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.
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.
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.
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 uses one or two stage re-heater.
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.
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.
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.
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 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.
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.
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.
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.
Also published on Medium.