{"id":16074,"date":"2017-12-09T15:10:50","date_gmt":"2017-12-09T15:10:50","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=16074"},"modified":"2022-11-03T12:49:46","modified_gmt":"2022-11-03T12:49:46","slug":"properties-steam-what-is-steam","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/materials-nuclear-engineering\/properties-steam-what-is-steam\/","title":{"rendered":"Properties of Steam – What is Steam"},"content":{"rendered":"
<\/div>\n

What is steam<\/h2>\n

Steam<\/strong> is an invisible gas<\/strong> consisting of vaporized water, which is formed when water boils. When steam is visible, it contains the visible mist of water droplets. Such steam is referred to as \u201cwet steam<\/strong>\u201d, but \u201cdry steam<\/strong>\u201d is always invisible. At lower pressures, such as in the upper atmosphere or the condenser of thermal power plants, steam can exist at a lower temperature than the nominal 100 \u00b0C at standard temperature and pressure.<\/p>\n

\"Phase<\/a>
Phase diagram of water.
Source: wikipedia.org CC BY-SA<\/figcaption><\/figure>\n

Since water and steam<\/strong> are common media used for heat exchange <\/strong>and energy conversion,<\/strong> steam is generated largely by energy systems, such as in thermal power plants. \u00a0As is typical in all conventional thermal power plants, the heat is used to generate steam which drives a steam turbine connected to a generator that produces electricity. Note that modern steam turbines are used to generate more than 80% of the world\u2019s electricity.<\/p>\n

Steam is generally categorized according to the vapor\/(liquid + vapor) fraction. This fraction is a very important parameter of steam, and it is known as vapor quality.<\/p>\n

See also: Properties of Water<\/a><\/p><\/div><\/div>

\u00a0
<\/span>Pressurizer: steam-liquid equilibrium<\/div>
\n
\"pressurizer\"<\/a>
A pressurizer is a key component of PWRs.<\/figcaption><\/figure>\n

A pressurizer<\/strong> is a component of a pressurized water reactor<\/a>. Pressure in the primary circuit<\/strong> of PWRs is maintained by a pressurizer<\/strong>, a separate vessel connected to the primary circuit (hot leg), and partially filled with water heated to the saturation temperature<\/strong> (boiling point) for the desired pressure by submerged electrical heaters<\/strong>. The temperature in the pressurizer can be maintained at 350 \u00b0C (662 \u00b0F), which gives a subcooling margin (the difference between the pressurizer temperature and the highest temperature in the reactor core) of 30 \u00b0C. Subcooling margin is a very important safety parameter of PWRs since the boiling in the reactor core must be excluded. The basic design of the pressurized water reactor<\/strong> includes such a requirement that the coolant (water) in the reactor coolant system must not boil. To achieve this, the coolant in the reactor coolant system is maintained at a pressure sufficiently high that boiling does not occur at the coolant temperatures experienced while the plant is operating or in an analyzed transient.<\/p>\n

Functions<\/h2>\n

Pressure<\/strong> in the pressurizer is controlled by varying the temperature of the coolant in the pressurizer. For these purposes, two systems are installed. Water spray system<\/strong> and electrical heaters system<\/strong>. The volume of the pressurizer (tens of cubic meters) is filled with water on saturation parameters and steam. The water spray system (relatively cool water \u2013 from the cold leg) can decrease the pressure in the vessel by condensing the steam<\/strong> on water droplets sprayed in the vessel. On the other hand, the submerged electrical heaters are designed to increase the pressure by evaporating the water<\/strong> in the vessel. Water pressure in a closed system tracks water temperature directly; as the temperature rises, the pressure goes up.<\/p><\/div><\/div>

<\/span>Steam Generator - operating conditions<\/div>
\n
\"Steam<\/a>
Steam Generator – vertical<\/figcaption><\/figure>\n

Steam generators are heat exchangers<\/strong> that convert feedwater into steam<\/strong> from heat produced in a nuclear reactor core<\/a>. The steam produced drives the turbine. They are used in most nuclear power plants, but there are many types according to the reactor type<\/a>.<\/p>\n

The hot primary coolant (water 330\u00b0C; 626\u00b0F; 16MPa<\/strong>) is pumped into the steam generator<\/strong> through the primary inlet. High pressure of primary coolant is used to keep the water in the liquid state. Boiling of the primary coolant shall not occur.<\/strong> The liquid water flows through hundreds or thousands of tubes (usually 1.9 cm in diameter) inside the steam generator. The feedwater (secondary circuit) is heated from ~260\u00b0C 500\u00b0F<\/strong> to the boiling point of that fluid (280\u00b0C; 536\u00b0F; 6,5MPa)<\/strong>. Heat is transferred through the walls of these tubes to the lower pressure secondary coolant located on the secondary\u00a0side of the exchanger where the coolant evaporates to pressurized steam (saturated steam 280\u00b0C; 536\u00b0F; 6,5 MPa)<\/strong>. The pressurized steam leaves the steam generator through a steam outlet and continues to the steam turbine. Heat transfer is accomplished without mixing the two fluids to prevent the secondary coolant from becoming radioactive. The primary coolant leaves (water 295\u00b0C; 563\u00b0F; 16MPa)<\/strong> the steam generator through the primary outlet and continues through a cold leg to a reactor coolant <\/a>pump<\/a> and\u00a0then into the reactor.<\/p><\/div><\/div><\/div>

<\/div><\/div><\/div>
<\/div>\n

Vapor Quality – Dryness Fraction<\/h2>\n

\"wet-steam-vapor-liquid-mixture-min\"<\/a>As seen from the phase diagram of water<\/strong>, in the two-phase regions (e.g.,, on the border of vapor\/liquid phases), specifying temperature alone will set the pressure, and specifying pressure will set the temperature. But these parameters will not define the volume and enthalpy because we will need to know the relative proportion of the two phases<\/strong> present.<\/p>\n

The mass fraction<\/strong> of the vapor in a two-phase liquid-vapor region is called the vapor quality<\/strong> (or dryness fraction), x<\/em>, and it is given by the following formula:<\/p>\n

\"vapor<\/a><\/p>\n

The value of the quality ranges from zero to unity<\/strong>. Although defined as a ratio, the quality is frequently given as a percentage. From this point of view, we distinguish between three basic types of steam. It must be added, at x=0, we are talking about the saturated liquid state<\/strong> (single-phase).<\/p>\n