When it comes to use and transfer thermal energy, steam is the most economical and popular mode of conveying energy. For various application of steam click here. Steam is used to run powerplants and various process industries like chemical, textile, paper, fertilizer, petrochemical etc. Therefore it is important to understand concept of steam and how does it work, In this article we will discuss about what is steam and why it is important for industries.
What is Steam
Water exists in all the three forms solid, liquid and gas. Steam is gaseous form of water, it is formed by heating water till it gets evaporated. Steam is formed after adding heat in water till it reaches saturation temperature. Above saturation temperature water ceases to exist and changes its phase form water to steam. During the phase change it requires large amount of heat at saturation temperature. This heat is known as latent heat of vaporization. For water latent heat of vaporization is 540 Kcal/kg at atm pressure, at this pressure saturation temperature is 100 Deg C. Latent heat of vaporization and saturation temperature changes with the changes in pressure condition. The amount of heat needed to raise the temperature of 1 gram of a substance by 1 degree Celsius (°C). is specific heat of water. Specific heat and latent heat are also known as liquid enthalpy and vapor enthalpy.
Why steam is important for industries :
Steam is having various advantages in comparison to any other mode of thermal energy usage, Lets discuss few properties which makes it most popular.
High Specific and latent heat : Steam is having is highest specific and latent heat at atmospheric pressure ie 1 Kcal/ Kg and 540 Kcal/kg respectively. This allows steam to be used for thermal energy transfer effectively. In process industry water is heated and steam is generated at boilers. Steam is transferred thogh pipe and used to heat process, During process heating steam condenses back to water releasing latent heat of vaporization to process. Condensed water is collected and sent to boiler for further production of steam. The change of phase is done at saturation temperature corresponding to the pressure conditions. The temperature of saturated steam and saturated liquid is the same at corresponding pressure. Superheat is the addition of heat to dry saturated steam without increase in pressure. The temperature of superheated steam, expressed as degrees above saturation corresponding to the pressure, is referred to as the degrees of superheat. Superheated steam is mostly used for power generation in turbine. Fig-1 Shows the steam phase diagram with liquid and superheated steam region. Water is in two phase between saturated liquid and saturated vapor line. Enthalpy addition for changing water from liquid to vapor phase at constant temperature is Latent heat of vaporization, shown as horizontal line in fig-1
High heat transfer coefficient As the steam is used mostly for thermal energy requirement in the process. Higher heat transfer coefficient helps in designing lower size heat exchanger. Heat transfer coefficient determines how well heat can be exchanged on surface or heat exchanger. Higher the heat transfer coefficient lower the size of heat exchanger thus economical design and lower investment requirement.
Ease in Distribution and control : Steam produced at boiler can travel long distance without support of additional energy. specific volume of steam is low at higher pressure therefore steam is transported at high pressure and used at lower pressure. Distribution of steam is easy as only right size pipeline network needs to be created. Condensate formed during steam transportation needs to removed from the pipeline as it may create water hammer in pipeline. Different type of traps are used to separate out condensate from steam network. Condensate water is recycled to boiler to form steam. Control valve is used for controlling flow and pressure in steam distribution network.
Low Cost Distribution piping : As specific volume of steam is low at higher pressure, steam is transported and higher pressure. This makes lower pipeline size and capital cost requirement for steam distribution. Steam piping design is considered at 50-70 m/sec velocity for super heated steam, 30-40m/sec for Saturated steam and 20-30 m/sec for wet steam. Higher velocity in pipeline helps in further reduction of pipe sizing.
Inert : Steam is non reactive with most of the chemicals which makes steam as safer to use.
Low Cost : As steam can be produced with any cheaper fuel like coal or agrowaste, It is more favorable for industries.