Terms used related to Steam


Terms used related to Steam

The terms used related to steam are given below.

Absolute pressure – It is gauge pressure plus atmospheric pressure.

Atmospheric pressure – It is the normal pressure of the atmosphere on the sea level at 0 deg C.

Boiling point – It is the temperature (100 deg C) at which water boils. Water boils when its absolute pressure reaches the vapour pressure.

Condensation – It is defined as the change in the physical state of water vapour to liquid water. It is reverse of vaporization.

Condensate – It is the liquid phase produced by the condensation of steam.

Density of steam – It is the mass of steam per unit of volume of steam at the given pressure and temperature. It is the reciprocal of the specific volume.

Dry saturated steam – On heating the wet steam, the water particles, which are in suspension, starts evaporating gradually and at a particular moment the final particles just evaporates. The steam at that moment is called dry steam or dry saturated steam. The resulting volume is known as specific volume of dry steam. This steam does not obey the gas laws.

Dryness fraction of saturated steam – It is a measure of quality of wet steam. It is the ratio of the mass of dry steam to the mass of total wet steam.

Enthalpy – Enthalpy of a system is defined as the mass of the system (m) multiplied by the specific enthalpy (h) of the system and can be expressed as H = m h, where H is enthalpy in kJ, m is mass in kg, and h is specific enthalpy in kJ/kg.

Enthalpy is the total amount of heat received by 1 kg of water from O deg C at constant pressure to convert it to desired form of steam. It is the sum of the internal energy and work done at constant pressure process, which is equal to change in enthalpy.

Enthalpy of evaporation (vaporization) – The enthalpy of vaporization is the energy required to turn water into the gaseous form when it increases in volume by 1,600 times at standard temperature and pressure. It is same as latent heat of vaporization.

External work of evaporation – It is the fraction of the latent heat of vaporization which does an external work in moving the piston at constant pressure due to increase in volume. Its unit is kj/kg.

Evaporation – It is vaporization of water that occurs from the surface of water. Water is converted into water vapour which is not saturated with water.

Flash steam – When hot condensate under pressure, is released to lower pressure, part of it is re-evaporated, becoming what is known as flash steam. The term is traditionally used to describe steam issuing from condensate receiver vents and open ended condensate discharge lines from steam traps.

Internal (True) latent heat (internal energy of steam) – It is the energy required to change the phase. Hence, it is the actual heat energy stored in the steam above O deg C. It may be calculated by subtracting the external work of evaporation from the enthalpy. Its unit is kj/kg.

Latent heat of vaporization – It is the heat which causes a phase/state change of water without raising its temperature. 2 257 kJ of energy is required to evaporate 1 kg of water at 100 deg C into 1 kg of steam at 100 deg C.

Mass density (or density) of the steam – It is the specific mass of the steam in a volume of 1 cubic meter.

Mollier diagram – This is the thermodynamic phase diagrams for water/steam. The Mollier diagram is a small portion of data from the steam tables graphed onto enthalpy-entropy coordinates.  It presents the region that is commonly found in propulsion plant steam systems. A Mollier diagram is shown in Fig 1.

Mollier diagram

Fig 1 Mollier diagram

Priming – It is the representation of wetness fraction in percentage.

Process steam – It is the steam used in various industrial processes mainly to utilize its heat and moisture. The most common operational end uses employed for process steam include stripping, fractionation, process heating and cooling, quenching, dilution, vacuum draw, pressure regulation, injection and source of process water etc.

Quality of steam – It is the representation of dryness fraction in percentage.

Saturated steam – Once the boiling point is reached, the water’s temperature ceases to rise and stays the same until all the water is vaporized. The water goes from a liquid state to a vapour state by receiving energy in the form of latent heat of vaporization. As long as there’s some liquid water left, the steam’s temperature is the same as the temperature of liquid water. This steam is then called saturated steam.

Sensible heat – It is the heat which increases the temperature of water but do not change its phase/state.

Specific enthalpy – Specific enthalpy is a property of the fluid and can be expressed as h = u + pv where u is internal energy in kj/kg, p is absolute pressure in N/Sqm, and v is specific volume in Cum/kg.

Specific enthalpy of steam – It is the total heat contained in 1 kg of steam. It is the sum of enthalpy of the various states, liquid (water) and gas (vapour).

Specific heat of steam – It is the quantity of heat necessary to increase the temperature of one Celsius degree on a unit of mass of 1 kg of steam.

Steam – It is the technical term used for water vapour, the gaseous phase of water, which is formed when water boils. Technically speaking water boils at a temperature of 100 deg C at atmospheric

Specific volume of steam – It is the volume occupied in cubic meter by 1 kg of steam.

Steam boiler – A steam boiler is an enclosed container where water is heated under controlled conditions to convert it into steam. Boiler is basically a heat exchanger where heat is transferred to water. It is also sometimes referred to steam generator.

Steam drum – A steam drum is a standard feature of a water tube boiler. It is a reservoir of water/steam at the top end of the water tubes. The drum stores the steam generated in the water tubes and acts as a phase separator for the steam/water mixture.

Steam engine – It is a heat engine that performs mechanical work using steam as its working fluid.

Steam generator – It is another name of steam boiler.

Steam phase diagram – It is the data provided in the steam tables whichn is expressed in the graphical form.

Steam pressure – The conventions of low, medium and high pressure steam vary as defined by the various steam users around the country.  Typically, steam below 3.5 bar gauge is termed as low pressure steam. Steam above 3.5 bar g but below 17.5 bar gauge is termed as medium pressure steam. Steam above 17.5 bar gauge is termed as high pressure steam. Some users define their steam above 40 bar gauge as ultra high pressure steam.

Steam tables – Steam tables are the compilation of experimental results of the thermodynamic properties (viz. specific volume, internal energy, sensible heat, latent heat, saturation temperature etc.) of 1 kg of steam in a tabular column. These are available either on pressure basis or on temperature basis. These tables are useful for steam engineering calculations, as vapors do not obey gas laws. The pressures in the steam tables are in bar (absolute). In case of gauge pressures, they must be converted in to absolute pressure by adding atmospheric pressure to them. All the values given in the steam tables are reckoned above O deg C. If the initial temperature of water is other than O deg C, the enthalpy of steam will be calculated from the steam tables by deducting the amount of heat contained initially by the water.

Steam trap – It is a device used to discharge condensate and non condensable gases with a negligible consumption or loss of live steam. Most steam traps are nothing more than automatic valves. They open, close or modulate automatically.

Steam turbine – It is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Since the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator.

Superheated steam – When all the water is vaporized, any subsequent addition of heat raises the temperature of steam. Steam heated beyond the saturated steam level is called superheated steam. Superheated steam obeys gas laws.

Super saturated steam – The steam having lesser temperature and greater density with respect to the steam table values for a particular saturation pressure is called super saturated steam. This condition is obtained when it is cooled by its own expansion in a nozzle. It is very unstable and the steam soon resumes the saturated condition.

Unsaturated steam – This is the most common form of steam. It usually contains wetness from non-vaporized water molecules that are carried over into the distributed steam. It is also known as wet steam.

Water vapour – It is the gaseous phase of water. Water vapour  can be produced by the evaporation or by boiling of liquid water. Water vapour is invisible. Under typical atmospheric conditions, water vapour is continuously generated by evaporation and removed by condensation. It is lighter than air.

Weight of steam – It is the weight of steam in kg/Cum.

Wetness fraction – It is a measure of quality of wet steam. It is the ratio of the mass of water vapor to the mass of total wet steam.

Wet steam – It is the mixture of water and steam in which both are at saturation temperature. If additional heat is added to the wet steam at constant pressure, the temperature remains constant until all water is evaporated.