Properties of Steam – What is Steam

What is steam

Steam is an invisible gas 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 “wet steam“, but “dry steam” is always invisible. At lower pressures, such as in the upper atmosphere or in the condenser of thermal power plants, steam can exist at a lower temperature than the nominal 100 °C at standard temperature and pressure.

Phase diagram of water
Phase diagram of water.
Source: wikipedia.org CC BY-SA

Since water and steam are a common media used for heat exchange and for conversion of energy, steam is generated on a large scale by energy systems, as, for example, in thermal power plants.  As is typical in all conventional thermal power plants the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity. Note that, modern steam turbines are used to generate more than 80% of the world’s electricity.

Steam is generally categorized according to the vapor/(liquid + vapor) fraction. This fraction is very important parameter of steam and it is known as the vapor quality.

See also: Properties of Water

 
Pressurizer: steam-liquid equilibrium
pressurizer
A pressurizer is a key component of PWRs.

A pressurizer is a component of a pressurized water reactor. Pressure in the primary circuit of PWRs is maintained by a pressurizer, a separate vessel that is connected to the primary circuit (hot leg) and partially filled with water which is heated to the saturation temperature (boiling point) for the desired pressure by submerged electrical heaters. Temperature in the pressurizer can be maintained at 350 °C (662 °F), which gives a subcooling margin (the difference between the pressurizer temperature and the highest temperature in the reactor core) of 30 °C. Subcooling margin is very important safety parameter of PWRs, since the boiling in the reactor core must be excluded. The basic design of the pressurized water reactor includes such 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.

Functions

Pressure 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 and electrical heaters system. Volume of the pressurizer (tens of cubic meters) is filled with water on saturation parameters and steam. The water spray system (relatively cool water – from cold leg) can decrease the pressure in the vessel by condensing the steam on water droplets sprayed in the vessel. On the other hand the submerged electrical heaters are designed to increase the pressure by evaporation the water in the vessel. Water pressure in a closed system tracks water temperature directly; as the temperature goes up, pressure goes up.

Steam Generator - operating conditions
Steam Generator - vertical
Steam Generator – vertical

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

The hot primary coolant (water 330°C; 626°F; 16MPa) is pumped into the steam generator through 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. 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°C 500°F to the boiling point of that fluid (280°C; 536°F; 6,5MPa). Heat is transferred through the walls of these tubes to the lower pressure secondary coolant located on the secondary side of the exchanger where the coolant evaporates to pressurized steam (saturated steam 280°C; 536°F; 6,5 MPa). The pressurized steam leaves the steam generator through a steam outlet and continues to the steam turbine. The transfer of heat is accomplished without mixing the two fluids to prevent the secondary coolant from becoming radioactive. The primary coolant leaves (water 295°C; 563°F; 16MPa) the steam generator through primary outlet and continues through a cold leg to a reactor coolant pumpand then into the reactor.

Vapor Quality – Dryness Fraction

wet-steam-vapor-liquid-mixture-minAs can be seen from the phase diagram of water, 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 present.

The mass fraction of the vapor in a two-phase liquid-vapor region is called the vapor quality (or dryness fraction), x, and it is given by following formula:

vapor quality

The value of the quality ranges from zero to unity. 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 saturated liquid state (single-phase).

This classification of steam has its limitation. Consider the behavior of the system which is heated at the pressure, that is higher than the critical pressure. In this case, there would be no change in phase from liquid to steam. At all states there would be only one phase. Vaporization and condensation can occur only when the pressure is less than the critical pressure. The terms liquid and vapor tend to lose their significance. At pressure, that is higher than the critical pressure,  water is in special state, that is known as supercritical fluid state.

See also: Saturation

See also: Supercritical Fluid

See also: Throttling of Steam

 
Saturated and Subcooled Liquid
Phase diagram of water
Phase diagram of water.
Source: wikipedia.org CC BY-SA

As can be seen from the phase diagram of water, 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.

  • The saturation vapor curve is the curve separating the two-phase state and the superheated vapor state in the T-s diagram.
  • The saturated liquid curve is the curve separating the subcooled liquid state and the two-phase state in the T-s diagram.

If a water exists as a liquid at the saturation temperature and pressure with quality of x = 0, it is called a saturated liquid state (single-phase). If the temperature of the liquid is lower than the saturation temperature for the existing pressure, it is called either a subcooled liquid or a compressed liquid . The term subcooling refers to a liquid existing at a temperature below its normal boiling point.  For example, water normally boils at 100°C (at atmospheric pressure); at room temperature 20°C the water is termed “subcooled”. Analogically the subcooling is defined also in nuclear engineering but for another purpose.

For example, the temperature in the pressurizer can be maintained at 350 °C (662 °F), which gives a subcooling margin (the difference between the pressurizer temperature and the highest temperature in the reactor core) of 30 °C. Subcooling margin is very important safety parameter of PWRs, since the boiling in the reactor core must be excluded.

subcooled-liquid-min

Wet Steam - Vapor-liquid Mixture
wet-steam-vapor-liquid-mixture-minWet steam is characterized by the vapor quality, which ranges from zero to unity – open interval (0,1). When the vapor quality is equal to 0, it is referred to as the saturated liquid state (single-phase). On the other hand, when the vapor quality is equal to 1, it is referred to as the saturated vapor state or dry steam (single-phase). Between these two states, we talk about vapor-liquid mixture or wet steam (two-phase mixture). At constant pressure, an addition of energy does not changes the temperature of the mixture, but the vapor quality and specific volume changes. In the case of dry steam (100% quality), it contains 100% of the latent heat available at that pressure. Saturated liquid water, which has no latent heat and therefore 0% quality, will therefore only contain sensible heat.
engineering thermodynamics
Rankine Cycle – Thermodynamics as Energy Conversion Science

Typically most of nuclear power plants operates multi-stage condensing steam turbines. In these turbines the high-pressure stage receives steam (this steam is nearly saturated steam – x = 0.995 – point C at the figure) from a steam generator and exhaust it to moisture separator-reheater (point D). The steam must be reheated in order to avoid damages that could be caused to blades of steam turbine by low quality steam. The reheater heats the steam (point D) and then the steam is directed to the low-pressure stage of steam turbine, where expands (point E to F). The exhausted steam is at a pressure well below atmospheric, and is in a partially condensed state (point F), typically of a quality near 90%.

Specific Enthalpy of Wet Steam

The specific enthalpy of saturated liquid water (x=0) and dry steam (x=1) can be picked from steam tables. In case of wet steam, the actual enthalpy can be calculated with the vapor quality, x, and the specific enthalpies of saturated liquid water and dry steam:

hwet = hs x + (1 – x ) hl              

where

hwet = enthalpy of wet steam (J/kg)

hs = enthalpy of “dry” steam (J/kg)

hl = enthalpy of saturated liquid water (J/kg)

As can be seen, wet steam will always have lower enthalpy than dry steam.

Specific Entropy of Wet Steam

Similarly, the specific entropy of saturated liquid water (x=0) and dry steam (x=1) can be picked from steam tables. In case of wet steam, the actual entropy can be calculated with the vapor quality, x, and the specific entropies of saturated liquid water and dry steam:

swet = ss x + (1 – x ) sl              

where

swet = entropy of wet steam (J/kg K)

ss = entropy of “dry” steam (J/kg K)

sl = entropy of saturated liquid water (J/kg K)

Specific Volume of Wet Steam

Similarly, the specific volume of saturated liquid water (x=0) and dry steam (x=1) can be picked from steam tables. In case of wet steam, the actual specific volume can be calculated with the vapor quality, x, and the specific volumes of saturated liquid water and dry steam:

vwet = vs x + (1 – x ) vl              

where

vwet = specific volume of wet steam (m3/kg)

vs = specific volume of “dry” steam (m3/kg)

vl = specific volume of saturated liquid water (m3/kg)

Example:

A high-pressure stage of steam turbine operates at steady state with inlet conditions of  6 MPa, t = 275.6°C, x = 1 (point C). Steam leaves this stage of turbine at a pressure of 1.15 MPa, 186°C and x = 0.87 (point D). Calculate the enthalpy difference between these two states.

The enthalpy for the state C can be picked directly from steam tables, whereas the enthalpy for the state D must be calculated using vapor quality:

h1, wet = 2785 kJ/kg

h2, wet = h2,s x + (1 – x ) h2,l  = 2782 . 0.87 + (1 – 0.87) . 790 = 2420 + 103 = 2523 kJ/kg

Δh = 262 kJ/kg

Dry Steam - Saturated Vapor
Phase diagram of water
Phase diagram of water.
Source: wikipedia.org CC BY-SA

Dry steam, or saturated steam,  is characterized by the vapor quality, which is equal to unity. When the vapor quality is equal to 0, it is referred to as the saturated liquid state (single-phase). On the other hand, when the vapor quality is equal to 1, it is referred to as the saturated vapor state or dry steam (single-phase). Between these two states, we talk about vapor-liquid mixture or wet steam (two-phase mixture). At constant pressure, an addition of energy does not changes the temperature of the mixture, but the vapor quality and specific volume changes. In the case of dry steam (100% quality), it contains 100% of the latent heat available at that pressure. Saturated liquid water, which has no latent heat and therefore 0% quality, will therefore only contain sensible heat.

Typically most of nuclear power plants operates multi-stage condensing steam turbines. In these turbines the high-pressure stage receives steam (this steam is nearly saturated steam – x = 0.995 – point C at the figure) from a steam generator and exhaust it to moisture separator-reheater (point D). The steam must be reheated in order to avoid damages that could be caused to blades of steam turbine by low quality steam. The reheater heats the steam (point D) and then the steam is directed to the low-pressure stage of steam turbine, where expands (point E to F). The exhausted steam is at a pressure well below atmospheric, and is in a partially condensed state (point F), typically of a quality near 90%.

dry-steam-saturated-vapor-min

Superheated Steam
Phase diagram of water
Phase diagram of water.
Source: wikipedia.org CC BY-SA

As can be seen from the phase diagram of water, 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.

  • The saturation vapor curve is the curve separating the two-phase state and the superheated vapor state in the T-s diagram.
  • The saturated liquid curve is the curve separating the subcooled liquid state and the two-phase state in the T-s diagram.

If a steam exists entirely as vapor at saturation temperature, it is called saturated vapor or saturated steam or dry steam. The dry saturated vapor  is characterized by the vapor quality, which is equal to unity. Superheated vapor or superheated steam is a vapor at a temperature higher than its boiling point at the absolute pressure where the temperature is measured. The pressure and temperature of superheated vapor are independent properties, since the temperature may increase while the pressure remains constant. Actually, the substances we call gases are highly superheated vapors.

engineering thermodynamics
Rankine Cycle – Thermodynamics as Energy Conversion Science

The process of superheating of water vapor in the T-s diagram is provided in the figure between state E and saturation vapor curve. As can be seen also wet steam turbines use superheated steam especially at the inlet of low-pressure stages. In order to evaluate the cycle thermal efficiency the enthalpy must be obtained from the superheated steam tables.

The process of superheating is the only way to increase the peak temperature of the Rankine cycle (and to increase efficiency) without increasing the boiler pressure. This requires the addition of another type of heat exchanger called a superheater, which produces the superheated steam.

In the superheater, further heating at fixed pressure results in increases in both temperature and specific volume. The process of superheating in the T-s diagram is provided in the figure between state E and saturation vapor curve.

Typically most of nuclear power plants operates multi-stage condensing steam turbines. In these turbines the high-pressure stage receives steam (this steam is nearly saturated steam – x = 0.995 – point C at the figure) from a steam generator and exhaust it to moisture separator-reheater (point D). The steam must be reheated or superheated in order to avoid damages that could be caused to blades of steam turbine by low quality steam. High content of water droplets can cause the rapid impingement and erosion of the blades which occurs when condensed water is blasted onto the blades. To prevent this, condensate drains are installed in the steam piping leading to the turbine. The reheater heats the steam (point D) and then the steam is directed to the low-pressure stage of steam turbine, where expands (point E to F). The exhausted steam is at a pressure well below atmospheric, and is in a partially condensed state (point F), typically of a quality near 90%.

Properties of Steam – Steam Tables

Water and steam are a common fluid used for heat exchange in the primary circuit (from surface of fuel rods to the coolant flow) and in the secondary circuit. It used due to its availability and high heat capacity, both for cooling and heating. It is especially effective to transport heat through vaporization and condensation of water because of its very large latent heat of vaporization.

A disadvantage is that water moderated reactors have to use high pressure primary circuit in order to keep water in liquid state and in order to achieve sufficient thermodynamic efficiency. Water and steam also reacts with metals commonly found in industries such as steel and copper that are oxidized faster by untreated water and steam. In almost all thermal power stations (coal, gas, nuclear), water is used as the working fluid (used in a closed loop between boiler, steam turbine and condenser), and the coolant (used to exchange the waste heat to a water body or carry it away by evaporation in a cooling tower).

Properties of water - steam tables
Steam Tables – common parameters in energy systems

Water and steam are a common medium because their properties are very well known. Their properties are tabulated in so called “Steam Tables”. In these tables the basic and key properties, such as pressure, temperature, enthalpy, density and specific heat, are tabulated along the vapor-liquid saturation curve as a function of both temperature and pressure. The properties are also tabulated for single-phase states (compressed water or superheated steam) on a grid of temperatures and pressures extending to 2000 ºC and 1000 MPa.

Further comprehensive authoritative data can be found at the NIST Webbook page on thermophysical properties of fluids.

See also: Steam Tables

Special Reference: Allan H. Harvey. Thermodynamic Properties of Water, NISTIR 5078. Retrieved from https://www.nist.gov/sites/default/files/documents/srd/NISTIR5078.htm

 
Chart: Absolute pressure as a function of temperature of water
Water: Absolute pressure as a function of temperature
Water: Absolute pressure as a function of temperature
Chart: Density as a function of temperature of water
Chart - density - water - temperature
Density as a function of temperature of water
Chart: Dynamic viscosity as a function of temperature of water
Dynamic viscosity as a function of temperature of water
Chart: Dynamic viscosity as a function of temperature of water
Source: wikipedia.org CC BY-SA

Examples

 
Evaporation of water at atmospheric pressure
Calculate heat required to evaporate 1 kg of water at the atmospheric pressure (p = 1.0133 bar) and at the temperature of 100°C.

Solution:

Since these parameters corresponds to the saturated liquid state, only latent heat of vaporization of 1 kg of water is required. From steam tables, the latent heat of vaporization is L = 2257 kJ/kg. Therefore the heat required is equal to:

ΔH = 2257 kJ

Note that the initial specific enthalpy h1 = 419 kJ/kg, whereas the final specific enthalpy will be h2 = 2676 kJ/kg. The specific enthalpy of low-pressure dry steam is very similar to the specific enthalpy of high-pressure dry steam, despite the fact they have different temperatures.

Evaporation of water at high pressure
Calculate heat required to evaporate 1 kg of feedwater at the pressure of 6 MPa (p = 60 bar) and at the temperature of 275.6°C (saturation temperature).

Solution:

Since these parameters corresponds to the saturated liquid state, only latent heat of vaporization of 1 kg of water is required. From steam tables, the latent heat of vaporization is L = 1571 kJ/kg. Therefore the heat required is equal to:

ΔH = 1571 kJ

Note that the initial specific enthalpy h1 = 1214 kJ/kg, whereas the final specific enthalpy will be h2 = 2785 kJ/kg. The specific enthalpy of low-pressure dry steam is very similar to the specific enthalpy of high-pressure dry steam, despite the fact they have different temperatures.

Evaporation of water at high pressure - Energy balance in a steam generator
Steam Generator - vertical
Steam Generator – vertical

Calculate the amount of primary coolant, which is required to evaporate 1 kg of feedwater in a typical steam generator. Assume that there are no energy losses, this is only idealized example.

Balance of the primary circuit

The hot primary coolant (water 330°C; 626°F; 16MPa) is pumped into the steam generator through primary inlet. The primary coolant leaves (water 295°C; 563°F; 16MPa) the steam generator through primary outlet.

hI, inlet = 1516 kJ/kg

=> ΔhI = -206 kJ/kg

hI, outlet = 1310 kJ/kg

Balance of the feedwater

The feedwater (water 230°C; 446°F; 6,5MPa) is pumped into the steam generator through the feedwater inlet. The feedwater (secondary circuit) is heated from ~230°C 446°F to the boiling point of that fluid (280°C; 536°F; 6,5MPa). Feedwater is then evaporated and the pressurized steam (saturated steam 280°C; 536°F; 6,5 MPa) leaves the steam generator through steam outlet and continues to the steam turbine.

hII, inlet = 991 kJ/kg

=> ΔhII = 1789 kJ/kg

hII, outlet = 2780 kJ/kg

Balance of the steam generator

Since the difference in specific enthalpies is less for primary coolant than for feedwater, it is obvious that the amount of primary coolant will be higher than 1kg. To produce of 1 kg of saturated steam from feedwater, about 1789/206 x 1 kg =  8.68 kg of primary coolant is required.

 
References:
Reactor Physics and Thermal Hydraulics:
  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. Todreas Neil E., Kazimi Mujid S. Nuclear Systems Volume I: Thermal Hydraulic Fundamentals, Second Edition. CRC Press; 2 edition, 2012, ISBN: 978-0415802871
  6. Zohuri B., McDaniel P. Thermodynamics in Nuclear Power Plant Systems. Springer; 2015, ISBN: 978-3-319-13419-2
  7. Moran Michal J., Shapiro Howard N. Fundamentals of Engineering Thermodynamics, Fifth Edition, John Wiley & Sons, 2006, ISBN: 978-0-470-03037-0
  8. Kleinstreuer C. Modern Fluid Dynamics. Springer, 2010, ISBN 978-1-4020-8670-0.
  9. U.S. Department of Energy, THERMODYNAMICS, HEAT TRANSFER, AND FLUID FLOW. DOE Fundamentals Handbook, Volume 1, 2 and 3. June 1992.

See above:

Materials