Types of Superheater Boiler

Types of Superheater Boiler – Superheater is a subcritical boiler component that serves to heat the saturated steam, at constant working pressure, so it becomes superheated steam. In its development since the beginning of the 20th century, along with various boiler design races, some engineering experts patented the design of different superheater. Here are the types of superheater boiler, according to the patents:

1. Radiant Superheater

Radiant superheater is a superheater positioned in the boiler’s combustion chamber, so the superheater pipes instantly absorb the radiant heat from the combustion inside the furnace. In modern water-tube boilers, these superheater radiant pipes are placed hanging over the top of the boiler furnace. These pipes will absorb the second greatest heat energy, after the wall tubes (raiser/evaporator tube).

Types of Superheater Boiler
Radiant Superheater


At radiant superheater, the more steam flows in the superheater pipes of radians, the steam temperature output are decreasing.

2. Convective Superheater

As the name implies, the convective superheater is the superheater pipes of the boiler which placed in the flow of the flue gases that still contain heat. These convective superheater pipes will absorb heat from combustion exhaust gases convectionally. This concept aims primarily to maximize heat absorption from combustion.

In contrast to the radian superheater, the characteristic of the convection superheater is that the more steam flows in the convection superheater pipes, the superheater steam temperature output increased.

3. Separately Fired Superheater

Separately fired superheater is a superheater that is placed separately from the main boiler, which has own separate combustion system with the main boiler. This superheater design is not like the radians or convection types that still use the combustion heat inside the furnace, but instead put additional burners in the area of ​​superheater pipes. This type of superheater is not popularly used, and even tends to be extinct due to efficiency of combustion ratio with steam quality that is not better than other superheater types.

Separately Fired Superheater
One of the patented designs of Separately Fired Superheater

4. Combination Radiant and Convection Superheater

The last superheater type is the most popular, and still applied today. This superheater simultaneously combines two opposite characteristics between radiant and convection superheater, resulting in more homogeneous superheated steam temperature output in various steam flow. The graph below will explain these characteristics.

Types of Superheater Boiler
Variation of Steam Flow vs. Superheated Steam Final Temperature

In modern subcritical boilers, the superheater component of radians is subdivided into several stages. As in the subcritical boiler diagram below for example, after passing the Primary Superheater which is a convection superheater, steam is streamed sequentially to the Platent Secondary Superheater, Intermediate Secondary Superheater, then the last is the Final Secondary Superheater. This design aims to maximize the absorption of radiant heat from combustion inside the furnace.

Subcritical Boiler

What is Reheater on Boiler?

What is Reheater on Boiler? – Reheater is a part of boiler which to reheat steam output from the first level of steam turbine. Reheated steam will again absorb the heat energy from boiler to be used in the next level steam turbine. Reheater is one way to improve the thermal efficiency of the Rankine Cycle. Visit the following article to find out how to improve the thermal efficiency of the Rankine Cycle.

Let us consider the following Rankine T-s diagram.

What is Reheater on Boiler?

The T-s diagram above is the ideal, standardized Rankine Cycle diagram, without the use of the reheater concept. Superheated steam produced by the boiler only once flows the turbine blades, and ends with a condensation process in the condenser. Next let’s compare it with the Rankine Cycle with the reheater system.

What is Reheater on Boiler?



The Rankine Cycle equipped with a reheater, will have at least two levels of steam turbine: high pressure turbine and low pressure turbine. High pressure steam turbine output steam, commonly known as cold reheat steam, has lower pressure and temperature of main steam, before entering the turbine. Although the cold reheat pressure is drop significantly, it still has not reached its saturation point. So, if this steam reheated, it will return to superheated steam. Therefore the cold reheat steam that exits the high pressure turbine, will be reheated back into the boiler.

Modern boilers equipped with reheater pipes, specially designed so that the reheated steam absorb heat just like the main steam. The reheated steam, also called hot heat reheat steam, will reach the same temperature as the main steam vapor. To achieve this, the reheater pipes will be placed not far from the final pipes of the superheater. More specifically, let us refer to the following supercritical boiler diagram.

Boiler Supercritical

Seen in above boiler diagram, reheater pipes are divided into two heating stages. First the cold reheat steam passes through the Low Temperature Reheater pipes, which shown by the name “LTRH” at the diagram. Furthermore, LTRH output steam flows to the pipes called Final Reheater. The steam produced by the Final Reheater is then called Hot Reheat steam. Hot Reheat steam then goes to the low pressure steam turbine, so that the heat energy contained in it is converted into mechanical energy of the turbine shaft rotation.

Theoretically, the addition of one stage reheater utilization will increase the thermal efficiency of the rankine cycle by 3-4%, the addition of two stages of the reheater increases the efficiency by 1.5-2%, the addition of the three-stage reheater increases efficiency by 0.75-1%, and so on. Commonly, modern boiler only use one or two stage reheater.

How to Calculate Thermal Efficiency of Rankine Cycle

How to Calculate Thermal Efficiency of Rankine Cycle – The thermal efficiency of the rankine cycle is the ratio between the work produced by the steam turbine that has been reduced by the pump work, with the incoming heat energy from the boiler. Before further discussing the thermal efficiency of the rankine cycle, it is easier to understand by discuss the processes first.

How to Calculate Thermal Efficiency of Rankine Cycle



The rankine cycle is one form of energy conservation laws. The source of energy that exists on earth is converted into another beneficial form of energy to humans. Heat energy is used as the energy source of the rankine cycle process. This energy can be taken from the burning of fossil fuels, geothermal usage, or from nuclear reactions.

The heat energy from the above sources is transferred to the working fluid, such as water. When the fuel used is coal then this process occurs in the boiler. Through the diagram above D-E-A-F, the D-E line, water is still in liquid form, on the E-A line boiling process occurs and the water phase is liquid and steam mixture, while on the A-F line the water working fluid has water-vapor phase and got further heating process to obtain superheated point. The calorific value absorbed by water vapor can be calculated using the following formula:

Qin = m (hF – hD)

The superheated steam produced by the boiler then goes to the steam turbine. Heat energy from water vapor is then converted into motion energy, shown by the F-G line in the image above. The reduction of the enthalpy can be used to calculate the magnitude of the motion energy produced by the steam turbine using the following formula:

Wout = m (hF – hG)

The steam coming out from the steam turbine enters the condenser to be condensed back into liquid phase. Here we can see there is amount of heat energy not converted into motion energy, because the energy is used to convert the water into steam (latent heat). The decreases of the enthalpy (G-C line) can be used to calculate the thermal energy of condensed water using the following formula:

Qout = m (hG – hC)

The next process is pumping the condensate water to increase its pressure before entering the boiler. Shown by the C-D line, water does not experience much increase in enthalpy. This means that the energy given to the air is not too significant. Incoming energy values ​​can be calculated using the following formula:

Win = m (hD – hC)

So now we can calculate the thermal efficiency by using the formula below:

\eta _{thermal}=\dfrac {W_{out}-W_{in}}{Q_{in}}

eBook Rankine Efficiency Cycle Calculations:

Working Principle of Supercritical Steam Generators

Working Principles of Supercritical Steam Generator – Supercritical boilers have been popular since the mid-20th century. Since then until now, this boiler is very popularly applied to power plants, replacing subcritical boilers. Supercritical boilers have several advantages not possessed by subcritical boilers:

  1. The efficiency of the supercritical boiler is higher because to generate the same heat energy, it takes less fuel than the subcritical boiler.
  2. The exhaust emissions, especially carbon dioxide, are relatively lower than those of subcritical boilers.
  3. The size of the boiler is relatively smaller than the subcritical boiler with the same output.

Working Principle of Supercritical Steam Generators
T-s diagram of Supercritical Steam Generators

Supercritical Steam Generators



One basic concept of supercritical boilers is the working pressure and temperature above the critical point of water. If you already understand what is a critical point, then you should be know that above the critical point, water has a very different character than the other phase. Known as supercritical water, this phase has an indistinguishable characteristic of whether liquid water, or gas water (read the following article about supercritical steam). Supercritical water no longer has a boiling point, no longer having a saturation vapor phase, so that the pressurized water above 221 MPa if kept heated to above 374°C will immediately turn the phase into a supercritical steam, without passing the mixed phase between water and steam as in the boiler subcritical. This concept makes supercritical boiler components slightly different from subcritical boilers. Also because of this phenomenon, the term of boiler is not appropriate to use, since the water never actually boiled at supercritical boiler. We use supercritical steam generators term instead of supercritical boilers.

From those concept, there is already a fundamental difference between subcritical boilers and supercritical steam generators. In a subcritical boiler before the water completely changes phase to superheated steam, water passes through the saturation phase. Therefore, the subcritical boiler required a steam drum component as a separator between liquid water with saturation vapor that can be heated further into superheated steam. While the formation of water vapor in the supercritical steam generators does not pass through the saturated steam phase, it can be ascertained that supercritical steam generators do not require steam drum. It is also one of the economic advantages of supercritical steam generators.

h-P Diagram of Supercritical Steam Generators Sliding Pressure
h-P Diagram of Supercritical Steam Generators Sliding Pressure

However, supercritical steam generators do not completely negate the separation system of liquid water with steam. In the initial condition of boiler ignition, the boiler is still working at the pressure below the critical point. In this condition, water heating will surely pass the phase of saturation, so the steam separator component is needed. To ensure continuous flow of water inside the boiler evaporator as long as the working pressure is below the critical point, a boiler circulation pump is occupied. Gradually, the working pressure will be increased (sliding pressure) until it reaches the ideal pressure above the critical point. If the working pressure of the water is above the critical point (generally designed boiler load is more than 30%), the separator will automatically direct the fluid to the low temperature superheater, and no longer recirculated to the evaporator via boiler circulating pump. Boiler circulating pump stop working at this state. Under these conditions, the supercritical steam generators fully enters a once-through boiler flow process. Thus, the amount of water flow into the boiler via economizer is fully controlled by the boiler feed water pump.

Working Principle of Supercritical Steam Generators
Working Principle of Supercritical Steam Generators

If we look again, supercritical steam generators will experience two kinds of circulatory systems:

  1. Wet mode. Wet mode occurs when the boiler load is still below 30%, or in other words the water pressure is still below the critical point. In this condition, because water will still experience saturation phase, there will be circulation process inside the boiler so it is similar to that happened in subcritical boiler where the amount of water circulation passing boiler evaporator will be more than the amount of superheated water vapor produced. If referring to the example of the above boiler illustration, then the water flow will be like the scheme below.
    Wet Mode Supercritical Steam Generators Flow Chart
  2. Dry Mode. Dry mode occurs when the boiler load is above 30%, and the fluid pressure is above the critical point. In this condition the boiler will no longer pass through the saturation phase, so the separator and boiler circulation pump will stop working. Supercritical boilers will experience a single fluid flow (once-through) in the sense that all water entering the boiler will only pass once through the boiler pipes in the absence of recirculating porosity through the evaporator again. Supercritical boiler flow scheme will be as below.
    Dry Mode Supercritical Steam Generators Flow Chart
    Supercritical Boiler Circulation during High Load Dry Mode

Free E-book:

  1. E-book 1
  2. E-book 2

What is Rankine Cycle?

What is Rankine Cycle? Rankine cycle is a theoritical cycle which converts heat energy into work. Developed by William John Macquorn Rankine in the 19th century, it has been widely used to steam engines. Currently, the rankine cycle is used in power plants and produces 90% of the world’s electricity.

What is Rankine Cycle
Basic System of Rankine Cycle
What is Rankine Cycle?
Rankine Cycle T-s Diagram


Water becomes the closed-loop rankine cycle’s working fluid. It means that water at the end of the cycle process, back to use into the first step of the cycle. In the rankine cycle, this water undergoes four processes according to the image above:

  1. C-D Process: The working fluid is pumped from low to high pressure, and in this process the working fluid is still at liquid phase. This process is called isentropic-compression because when pumped, ideally there is no entropy change.
  2. D-F Process: High pressure water from C-D process, pumped into the boiler. Water is isobaric heated (constant pressure) at the boiler. Boiler use various heat sources from outside such as coal, diesel, or nuclear reactions. In the boiler, water undergoes a phase change from the liquid, a mixture of liquid and steam (saturated steam), and 100% dry vapor (superheated steam).
  3. F-G Process: This process occurs in a steam turbine. The steam from the boiler enters the turbine and undergoes an isentropic-expansion process. The energy stored in water vapor is converted to motion energy in the turbine.
  4. G-C Process: The steam coming out from the steam turbine enters the condenser to condensed at isobaric pressure. The steam changed its phase back to a liquid so it can be reused in the cycle process.

The cycle described through the T-S diagram above is the most basic and simple Rankine Cycle. In its practical use, there are several process modifications to obtain higher thermal efficiency. Such as the use of preheated water prior to entering the boiler, as well as the use of reheated steam from the first turbine (so called high-pressure turbine) so that it can be used again to enter the second steam turbine (intermediate pressure turbine).

Rankine Cycle with Water Preheater and Reheater System
Rankine Cycle with Water Preheater and Reheater System

In the illustration above, the condensate water pumped by the condensate extraction pump from the condenser, through the low pressure preheater system, before flowing to the deaerator/feedwater tank. As well as the water pumped by feedwater pump from the feedwater tank, also passing the high pressure preheater system before going to the boiler. The heat source used by the preheater is derived from the steam extraction taken from the steam turbine at certain stages.

Temperature-Entropy Diagram For Rankine Cycles with Water Preheater and Reheater System
Temperature-Entropy Diagram For Rankine Cycles with Water Preheater and Reheater System


Another difference with the conventional rankine cycle is the reheating of steam coming out from the high-pressure turbine by the reheater boiler to re-gain the superheater phase. The reheated steam than use as the energy source of the intermediate-pressure turbine.

There is also a bypass system, so steam not to be passed to the steam turbine. The superheater steam from the boiler does not enter the turbine and bypasses back into the reheater boiler. The steam coming out of the reheater boiler is bypassed for direct entry to the condenser. The function of this bypass system is as a protection system of the rankine cycle so that it can avoid severe damage. And also used at the beginning of the start-up process of the cycle system and even the process of turning it off.

Superheater Working Principle

Superheater Working Principle – Superheater is a subcritical boiler’s component that heat the saturated vapor, at constant pressure, so it becomes superheated steam. Superheater technology has been used since the use of steam engines early 20th century. The main purpose is to increase the heat energy contained by the steam, so that increasing the thermal efficiency of the engine. Until now the use of superheater is still very popular, especially in large water-tube boiler steam power plant.

Superheater Working Principle



Picture above is a simplified of a subcritical water-tube boiler. This water-tube boiler is composed by two water tanks on the bottom and top. Both tanks are connected with pipes that we know as the raiser tube. The heat from the combustion will first pass through the raiser tube, heat the water inside the pipe. Water than reaches its saturation point and turns the phase into saturated steam.

Saturated steam is still mixed with liquid water so it needs a mechanism to separate the saturated steam with water. This is the function of the top side tank. This tank is commonly known as steam drum. The liquid water will remain in the steam drum and will be recirculated by the raiser tube. While the saturated steam will exit the steam drum and go to the superheater pipes. The superheater pendant will absorb heat by convection and radiation from the flue gas of combustion, until saturated steam dried and become superheated steam. Superheated steam have a greater heat energy content than saturated vapor.

Above is a much more complex and modern subcritical boiler scheme. This boiler is very popular used in steam power plants. The concept is not much different from the previous subcritical boiler principle. The superheater components in modern subcritical boilers made into several levels to fulfill the needs of the quality and quantity of superheated steam produced. In the diagram the superheater is shown by red pipes.

The subcritical boiler’s combustion chamber is composed of vertical raiser tubes that will circulate the water from and to the steam drum. In modern subcritical boilers, only one water tank is used as a steam drum on the upper side of the boiler.



The water in the raiser tube will absorb the heat directly from the combustion process. The water from the raiser tube goes back to the steam drum, and will be separated between the saturated steam phase and the liquid water. Liquid water will be re-circulated through the raiser tube, while the saturated vapor go out to the first stage superheater pipe (primary superheater). Primary superheater is also commonly known as Low Temperature Superheater (LTSH). LTSH pipes absorb heat conventionally from combustion exhaust gases.

From LTSH, the steam will pass consecutively the Secondary Superheater Platform, Intermediate Secondary Superheater, and the Final Secondary Superheater. This steam produced by Final Secondary Superheater is called superheated steam or dry vapor. One phase of water that actually gas phase. It contains no moisture at all, and stores very high heat energy, much higher than the saturated vapor.

Understanding Superheater and Supercritical Boiler

Superheater Boiler

The superheater boiler produces superheated or dry water vapor. This steam stores more heat energy than saturated water steam, characterized by higher enthalpy values. The steam produced by conventional boilers generally only reaches the saturated phase, but in this superheater boiler, saturated steam will be heated further to reach the superheated phase. In addition to storing greater heat energy, superheater boiler removes the dampness of saturated steam.

The main advantage of using a superheater boiler is to increase the boiler efficiency, so it can reduce fuel and water consumption. But on the other hand, there are additional costs required for the erection and maintenance. Since the superheater boiler works at high pressure and temperature, it is using higher pipe quality than conventional boilers.

Superheater Boiler

The superheater boiler at the beginning of its emergence is used on steam trains. And then more widely used for the needs of steam power plants. The size also depends on the needs of steam consumption, 640 megawatts of power plants for example using a superheater boiler with steam production of about 1800 tons per hour.



Supercritical Boilers

Supercritical boilers produce supercritical steam. This boiler is called supercritical because it operates above the critical pressure and temperatures, which is 3,200 psi and 647 Kelvin. In contrast to a superheater boiler that requires a device to separate water vapor with a mixture of steam and water (usually called steam drum), supercritical boilers do not need it. During the process of supercritical steam formation there will be no transition phase from liquid water to steam. This leads to less fuel consumption, and further reduces CO2 gas emission. Actually the term boiler is not appropriately used in supercritical boilers, because in the process does not occur boiling process in it. So that supercritical boiler better known as supercritical steam generator.

Supercritical Boiler

What is Supercritical Steam?

What is supercritical steam? While a supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. So, supercritical steam is water at a temperature and pressure above its critical point (>647.096 K, >22.064 MPa), where distinct liquid water and steam phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be ‘fine-tuned’.

Above the critical point there is a state of matter that is continuously connected with (can be transformed without phase transition into) both the liquid and the gaseous state. It is called supercritical fluid. All differences between liquid and vapor disappear beyond the critical point.

What is Supercritical Steam?

This can be seen in the water phase diagram above, beyond the critical point (>647.096 K, >22.064 MPa) in the liquid-vapor space on the right. At the critical point there is about 30% free monomeric H2O molecules and about 17% hydrogen bonding. As with other supercritical fluids. supercritical water has no surface tension with its gas or liquid phase or any other supercritical phase as no such interfaces exist. Above 647.096 K, water vapor cannot be liquefied by increasing pressure.



The properties of supercritical water are very different from ambient liquid water. For example, supercritical water is a poor solvent for electrolytes. However, it is such an excellent solvent for non-polar organic molecules, due to its low relative permittivity (dielectric constant) and poor hydrogen bonding, that many are completely miscible. Viscosity and dielectric both decrease substantially whereas auto-dissociation increases substantially. The physical properties of water close to the critical point (near-critical) are particularly strongly affected by density. An extreme density fluctuation around the critical point causes opalescent turbidity. At the critical point, only one phase exists. The heat of vaporization is zero.

As pointed out in numerous studies, water’s properties exhibit dramatic changes under supercritical conditions: the fraction of hydrogen bound (HB) molecules greatly decreases with respect to ambient P and T, and there appears to be a consensus on the persistence of some hydrogen bounds up to at least 600 °C and 134 MPa (3). The hydrogen bound network, where present, is substantially distorted. Whether supercritical water is homogeneous (or composed of patches of clearly defined HB regions and non-HB ones), and over which length-scale possible density heterogeneities might appear, are still matters of debate. The presence of inhomogeneous patterns in the density has long been a contentious topic for the liquid at ambient conditions as well. Again, a tenuous consensus is forming in the scientific community around the idea that possible heterogeneities represent transient rather than equilibrium states of the liquid or supercritical fluid.

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.

What is Saturated Steam?

What is Saturated Steam? – Saturated steam is a condition where water vapor is at the equilibrium of pressure and temperature equal to liquid phase water. Saturated steam becomes a phase transition between liquid phase of water to its pure gas phase, or commonly known as superheated steam. When the water is in this phase of transition, there is mixing between the liquid phase of water with gas phase of water (saturated steam) in proportion to the amount of latent heat absorbed by the fluid.

Saturated steam 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% compared to its liquid phase, that is the end of the phase of saturated steam. The process of reaching almost 100% of the vapor phase occurs at a constant pressure and temperature. Furthermore, if thermal energy continues to be fed to saturated steam, there will be an increase in fluid temperature and encourage steam to turn the phase into superheated steam.

Phase Diagram of Water



According to the water phase diagram above, the phase of saturated steam can only form along the saturated curve. The lower limit of the saturated curve is the triple point, while the upper limit of the curve is the critical point. Water in more than triple-point conditions will not experience a phase of saturated steam. Water that has a pressure above 22.1 kPa, if it continues to be heated will immediately turn the phase into supercritical steam.

The mixture between water vapor and liquid water in the saturated steam can be determined in amount by using a saturated steam diagram. This diagram uses pressure as the Y axis and the enthalpy as the X axis. This saturated steam diagram is made of a curve. Half of the curve from the lowest point to the top is called the saturated water curve. This part curve becomes the boundary between liquid water with the saturated steam phase. For the right curve from the top of the curve to the lowest point is called the saturated steam curve. This curve becomes the boundary between the phase of saturated steam and superheated steam phase. Right at the vertex of the curve is a critical point, the same point as the critical point in the phase diagram of water.

What is Saturated Steam

Since the saturated steam is in constant pressure, a certain amount of saturated steam is represented by a horizontal straight line connecting a point on the saturated water curve to another point on the saturated steam curve. The point on the saturated water curve (hf) shows the enthalpy value of saturated water, ie how much heat energy required for water at pressure P per one unit of mass can reach saturated water. While the point on the saturated steam curve (hg) is the total enthalpy value required so that the water reaches 100% of steam.

The simple relationship is:
hg – hf = hfg

Where:
hf = enthalpy saturated water
hg = enthalpy saturated steam
hfg = difference of enthalpy required saturated water to achieve saturated steam

In other cases, the enthalpy value given to water is not as large as hg, ie only by hmix. The hmix point is anywhere along the horizontal line. In this case the saturation vapor is a mixture of vapor with water whose ratio can be easily determined using the following equation:
x=\dfrac{h_{mix}}{h_{fg}}

So:
hmix = hf + x . hfg

Where:
x = comparison of the amount of water in the overall vapor mixture of saturation
hmix = enthalpy mixture