Steam Boiler: Economizer

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The Performance of Economizer

The performance of economizer should be known related to steam boiler efficiency. Economizer is equipment for heat transfer with shaped tubular which is used to heat boiler feedwater from BFP (Boiler Feedwater Pump) before entering the steam drum. The term economizer is taken from the usefulness of this equipment is to save or to economize fuel use in taking the heat (recovery) of the exhaust gas before being discharged into atmosphere.

Economizer can be used to utilize the heat of exhaust gas for preheat boiler feedwater. Any reduction in exhaust gas temperature of 220 C or preheater there is 1% savings in fuel of the steam boiler. Any increase in feedwater temperature of 60 C through the economizer or 200 C rises in temperature of combustion air through the air preheater will get 1% fuel savings in the boler.

The performance of economizer is determined by the fluid that has a low coefficient of heat transfer, namely gas. Heat transfer rate can be improved by increasing the total heat transfer coefficient by regulating the arrangement of tubing / fin property and increase contact area of heat transfer. Response which is generated by the economizer is heat transfer effectiveness and operating costs. Effectiveness of heat transfer is the amount of energy that can be taken from the total energy that can be absorbed.

The greater efficiency of heat transfer in the economizer result in the heat of remaining gas will be drawn more and more. The greater effectiveness of heat transfer that occurs can make this equipment will be more efficient. Economizer operation costs are determined by fan power and pump power. Fan is used to flow combustion air to the steam boiler through economizer. The more loops and more complex arrangement of economizer tubing, so the fan power required will more increase.

While the pump is used to flow the boiler feedwater to steam drum through the economizer. The longer and more loops in the economizer will result in the required pump power increases. The optimum response is obtained by using the design factors that affect the performance of economizer as follows:

Outer diameter of tubing, it is the size diameter of tube used in arranging economizer. The larger diameter of tube will result in diminishing on the effectiveness of heat transfer.
Transverse spacing states distances between the aligned tubes towards the wide of economizer. The wider spacing between the tubes resulted in the heat induction process of economizer diminishing thus the effectiveness of heat transfer decreases.
How close density among fins is the number of fin per inch that can be arranged to incorporate some of the tubes in the economizer. The more arranged fin resulting in heat transfer is ineffective because the farther distance between the tubes.
The performance of economizer is very sensitive to factor of feedwater noise temperature. This is because when the feedwater temperature is not good so it will result in increased operating costs.

Economizer as Supporting Instrument in Steam Boiler

Economizer has similar function with deaerator as supporting instrument in steam boiler, though only enhancements, the usefulness of economizer can make working process of steam boiler will be more efficient. Where we know the burning water in economizer is only utilizing exhaust gases from combustion in the boiler by not adding fuel to heat water in it. It is not only deaerator and economizer alone that working as supporting heater, but many other heaters can also be used in an industrial system that can convert the water into steam in the power plant.

Figure 1: Graph the Using of Economizer

The graph above shows the advantages and disadvantages of using the economizer for preheating. Clearly seen without using a boiler economizer so work efficiency of steam boiler will decrease, in the sense that without heating which is assisted by the economizer, the boiler must work longer in the production of steam and in addition boiler will require more fuel to reach the hot temperature steam that has been determined. In addition, if the boiler still forced to work much more quickly it will damage the tubes within the boiler itself. Once this occurs then a plant will experience huge losses in the boiler operation because the use of too much fuel and the resistance of a tool will quickly decline and need to replace equipment.

However, if a boiler using the economizer and some other auxiliary heater to heating water in the heating process before it burned, it will further enhance the work efficiency of the boiler itself, because the water temperature before being burned in the boiler is already quite high, it means that heating water into steam inside the boiler does not take long time and do not use that much fuel to achieve a predetermined standard temperature. Furthermore the operating costs can be more efficient and can indirectly benefit to power plant performance. Besides maintenance or maintenance of equipment or replacement equipment can be carried out much longer.

It is apparent that by using a boiler economizer can increase the capacity of boiler and can also make efficient in the process of combustion, convert water into steam in the boiler, so it will doing more fuel savings far enough difference if compared to the boilers which running without economizer.

Deaerator-Economizer as Feedwater Heater in Steam Boiler

Use of deaerator and economizer as auxiliary instruments in warm feedwater before the feedwater is burned in a boiler. Water is obtained from the raw water that has been in treatment to conform to the standards set supplied to deaerator with the aim of separation of the gases dissolved in water and separating minerals contained in water in order to keep all the tubes that pass through can avoid corrosion. In addition, in the deaerator water went through the process of preheating the steam heated by the rest coming from turbine generator. The function of the deaerator is as a gas separator-gas dissolved in water and heats the boiler feed water before it was burned in the boiler.

Economizer is shaped tubular heat transfer equipment used to heat boiler feed water before entering the steam drum. The term economizer is taken of the usefulness of such tools, namely to conserve fuel by taking the hot flue gas before being discharged into atmosphere. An economizer can be used to utilize the exhaust heat to preheat boiler feed water. Any reduction in exhaust gas temperature through the economizer or preheater is 1% saving of fuel in the boiler. Any increase in temperature of feed water through the economizer or the air temperature rise through the combustion air preheater, there is a 1% fuel savings in the boiler.

Figure 1: Mechanism of Deaerator and Economizer

Economizer performance is determined by the fluid having a low coefficient of heat transfer gas. Heat transfer speed can be improved by increasing the total heat transfer coefficient by regulating the composition of tubing / fin properties and increase the contact area of heat transfer. The response generated by the economizer is heat transfer effectiveness and operating costs. Effectiveness of heat transfer is the amount of energy that can be drawn from the total amount of energy that can be absorbed. The greater efficiency of heat transfer in the economizer, heat the remaining gas that is picked will be many more.

The greater effectiveness of heat transfer that occurs, then the tool is more efficient. Economizer operation costs are determined by fan power and pump power. Fan used to flow combustion air to the boiler through the economizer. The more loops and more complex arrangement of economizer tubing on the fan power required increases. Pumps used to drain the boiler feedwater to the steam drum through the economizer. The longer and more loops in the economizer, the required pump power increases. The optimum response is obtained using the design factors that affect the performance of economizer as follows:

Outside diameter tubing, the diameter of the tube size used in preparing the economizer. The larger the diameter of the tube will result in diminishing the effectiveness of heat transfer.
Transverse spacing, which express the distance between the tubes parallel to the direction of the width of economizer. The wider spacing between the tubes resulted in the induction process of economizer heat decreases, thus decreasing the effectiveness of heat transfer.
Fin density, the number of fins per inch that can be structured to incorporate some of the tubes in the economizer. The more structured fin will result in heat transfer is not effective because the distance between the tube will be farther.

Component of Economizer

Economizer has a role as a heat exchanger to increase temperature of feedwater before to be supplied into steam drum. Economizer consists of similar component with other heat exchanger but on the certain parts has different task and function. The following below are the component of economizer

- Shell, is one of parts in economizer which be like a house for tubes and header. Between tubes and header, there are fluids that can receive and release heat in accordance with occurred mechanism.

- Tubes, is the most important component of economizer in the process of heat transfer. Fluids flow both inside and outside tube. The flowing fluid has different capacity, temperature, density and pressure. Position of tube must be maintained to prevent vibration, so on certain design, tubes is supported with baffle. Tube should also be able to transfer heat between the fluids inside tube and outside tubes.

Determination material of tube should follow these requirements; tube should have capability to transfer high heat, has resistance to heat, corrosion, and erosion and has well other properties. Capability to receive and release heat is also affected by amount of surface area.

- Baffle, is one of component in economizer that has some roles such as to support tube and maintain its position, baffle also has function to prevent vibration on the tube and control and direct the flow of fluid outside tubes.

Baffle can be classified based on construction as follow: segment baffle, rod baffle, horizontal baffle and impingement baffle. These types of baffle are usually used independently but in the special needs, one type and other type can be combined.

- Header, is the component of economizer as a water collector and distribute water to each tubes evenly. Header has two types; upper header and lower header. Tubes are connected into header, so header has size and mount of hole in accordance with tubes condition.

- Tie rods and spacer, are one of part economizer to maintain position one baffle and other baffle so vibration can be prevented.

Mechanism of Economizer

Economizer is one of steam boiler’s equipment which is used to heat feedwater before it is supplied into steam drum. Economizer is the heat exchanger equipment to increase boiler efficiency by absorbing heat recovery of flue gases. The lower temperature of flue gas out from stack, the heat loss will be less and the fuel which is needed to convert water into steam will be also less in certain circumstances. So it can be said that economizer can save the fuel efficiently. Economizer will make temperature of feedwater higher, so steam boiler can produce steam easily.

The heating process of water in the economizer is like an heat exchanger, water is distributed inside tubes and flue gas outside tubes. The hot flue gas outside tubes flow and making contact with outside surface tube, so the heat transfer occur between hot flue gas and surface metal tubes by convection. Water inside tubes receives heat transfer from metal tubes, so temperature of water is higher than before enter economizer. Contact of flue gas flow to feedwater flow is regular and perpendicular. It is controlled and regulated by setting the flue gas flow.

Furthermore, feedwater that has been heated in economizer is sent to steam drum through connecting pipes. In the steam drum, feedwater will be boiler water and be heated again in water wall tubes to be saturated steam and superheater will make it as superheated steam.

The advantages of using economizer as initial heater are as follow:

- The metal wall of steam drum will not easy to be contracted, shrivel and broken because the incoming water is not cold condition so the cost maintenance will be smaller.

- Increase steam boiler efficiency and decrease heat loss.

- Save the fuel needed to perform combustion process

- The size of furnace can be smaller because the requirement heat surface is less than without using economizer.