Chapter 1 Properties of Steam PDF

Chapter 1 PROPERTIES OF STEAM Department of Mechanical & Industrial Engineering, 1 MIT. Manipal  Content Definition of steam Application of steam Property of a substance Formation of steam Relation of pressure on boilin...

Chapter 1 PROPERTIES OF STEAM Department of Mechanical & Industrial Engineering, 1 MIT. Manipal  Content Definition of steam Application of steam Property of a substance Formation of steam Relation of pressure on boiling point of water Enthalpy equations Department of Mechanical & Industrial Engineering, 2 MIT. Manipal  What is Steam? Vapour form of water is called STEAM. Water in solid phase: ICE Water in liquid phase: WATER Water in gaseous phase: STEAM. Department of Mechanical & Industrial Engineering, 3 MIT. Manipal  Applications of Steam 1. Steam is used as a working fluid in Steam Turbines & in Steam Engines. Department of Mechanical & Industrial Engineering, 4 MIT. Manipal  Applications of Steam 1. Steam is used as a working fluid in Steam Turbines & in Steam Engines. Department of Mechanical & Industrial Engineering, 5 MIT. Manipal 2. Used in Food Processing Industry for Cooking, Pasteurizing and for Sanitization of Food Processing Equipment. Department of Mechanical & Industrial Engineering, 6 MIT. Manipal 3. Used in Industries for Process Heating. Department of Mechanical & Industrial Engineering, 7 MIT. Manipal 4. Used in Hospitals for Sterilization. Department of Mechanical & Industrial Engineering, 8 MIT. Manipal 5. Used for Cleaning and in Health Clinics. Department of Mechanical & Industrial Engineering, 9 MIT. Manipal Steam as a working fluid in steam turbines. Department of Mechanical & Industrial Engineering, 10 MIT. Manipal  What is meant by Property of a substance? It is any variable that defines the characteristics/Physical condition of a substance. Temperature: 0 C Density: 916 kg/m3 Pressure: 1 bar Mass: 2 kg Properties of Ice Department of Mechanical & Industrial Engineering, 11 MIT. Manipal  Why study property of steam? ICE Temperature: 0C As property changes, the physical WATER Temperature: 30C characteristics / condition also changes. STEAM Temperature: 100C Department of Mechanical & Industrial Engineering, 12 MIT. Manipal  Why study property of steam? A change in property, changes the characteristics of water/ steam. As a result, the associated energy levels in water / steam will also change. Steam has higher energy levels Department of Mechanical & Industrial Engineering, 13 MIT. Manipal  Why study property of steam? An understanding of various properties of steam helps an engineer: – To control the working of devices handling steam (steam engines, steam turbines, steam boilers). – To control as well as improve operational efficiency of steam operated devices. Steam Steam Turbine wheel Department of Mechanical & Industrial Engineering, 14 MIT. Manipal FORMATION OF STEAM AT CONSTANT PRESSURE CONDITION Department of Mechanical & Industrial Engineering, 15 MIT. Manipal  FORMATION OF STEAM AT CONSTANT PRESSURE Consider 1 kg of water at 0oC Frictionless piston taken in a cylinder fitted with a freely moving frictionless piston Cylinder (of negligible weight) as shown in figure. Pressure “P” Water One kg of water at 0oC Since, ‘W’ is kept constant, the pressure applied on water also remains constant. This weight can be chosen in such a way that a required pressure is to be applied is obtained. Department of Mechanical & Industrial Engineering, 16 MIT. Manipal  FORMATION OF STEAM EXPERIMENT AT CONSTANT PRESSURE Steam Steam Steam Water Water Water Fig.1.2A Fig.1.2B Fig.1.2C Fig.1.2D Fig.1.2E Water below Water at Mixture at Steam at Steam above boiling point boiling point boiling point boiling point boiling point (Unsaturated (Saturated (Wet Steam) (Saturated (Superheated water) water) steam or Dry steam) steam) Department of Mechanical & Industrial Engineering, MIT. Manipal  The initial condition of water at 0oC is represented by the point “A” on the Temperature – Enthalpy graph. Enthalpy is a measure of heat energy in the water/steam. Temperature (ToC) A Enthalpy (h) Department of Mechanical & Industrial Engineering, 18 MIT. Manipal  Temperature Superheated steam Tsup D Degree of Saturated Saturated Superheat water steam B C Ts Wet steam A hf hfg Enthalpy Sensible Latent Heat Amount of Heat hg Superheat Department of Mechanical & Industrial Engineering, 19 MIT. Manipal Saturation temperature(Ts): It is defined as the temperature at which the water begins to boil at the stated pressure. Sensible heat (hf): (Enthalpy of saturated water) It is the amount of heat required to raise the temperature of 1 kg of water from 00C to the saturation temperature Tsat0C at a given constant pressure. Latent heat (hfg): (Enthalpy of evaporation) It is the amount of heat required to evaporate 1 kg of water at the saturation temperature in to 1 kg of dry steam at the same saturation temperature at a given constant pressure. Department of Mechanical & Industrial Engineering, 20 MIT. Manipal Superheated Temperature(Tsup): It is the temperature of the steam above the saturation temperature at a given constant pressure. Amount of superheat (AOS): (Enthalpy of superheat) It is the amount of heat required to increase the temperature of 1 kg of dry steam from its saturation temperature to any desired higher temperature at the given constant pressure. Degree of superheat (DOS): It is the difference between the superheated temperature and the saturation temperature. Department of Mechanical & Industrial Engineering, 21 MIT. Manipal  Different states of steam The steam as it is being generated can exist in three different states, 1. Wet steam 2. Dry steam 3. Superheated steam Department of Mechanical & Industrial Engineering, 22 MIT. Manipal  Wet Steam: A wet steam is defined as a two-phase mixture of finely divided water particles and dry steam in thermal equilibrium (means both are at same temperature) at the saturation temperature corresponding to a given stated pressure. Department of Mechanical & Industrial Engineering, 23 MIT. Manipal  How to specify the relative quantity of steam in Wet steam? A parameter called Dryness Fraction is used for this purpose. It indicates the fraction of vapor content in a given wet steam. Mass of Dry Steam present in Wet Steam Dryness fraction, x  Total Mass of Wet Steam mg x m f  mg For dry steam, x=1. For saturated water, x=0 mg For wet steam, 01 mf Department of Mechanical & Industrial Engineering, 24 MIT. Manipal  Advantages of Superheated Steam High Energy Content: Its capacity to do the work will be higher. Minimising Chances of Corrosion: No problems like rusting or corrosion of turbine blades / engine cylinder. Department of Mechanical & Industrial Engineering, 25 MIT. Manipal  Disadvantages of Superheated Steam Difficulty in Lubrication: The lubricant may get burnt at that high temperature. Additional Cost: Additional cost of Super heater thereby increasing initial investment. Department of Mechanical & Industrial Engineering, 26 MIT. Manipal Enthalpy equations a) Enthalpy of Unsaturated water (hw): hw = m Cp (Tw-0) kJ/kg Where, m = Mass of water in kg,Cp = Specific heat of water = 4.187 kJ/kg K and Tw= Temperature of feed water in ºC Temperature D B C Ts Enthalpy of unsaturated water Enthalpy A hf Sensible Heat Department of Mechanical & Industrial Engineering, 27 MIT. Manipal Enthalpy equations b) Enthalpy of saturated water OR Sensible heat (hf): hf = m Cp (Ts-0) kJ/kg Where, m = Mass of water in kg,Cp = Specific heat of water =4.187 kJ/kg K and Ts= Saturation temperature in ºC Temperature Tsup D Degree of Superheat B C Ts Enthalpy A hf hfg Sensible Latent Heat Amount of Heat hg Superheat Department of Mechanical & Industrial Engineering, 28 MIT. Manipal Enthalpy equations c) Enthalpy of Dry saturated Steam (hg): hg = hf + hfg kJ/kg Where hg = Enthalpy of dry steam in kJ/kg. hf = Sensible heat in kJ/kg. hfg = Enthalpy of vaporization or latent heat in kJ/kg. Temperature Tsup D Degree of Superheat B C Ts A Enthalpy hf hfg Sensible Latent Heat Amount of Heat hg Superheat Department of Mechanical & Industrial Engineering, 29 MIT. Manipal d) Enthalpy of Wet Steam (h): h= hf + x.hfg kJ/kg Where x = Dryness fraction of wet steam 0˂x˂1 Temperature Tsup D Degree of Superheat B C Tsat Enthalpy A hf hfg Sensible Latent Heat Amount of Heat Superheat Department of Mechanical & Industrial Engineering, 30 MIT. Manipal e) Enthalpy of Superheated Steam (hsup): hsup = hf + hfg + Csup (Tsup - Tsat) kJ/kg where hsup = Enthalpy of superheated steam in kJ/kg Csup = Specific heat of superheated steam =2.25 kJ/kg K Tsup = Superheated temperature in °C & Tsat = Saturation temperature in °C Temperature Tsup D Degree of Superheat B C Tsat A Enthalpy hf hfg Sensible Latent Heat Amount of Heat Superheat hg Department of Mechanical & Industrial Engineering, 31 MIT. Manipal f) Amount of superheat (AOS): (Enthalpy of superheat) AOS = Csup (Tsup - Tsat) kJ/kg Temperature Tsup D Degree of Superheat B C Ts A Enthalpy hf hfg Sensible Latent Heat Amount of Heat Superheat Department of Mechanical & Industrial Engineering, 32 MIT. Manipal g) Degree of superheat (DOS): DOS = (Tsup - Tsat) 0C Temperatur Temperature Tsup D Degree of Superheat Tsat B C A Enthalpy hf hfg Sensible Latent Heat Amount of Heat Superheat Department of Mechanical & Industrial Engineering, 33 MIT. Manipal a) Enthalpy of Dry saturated Steam: hg = hf + hfg kJ/kg b) Enthalpy of Wet Steam: h = hf + x.hfg kJ/kg c) Enthalpy of Superheated Steam: hsup = hf + hfg + Csup (Tsup - Tsat) kJ/kg d) Degree of superheat (DOS): DOS = (Tsup - Tsat) 0C e) Amount of superheat (AOS): AOS = Csup (Tsup - Tsat) kJ/kg Department of Mechanical & Industrial Engineering, 34 MIT. Manipal  Critical Temperature & Pressure At a particular pressure, water is Temperature Pc directly converted into dry steam without going through the phase of P3 vaporization (or boiling). i.e., hfg = 0. C Tc This point is called critical point ‘C’. P2 Pc = 221.2 bar Tc = 374.150C P1  Boiling will not happen at and above critical point. Constant pressure line  At the critical point, only one phase exists. Enthalpy Department of Mechanical & Industrial Engineering, 35 MIT. Manipal