Steam Boiler: July 2011

Pressure Gauge with Bellow

Pressure gauge with bellow is the pressure gauge that is equipped with bellow. Bellow is thin metal which is shaped like an accordion bellow. Same as pressure gauge bourdon tubes, bellow also expands when the inside pressure is greater than outside pressure.

Any changes in inside pressure of bellow will produce mechanical motion backward or forward. Movement of backward and forward through tuns and twister tube then to be converted into the form of circular movement of pointer on scale numbers that have been calibrated as shown in Figure 1 below.

Figure 1: Pressure Gauge with Bellow

Pressure Gauge-Bourdon Tube

Pressure gauge with bourdon tube is a pressure gauge with is equipped with metal which easy to expands and hole is created on the metal. One end is created with closed condition and other end is made open to be connected with the pressure which then to be measured how much the pressure it is. This end is called as socket.

Generally the working principle of the pressure gauge with bourdon tube is the pressure will be measured enter into bourdon tube through socket, this pressure will result bourdon tube expands, so it will cause mechanical movement on the closed end. This movement is then forwarded to a wheel arrangement - the gear lever through liaison which is called as Lever. This gear consists of two gears namely: motion gears and pinion gears.

Motion gears is directly connected with lever, while the pinion gears is welded with pointer so when the bourdon tube moves, the pointe will also move as shown in Figure 1 below.

Figure 1: Pressure Gauge with Bourdon Tube

Definitions for Pressure in Boiler

Pressure is the force that occurs for each unit area in the plane, the unit of pressure which is widely used is kg/m2, Newton/m2, lb/inc2, kip/ft2 and so on. Besides that, there are units of pressure that are known and used in steam boiler or power plant such as atmospheric, Bar, psig).

In addition to a wide - range unit used for pressure, the pressure can be also classified based in its definition either for liquid, gas, vapor or steam. There are three types of pressure:

Absolute pressure

Absolute pressure is the actual pressure of fluid

Pressure Gauge

Pressure gauge is measurement of how much the pressure of fluid compared to the outside air pressure (atm).

Vacuum Pressure

Vacuum pressure is equal to the pressure gauge but it is smaller than the atmospheric pressure (atm).

The differences of absolute pressure and pressure gauge can be seen from its unit. Absolute pressure can be expressed as Psi (Pound square inch), then the pressure gauge is expressed in psig (pound / square inch gauge).

Pressure gauge consists of main elements of material which easy to expand and still use manual systems and mechanical motion can be classified as follow:

- Pressure Gauge with Bourdon Tube

- Pressure Gauge with Below

- Pressure Gauge with Diaphragm

Strainer in Steam and Condensate System

Shutdown, repair, and downtime of steam boiler or power plant should be reduced. This matter can be done by maintain steam and condensate systems in well performance. Plant or boiler damage is often caused by impurities in the pipeline such as scale, corrosion, compounds on the connection, welding metals and other solids, which can enter into piping system . Strainer is device that captures solids in liquids, gases or steam. Strainer protects equipment from harmful influences, thus reducing the time of shutdown and repair. Strainer should be installed at upstream of each steam trap, at flow meter and at control valve.

Strainers in steam and condensate system can be classified into two main types according to shape and composition of body as follow (see Figure 1):

Y-type Strainer
Type Basket Strainer.

Figure 1: Y-Type Strainer and Basket Strainer Type

Y-type strainer is common standard and widely used everywhere especially for fluid steam or gas. The body is cylinder and compact-shaped, very strong and can handle high pressure. This device can be grouped as pressure device because the Y-type strainers are capable to handling pressures up to 400 barg. At that pressure, steam usually has high temperature, then to overcome this condition, strainer is made from unusual materials such as chrome-molybdenum steel.

Y-type strainer has capacity to eliminate dirt lower than the type basket strainer. Furthermore the Y-type trainer requires more frequent to cleaning. In steam systems, this is not a problem, except when level of corrosion is high.

If in the steam and condensate system there are a number of significant impurities, then a blowdown valve can be usually installed on the strainer cover. This will create strainer has steam pressure steam to clean without having to shut down the plant.

At Y-type strainer where flow of steam / gas is distributed horizontally, pocket shall be mounted in the horizontal plane. This way to prevent water gathered in the pocket, helping to prevent entrainment of water droplets which can cause erosion and affect the process of heat transfer. Pocket forms must be led down vertically.

Installation sometimes does not always in the horizontal direction, but allows also to be mounted vertically in steam and condensate system which has flow direction of fluid vertically from top to bottom. The dirt will flow naturally down into the pocket. Installation of strainers cannot be done if the fluid flows upwards (from bottom to top) because the existence of pockets in the strainer will be useless.

Figure 2: Horizontal Strainer and Vertical Strainer

Steam Distribution Factors in Boiler

Steam pressure distribution in boiler is influenced by many factors and limited by:

- The maximum safe working pressure in boiler

- The minimum required pressure in the power plant

When the steam through pipe distribution, then steam cannot avoid the loss of pressure due to some factors such as:

- Friction resistance inside pipe.

- Condensation that occurs inside pipe when the heat is transferred to the environment.

Therefore, when determining the initial distribution of pressure, tolerance should be provided due to this pressure loss. One kilogram of steam at higher pressure has smaller volume than at low pressure. So, if the steam generated in the boiler at high pressure and is distributed at high pressure, then the size of distribution pipe will be smaller.

Generation and distribution of steam at high pressure provides significant benefits as follow:

- Capacity of heat storage in the boiler will be increase.

- Increase boiler efficiency in dealing with fluctuating load

- Minimize the risk of wet steam and dirty steam occurs.

- Required pipe size to flow steam will be smaller, so the investment cost for the pipeline, supporting materials, insulation materials and labor are lower.

- At high pressure steam distribution system, steam pressure reduction is required in each zone or point of use on the system to adjust with the maximum pressure which is required by user. The reduction pressure will also generate drier steam at the point of use.

The following below are important component can influence steam distribution factor in boiler:

- Piping system

- The expenditure

- Line branch

- Strainers

- Filter

- Separator

- Steam traps

- Air vents

Steam Distribution System

Steam distribution system has important relationship between steam generator and steam users. There are various methods for carrying steam from the central source to the point of use. Central source may be boiler room or expenditure of the cogeneration plant. Boilers can use primary fuel, or boiler that uses waste heat from the exhaust gas of high-temperature processes, machines or even the incinerator. Whatever the source, an efficient steam distribution system is very important to be designed so that the distribution of steam produced has good steam quality both pressure and temperature which are needed to all of equipment that use steam.

Figure 1: Steam Distribution System

Installation and maintenance of steam systems are important and should be considered to start design phase. Understanding of the basic steam circuits or 'loops and steam condensate' is very required. When the steam condenses in the process, the condensate must be distributed back into water supply pipes of boiler. Although the condensate has very small volume compared with steam, but can cause pressure steam flowing through the pipes down.

Steam generated in the boiler must be carried through the working pipe to the point where the heat energy is required. At first there were only one or more main pipes, or 'steam pipeline', which carries the steam from boiler towards plant that uses steam. The smaller branches pipes carry steam to the respective equipment. When the main isolation valve of boiler is opened, steam is immediately across from the boiler to steam pipe that has low pressure. The working pipe is initially colder than steam will be hotter when steam flows through pipe that has higher temperature. Heat transfer occurs from the steam to wall pipe, therefore pipe should be wrapped with insulation so as not to endanger the personnel who work around it.

Steam that contact with cooler pipes will begin to condenses. At start-up, the rate of condensation will have maximum value, this happens because the large difference of temperature between steam and pipe. The rate of condensation is usually called as 'initial load'. At the time temperature pipe rise to be hotter, the difference temperature between steam and pipe is minimal, but the condensation still occurred because pipe is still transferring heat to the surrounding air. The rate of condensation is called as ‘running load'. The result of condensation (condensate) falls down to the bottom of pipe due to gravity and is carried by the flow of steam. Condensate be trapped by the steam trap which has gradient angle of pipeline so that the flow of steam containing condensate will drop and then released into water pipe of boiler or disposal.

Steam Quality Parameters

The following below are parameters in determining the quality of steam produced by boiler:

- The amount of steam which is generated by boilers in accordance with required amount to drive turbine generator. The amount of steam produced is usually in the unit form of Kg/hour or tons/hour. Therefore the heat transfer should be maintained in order to generate expected amount of steam.

- Steam produced must have required temperature and pressure so as to drive turbine generators. Setting temperature of steam can be done by desuperheater while setting pressure of steam can be performed by relieve valve or safety valve. If the steam pressure and temperature are met then it means that one of the parameters in the determination of steam quality is met.

- The steam which is produced should be free from air and condensed gases because air and moisture can inhibit heat transfer in steam boiler.

- Steam which is produced by boilers must be clean; there should be no crust (e.g., corrosion or sediment carbonate) or impurities that can increase the rate of erosion in the pipe, orifice and valve.

- Steam is produced must be dry or superheated. The presence of water droplets in the steam will reduce the actual enthalpy of evaporation, and will also lead to scaling on the pipe wall and damage to turbine blades.

Phase Steam Diagram

The data provided in the steam table can also be expressed in graph form. Figure 1 illustrates the relationship between enthalpy and temperature at various pressures, and known as the phase steam diagram.

Figure 1: Phase Steam Diagram

When water is heated from 0° C to saturation temperature, its condition follow the line of saturated liquid until receives all of liquid enthalpy, hf, (A - B). If heat is added further, it will change the phase steam diagram to the saturated steam enthalpy and continually improving while remaining at saturation temperature, hfg, (B - C).

If the dryness of mixture of steam and water increase, the condition moves from the saturated liquid line to the saturated steam line. Therefore at the middle point between both conditions, the dryness fraction (x) is 0.5. The same condition occurs on the saturated steam line which has steam 100% dry.

Upon receiving enthalpy of evaporation it will reach the saturated steam line. If heating is continued after this point, the temperature of steam will begin to rise to supersaturated (C - D). The lines of saturated liquid and saturated steam cover areas where there is mixture of steam/water-wet steam. In the area of left side of saturated liquid line, there is only water, and on the right side of the saturated steam line there is only supersaturated steam. The point where the saturated liquid line and saturated steam meet is known as the critical point.

If the pressure raises towards the critical point the enthalpy of evaporation decreases, until it becomes zero at the critical point. This indicates that the water turns directly into saturated steam at the critical point. Gas may just exist above the critical point. Gaseous state is the most diffused situation where the molecule is almost has unlimited movement and the volume increases without limit when the pressure is reduced.

The critical point is the highest temperature where fluid is in liquid form. Giving the pressure at constant temperature below the critical point will not cause phase steam diagram change. Even so, giving the pressure at constant temperature below the critical point will result in melting of steam when passing superheated area to wet steam. Critical point occurs at temperature of 374.15 ° C and 221.2 bars steam pressure. Above this pressure are called supercritical steam and there is not boiling point can be applied.

Waste Heat Boiler

Waste heat boiler is one of type boiler based on method of combustion. When waste heat at medium or high temperature is available, so waste heat boiler can be installed economically. If waste heat boiler needs more steam from the steam which is generated using the hot exhaust gases, can be used an additional burner that uses fuel. If the steam does not directly be used, steam can be used to produce electric power using steam turbine generators. It is widely used in heat recovery from flue gases of gas turbines and diesel engines.

Figure 1: Waste Heat Boiler

Thermic Fluid Heater Boiler

Currently, thermic fluid heater boiler has been widely used in various applications for indirect heating process. By using petroleum fluids as heat transfer medium, these heaters provide constant temperature. Combustion system consists of fixed grate with mechanical draft arrangements.

Thermic fluid heater boiler modern with oil-fired consists of double coil, construction of three passes and is fitted with pressure jet system. Thermic fluid, which acts as a heat carrier, heated in the heater and circulated through the user's equipment. Fluid transfer heat through heat exchanger to the process, then the fluid is returned to the heater.

Thermic fluid flow at the user end is controlled by a control valve that is pneumatically operated, based on the operating temperature. Heater operates at high fire or low fire depending on the return oil temperature which varies depending on system load.

The advantages of thermic fluid heater are:

- Closed operating system with minimum losses as compared to steam boilers.

- Operating system is not pressurized even for temperatures around 250 C compared to the needs of the steam pressure of 40 kg/cm2 in a similar steam system.

- Automatic control settings, which provide operational flexibility.

- Thermal efficiency is good because there is no heat loss caused by blowdown, discharge condensate and flash steam.

The overall economic of thermic fluid heater boiler depend upon the specific application and the reference basis. Thermic fluid heater boiler with coal-fired which has range thermal efficiency 55-65%. Thermic fluid heater boiler is most comfortable to use than the most common boiler. Incorporation of heat recovery devices in the exhaust gas will further enhance the thermal efficiency.

Figure 1: Thermic Fluid Heater Boiler

Boiler Heat Balance

Boiler performance parameters such as efficiency and evaporation ratio, decreases with time due to poor combustion, surface fouling of heat transfer, poor operation and bad maintenance. Even for a new boiler, for reasons such as poor fuel quality and water quality can lead to poor performance of the boiler. Boiler heat balance can help in identifying heat loss that can or cannot be avoided. Boiler efficiency tests can help in finding the deviation efficiency of the best boiler efficiency and target the problem areas for corrective action.

The process of combustion in the boiler can be described in terms of flow energy diagram. This diagram illustrates graphically how the incoming energy from the fuel is converted into useful flow energy and the flow of heat and energy loss. Thick arrow indicates the amount of energy contained in each stream.

Figure 1: Diagram of Boiler Energy Balance

Boiler heat balance is balance of the total incoming energy to the boiler and that leaves the boiler in different form. The following figure provides a range of loss that occurred for steam generation.

Figure 2: Heat Loss in Coal-Fired Boiler

Boiler Fuel

The fuel can be defined technically as any material that can burn. While commercially, the fuel can be called as any material that has specific calorific value and is able to react with oxygen in air to produce heat. Generally, fuel can be classified into three main types, namely:

Boiler fuel solid
Boiler fuel liquid
Boiler fuel gas

Based on the occurrence, fuel can be differentiated into natural fuels and artificial fuels. The following below is the differences of boiler fuel based on natural fuel and artificial fuel:

Boiler fuel solid

Natural: Wood, peat, lignite, bituminous, anthracite

Artificial: wood charcoal, coke, briquettes, bagasse, palm oil waste, coconut shell

Boiler fuel liquid

Nature: Crude Oil

Artificial: Gasoline, kerosene, fuel oils

Boiler fuel gas

Nature: The gas methane (CH₄), Ethane gas (C₂H₆), carbon monoxide (CO), LNG, LPG

Artificial: Coal gas, water gas, Raymond gas, high gas furnace, coke oven gas, producer gas.

A steam boiler requires heat source at high enough temperatures to produce steam. Fossil fuels used for generating steam are usually burned directly in the furnace boiler, although the heat for the steam boiler may also be in the form of residual heat from another process.

Combustion can be defined as rapid chemical combination of oxygen with combustible elements of fuel. There are only three important chemical elements which can be burned, namely: carbon (C), hydrogen (H) and sulfur (S). Sulfur usually have little meaning as a source of heat but can be an essential element in terms of problems of corrosion and pollution.

Any fuels which contain hydrogen will produce water (H₂O) as one of product results. The water produced can be a liquid, gas or mixture of two phases. If the water formed during combustion of hydrogen in the boiler fuel can be condensed, the amount of heat that can be obtained will be greater than if the water formed in gaseous form.

Therefore, there are two kinds of combustion value or calorific value:

Higher Heating Value (HHV)

When water vapor of combustion is condensed that must take into account of latent heat of evaporation. The value of the boiler fuel liquid combustion is useful for the calculation of heat loss which can be calculated by using the equation:

HHV = 14500 C + 62000(H₂ – (O/8) + 4000 S

Where:

HHV = Higher Heating Value

C =% carbon in boiler fuel

H =% hydrogen in boiler fuel

O =% oxygen in boiler fuel

S =% of sulfur in boiler fuel

Lower Heating Value (LHV)

When water vapor of combustion is condensed and appears entirely in the form of gas so it does not take into account of latent heat evaporation. Lower Heating Value of boiler fuel liquid is useful for the calculation of heat loss which can be calculated by using the equation:

LHV = HHV - 9720 H₂ - 1110W

Where:

LHV = Lower Heating Value

H₂ = percentage of hydrogen in the boiler fuel

W = content of water steam contained in air

Boiler Feed Water Treatment

Water used in boilers is water which the content of mineral had been released and filtered (demin water) because the mineral content in water can damage the parts of boiler. To eliminate the mineral content in water, can use the flocculation and clarification steps. Flocculation is the process of removal of particles suspended in water either large or colloidal compounds are suspended in water. Content levels of these particles are expressed as turbidity. The flocculation steps are:

Filtration process

Filtration is the process of filtering large-sized dirt. The filtered impurities are organic compounds, fine particles, color compounds and microorganisms.

Coagulation

Coagulation is done to remove dirt impurities form compounds that are ionic.

Water generated from the process above is called with demineralized water (demin water). But in demin water still contained dissolved gases such as carbon dioxide and oxygen can cause corrosion of pipes and tubes. To eliminate the solution of these gases is performing deaeration process. Deaeration process is performed in the deaerator process on the two stages, namely:

Mechanically

Deaeration process mechanically is done by stripping process with low steam (LS). This can eliminate the solution of oxygen and carbon dioxide by up to 0.007 ppm.

Chemically

Deaeration processes chemically are carried out by injecting solution of hydrazine (N2H4). The end result of deaeration process is called Boiler Feed Water (BFW) which is then used as feed water to the steam boiler.

Circulation Water in Steam Boiler

Circulation of water in the pipes/tubes in steam boiler is a very important thing to be designed. Boiler must be designed in such a way that avoided the formation of steam and out of the water wall tubes from steam drum. In other words, should not be going back-flow.

To get even warming on all parts of the boiler, especially in boiler water tubes, then the perfect water circulation must be maintained to prevent air bubbles and steam in the wall tubes and termination expenses of steam from tubes. The occurrence of bubbles on the wall tubes and the cessation of vaporization can lead to corrosion and salt concentrations which can damage the wall tubes.

The circulation of water and steam in the boiler occurs because:

1. Difference of density between water and steam.

2. The existence of mixture of water and steam.

The type of water circulation in the steam boiler can be classified into two types, namely:

Natural Circulation

In this circulation, the water flowing from upper drums through downcomers which is located in the boiler is relatively cold, flow down to mud drum. From mud drum, water flow back into the steam drum after passing through the evaporator tubes.

Forced circulation

In this forced circulation, fluid is pumped through the evaporator. This causes the boiler can work with very high pressures.

Convection Heat Transfer in Liquid Phase

Convection heat transfer with heat flux (φ) constant is calculated using the following equation:

q_conv = φ . A

Where:

A = Area of the heated surface (m²). In the pipe, cross-sectional area which is heated is π.D.z

φ = Heat flux on the surface of pipe (Watt/m²)

q_conv = Convection heat transfer (Watt)

z = length of pipe (m)

So for a pipe with diameter D, The value of heat transfer that occurs is:

q_conv = φ . π . D . z

While the heat transfer on the fluid inside pipe is:

q_conv = W_f . c_pf . (T_f (z) – T_fi)

Where:

W_f = mass flow rate in the liquid phase (kg/s)

c_pf = coefficient of heat convection in the liquid phase (J/kg⁰C]

T_f(z) = local fluid temperature in the pipe (⁰C)

T_fi = temperature of fluid enter pipe [⁰C ]

So the heat balance on pipe is by combining equations above to be following equation:

φ . π . D . z = W_f . c_pf . (T_f (z) – T_fi)

Mass flow rate (W_f) is often made in mass velocity (G) the relationship between both of them is as following equation:

G = (4. W_f) / (π . D²)

So by rearranging the equation above and combine them can be obtained equation below to calculate distribution the local heat fluid of along pipe.

T_f (z) = T_fi + ((4 . φ . z) / (G . C_pf . D))

Pipe wall surface temperature is the temperature of local fluid coupled with the difference of wall temperature and the local temperature:

T_w = (T_f (z) + ΔT_f )

Where:

ΔT_f = φ / h_fo

So the equation will be:

T_w = (T_f (z) + (φ / h_fo))

h_fo is calculated from Nusselt number as following equation:

Nu_D = (h_fo . D) / k_f

Where:

Nu_D = Nusselt number

h_fo = coefficient of convection fluid (W/m^{2 0}C)

k_f = thermal fluid conductivity (W/m⁰C)

D = pipe diameter (m)

Nusselt number for laminar flow in pipe:

Nu_D = 0.17 Re^0.33 Pr_f^0.43 (Pr_f/Pr_w)^0.25 ((D³ρ_f³gβΔT)/(μ_f²))^0.1

applies to z/D > 50 and Re < 2000, while for turbulent flow in a pipe used Dittus-Boelter equation, which applies to z/D > 10 and Re > 3000.

Nu_D = 0.023 Re^0.28 Pr_f^0.4

Boiler Type O and Type A

Boiler Type O

Boiler type O has 2 drums, they are steam drum and mud drum. The composition of the convection tubes and water wall tubes forming the letter O. Combustion chamber is placed in the middle between convection tubes and water wall tubes. Boiler type O can be shown in the Figure 1 below:

Figure 1: Boiler Type O

Legend:

1 : Furnace (combustion chamber)

2 : Steam drum

3 : Mud drum

Boiler Type A

Boiler type A has 3 drums, they are steam drum and two mud drums. Boiler type A can be shown in the Figure 2 below:

Figure 2: Boiler Type A

Legend:

1 : Furnace (combustion chamber)

2 : Steam drum

3 : Mud drum

Water Tube Boiler

In water tube boilers, boiler feed water flowing through the tubes into steam drum, water drum and header. Water is heated by gas burners or other fuel combustion to form steam and to be distributed into steam drum. Water tube boiler is selected if its steam and the steam pressure are very high as in the case of boilers for power generation.

Water tube boiler which is very modern designed with steam capacity of 4500-12000 kg / hour, with a very high pressure. Lots of water tube boilers are constructed in a package if used fuel oil and gas. For water tube that uses solid fuel, is not commonly designed package.

Characteristics of water tube boilers are the following below:

- Forced, induced and balanced draft help to improve combustion efficiency

- Less tolerant of water quality resulting from water treatment plant.

- Allows for the higher thermal efficiency.

Figure 1: Water Tube Boiler
(Source: United Nation Environment Program, 2008)

Characteristic Output of ESP (Electrostatic Precipitator)

Characteristic output of ESP (Electrostatic Precipitator) is a comparison between voltage and current density. Characteristic voltage and current generated in the control system is a combination of high voltage rectifier transformer and precipitator as a supplier of high voltage DC. Dust that passes through the precipitator plate can be likened to the particle charge carriers from the negative plate to positive plate.

Figure 1: Characteristic of ESP (Electrostatic Precipitator)

From the Figure 1 above show that the output voltage and current density as a characteristic, dust deposition efficiency depends on the magnitude of the voltage difference on each plate precipitator. On the other hand, through the addition of the operating voltage potential output will cause a magnetic field that would cause a greater potential as well.

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 as Supporting Instrument in Power Plant

In this deaerator water will be heated to a temperature of 100^oC -105^oC, temperature of water is initially 30^oC - 50^oC. After going through the process of preheating the feed water then flowed into the economizer to be heated back up to the level of temperature 150^oC - 160^oC in which the heating in the economizer using exhaust gases from combustion in the steam boiler or chain grate before the gas was discharged through a chimney or stack.

After to be heated up in economizer, water flowed into the drum boiler before the water is burned in water wall tubes boiler. Then the water inside the boiler to be burned at a temperature of 400^oC - 459^oC, at this form of water has been turned into full steam. But at this level the water can not be used to turn turbines, and therefore at this level after the water turns into steam, steam will be distributed into the superheater to raise the temperature of the steam itself to the level of 500^oC - 600^oC.

Steam at this level is ready to turn turbines and generators to produce electricity play. The remaining steam turbines were going to play back streamed to the deaerator in order to preheat the water in it, that's so deaerator and economizer cycles of use as a supporting instrument in warm water until it becomes steam. We know the function of deaerator is to remove the gases contained in the boiler feed water, after the purification process of water (water treatment).

In addition deaerator heater also serves as the initial water filling the boiler before it is inserted into the boiler. Deaerator works based on the nature of oxygen solubility in water decreases with an increase in temperature. If water from water treatment directly burned in a boiler, it will cause severe corrosion because the water still contains gases that can cause corrosion and so on.

Likewise, if the water is burned directly in steam boilers will not rule out going to use fuel that is not less, because water from water treatment temperature is 30^oC - 50^oC and burned in a boiler with a target temperature of the water into steam at 400^oC and above. From the small sample above shows clearly that the pre-heating the water is very useful for saving fuel.

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.

Heating Process in Steam Boiler

With rising temperatures and water near its boiling conditions, some molecules gain enough kinetic energy to reach the speed that made him any time off from the liquid into the space above surface, before falling back into the liquid. Further heating causes greater excitation and the number of molecules with enough energy to leave the liquid increases. Taking into account the molecular structure of liquid and vapor, it makes sense that the density of steam is smaller than water, because the steam molecules are far apart from one another. Space that suddenly occurs above the water surface becomes filled with dense steam molecules.

If the number of molecules leaving the liquid surface is larger than the re-entry, then the water evaporates freely. At this point the water has reached boiling point or saturation temperature, which is saturated with heat energy. If the pressure is fix, adding more heat does not cause further increase in temperature but causes water to form saturated steam.

The temperature of boiling water with the saturated steam in the same system is same, but the heat energy per unit mass is larger in the steam. Saturated temperature at atmospheric pressure is 100 ° C. However, if the pressure increases, then there will be the addition of more heat that the increase in temperature without phase changes. Therefore, the increase in pressure will effectively increase the enthalpy of saturated water and temperature. The relationship between saturation temperature and pressure are known as saturated steam curve (Figure 1).

Figure: Saturated Steam Curve

Water and steam can exist simultaneously on a variety of pressure on this curve; both will be at saturation temperature. Steam saturation curve in the above condition is known as superheated steam / steam through the saturated:

- Temperature above saturation temperature is called the degree of saturated steam.

- Water on the conditions under the curve is called sub-saturated water.

If the steam flows from the boiler at the same speed with which it produces, the addition of heat will further increase the rate of production. If the same steam leaving the boiler is not retained, and the amount of incoming heat is maintained, the energy flow to the boiler will be greater than the energy that flows out. This excess energy will raise the pressure, which in turn will lead to saturation temperature increases, because the temperature is related to pressure saturated steam.

In this case the water in a boiler is combustion, water through the economizer which has flowed through the heating inside the drum boiler (steam shelter) and then burned in a boiler to be heated further up to a wet steam. The temperature inside the boiler is approximately 400^oC - 459^oC. Combustion of fuel coal and assisted with air to maintain combustion stability in combustion system. Combustion control system connects the control of heat input to the boiler with the ratio of air / fuel entering the combustion chamber. This control system must be able to ensure sufficient amount of air available for combustion of a fuel efficiently without causing smoke and with minimum particulate discharge from the chimney. After this process within the boiler, steam flow and then proceed to the superheater to be his dry steam, steam temperature was about 520^oC - 600^oC and ready to turn turbines.

Feed Water Process of Boiler

A boiler is a closed vessel where the combustion heat flowed into the water until it forms a hot water or steam. Hot water or steam at a certain pressure and then used for transferring heat to a process. Water is a useful and inexpensive medium for transferring heat to a process. If water is boiled into steam, its volume will increase by about 1,600 times, producing a force that gunpowder is explosive, so the boiler is a device that must be managed and maintained very well.

Boiler system consists of: feed water systems, steam systems and fuel systems. Water system provides water to the boiler automatically as needed steam. Various valves are provided for purposes of maintenance and repairs. Steam system collects and controls the production of steam in the boiler. Steam is directed through a piping system to the user's point.

The entire system, steam pressure is set using a valve and monitored with a pressure monitor. The fuel system is all the equipment used to provide fuel to generate the necessary heat. Equipment required on the fuel system depends on the type of fuel used on the system. Water supplied to the boiler is converted into steam is called feed water. Two sources of feed water are: condensate or steam is returned from the process and make up water (treated raw water) which must come from outside the boiler room and plant processes. To obtain a higher efficiency boiler, an economizer for heating feed water using waste heat in flue gases.

The raw material used to make steam is clean water. Water from the Reverse Osmosis (RO) that has been processed in the stream using a pump to the deaerator tank to the level specified. Warming in the deaerator is to use the residual steam from the turbine playback. In this case there is some stage or stages of the circulation of steam to preheat deaerator.

Phase 1

Residual steam from the steam turbine rotating returned directly to the deaerator to reheat the water contained in the deaerator tank. The remaining steam is directly due to differences in pressure and flow density of water and steam, because of differences in the density of the steam was inclined towards a greater density of water. Circulation at this stage continuously like that.

Phase 2

The remaining steam turbine of the player falls into the condenser (cooling process). At this stage the remaining pedinginan steam assisted by seawater. After going through this cooling process, the steam turns into water again later in the stream to the LPH (Low Pressure Heater) to be heated again. Once the water is almost hot LPH earlier in the stream again to the deaerator for heating up. when heated in hot water deaerator was not directly in the stream to the economizer, but the water in the stream prior to HPH (High Pressure Heater) to be heated more and after that then flowed into the economizer. Help some heaters on stage 2 this is just a step in the maintenance of the instrument where it has be set in such a way as to guard. It also can be used as a safety if any of the one-stage system of the stage had been damaged, other than that it's step by step depending on the type of turbine used