired plants to prevent acid corrosion, while condensing economisers, used in natural gas plants, improve efficiency by cooling flue gases below their condensation point.

Applications and Benefits

It is used in all modern plants. The use of economizer results in saving fuel consumption, increases steaming rate and boiler efficiency.

Some of the common applications of economizer are given below:

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

• Air-side economizers HVAC (Heating, Ventilation and Air Condition) can save energy in buildings by using cool outside air as a means of cooling the indoor space.

• Refrigeration: This is commonly used in industrial refrigeration where vapor compression refrigeration is essential. Systems with economizers aim to produce part of the refrigeration work on high pressures, condition in which gas compressors are normally more efficient.

Advantages and Benefits of Economizer

The advantages of an economizer include:

• It recovers more heat of flue gases which normal air pre-heater can not do.

• Due increase in fuel prices, all power plants are facing pressure for increasing boiler efficiency.So by using economizer, this pressure can be minimized.

• Power plants where it is not used, large quantity of water is required to cool the flue gas

• before desulphurization which is minimized by using economizers.

• The efficiency of power plant reduced when steam air pre-heater required steam.