Economiser  ( Sometimes Called Economizer )

                      Boiler Economizer is the Energy improving device that helps to reduce the cost of operation by saving the fuel.

                     In boilers, economizers are heat exchange devices that heat fluids, usually water, up to but not normally beyond the boiling point of that fluid. Economizers are so named because they can make use of the enthalpy in fluid streams that are hot, but not hot enough to be used in a boiler, thereby recovering more useful enthalpy and improving the boiler's efficiency. They are a device fitted to a boiler which saves energy by using the exhaust flue gases from the boiler to preheat the cold water used to fill it (the feed water).

                      Boilers are generally designed to produce steam from water. Water is converted to 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, while Latent heat is the amount of heat required to change the state from Liquid to Vapor at constant temperature and pressure.

                     If the exhaust gases which are leaving the Boiler at such high temperature is made to pass through the Economiser in order to provide the required sensible heat to the water by increasing its temperature, it will reduce the heat load on the boiler to the greater extent.

The function of Boiler Economizer

                      The Economizer in Boiler works on the principle of Heat Transfer. Heat transfer usually takes place from high temperature to low temperature. In the case of Boilers, flue gases or exhaust from the boiler outlet are at high temperature and water that needs to be preheated is at low temperature. So, this temperature difference between water and flue gases helps to increase the feed water temperature.Depending on the type of operations, design of Economizers can be smoke tube type or water tube type.
                 In smoke tube type flue gases are inside the tubes and water is on the shell side while in the water tube type, water is in the tube and flue gases are on the shell side.

                 Adding an economizer will increase boiler efficiency by 10-15%. In addition, the reduced amount of energy required for heating results in reduced fuel consumption and cost.

How does it Work?

In a boiler system, an economizer captures the heat of the flue gas and transfers it to the feed water. The process is simple. As the flue gas exits the boiler, it enters an economizer with finned tubes. The finned tubes absorb the heat and raises the temperature of the boiler feed water, lowering the needed energy input.

Applications of Economizer

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

1.   It recovers more heat of flue gases which normal air pre-heater can not do.
2.   Due increase in fuel prices, all power plants are facing pressure for increasing boiler efficiency. So by using economizer, this pressure can be minimized.
3.   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.
4.   The efficiency of power plant reduced when steam air pre-heater required steam.

Types of Economizers Based on Boiler Efficiency

                  Throughout this article, we have been having a qualitative discussion on the economical aspect of the economizer, let us now see how the different types of economizers can be judicially assembled along with the boiler, in order to perceive maximum boiler efficiency. They are broadly classified into two types, as has been described below.

Non condensing Economizer

             The most widely used one, in a thermal power plant is the non-condensing economizer. These are basically heat ex-changer 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. They have the ability to reduces 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 250o F (120oC).

           You can well understand form the discussion above that, cooling the flue gas below 250o F 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 impurity. And the flue gas thus formed by burning this coal, results in the formation of sulphurous compounds as by product. Now if this flue gas is allowed to cool below 250o F, condensation of the gaseous compounds result in the formation of sulphuric acid, which is considered extremely corrosive against the metal surface. Since the installation and maintenance cost of a power plant is huge, it is note-worthy that a non-condensing economizer be installed to limit the cooling capacity of the flue gas to about 250o F, i.e above the condensation temperature and increase the overall boiler efficiency by about 3 to 6 %.

Condensing Economizer

                         The condensing economizers are mainly used in natural gas fired thermal power plants, as they have the ability to improve the waste heat recovery by cooling the flue gas below its condensation temperature, which is about 80o F (25oC). This particular variant of economizers result in greater efficiency of around 10 to 15% and more economical operation as it reclaims both sensible heat 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 %. But the condensing variant with greater value of efficiency can only be used, when the flue gas does not contain any sulphurous, nitrate or other corrosive compounds.

Types of Economizer

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,
1.   High optimum efficiency due to proper contact of gills with tubes.
2.   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,
1.   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,
1.   These are very efficient in recovering the heat from gases.
2.   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,
1.   Proper care is taken for making the contact of fins with tubes for perfect heat transfer.
2.   These are used mainly used by Thermal Power Plants.

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