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Superheater
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Superheater
A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, in some steam engines, and in processes such as steam reforming. There are three types of superheaters: radiant, convection, and separately fired. A superheater can vary in size from a few tens of feet to several hundred feet (a few metres to some hundred metres).
In many applications, a turbine will make more efficient use of steam energy than a reciprocating engine. However, saturated ("wet") steam at boiling point may contain, or condense into, liquid water droplets, which can cause damage to turbine blades. Therefore, steam turbine engines typically superheat the steam, usually within the primary boiler, to ensure that no liquid water enters the system and damages the blades.
In a steam engine, the superheater further heats the steam generated by the boiler, increasing its thermal energy and decreasing the likelihood that it will condense inside the engine. Superheaters increase the thermal efficiency of the steam engine, and have been widely adopted. Steam which has been superheated is known as superheated steam, and non-superheated steam is called saturated steam or wet steam. From the early 20th century, superheaters were applied to many steam locomotives, to most steam vehicles, and to stationary steam engines. It is still used in conjunction with steam turbines in electrical power generating stations throughout the world.
In steam locomotive use, by far the most common form of superheater is the fire-tube type. That takes the saturated steam supplied in the dry pipe into a superheater header mounted against the tube sheet in the smokebox. The steam is then passed through a number of superheater elements, which are long pipes placed inside the larger diameter fire tubes, called flues. Hot combustion gases from the locomotive's fire pass through the flues and, as well as heating the water in the surrounding boiler, they heat the steam inside the superheater elements they flow over. The superheater element doubles back on itself so that the heated steam can return. Most do that twice at the fire end and once at the smokebox end, so that the steam travels a distance of four times the header's length while being heated. At the end of its journey through the elements, the superheated steam passes into a separate compartment of the superheater header and then to the cylinders of the engine.
The steam passing through the superheater elements cools their metal and prevents them from melting, but when the throttle closes that cooling effect is absent, and so a damper closes in the smokebox to cut off the flow through the flues and prevent them being damaged. Some locomotives, particularly on the London and North Eastern Railway, were fitted with snifting valves, which admitted air to the superheater when the locomotive was coasting. That kept the superheater elements relatively cooler and the cylinders warm. The snifting valve can be seen behind the chimney on many LNER locomotives.
A superheater increases the distance between the throttle and the cylinders in the steam circuit and thus reduces the immediacy of throttle action. To counteract that, some later steam locomotives were fitted with a front-end throttle, placed in the smokebox after the superheater. Such locomotives can sometimes be identified by an external throttle rod that stretches the whole length of the boiler, with a crank on the outside of the smokebox. That arrangement also allows superheated steam to be used for auxiliary appliances, such as the dynamo and air pumps. Another benefit of the front-end throttle is that superheated steam is immediately available. With a dome throttle, it takes some time before the super heater actually provides an efficiency benefit.
Locomotives with superheaters are usually fitted with piston valves or poppet valves, because it is difficult to keep a slide valve properly lubricated at high temperature.
The first practical superheater was developed in Germany by Wilhelm Schmidt during the 1880s and 1890s. The Prussian S 4 locomotive, with an early form of superheater, was built in 1898, and more were produced in series from 1902. The benefits of the invention were demonstrated in the UK by the Great Western Railway (GWR) in 1906. The GWR Chief Mechanical Engineer, G. J. Churchward, believed that the Schmidt type could be bettered, and the design and testing of an indigenous Swindon type was undertaken, culminating in the Swindon No. 3 superheater in 1909. Douglas Earle Marsh carried out a series of comparative tests between members of his I3 class using saturated steam and those fitted with the Schmidt superheater between October 1907 and March 1910, proving the advantages of the latter in terms of performance and efficiency.
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Superheater
A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, in some steam engines, and in processes such as steam reforming. There are three types of superheaters: radiant, convection, and separately fired. A superheater can vary in size from a few tens of feet to several hundred feet (a few metres to some hundred metres).
In many applications, a turbine will make more efficient use of steam energy than a reciprocating engine. However, saturated ("wet") steam at boiling point may contain, or condense into, liquid water droplets, which can cause damage to turbine blades. Therefore, steam turbine engines typically superheat the steam, usually within the primary boiler, to ensure that no liquid water enters the system and damages the blades.
In a steam engine, the superheater further heats the steam generated by the boiler, increasing its thermal energy and decreasing the likelihood that it will condense inside the engine. Superheaters increase the thermal efficiency of the steam engine, and have been widely adopted. Steam which has been superheated is known as superheated steam, and non-superheated steam is called saturated steam or wet steam. From the early 20th century, superheaters were applied to many steam locomotives, to most steam vehicles, and to stationary steam engines. It is still used in conjunction with steam turbines in electrical power generating stations throughout the world.
In steam locomotive use, by far the most common form of superheater is the fire-tube type. That takes the saturated steam supplied in the dry pipe into a superheater header mounted against the tube sheet in the smokebox. The steam is then passed through a number of superheater elements, which are long pipes placed inside the larger diameter fire tubes, called flues. Hot combustion gases from the locomotive's fire pass through the flues and, as well as heating the water in the surrounding boiler, they heat the steam inside the superheater elements they flow over. The superheater element doubles back on itself so that the heated steam can return. Most do that twice at the fire end and once at the smokebox end, so that the steam travels a distance of four times the header's length while being heated. At the end of its journey through the elements, the superheated steam passes into a separate compartment of the superheater header and then to the cylinders of the engine.
The steam passing through the superheater elements cools their metal and prevents them from melting, but when the throttle closes that cooling effect is absent, and so a damper closes in the smokebox to cut off the flow through the flues and prevent them being damaged. Some locomotives, particularly on the London and North Eastern Railway, were fitted with snifting valves, which admitted air to the superheater when the locomotive was coasting. That kept the superheater elements relatively cooler and the cylinders warm. The snifting valve can be seen behind the chimney on many LNER locomotives.
A superheater increases the distance between the throttle and the cylinders in the steam circuit and thus reduces the immediacy of throttle action. To counteract that, some later steam locomotives were fitted with a front-end throttle, placed in the smokebox after the superheater. Such locomotives can sometimes be identified by an external throttle rod that stretches the whole length of the boiler, with a crank on the outside of the smokebox. That arrangement also allows superheated steam to be used for auxiliary appliances, such as the dynamo and air pumps. Another benefit of the front-end throttle is that superheated steam is immediately available. With a dome throttle, it takes some time before the super heater actually provides an efficiency benefit.
Locomotives with superheaters are usually fitted with piston valves or poppet valves, because it is difficult to keep a slide valve properly lubricated at high temperature.
The first practical superheater was developed in Germany by Wilhelm Schmidt during the 1880s and 1890s. The Prussian S 4 locomotive, with an early form of superheater, was built in 1898, and more were produced in series from 1902. The benefits of the invention were demonstrated in the UK by the Great Western Railway (GWR) in 1906. The GWR Chief Mechanical Engineer, G. J. Churchward, believed that the Schmidt type could be bettered, and the design and testing of an indigenous Swindon type was undertaken, culminating in the Swindon No. 3 superheater in 1909. Douglas Earle Marsh carried out a series of comparative tests between members of his I3 class using saturated steam and those fitted with the Schmidt superheater between October 1907 and March 1910, proving the advantages of the latter in terms of performance and efficiency.