What is Superheated Steam?

What is superheated steam? The answer is very simple: superheated steam is the real gaseous phase of water. If saturated steam is a water vapor that still mixed with liquid water, or even supercritical steam that is indistinguishable with supercritical water, the superheated steam is dry H2O gas that has no moisture content at all.

What is Superheated Steam?

To understand how water can turn its phase into superheated steam, we just need to understand the phase diagram of water above. Superheated steam becomes one of the water phases besides solid, liquid, saturated steam, and supercritical. Also according to the diagram, superheated steam can be derived from liquid water, solid water (ice), and also supercritical depending on environmental conditions.

Water Liquid Phase change to Superheated Steam

Liquid water can turn the phase into superheated steam after crossing the saturation curve line. To get past the saturation curve line, there are several ways that can be done. First with fixed pressure, the environmental temperature is increased. Second at a fixed temperature, the environmental pressure is lowered. Finally, room pressure is lowered along with rising room temperature. But all these processes must meet one condition, the line of change of environmental conditions must cut the saturation curve.

Solid Water Phase (Ice) change to Superheated Steam

Ice, can directly change the phase to dry superheated steam without having to pass through the liquid phase, and vice versa superheated steam can directly change the phase to ice without having to pass through the liquid phase. This change of phase must be at a lower pressure than the triple point, ie 0.61 kPa, or 0.006 atm, almost vacuum pressure. This principle explains how hail can occur. When the cloud above the Earth’s atmosphere which has vacuum pressure, it cools rapidly so that the water vapor changes instantly into ice.

Supercritical Steam Phase Change becomes Superheater Steam

Supercritical steam can also turn the phase into superheated steam by lowering the pressure. Supercritical steam has pressure and working temperature above the critical point, which is more than 22.1 MPa and 374 ° C. Thus, suppose a certain amount of supercritical steam is at 25 MPa and a temperature of 600°C decreases the pressure to 18 MPa, the steam will turn into superheated steam.

Types of Steam – Simply Determined by Phase Diagram of Water

Types of Steam — As we known from the previous article, water has three main phases: solid, liquid, and gas; which we can easily determine through various conditions of temperature and pressure in the phase diagram of water. However, water vapor themselves are actually still subdivided into three: saturated steam, superheated steam, and supercritical.

Saturated Steam

Saturated steam is a condition in which water vapor has an equilibrium pressure and temperature with liquid water. In other words, saturated steam is a wet steam, which still mixed with liquid water-phase molecules.

This saturation vapor begins to form just as the water reaches its boiling point, until all the energy from latent heat is absorbed by water. While all latent heat has been absorbed by water, and the amount of vapor phase has reached almost 100% of its, that is the end of the vapor phase of saturation. The process of reaching almost 100% of the vapor phase occurs at stable pressure and temperature. Furthermore, if thermal energy continues to being absorbed by saturated steam, there will be an increase in temperature and encourage steam to turn the phase into superheated steam.

In the phase diagram of water, saturated steam phase is shown only as a line of saturation curves which is the boundary between liquid water and superheated steam.

Types of Steam

Superheated Steam

Superheated steam is a phase of water that has passed through the saturation phase by absorbing more heat energy, so that the entire water fluid has a pure gas phase. Superheated steam contain high thermal energy, and is very popular used as an energy source of power plant steam turbine.

The dry and pure phase characteristics of this gas become the absolute requirement of steam before it can enter the steam turbine. This is because superheated steam is able to avoid the damage caused by the erosion of wet moisture as in saturated steam.

Supercritical Steam

Supercritical steam is a water phase condition above the critical point of water, which is 22.1 MPa and 374°C. There are two characteristics of supercritical steam: water has no latent heat value, as well as the same volume-specific value between water and supercritical steam. It can be said that this supercritical steam phase has an indistinguishable phase between liquid nor gas.

Supercritical steam is also used at modern steam turbine power plants which is pursuing high efficiency. The main challenge of using this supercritical steam is on the design of boiler tube materials. The boiler tube should be very resistant to thermal stress causing by the generation process of supercritical steam.

Phase Diagram of Water: Definition and Explanation

Phase diagram is a diagram showing the physical phase changes of a substance under various conditions of temperature and pressure. Phase diagram of water is a pressure-temperature diagram showing the changes of water physical phase under various conditions. A simple water phase diagram is shown in the figure below.

Phase Diagram of Water

The phase diagram, whatever the substance, generally uses the pressure parameters for the Y axis and the temperature parameters for the X axis. The diagram are formed by three curves which become the boundary of the three phases of water physics. Those curves become an equilibrium line of two different water phase that could be exist at the same pressure and temperature.

The first curve becomes the boundary between the solid phase and the gas phase, the second curve being the boundary between the solid phase and the liquid, while the third curve becomes the boundary between the liquid phase and the gas. This third curve also known as saturated line. The phase of water at this line is saturated steam. Along the saturation line, regardless of the magnitude of pressure and temperature, water and steam may be present in equilibrium.

So, if we examine in more detail, the phase of water vapor alone can be classified again into three: saturated steam, superheated steam, and supercritical steam.

The three curves that form the water phase diagram meet at a point known as a triple point. The triple point is a point where the three phases of water physics, ie solid, liquid, and gas, can be in one equilibrium condition. This condition is at a pressure of 0.61 kPa and 0.010°C. Seen in the diagram above, below the triple point, water has no liquid phase. Under this triple point the solid water (ice) will instantly evaporate into gas if there is a rise temperature at constant pressure.

Above the triple point, there are two branching curves with each function. One curve limits between solid and liquid phases, and the other curves constrain between the liquid phase and the gas. At the end of the liquid-gas curve there is a point we know as a critical point. Critical point is a point that becomes the end of the liquid and gas phase equilibrium curve so that it can be in the same condition of pressure and temperature. Critical point of water is at 22.1 MPa and 374°C.

Water vapor at pressures and temperatures over critical points can not be condensed only by increasing the pressure alone. Or another example, if we heat water at a constant pressure of 25 MPa, liquid water will not experiencing saturated steam phase – which is a mixture of water and water vapor – but will instantaneously turn the phase into supercritical vapors. This concept is the basis for the use of supercritical boilers in modern steam power plants.

Steam Boiling Curve – How to simply understand it?

The boiling point of a liquid or also known as the saturation temperature is the temperature at which the vapor pressure of the liquid is equal to the ambient pressure of the liquid. At this point the liquid will turn the phase into steam. The saturation temperature of pure water at atmospheric pressure is 100°C. At this point the water will turn the phase into steam by forming steam bubbles.

The saturation temperature becomes a unique function of pressure. The higher the pressure around the water, the higher the boiling point will. And also its vice versa. This is because the water pressure will affect the characteristics – such as the enthalpy of water, latent heat, and steam enthalpy – of steam formed at that pressure.

At a critical pressure condition of 3200 psi (22.1 MPa) for example, the latent heat required to form water vapor becomes zero, and in this condition there will be no bubbles of vapor during evaporation. So that the transition process changes the water phase into water vapor under these conditions will occur more smoothly. On the basis of this phenomenon, we know a boiler technology called super and ultra-super critical boiler. Those boilers works by circulating water on boiler pipes with a pressure above the critical pressure of 22.1 MPa (221 bar).

Steam Boiling Curve
Steam Boiling Curve

Let me introduce you a curve called boiling curve. This curve will explain to us the characteristics of the water boiling process. Research for this curve is done by dipping a hot metal into a certain amount of water. The rate of heat transfer per unit area or called the heat flux fills the Y axis of the curve. While the X axis is filled by the temperature differential between the metal surface with the surrounding water.

From point A to B, convection heat transfer will cool the metal so that the boiling process will be retained. As it passes slightly through point B, it is known as the early boiling process, where the water temperature will rapidly adjust to the metal surface temperature and closer to its saturation temperature. Water vapor bubbles begin to form on the metal surface. Periodically the bubbles will collapse (smaller) because it interacts with other water. This phenomenon is called a subcooled boiling, and is characterized by points B and S on the curve. In this process, the heat transfer rate is high enough, but there is still no amount of water vapor. From point S to C, the water temperature has reached the saturation temperature more evenly. The vapor bubbles no longer collapse and shrink, it gets bigger and more bubbles form. This area curve is commonly called nucleate boilling region, which has a fast heat transfer rate, and the metal surface temperature is slightly higher than the water saturation temperature.

Approaching point C, the evaporation surface will be wider. At this time the process of steam formation occurs very quickly, causing the vapor to form as blocking the water to approach the metal surface. The metal surface becomes isolated by a kind of layer made of water vapor, resulting a reduction in heat transfer rate. This process (C-D) is known as critical heat flux (CHF), where the process of heat transfer from metal to water becomes slow because of the film layer formed.

Further, as illustrated by point D to E, it is called the unstable film boilling process. At this process, the metal-fluid contact surface temperature does not increase. Consequently, the heat transfer performance per area and energy transfer process are decreasing. From point E passing D ‘to F, the vapor insulation layer on the metal surface becomes very effective. So that the heat transfer from the metal surface through the film layer occurs by way of radiation, conduction, and micro-convection to the water surface adjacent to the film layer. In this phase the evaporation process continues with the marked formation of water vapor bubbles. This phase is known as stable film boiling.

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Formation of Water Vapor In Boiler Pipe

The process of forming water vapor in the water pipe boiler is theoretically referring also to the boiling curve. In more detail, the process of vapor formation you can see the process in the another curve above. What distinguishes from the usual process of moisture formation is, the process of vapor formation in a water pipe boiler occurs in the flow of water at a certain flow rate. This process is known as forced convection boiling, which is a more complex process involving two-phase fluid flow, gravity, material phenomena, and heat transfer mechanisms.

The second picture above is a process of boiling water on long pipe sections and heated evenly. As it approaches point (1), water enters the pipe and becomes convectionally a pipe cooling medium. Just at the point (1) begin to form water vapor bubbles, a sign that the initial boiling process begins. At point (2), more water vapor bubbles are formed and create bubbly flow.

Between points (2) and (3), more vapor bubbles are gathered to form larger bubbles. This flow known as intermediate flow has a phase called saturated nucleate boiling.

At point (3), the water temperature is higher and reaches its saturation temperature and reaches the nucleate boiling region phase. In this phase a mixture of water with vapor begins to form a bubbling stream, and forms an annular flow. This phenomenon as a result of the complex interaction between the surface tension force, the phenomenon of two surfaces, the drastic reduction of pressure, the water-vapor mass, and the momentum effect of the boiling process on the surface of the pipe.

The heat transfer process continues so that after passing through point (3) the annular flow is enlarged and the film water layer is formed on the pipe wall. Subsequent heat transfer occurs by conduction and convection by passing through the film layer, so that the evaporation process occurs at the water layer coat with water vapor. This heat transfer mechanism is called convection boiling, which also produces a high heat transfer rate.

At point (4) the heat transfer process reaches the CHF (Critical Heat Flux), where the film water layer on the pipe wall is replaced with a film layer of water vapor. In this phase there are several possible phenomenon risks:

  1. Increase the metal temperature of the pipe so it can damage the pipe.
  2. Heat transfer loss. And,
  3. Temperature fluctuations are very likely to cause thermal fatigue failures.

From point (4) to (5) is called post-CHF heat transfer, which occurs very complex. After point (5), all water has been evaporated and turned phase into water vapor.

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Single-lead Ribbed Tube

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Multi-lead Ribbed Tube

Some of the losses that may occur during the CHF heat transfer phase above, resulted in innovations with the development of threaded boiler pipes. There are two types of threaded boiler pipes, namely single-lead ribbed tube type and multi-lead ribbed tube. These screw pipes improve the performance of CHF, with the side effects of pressure drop that can still be tolerated but can eliminate other more dangerous side effects. The pipe thread causes a rotating stream that produces centrifugal force. The centrifugal force of the fluid against the wall of the pipe will prevent the formation of film layers to form a high quality water vapor with high heat flux.

What is Boiler? – A Simply Explanation

What is Boiler?

Boiler or also known as Steam Generator is a closed vessel in which contains water to be heated. The thermal energy of the boiler’s water vapor is then used for various purposes, such as for steam turbines, room heaters, steam engines, and so on. In the term of energy conversion process, the boiler has a function to convert chemical energy stored in the fuel into heat energy transferred to the working fluid.

Pressurized boilers generally use steel materials with certain specifications that have been specified in the ASME standard, primarily for the use of boilers in large industries. In recorded history various types of materials are used as boiler materials such as copper, brass, and cast iron. However, these materials have long been abandoned for economic reasons as well as material resilience that is not in accordance with industry needs.

The heat given to the fluid in the boiler comes from the combustion process with various types of fuel that such as wood, coal, diesel, petroleum, and gas. With the advancement of technology, nuclear is also used as a source of heat in the boiler.

Here are some examples of types of boilers:

1. “Boiler Pot” or “Haycock Boiler”

It is the simplest boiler in history. It began to be introduced in the 18th century, using large water volumes but can only produce at low pressure. This boiler uses wood or coal as its fuel. This type of boiler does not last long because its efficiency is very low.

2. Fire-Tube Boiler

In subsequent developments comes the design of fire-tube boiler. This boiler has 2 main parts in it, the tube/pipe side and the barrel side. The barrel side contains water, while the pipe side is the place of burning.

what is boiler
Fire-tube Boiler

Fire-tube boilers usually have a low vapor production speed, but have a larger reservoir of water vapor.

3. Water-Tube Boiler

Just like a fire-tube boiler, a water-tube boiler also consists of two main parts which is pipes and barrels. But the side of the pipe is filled with water while the barrel side becomes the place of the burning process. This type of boiler has a high velocity in producing water vapor, but does not have much water vapor reserves in it.

Water-tube Boiler

4. Combination of Fire-tube with Water-tube Boiler

This type of boiler is a combination of a fire-tube boiler with a water-tube. A firebox in it contains pipes filled with water, the resulting water vapor flows into the barrel with a fire-pipe inside. This type of boiler is used to be the locomotive engine, but not very popular in history.

What is Steam Turbine Degree of Reaction (Reaction Ratio)?

One of steam turbine components are the turbine blades that act as nozzles. Those nozzles, either on the stator or rotor side, convert heat of the steam into kinetic energy. The nozzled shape of turbine blade on the rotor side also serves to convert the steam kinetic energy into mechanical energy as rotation of the rotor.

The convertion of heat energy to kinetic energy is always followed by enthalpy drop at isentropic condition. This enthalpy drop may occur on the side of the stator or rotor blade depending on the turbine design.

Reaction Ratio (also known as the

Simply put, the Reaction Ratio becomes a number indicating the type of a turbine whether it is an impulse turbine, reaction turbine or mixture.

Reaction Ratio (R) = \dfrac {\Delta h_{rotor}}{\Delta h_{stage}}

Where Δhrotor is the amount of decreased enthalpy converted to kinetic energy on the side of the rotor blade, and Δhstage is the total amount of the enthalpy drop in one stage.

steam turbine degree of reaction
Speed Profile and Vector ​​of Impulse and Reaction Turbine

Reaction Ratio (R) = 0
At R = 0 means 100% enthalpy drop due to change into kinetic energy occurs on the stator blades. This process is a pure impulse process characterized by constant pressure at the point before and after the rotor blade, the steam stream only changes the direction only. The rotor blades alter the direction of the steam impulse directed at it and transfer high torque to the turbine shaft. Therefore, this type of turbine is also called an impulse turbine.

The advantage of this type of turbine is the large drop of enthalpy on a single stage of the blades, so the generation of energy by one turbine is greater. So the number of stages from the turbine will be less, and the turbine size will be shorter. But the disadvantage of this type is the loss of too much steam flow due to the larger flow velocity.

Reaction Ratio (R) = 0.5
A turbine of design R = 0.5 means that half of the enthalpy drop at one stage of the turbine blade occurs on the side of the stator blade, and the other half occurs on the turbine rotor blade. Turbine with this design is also called reaction turbine. Decrease in pressure and enthalpy of steam occurs on the stator side and turbine rotor. The steam pressure in the rotor blade inlet is larger than the outlet side. The flow of the steam is not only accelerated on the stator side, but also on the rotor side of the turbine.

The difference in steam pressure on the rotor blade side, causing the axial force on the whole turbine. An axial force means a force that is in line with the direction of the shaft. The axial force of the turbine rotor is opposite to the direction of the steam stream, and is also called axial thrust. Axial thrust must be compensated by the use of thrust bearing or against the force using a balance piston.

The advantage of using this type of turbine is the loss of steam flow due to the increase in flow velocity at every small stage. But the disadvantage is that the longer the turbine design, because the need for more stage than the impulse turbine.

In practice, turbines with a design R = 0.7 are more widely used at present. This means more enthalpy drops occur on the rotor side of the turbine than the stator side.

Steam Turbine Working Principle

The steam turbine working principle lies in the change of heat energy contained in steam which is converted into mechanical energy transmitted to the turbine rotor. This happens in several different steam turbine stages. Each turbine stage always consists of a stationary circular blade and a rotating blade.

Heat energy in steam is shown by the amount of enthalpy (h).

h = u + p.V
u = internal energy, p.V = work flow

Steam Turbine Working Principle
Converting Heat Energy from Steam into Kinetic Energy

First, heat energy must be converted to kinetic energy, this process occurs in the nozzle (see picture above). In steam turbines, nozzles are mounted on the sides of the turbine stator and also at the rotor blades, hereinafter known as reaction stage. In this nozzle, water steam increases the speed (kinetic energy increase), and this acceleration causes differential pressure between the upstream sides nozzle and downstream nozzle.

Second, the kinetic energy is transformed into a rotary energy of the turbine rotor that occurs only on the rotating blade (rotor side).

Velocity Vector On Steam Turbine Reaction Stage


Stages on the turbine has a speed difference, as shown in the picture above. At each level, a velocity triangle is drawn, one on the rotating inlet side of the blade, and the second at the outlet side. The absolute speed (c) in the inlet and outlet have different magnitude, since the kinetic energy of water vapor is converted to mechanical energy in the rotor.

Boiler Efficiency Calculation: a complete guide

Boiler efficiency is a quantity that indicates the relationship between input energy entering the boiler with output energy produced by the boiler. However, the boiler efficiency calculation can be defined in three ways:

  1. Combustion Efficiency
  2. Thermal Efficiency
  3. Fuel-to-Steam Efficiency

Boiler Combustion Efficiency

Combustion boiler efficiency generally describes the ability of a burner to burn the entire fuel into the boiler combustion chamber (furnace). The efficiency of this type is calculated from the amount of fuel that does not burn along with the amount of air combustion air (air excess). Burning boiler can be said to be efficient if there is no remaining fuel in the end outlet of boiler combustion chamber, so does the number of residual air.

To obtain high combustion efficiency, burner and boiler combustion chamber must be designed as optimal as possible. On the other hand differences in the use of fuels also affect the efficiency of combustion. It is known that liquid fuel and gas (LNG and HSD) produce higher combustion efficiency than solid fuels such as coal.

boiler efficiency

Calculating the combustion efficiency of the boiler is not difficult, we just need to reduce the total amount of heat energy released by thermal energy burning that passes out through the stack (chimney) divided by the total heat energy.

\eta_{combustion}=\dfrac {Q_{in}-Q_{losses}}{Q_{in}}\times100\%


\eta_{combustion} : boiler combustion efficiency (%)

Q_{in} : The total heat of combustion energy (calories; Joule)

Q_{losses} : Heat energy passing out through the chimney (calories; Joule)

The only difficult thing in calculating the combustion efficiency is how to pursuit the optimal number. Combustion efficiency is characterized by the overall fuel burning in the combustion chamber. While the control parameters are used to ensure the overall fuel burning, is the amount of air combustion (air excess) coming out through the stack. The more the amount of air excess coming out through the chimney, then the more likely the amount of unburned fuel can pass through the chimney. But you should remember that the more the amount of excess water passing through the chimney, the amount of heat energy escaping the rest of airborne is also growing. Therefore there is an optimum number of excess amounts of air, so that the boiler combustion efficiency can obtain the most optimal number.

Appears in the graph illustration above that the higher the amount of air (oxygen) passing through the stack, the smaller amount of fuel including carbon monoxide burned imperfectly. But as we have discussed above, the higher the amount of air excess, so the combustion efficiency chart is going to go back down, since the heat energy was coming away with the rest of the air. Then certainly there is an optimum value of the air excess to obtain the best combustion efficiency. As an illustration, the optimum value of air excess to the combustion of natural gas is 5 to 10%, liquid fuel at the rate of 5 to 20%, and 15 to 60% for coal combustion.

Boiler Thermal Efficiency

Boiler thermal efficiency shows how the performance in terms of its function as a heat exchanger. The efficiency calculation will show how effective the transfer of heat energy from the combustion process of fuel into the air. However, the efficiency calculation is not very accurate, because it does not account for the loss of heat radiation and convection that are not absorbed by water. In addition, the calculation of the thermal efficiency of the boiler cannot be used for economic analysis, because these calculation doesn’t take notice carefully the amount of fuel consumed. On this basis we will not discuss more about the calculation of the thermal efficiency of the boiler.

One that is considered the most effective way to determine the performance of broiler more closely is to count Efficiency of Fuel-to-Steam (commonly referred to as Fuel Efficiency). In addition to considering the effectiveness of the boiler as a heat exchanger (thermal efficiency), the calculation of boiler’s fuel efficiency also notices losses due to radiation heat transfer and convection. The calculation of boiler’s fuel efficiency boiler should consider the consumption amount of fuel used, so it is very appropriate to be used as a point of boiler economic analysis.

Direct Method

There are two popular methods for calculating the fuel efficiency of the boiler; the direct method and the indirect one. The direct method, known as method of input-output, is done by comparing directly the heat energy absorbed by the water so that the change phase into a vapor (output energy) with the thermal energy generated by burning fuel in the boiler’s combustion chamber (input energy). Simple formulation of calculation using the direct method can be described as follows:

\eta_{fuel}=\dfrac {Q_{steam}}{Q_{fuel}}\times 100\%
=\dfrac {Q\times \left( h_{g}-h_{f}\right) }{q\times GCV}\times 100\%


\eta_{fuel} : Boiler Fuel Efficiency (%)

Q_{steam} : Total heat energy absorbed by water vapor (calories; Joule)

Q ​: Discharge of water vapor out of boiler (kg / h)

h_{g} ​: Steam enthalpy out of boiler (kcal/kg)

h_{g} ​: Water enthalpy entering boiler (kcal/kg)

Q_{fuel} ​: The heat energy produced by fuel burning (calories; Joule)

q : Debit of fuel requirement (kg/h)

GCV ​: Gross Calorific Value (kcal/kg)

In the direct method, there are some parameters that should be measured with precision in order to get an accurate calculation results. These parameters include:

  1. Water discharge (feedwater) into the boiler
  2. Water discharge desuperheater
  3. Overall secondary flow rate as boiler blowdown, auxiliary steam, and so forth.
  4. Pressure and temperature of the entire flow of a working fluid such as water entry, exiting steam superheater, entering and exiting reheater steam, auxiliary steam, and others.
  5. Debit fuel needs
  6. Calorific value (heating value) fuels
  7. Other incoming energy

The following table shows the advantages and disadvantages of the methods of direct and indirect in the calculation of Boiler Fuel Efficiency.

Advantages Disadvantages
Direct Method
The primary parameters of the definition of Boiler Fuel Efficiency (input-output) are calculated directly. Debit and fuel heating value, as well as debit and steam water properties, should be calculated as accurately as possible to minimize inaccuracies.
Only requires a little calculation. Not able to demonstrate the potential causes of inefficiency.
Does not require the value assumption of immeasurable loss. Must use indirect methods to assess the accuracy of the calculation.
Indirect Method
The primary calculations such as flue gas analyzer and exhaust gas temperature can be done very accurately. Requiring more calculations than the direct method.
Calculations correction can be done to pursue the existing standards or to the fulfillment of the warranty. Not able to provide the data capacity and output automatically.
Has a low level of uncertainty, because the calculation of the loss reflects only a small fraction of the total available energy conversion. Some loss points can’t be measured so that its value should be assumed.
Biggest loss source can be known.
Error calculation rate is relatively low.

Indirect Method

What is meant by the indirect calculation of Boiler Fuel Efficiency is a calculation that does not directly involve the formulation of the main components of the input and output boiler efficiency energy, but by calculating the losses that exist. Let us consider the following formula:

\eta_{fuel}=\dfrac {output}{input}\times 100\%

And if:

input + credits = output + losses

And then:

\eta_{fuel}=\left[ \dfrac {input-losses+credit}{input}\right] \times 100\%
=\left[ 1-\dfrac {\left( losses-credit\right) }{input}\right] \times 100\%

The definitions of inputs, credit, outputs, as well as the losses are in accordance with the following illustration which we have quoted from the standard book of ASME.

In accordance with the formula above, the calculation of the efficiency of indirect done by reverse the focus to the parameters of losses and credit energy. What meant by credit energy is secondary energies entering the boiler in addition to the primary energy of the fuel. While losses are parameters of wasted energy that are not converted into heat energy in the steam. Calculation instruction and measurement of these parameters are elaborated clearly through standardization issued by the American Society of Mechanical Engineers (ASME).

The indirect method is done with great detail on each measured parameter, so that the level of accuracy is considered much better than the direct method. But of course the indirect method requires a greater cost because it requires special equipment. For this reason many experts consider the indirect method is more suitable for use in large-scale boilers and certainly not very suitable used to calculate the efficiency of small boilers.

One parameter that should be taken into account in the both methods (direct and indirect) is the input energy from the fuel. In the indirect method, the input energy symbolized by the QrF is formulated as follows:

QrF = MrF \times HVF


MrF = The flow rate of input fuel (kg/s)

HVF = The heating value of fuel (J/kg)

The above formula looks identical to the formulation of the input energy in the calculation of the direct method’s efficiency. One important component coming into the formula above is the heating value of fuel. As we discussed in another article about the heating value, that there are two types of heating value, namely the higher heating value (HHV) and the lower heating value (LHV). Both are equally reflect the calorific value contained in the fuel, but both have a distinct difference in value. In most of the fuel, the HHV value tends to be bigger than the LHV. So if it is associated with the calculation of the efficiency of the boiler, the boiler efficiency value using HHV as a reference would be relatively small compared with the calculation of Boiler Fuel Efficiency using LHV as a reference.


*Original article: Cara Menghitung Efisiensi Boiler
*Translated by: Todi