Most boilers heat water until it boils, and then the steam is used at saturation temperature (i.e., saturated steam). Superheated steam boilers boil the water and then further heat the steam in a superheater. This provides steam at much higher temperature, and can decrease the overall thermal efficiency of the steam plant due to the fact that the higher steam temperature requires a higher flue gas exhaust temperature. However, there are advantages to superheated steam. For example, useful heat can be extracted from the steam without causing condensation, which could damage piping and turbine blades.
Superheated steam presents unique safety concerns because, if there is a leak in the steam piping, steam at such high pressure/temperature can cause serious, instantaneous harm to anyone entering its flow. Since the escaping steam will initially be completely superheated vapor, it is not easy to see the leak, although the intense heat and sound from such a leak clearly indicates its presence.
The superheater works like coils on an air conditioning unit, however to a different end. The steam piping (with steam flowing through it) is directed through the flue gas path in the boiler furnace. This area typically is between 1300-1600 degrees Celsius (2500-3000 degrees Fahrenheit). Some superheaters are radiant type (absorb heat by radiation), others are convection type (absorb heat via a fluid i.e. gas) and some are a combination of the two. So whether by convection or radiation the extreme heat in the boiler furnace/flue gas path will also heat the superheater steam piping and the steam within as well. It is important to note that while the temperature of the steam in the superheater is raised, the pressure of the steam is not: the turbine or moving pistons offer a "continuously expanding space" and the pressure remains the same as that of the boiler. The process of superheating steam is most importantly designed to remove all droplets entrained in the steam to prevent damage to the turbine blading and/or associated piping.
Supercritical steam generators (also known as Benson boilers) are frequently used for the production of electric power. They operate at "supercritical pressure". In contrast to a "subcritical boiler", a supercritical steam generator operates at such a high pressure (over 3200 PSI, 22 MPa, 220 bar) that actual boiling ceases to occur, and the boiler has no water - steam separation. There is no generation of steam bubbles within the water, because the pressure is above the "critical pressure" at which steam bubbles can form. It passes below the critical point as it does work in the high pressure turbine and enters the generators condenser. This is more efficient, resulting in slightly less fuel use and therefore less greenhouse gas production. The term "boiler" should not be used for a supercritical pressure steam generator, as no "boiling" actually occurs in this device.
The steam boiler is divided into one to three open radiation passes and a convection part. The radiation part includes the evaporator where saturated steam is produced. After passing through the radiation part, the flue gases enter the convection heating surfaces. There, they first transfer heat to super heaters and then to the economizer, and after that they go through to the flue gas cleaning system.
The superheater aims to increase the temperature of the condensed steam that comes from the evaporator. The temperature of the flue gases that head to the superheater does not exceed 630°C. The heating surface of the superheater is divided into three sections with intermediate water injection between the stages (de-superheaters) in order to control the steam temperature (max 400°C).The last section of the economizer aims to preheat the feed water prior to entering the boiler steam drum, and also to decrease the gas temperature to 160-220°C.
The radiation part of the boiler requires a space of up to 30 to 40 meters in height. The convection part of the boiler can be arranged either horizontally or vertically. The horizontal arrangement takes up approximately 20 meters more space than the vertical arrangement in the longitudinal direction. The arrangement of the convection section can significantly affect building costs and should be determined as early as possible.
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