Boiler Water and Steam Cycles - Understand the working"},"lengthSeconds":"961","ownerProfileUrl":"ht

This is the inside of a

boiler

. Here the

water

in the tubes lining the walls of the furnace absorbs the heat from burning fuel. The

water

is boiled producing

steam

that drives a turbine and also provides heat for other systems in the plant. The circulation of water in a

boiler

. necessary for two reasons: firstly, it provides a constant supply of water to convert to

steam

and secondly, it draws the intense heat of the boiler furnace away from the boiler tubes, preventing overheating and rupture of the tubes. in a typical drum type boiler: the water flow. the route includes an economizer the downcomer headers of the boiler drum and boiler tubes the water supplied to a boiler is called feedwater the first component that the feedwater passes through is the economizer the economizer uses heat from the flue gases that would otherwise flow out of the chimney to heat the feedwater heating the feedwater means that less fuel must be burned in the furnace to convert the water to steam in this boiler the feedwater heated flows from the economizer and enters the bottom of the boiler drum through the feedwater inlet.

The water continuously circulates inside the boiler through the downcomers, bottom headers and boiler tubes, a boiler It generally has two or more downspouts and two or more headers; However, for clarity, this illustration shows only one of each of the lower headers that supply water to the boiler tube bottoms. The boiler tubes that line the interior walls of the boiler furnace are commonly called walls. of water. The water walls are exposed to heat produced by burning fuel as water flows upward through the water walls. The heat converts some of the water into steam. The mixture of water and steam rises until it enters the boiler drum where it is separated into water and steam.

The steam rises to the top of the drum and the water remains at the bottom. The water is then available for recirculation through the boiler. water flow through some boilers occurs through natural circulation the key to

understand

ing natural circulation is that cold water is heavier or denser than hot water or steam in a boiler the mixture of water and steam in the water walls is hotter than the water flowing from the drum through the downcomers, this is because the water walls are heated by the fuel burned in the boiler furnace as the mixture of water and steam rises, the colder water from the downspouts flows to the lower headers and then to the water walls, the colder water is also heated. and rises creating a self-sustaining flow through the water side of the boiler.

The natural circulation of water in a boiler is affected by two factors: the amount of heat produced in the boiler furnace and the pressure at which the boiler operates, for example by raising the temperature in the boiler furnace. the natural circulation of water because it results in a greater temperature difference between the water walls and the downspouts. Increasing the second pressure factor has a negative effect on natural circulation because as the pressure increases, the density difference between water and steam decreases in some cases. Boilers Boiler water circulation pumps are used to increase circulation Circulation using pumps is often known as controlled circulation The pumps forced colder water from the drum through the downspouts to the lower headers and up through of water walls the increased flow over natural circulation provided by the pumps allows the water to absorb more heat, as a result, controlled circulation boilers can produce more steam for a given boiler size a boiler drum has two functions Mains: Receives heated feed water to compensate for the water that is converted to steam and leaves the boiler Also collects steam that is produced at the water walls This is a simplified cutaway view of a typical boiler drum The bottom of the drum contains water that will be circulated back through the boiler the top of the drum is used To collect steam, heated feedwater is introduced into the drum through a distribution pipe.

The drum is usually kept half full with water. In this example, the water and steam mixture returning to the drum from the water walls is directed against baffles called the drum. covers that run the length of the drum the covers direct the mixture of water and steam towards the moisture separators which are located at the top of the covers the moisture separators separate the water from the steam the water falls to the bottom of the drum where it is mixed with the feed water the steam rises to the top of the drum the steam could still have some water mixed in so in this example it is sent through dryers which are located at the top of the drum the dryers They remove as much water as possible before steam leaves the drum.

This is important because water coming out of the drum with steam could damage downstream components. Steam is discharged from the drum through steam outlet pipes between the time the steam leaves the boiler drum and the time it enters the condenser is influenced by components in the steam flow path to

understand

which are these components, you need a

working

knowledge of three basic concepts that are associated with the production and use of steam, they are saturation boiling temperature and superheated boiling. process of converting water into steam for any given pressure there is a corresponding temperature at which water boils the temperature at which water will boil at a given pressure is known as saturation temperature as the pressure increases the saturation temperature also increases For example, at sea level, the atmospheric pressure is fourteen point seven pounds per square inch.

At that pressure, the saturation temperature of water is 212 degrees Fahrenheit. At a pressure of 1,000 pounds per square inch, the saturation temperature of water is rises to 545 degrees Fahrenheit when boiling occurs as in this water wall both water and steam are present and at saturation temperature, no matter how much heat is added to the boiler furnace, the temperature in the water walls will never rise above of the saturation temperature whenever water is present after all water has been removed from the steam. The steam temperature can be raised above the saturation temperature. Steam that has been heated above the saturation temperature is called superheated steam.

Superheated steam is desirable because higher temperature steam can do more work on the turbine. The additional work from the same amount of steam production creates more work on the turbine. a more efficient generating unit, the steam in the drum of a typical drum type boiler is at its saturation temperature when the steam leaves this drum, passes through moisture separators and dryers that remove water from the steam and thus Prepare for superheating Once the water is removed, the steam is sent to two separate sets of boiler tubes called superheaters, where it is heated above its saturation temperature, the superheaters are positioned so that they are exposed to the heat of Combustion in the boiler furnace or hot flue gases, the superheater.

The tubes absorb heat and transfer it to the steam that passes through them. There are several ways to classify superheaters, for example, a superheater can be classified as a radiant superheater or a convection superheater. a radiant superheater is in a direct line of sight. with the flames in the boiler furnace and receives most of its heat by radiation from the flames a superheater can also be classified as a primary superheater or secondary superheater a primary superheater is a superheater through which the steam passes first It is generally a superheater convection and is generally in a cooler location than the secondary superheater.

The primary superheater is designed to raise the temperature of the steam from its saturation temperature to a specific temperature range. The secondary superheater raises the steam temperature again. The secondary superheater is usually located closer to the boiler heater so that it is exposed to higher temperatures. The secondary superheater may be a convection superheater or, as in this case, a radiant superheater. This boiler has an axial temperature or superheater located between the primary and secondary superheaters, the D superheater prevents the steam from getting so hot that it damages the superheater tubes if the steam gets too hot, the D superheater sprays water into the steam flow , this water immediately turns into steam and mixes with the steam already there, the result is a decrease in the temperature of the superheated steam of the secondary superheater.

The steam goes to the high pressure or HP section of the turbine. As the steam flows through the HP section, it gives up much of its energy and its temperature and pressure drop before passing to the other sections of the turbine, the steam returns to the boiler where its temperature increases as it flows through one or more overheat errors. Reheat bugs are very similar to superheaters, they use heat from the boiler furnace. or hot combustion gases to raise the temperature of the steam, for example, this boiler has a primary superheater into which the steam coming from the high pressure section of the turbine first enters, after the steam flows through the superheater primary, passes to a The secondary reheater and a temperer located at the beginning of the secondary reheater are used to control the final temperature of the superheated steam leaving the secondary reheater.

The temperature evaluator sprays water into the steam flow if the steam becomes too hot. In this example, the secondary superheater. It can also be classified as a radiant reheater because it is located in line of sight with the boiler flames and receives most of its heat by radiation from the flames. The primary reheater is located further from the flame and receives most of its heat by convection from the hot flue gases, so it can be classified as a convection reheater after reheating, the steam in this steam flow path It flows into the intermediate pressure or IP section of the turbine and then through the low pressure or LP section from the low pressure section the spent gas. the steam flows into the condenser inside the condenser the steam flows through tubes containing cooling water the water in the tubes absorbs the heat from the steam and the steam condenses back into water but collects at the bottom of the condenser and is pumped back to the boiler to start the cycle again the primary reheater is located further from the flame and receives most of its heat by convection from the hot flue gases, so it can be classified as a convection reheater at any time. given pressure, there is a corresponding temperature at which water will boil this temperature is called saturation temperature, for example, at standard atmospheric pressure which is 14.7 PSI, the saturation temperature of water is 212 degrees Fahrenheit when the pressure increases As it does in this boiler, the saturation temperature also increases to 1000 psi, the saturation temperature of water is 545 degrees Fahrenheit when the saturation temperatures of water at different pressures are plotted on a temperature versus pressure graph, a line showing the relationship between saturation temperature and pressure;

However, this relationship between temperature and pressure is only valid up to a certain point, that point which is called the critical point is 705 degrees Fahrenheit and 3206 PSI at or above the critical point, the densities of water and steam are the same, in In other words, there is no distinguishable difference between water and steam when the temperature and pressure of a boiler are above the critical point. Of the heat produced in the boiler can be used to increase the temperature of the steam, but boilers operating below the critical point always have to use some of their heat to convert water to steam, so all boilers operating below the critical point critical point have a built-in heater.

In inefficiency boilers, those operating below the critical point are called subcritical boilers, while boilers operating above the critical point are called supercritical boilers. A single pass boiler gets its name from the fact that water is pumped through it only once and is not recirculated many times. Passage boilers operate under supercritical conditions in a typical single pass supercritical boiler. Boiler feed pumps force feed water through a header at the bottom of the furnace and up through the water walls as the water flows through the water walls, itstemperature increases water. In the walls water is already under a very high pressure, typically around 3500 psi, which is well above the critical point pressure.

When the temperature increases above the critical point, the water becomes indistinguishable from steam. The section of the water walls where this happens is called the transition zone. There is no need for drum moisture separator dryers or a recirculation system because once the flow passes through the transition zone there is no water left to recirculate the vapor leaving the transition zone passes to the components in the path boiler steam flow, components. in the steam flow path are generally the same for supercritical and subcritical boilers. The main advantage of a single pass supercritical boiler is that it operates more efficiently than a subcritical boiler which has to recirculate water in the process of producing single pass supercritical steam.

The boiler requires less fuel than a drum boiler of the same size to produce the same amount of steam, making a supercritical boiler less expensive to operate; However, although single-pass supercritical boilers tend to be cheaper to operate, they are more expensive to build and maintain as special materials are needed to withstand supercritical temperatures and pressures. As an operator, you will be responsible for maintaining the correct steam pressure in your boiler. To do this, you need to know some of the factors that affect boiler steam pressure in a drum boiler. The steam pressure mainly depends on the firing rate, assuming that the steam demand does not change, it must maintain a constant firing rate to maintain a constant pressure.

If you reduce the firing rate in the boiler furnace, the boiler will produce less steam and the steam pressure will decrease. If you increase the firing rate in the boiler furnace more steam will be produced and the steam pressure will increase in a single pass boiler. The heat produced in the furnace has little effect on the steam pressure, because the pressure in this type of boiler is controlled primarily by the boiler feed. pumps if the pressure produced by the boiler feed pumps is reduced, the steam pressure will decrease if the pressure produced by the boiler feed pumps is increased, the steam pressure will increase reduce the combustion rate in the furnace a single pass boiler will only cause the temperature at the water walls to drop the pressure is not significantly affected