A deaerating feed tank (DFT), often found in steam plants that propel ships, is located after the main condensate pump and before the main feed booster pump. It has these three purposes:
Remove dissolved oxygen (“air”) from the condensate
Pre-heat the feedwater
Provide a storage/surge volume
Based on the relevant theoretical Rankine cycle diagram, there are four main processes, or steps:
The Rankine cycle is a model used to predict the performance of steam turbine systems. It was also used to study the performance of reciprocating steam engines. The Rankine cycle is an idealized thermodynamic cycle of a heat engine that converts heat into mechanical work while undergoing phase change. It is an idealized cycle in which friction losses in each of the four components are neglected. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. It is named after William John Macquorn Rankine, a Scottish polymath and Glasgow University professor.
1 to 2 Water pressure is raised from low to high (pump work in) (feedwater pump)
2 to 3 Water is heated to boiling (heat input) (steam generator)
3 to 4 Steam is expanded in the turbine (work out) (turbine)
In the practical implementation of a Rankine cycle, it is common to break the single pump (process 1 to 2) into three pumps: (in water flow order: condensate pump, feed booster pump and then feedwater pump).
A condensate pump is a specific type of pump used to pump the condensate (water) produced in an HVAC, refrigeration, condensing boiler furnace, or steam system.
Details
Dissolved oxygen is removed by injecting auxiliary exhaust steam into the upper portion of the tank (above the feed water level) at roughly the same location (elevation) that the condensate enters the tank. The two are put in close physical contact over a large surface area to maximize heat transfer. As the condensate is heated, the steam drives off any dissolved gasses. Since the steam is injected above the feed water level a steam blanket forms above the water to keep the non-condensable gasses from re-entering the feed water. There is a connection to the gland exhaust system on the upper portion of the DFT that withdraws the oxygen and other non-condensable gasses as they are driven from the condensate. Removing oxygen minimizes corrosion and improves the vacuum quality.
The water in the tank serves as a surge volume within the steam plant.
A surge volume allows the plant to change bells (power output level) without running the feed pump dry or flooding the turbines. Consider the plant running in a steady state condition.
The bell is increased, more power output demanded, the rate of feed is increased. This draws more water from the condenser, perhaps to the point of being dry and starving the boiler resulting in a loss of propulsion. This is until the water, converted to steam, provides its energy to the turbine and then is condensed in the condenser.
The bell is decreased, less power output demanded, the rate of feed is decreased. Since less water is drawn from the condenser the condensate level rises, covering more condenser tubes, reducing the ability of the condenser to maintain vacuum and, if the level is allowed to go high enough, vacuum could be lost and/or water could impinge (and damage) the turbine blades as the turbine normally sits directly above the condenser.
Related Research Articles
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force is transformed, by a connecting rod and flywheel, into rotational force for work. The term "steam engine" is generally applied only to reciprocating engines as just described, not to the steam turbine.
The Watt steam engine, alternatively known as the Boulton and Watt steam engine, was the first practical steam engine and was one of the driving forces of the industrial revolution. James Watt developed the design sporadically from 1763 to 1775 with support from Matthew Boulton. Watt's design saved significantly more fuel compared to earlier designs that they were licensed based on the amount of fuel they would save. Watt never ceased developing the steam engine, introducing double-acting designs and various systems for taking off rotary power. Watt's design became synonymous with steam engines, and it was many years before significantly new designs began to replace the basic Watt design.
The RBMK is a class of graphite-moderated nuclear power reactor designed and built by the Soviet Union.
In electric power generation a combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators. The principle is that after completing its cycle, the temperature of the working fluid in the system is still high enough that a second subsequent heat engine extracts energy from the heat that the first engine produced. By combining these multiple streams of work upon a single mechanical shaft turning an electric generator, the overall net efficiency of the system may be increased by 50–60%. That is, from an overall efficiency of say 34%, to possibly an overall efficiency of 62%, 84% Theoretical efficiency
Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time. Trigeneration or combined cooling, heat and power (CCHP) refers to the simultaneous generation of electricity and useful heating and cooling from the combustion of a fuel or a solar heat collector. The terms cogeneration and trigeneration can be also applied to the power systems generating simultaneously electricity, heat, and industrial chemicals – e.g., syngas or pure hydrogen.
A steam injector is typically used to deliver cold water to a boiler against its own pressure using its own live or exhaust steam, replacing any mechanical pump. This was the purpose for which it was originally invented in 1858 by Henri Giffard. Its operation was from the start intriguing since it seemed paradoxical, almost like perpetual motion, but its operation was later explained using thermodynamics. Other types of injector may use other pressurised motive fluids such as air.
A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system. This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feedwater is introduced back into the steam cycle.
A thermal power station is a power station in which heat energy is converted to electric power. In most of the places in the world the turbine is steam-driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankine cycle. The greatest variation in the design of thermal power stations is due to the different heat sources; fossil fuel dominates here, although nuclear heat energy; solar heat energy, biofuels and waste incineration are also used. Some prefer to use the term energy center because such facilities convert forms of heat energy into electrical energy. Certain thermal power stations are also designed to produce heat energy for industrial purposes, or district heating, or desalination of water, in addition to generating electrical power.
A deaerator is a device that removes oxygen and other dissolved gases from water, such as feedwater for steam-generating boilers. Dissolved oxygen in feedwater will cause serious corrosion damage in a boiler by attaching to the walls of metal piping and other equipment and forming oxides (rust). Dissolved carbon dioxide combines with water to form carbonic acid that causes further corrosion. Most deaerators are designed to remove oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or less, as well as essentially eliminating carbon dioxide.
A surface condenser is a commonly used term for a water-cooled shell and tube heat exchanger installed to exhaust steam from a steam turbine in thermal power stations. These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric pressure. Where cooling water is in short supply, an air-cooled condenser is often used. An air-cooled condenser is however, significantly more expensive and cannot achieve as low a steam turbine exhaust pressure as a water-cooled surface condenser.
A condensing steam locomotive is a type of locomotive designed to recover exhaust steam, either in order to improve range between taking on boiler water, or to reduce emission of steam inside enclosed spaces. The apparatus takes the exhaust steam that would normally be used to produce a draft for the firebox, and routes it through a heat exchanger, into the boiler water tanks. Installations vary depending on the purpose, design and the type of locomotive to which it is fitted. It differs from the usual closed cycle condensing steam engine, in that the function of the condenser is primarily either to recover water, or to avoid excessive emissions to the atmosphere, rather than maintaining a vacuum to improve both efficiency and power.
The steam-electric power station is a power station in which the electric generator is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser. The greatest variation in the design of steam-electric power plants is due to the different fuel sources.
A condensate polisher is a device used to filter water condensed from steam as part of the steam cycle, for example in a conventional or nuclear power plant. It is frequently filled with polymer resins which are used to remove or exchange ions such that the purity of the condensate is maintained at or near that of distilled water.
Boiler feedwater is an essential part of boiler operations. The feed water is put into the steam drum from a feed pump. In the steam drum the feed water is then turned into steam from the heat. After the steam is used it is then dumped to the main condenser. From the condenser it is then pumped to the deaerated feed tank. From this tank it then goes back to the steam drum to complete its cycle. The feed water is never open to the atmosphere. This cycle is known as a closed system or Rankine cycle.
The South African Railways Class 25 4-8-4 of 1953 was a condensing steam locomotive.
An evaporator, distiller or distilling apparatus is a piece of ship's equipment used to produce fresh drinking water from sea water by distillation. As fresh water is bulky, may spoil in storage, and is an essential supply for any long voyage, the ability to produce more in mid-ocean is important for any ship.
The Hygroscopic Cycle is a thermodynamic cycle converting thermal energy into mechanical power by the means of a steam turbine. It is similar to the Rankine cycle using water as the motive fluid but with the novelty of introducing salts and their hygroscopic properties for the condensation. The salts are desorbed in the boiler or steam generator, where clean steam is released and superheated in order to be expanded and generate power through the steam turbine. Boiler blowdown with the concentrated hygroscopic compounds is used thermally to pre-heat the steam turbine condensate, and as reflux in the steam-absorber.
The power loss factor β describes the loss of electrical power in CHP systems with a variable power-to-heat ratio when an increasing heat flow is extracted from the main thermodynamic electricity generating process in order to provide useful heat. Usually, the power loss factor refers to extraction steam turbines in thermal power stations, which conduct a part of the steam in a heating condenser for the production of useful heat, instead of the low pressure part of the steam turbine where is could perform mechanical work.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
