Thermodynamics of Phase Change from Liquid to Vapor Description of most bountiful State of Matter ….. P M V Subbarao Professor Mechanical Engineering Department I I T Delhi

Phase Diagram of Simple Substance

Use of Steam in Manufacturing of Wood Products Wood is normally exposed to temperatures between °C, to change physical and chemical changes. Dimensional stability and weather resistance are improved with heat treatment. On the other hand the mechanical strength properties are usually negatively affected by the treatment. Two different types of thermal modification processes used in industry. Using saturated steam Using superheated steam..

Principle of a Wood heat treatment plant

Wood Heat Treatment Process

Constant Pressure Heating of Liquid Let's consider the results of heating liquid water from 20°C, 1atm while keeping the pressure constant. Process 1-2: The temperature and specific volume will increase from the compressed liquid, or subcooled liquid, state 1, to the saturated liquid state 2. In the compressed liquid region, the properties of the liquid are approximately equal to the properties of the saturated liquid state at the temperature.

Process 2-3: At state 2 the liquid has reached the temperature at which it begins to boil, called the saturation temperature, and is said to exist as a saturated liquid. Properties at the saturated liquid state are noted by the subscript f and v 2 = v f. During the phase change as the pressure is kept constant, the temperature will remain constant (water boils at 100°C when the pressure is 1 atm or kPa). At state 3 the liquid and vapor phase are in equilibrium and any point on the line between states 2 and 3 has the same temperature and pressure.

Process 3-4: At state 4 a saturated vapor exists and vaporization is complete. The subscript g will always denote a saturated vapor state. Note : v 4 = v g. Process 4-5: If the constant pressure heating is continued, the temperature will begin to increase above the saturation temperature, 100 °C in this example, and the volume also increases. State 5 is called a superheated state because T 5 is greater than the saturation temperature for the pressure.

Constant Pressure Process

Saturation Properties of Water v g /v f

The Theory of Producing Steam Water and steam are typically used as heat carriers in heat treatment systems. Water boils and evaporates at 100°C under atmospheric pressure. By higher pressure, water evaporates at higher temperature - e.g. a pressure of 10 bar equals an evaporation temperature of ~ C. At a constant pressure of 10 MPa the saturation temperature is C.

TemperaturePressure Specific Volume, m3/kg v g /v f 0C0CMPa Saturated Liquid v f Saturated Vapour v g

Normal Boiling Points of Various Metals Metal Boiling temperature Metal Boiling temperature ( o C) Aluminum2467Molybdenum4651 Antimony1440Nickel2800 Chromium2670Plutonium3230 Cobalt2925Potassium760 Copper2575Rhodium3700 Gold2800Selenium700 Iron2870Silicon3280 Lead1750Silver2212 Magnesium1090Sodium884 Manganese2060Tantalum5365 Mercury357Thorium4800 Titanium3290Tin2600 Tungsten5550Uranium4140 Zinc910

Liquid-Vapour Equilibrium Mixture Wet vapour is a mixture of steam and water at same temperature and pressure. Saturation pressure is the pressure at which the liquid and vapor phases are in equilibrium at a given temperature. Saturation temperature is the temperature at which the liquid and vapor phases are in equilibrium at a given pressure. Saturation Pressure is function of temperature or vice versa. T = F(p) The Wagner-Ambrose equation

Equations for Saturation Conditions of Water Saturation Properties of Water :

Liquid-vapor Equilibrium for Metals : Zinc & Cadmium The boiling point of cadmium is 765  C at 1 atmosphere. The plot of log p(atm) against 1/T(K) is fairly linear, but the best line through the points requires a slight degree of curvature. The results are best expressed by the relation The boiling point of Zinc is  C at 1. Within the range studied, The plot of log p(atm) against 1/T(K) is truly linear

Many Constant Pressure Processes If all of the saturated liquid states are connected, the saturated liquid line is established. If all of the saturated vapor states are connected, the saturated vapor line is established. These two lines intersect at the critical point and form what is often called the “steam dome.” The critical point of water is o C, MPa

Density of Saturated Water

Density of Saturated Steam

Wet Vapour Pressure and temperature are not independent properties. Either p & V or T& V are independent pair. P & v or T & v can also be considered. A new property is defined for steam for ease of design. This is called Quality of wet steam.

Quality and Saturated Liquid-Vapor (Wet) Mixture Now, let’s review the constant pressure heat addition process for water shown in Figure. The state 3 is a mixture of saturated liquid and saturated vapor. How do we locate it on the T-v diagram? To establish the location of state 3 a new parameter called the quality x is defined as

The quality is zero for the saturated liquid and one for the saturated vapor (0  x  1). The average specific volume at any state 3 is given in terms of the quality as follows. Consider a mixture of saturated liquid and saturated vapor. The liquid has a mass m f and occupies a volume V f. The vapor has a mass m g and occupies a volume V g.

Specific Volume of Wet Mixture