Types of Steam Generators

Types of Steam Generators
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The advent of the steam generator, or boiler, transformed everyday life long before electronic innovations did, and arguably had a greater overall impact than more recent innovations such as as online commerce, social media and wireless technology. It is hard to appreciate now just how much of a game-changer it was to be able to get from place to place without either personal or animal (e.g., horse-drawn carriage) power.

On its face, the production of steam on purpose looks like a strange choice. Looking at the world the way a young child might, steam appears to be little more than an obligatory watery waste product of various processes involving heat generation, from cooking a box of pasta to warming the corridors of a building.

The best way to relate your mind to the value of properly harnessed steam is to picture what happens when something that has steam billowing from it is suddenly capped or otherwise physically prevented from emitting that steam – for example, clamping a lid tightly down on a pot of boiling water for even a second before releasing it.

The Basics and Origins of Steam Power

Steam is water vapor, or more generally, the gaseous form of the molecule. Water consists of hydrogen and oxygen atoms and has a molecular formula of H2O. Like other matter with a particular boiling point, water is able to enter the gaseous phase when it reaches that temperature (for water, 100 C, or degrees Celsius (212 F, or degrees Fahrenheit) and gets a tiny energy push so it can overcome its heat of vaporization, a sort of toll that matter usually has to pay to change between states (solid, liquid or gas).

Today, steam's most vital widespread role is in the generation of electrical power. But back in the late 1600s, it was discovered that it was easier to remove waste water from mines when it was condensed. In the process, it was discovered that the process of water condensing creates a vacuum (negative pressure in relation to whatever lies outside the area of condensation activity). This finding was eventually integrated into modern steam engines and generators.

What Do Steam Power Plants Generate?

There are various types of steam power plants, with the organization and other specific details of each depending on the ultimate purpose of the steam-generated power. In each case, steam is not the goal, but a means to a power-producing end.

Rather than simply releasing steam to the open air, with any pressure local differences quickly being ironed out owing to an unlimited air supply, it is trapped in some kind of space and its pent-up strength unleashed on human-supplied equipment.

In power plants, steam is created by the burning of fuel in a high-pressure environment – that is, a boiler. This is seen in chiefly coal-fired plants, although by the early 21st century these had come under heavy fire for both their direct polluting effects and their contribution to anthropogenic climate change. Steam is also used in nuclear power plants as well as in solar thermal power plants.

Steam Power Plant Components

Although the composition and construction of boilers can vary, their core components are largely the same and include the following:

  • Firebox: This chamber is where combustion occurs, and it houses the burners and various regulatory devices.
  • Burners: These inject a mixture of air and fuel (usually coal, fuel oil or natural gas) into the distribution system to optimize the blend for combustion.
  • Drums: These include a lower mud drum to collect mostly solid waste and an upper steam drum to collect the steam for placement into the distribution system.
  • Economizer: This device optimizes operational efficiency by preheat feedwater to a given temperature before it can enter the body of the boiler system.
  • Steam distribution system: This network of valves, tubes and connections is customized for the pressure levels of the steam being carried through the system. Steam leaves the boiler with enough pressure to power whatever process is downstream (e.g., electricity generation via a turbine).
  • Feedwater system: This critical element of a boiler ensures that the amount of water entering the system balances that leaving the system. This must be calculated in weight, not volume, since some of the water is steam and some is liquid.

Types of Steam Generators

Firetube. These are most often used in processes that need anywhere from from 15 to 2,200 horsepower (1 hp = 746 watts, or W). This type of boiler is cylindrical, with the flame in the furnace cavity itself and the combustion gases themselves kept inside a series of tubes. These come in two basic designs: dry back and wet back.

Watertube. In this arrangement, tubes contain steam, water or both, while the products of combustion pass around the outside of tubes. These often have multiple sets of drums, and because they use relatively little water, these boilers offer unusually fast steaming capabilities.

Commercial. These usually feature combinations of watertube, firetube and electric-resistance designs. They are popular in large buildings requiring a mostly constant temperature, such as schools and libraries, office and government buildings, airports, apartment complexes, college and other research laboratories hospitals, and so on.

Condensing. Condensing boilers can reach thermal efficiency levels of to 98 percent, compared to 70 to 80 percent attainable using standard boiler designs. Typical efficiency levels reach about 90 percent when the return water temperature is at 110 F or lower, and rise with decreasing water-return temperature thereafter.

Flexible watertube (flextube). This construction is particularly resistant to "heat shock," making it a natural option for heating uses. Flexible watertube boilers come in a wide range of fuel inputs and are well-suited for low-pressure applications using either steam or hot water. (Not all "boilers" actually boil water!) These are also quite easy to maintain, with easy access to their working parts from the outside.

Electric. These boilers are famously low-impact: clean, quiet, easy to install, and small in relation to their utility. Because nothing is actually burned (that is, there is no flame to worry about), electric boilers are marvelously simple. There are no fuels or fuel handling equipment in the mix, and hence no exhaust and no need for associated pipes and ports. In addition, these have heating elements that are easy to replace.

Heat recovery steam generator (HRSG). This is an innovative energy-recovery "heat exchanger" that recovers heat from a stream of hot gas passing by. These create steam that can be used to drive a particular a process or used to drive a steam turbine to power electricity generation using an electromagnet. HRSGs are built on a foundation of three primary components – an evaporator, a superheater and an economizer.

Steam as Nuclear Reactor Fuel

Nuclear power plants use energy not from the combustion of fuel but by the mechanical separation of its tiniest components. That is a very mild way of describing nuclear fission, in which atoms (in this case, those belonging to the element uranium) are broken into smaller atoms, releasing enormous amounts of energy.

The energy released by fission is captured and used to heat and boil water, and the resulting steam is used to power a turbine for the purpose of electricity generation.

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