Basic Mechanical Engineering (102001202) Chapter 3: Properties of Steam PDF

Basic Mechanical Engineering (102001202) Chapter – 3: Properties of Steam Prepared by :Prof. Bhavik A Ardeshana Mechatronics Department G H Patel College of Engineering & Technology (A Constituent College of CVM University) Content Introduction Difference...

Basic Mechanical Engineering (102001202) Chapter – 3: Properties of Steam Prepared by :Prof. Bhavik A Ardeshana Mechatronics Department G H Patel College of Engineering & Technology (A Constituent College of CVM University) Content Introduction Difference between Steam and Gas Steam Formation Types of Steam Enthalpy of Steam Specific Volume of Steam Steam Tables Types of Calorimeters 21-12-2020 Chapter-2 Properties of Steam 2 Introduction A perfect gas does not changes its phase during a thermodynamic process. However, a pure substance which is a homogeneous substance retains its chemical composition even through it undergoes a change in phase during a thermodynamics process. Water is one of the pure substance, which can exist in the three different phases, in solid phase as ICE, in liquid phase as WATER, and in the gaseous phase as STEAM. Of course in all its three different phases it retains the same chemical composition. 21-12-2020 Chapter-2 Properties of Steam 3 Introduction When ice is melts, its transforms from solid phase to liquid phase to form of water. Similarly when the water is heated beyond the boiling point, it starts evaporating and transform from the liquid phase to the gaseous phase to form steam which may be defined as the vapour of water. During this transformation, known as the vaporisation, it’s remain as a two phase mixture of water and steam. After the complete vaporisation, it exist purely in the gaseous phase as steam. Vapours do not obey Boyles law and Charles law, the behaviour of the vapours cannot be determined by this law’s. 21-12-2020 Chapter-2 Properties of Steam 4 21-12-2020 Chapter-2 Properties of Steam 5 Difference between Steam and Gas 21-12-2020 Chapter-2 Properties of Steam 6 Steam Formation 21-12-2020 Chapter-2 Properties of Steam 7 Steam Formation Consider a cylinder fitted with a piston which can move freely upwards and downwards in it. Consider 1 kg of water at 0° C under the piston (fig. 4.1). A weight w is placed over the piston so that it exerts constant pressure p on the water. This condition of water at 0° C is represented by the point A on the temperature – enthalpy graph as shown in fig. 4.2. Now if the heat is supplied to water, a rise in temperature will be noticed and this rise will continue till boiling point is reached. When the boiling point of water is reached, there will be a slight increase in the water as shown in fig. 4.1 (ii). The saturation temperature is defined as the temperature at which the water begins to boil at the stated pressure. This condition of water at temperature 𝑇𝑠 is represented by the point B on the graph. 21-12-2020 Chapter-2 Properties of Steam 8 Steam Formation (Temperature-Enthalpy Diagram) 21-12-2020 Chapter-2 Properties of Steam 9 Steam Formation The amount of heat required to raise the temperature of 1 kg of water from 0° C to the saturation temperature 𝑇𝑠 ° C at a given pressure is known as the sensible heat and denoted by ℎ𝑓. This heat is also called enthalpy of the liquid. Now, if supply of heat to water is continued it will initiate the evaporation of water while the temperature remains at the saturation temperature 𝑇𝑠 because the water will be saturated with heat and any further addition of heat changes only the phase from the liquid phase to the gaseous phase. This evaporation will be continued at the same saturation temperature 𝑇𝑠 until the whole of the water is completely into steam as shown in fig. 4.1 (iv). This point is represented by the point C on the graph. 21-12-2020 Chapter-2 Properties of Steam 10 Steam Formation This constant pressure and constant temperature heat addition is represented by the horizontal line BC on the graph. The heat being supplied does not show any rise of temperature but changes water into vapor state (steam) and is called latent heat or hidden heat or enthalpy of evaporation. It is denoted by ℎ𝑓𝑔. If the steam is in contact with water, it is called wet steam (fig. 4.1 (iii)). Again, if supply of heat to the saturated steam is continued at constant pressure there will be increase in temperature and volume of steam. The temperature of the steam above the saturation temperature at a given pressure is called superheated temperature. 21-12-2020 Chapter-2 Properties of Steam 11 Steam Formation During this process of heating, the dry steam will be heated from its dry state, and the process of heating is called superheating. The steam when superheated is called superheated steam. This superheating is represented by the inclined line CD on the graph. The amount of heat required to raise the temperature of dry steam from its saturation temperature to any required higher temperature at the given constant pressure is called amount of superheat or enthalpy of superheat. The difference between the superheated temperature and the saturation temperature is known as degree of superheat. 21-12-2020 Chapter-2 Properties of Steam 12 Advantages and disadvantages of superheated Steam: Advantages: 1) The superheated steam contains more heat energy compared to dry saturated steam or wet steam at a given pressure, hence its capacity to do the work will be higher. 2) It reduces the condensation in extreme cases when expanding in a steam turbine, thus giving better economy. 3) It improves the thermal efficiency of the boiler. Disadvantages: 1) High temperature of superheated steam causes problems in the lubrication of turbine. 2) Higher initial and maintenance cost for super heater. 21-12-2020 Chapter-2 Properties of Steam 13 Types of Steam 1) Wet Steam: A wet steam is a two-phase mixture of entrained water molecules and steam in thermal equilibrium at the saturation temperature corresponding to a given pressure. The quality of the wet steam is specified by the dry fraction which indicates the amount of dry steam present in the given quantity of wet steam and is denoted by x. Dryness fraction of steam: It is the ratio of the actual dry steam present in a known quantity of wet steam to the total mass of the wet steam. Let 𝑚𝑠 = mass of dry steam present in the given quantity of wet steam. 𝑚𝑤 = mass of superheated water molecules in the given quantity of wet steam. 21-12-2020 Chapter-2 Properties of Steam 14 Types of Steam 1) Wet Steam: The dryness fraction of wet steam is always less than 1 and for dry steam it is equal to 1. Wetness fraction of steam: It is the ratio of the mass of water particles present in a in a known quantity of wet steam to the total mass of the wet steam. Wetness fraction = 1 − 𝑥 Priming: When wetness fraction is expressed in percentage, it is known as priming. Priming = 100 1 − 𝑥 21-12-2020 Chapter-2 Properties of Steam 15 Types of Steam 2) Dry saturated steam: A steam at the saturation temperature corresponding to a given pressure and having no water molecules entrained in it is known as dry saturated steam or dry steam. Its dryness fraction will be unity. 3) Superheated steam: When a dry saturated steam is heated further at the given constant pressure, its temperature rises beyond its saturation temperature. The steam in this state is said to be superheated. 21-12-2020 Chapter-2 Properties of Steam 16 Enthalpy of Steam 1) Enthalpy of Liquid: 21-12-2020 Chapter-2 Properties of Steam 17 Enthalpy of Steam 2) Enthalpy of Dry Saturated Steam: 21-12-2020 Chapter-2 Properties of Steam 18 Enthalpy of Steam 3) Enthalpy of Wet Steam: 21-12-2020 Chapter-2 Properties of Steam 19 Enthalpy of Steam 4) Enthalpy of Superheated Steam: 21-12-2020 Chapter-2 Properties of Steam 20 21-12-2020 Chapter-2 Properties of Steam 21 Specific Volume of Steam: It is defined as the volume occupied by the unit mass of a substance. It is expressed in 𝑚3 Τ𝐾𝑔. The volume of water and steam increases with the increase in temperature. Specific Volume of Saturated Water (𝒗𝒇 ): It is defined as the volume occupied by 1 kg of water at the saturation temperature at a given pressure. See Fig. Specific Volume of Dry Saturated Steam (𝒗𝒈 ): It is defined as the volume occupied by 1 kg of dry saturated steam at a given pressure. See Fig. 21-12-2020 Chapter-2 Properties of Steam 22 Specific Volume of Steam: Specific Volume of Wet Steam (𝒗): 21-12-2020 Chapter-2 Properties of Steam 23 Specific Volume of Steam: Specific Volume of Superheated Steam (𝒗𝒔𝒖𝒑 ): 21-12-2020 Chapter-2 Properties of Steam 24 Throttling Process It is the type of expansion process, in which steam passes through a narrow passage and expands with a full of pressure without doing any external work.  ℎ1 = ℎ2 Initial enthalpy = Final Enthalpy Measurement of Dryness Fraction It is necessary to determine the quality of wet steam in order to ascertain the actual state of the wet steam. The dryness fraction of the steam is measured experimentally by calorimeter. Various types of calorimeters used for measuring dryness fraction of steam are as follows: 1) Bucket or Barrel calorimeter 2) Separating calorimeter 3) Throttling calorimeter 4) Separating and throttling calorimeter 21-12-2020 Chapter-2 Properties of Steam 27 Bucket or Barrel Calorimeter Construction Bucket calorimeter consists of a calorimeter which is placed on wooden blocks in a vessel. The vessel is large enough to provide an air space around the calorimeter. This air space provides insulation to prevent heat loss. The top cover is made of wood and it closes both the calorimeter and the vessel. This cover has two holes. Through one hole, the steam pipe is led into the calorimeter. The steam is distributed in the calorimeter by the holes in the bottom ring which is connected to the end of the steam pipe. The thermometer is inserted from the second hole to measure the temperature of water in the calorimeter. 21-12-2020 Chapter-2 Properties of Steam 28 Bucket or Barrel Calorimeter Working The calorimeter is placed in the vessel. The top cover is placed in position and the steam pipe is connected to main steam pipe. The steam comes in contact with water in the calorimeter when steam is passed through the water. It condenses and gives out its entire enthalpy of evaporation (latent heat) and part of its sensible heat. Due to heat transfer from steam to water in the calorimeter, the temperature of water increases. Condensation of steam will increase the mass of water. Sufficient quantity of steam should be blown in the calorimeter so that sufficient rise in temperature of water and thereby errors are reduced to minimum. Afterwards the steam cock is closed. 21-12-2020 Chapter-2 Properties of Steam 29 Separating Calorimeter Construction Separating calorimeter consists of two chambers, viz inner chamber and outer chamber. At the top of inner chamber perforated tray is provided where water droplet in the wet steam is separated due to its inertia. Separated droplet is collected in inner chamber while steam is condensed in barrel calorimeter. Working From main steam pipe certain quantity of steam is taken to the calorimeter through sampling tube. This steam strike against the baffle plates/perforated tray. Due to inertia of droplets and sudden change in direction, water droplets are separated from steam which is collected in inner chamber. Steam is condensed in barrel calorimeter. Quantity of water droplet separated can be read from scale and quantity of steam can be calculated from difference in mass of water of barrel calorimeter. 21-12-2020 Chapter-2 Properties of Steam 33 21-12-2020 Chapter-2 Properties of Steam 35 Throttling Calorimeter Construction Fig. shows throttling calorimeter which essentially consists of throttle valve, pressure gauge, thermometer and manometer. Through sampling tube steam is taken to throttle valve where steam is throttled from higher pressure to lower pressure. Pressure gauge is used to measure pressure before throttling and manometer is used to measure pressure after throttling. Thermometer is used to measure temperature after throttling. Working With full open steam stop valve, steam is allowed to throttle until steady pressure and temperature is reached. At steady state condition pressure before throttling (p1) and temperature after throttling is to be measured. 21-12-2020 Chapter-2 Properties of Steam 37 Limitation This calorimeter is used when the dryness fraction is greater than 0.95. To use this calorimeter condition of steam after throttling must be superheated. 21-12-2020 Chapter-2 Properties of Steam 40 Combined Separating and Throttling Calorimeter It is already stated that the dryness fraction of the steam can be found by using throttling calorimeter only if the dryness fraction is greater than 0.95. When the dryness fraction is less than 0.9, then part of water is removed first passing the steam through separating calorimeter and then it is passed through a throttling calorimeter with a combined arrangement of separating and throttling calorimeter as shown in Fig. Even load values of dryness fraction of steam can be easily determined. 21-12-2020 Chapter-2 Properties of Steam 41 Combined Separating and Throttling Calorimeter Construction This calorimeter has two calorimeters namely separating calorimeter and throttling calorimeter in series. Working In a separating and throttling calorimeter, the steam from sampling tube is first passed through the separating calorimeter where it is partly dried up and then it is further passed on to the throttling calorimeter from where it comes out as superheated steam. The steam coming out from throttling calorimeter is condensed in a condenser and the mass of the condensate coming out of the condenser is recorded. 21-12-2020 Chapter-2 Properties of Steam 42 21-12-2020 Chapter-2 Properties of Steam 44  Thank you 21-12-2020 Chapter-2 Properties of Steam 47