What is Fire-Tube Boiler? – Do you remember what boiler mean? Boiler is a vessel that serves to heat water. In principle, a pan is also a boiler, but it’s not a boiler of this kind that we will discuss. Long ago, mechanical engineers created boilers by simply increasing the size of the ‘pan’. Then gradually they design the boiler with more complex again. One important principle is that larger surface contact between the heat source and the heated water, the more steam produced by the boiler in equal size. Then came the idea of making pipelines in the gigantic ‘pan’, so the pipes flowed the fire – or at least the hot gases – through the amount of water housed by the giant ‘pan’. This is the forerunner of the fire-tube boiler. So the definition of a fire-tube boiler is one type of boiler that flows the heat of the combustion process into one or more pipes, which were inside a sealed container filled with water.
Fire-tube boiler is the simplest type of boiler. This boiler can be applied from low to medium water steam requirements. This is possible because the design is not complicated as the water-tube boiler. The size of the fire-pipe boiler is also relatively smaller, and allows it to be moved very easily. That advantages make this boiler is very popular when developed in conjunction with the steam engine. In the 19th century until the beginning of 20th century, fire-tube boilers were developed massively to meet the transportation needs of the time. Steam locomotive, naval ships, and early models of cars, became the most sophisticated models of fire-tube boiler transportation in that period.
As the name, the fire-tube boiler flows the hot gas to the water-coated tubes. Different pipeline designs from the various fire-tube boilers, are to maximize the heat absorption of the combustion gases. The water level inside the boiler tank, must be maintained to avoid overheating. On the other hand, this boiler is also equipped with a safety relief valve avoid the explosion from over pressure.
Many types of fire-tube boilers are also equipped with advanced steam heating systems to produce superheated steam. Nevertheless, the fire-tube boiler has limited steam production of only 2500 kg/h with a maximum pressure of 10 bars only.
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 use one or two stage reheater.
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:
The efficiency of the supercritical boiler is higher because to generate the same heat energy, it takes less fuel than the subcritical boiler.
The exhaust emissions, especially carbon dioxide, are relatively lower than those of subcritical boilers.
The size of the boiler is relatively smaller than the subcritical boiler with the same output.
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.
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.
If we look again, supercritical steam generators will experience two kinds of circulatory systems:
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.
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.
Supercritical Boiler Circulation during High Load Dry Mode
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).
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.
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:
Increase the metal temperature of the pipe so it can damage the pipe.
Heat transfer loss. And,
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.
Single-lead Ribbed Tube
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.
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.