Chapter 3 Pure Substance

Chapter 3 Pure Substance Pure Substance ► A substance that is uniform and invariable in chemical composition. ► A pure substance can exist in more than one phase, but its chemical composition must be the same in each phase. ► Examples: ► Drinking water with ice cubes can be regarded as a pure substance because each phase has the same composition. ► A fuel-air mixture in the cylinder of an automobile engine can be regarded as a pure substance until ignition occurs. THERMODYNAMIC TABLES EXAMPLE 3–1 Pressure of Saturated Liquid in a Tank A rigid tank contains 50 kg of saturated liquid water at 90 C. Determine the pressure in the tank and the volume of the tank P = p at T=90 C in saturated table = 70.183 kPa Specific volume: v = vf @ 90 C = 0.001036 m3/kg Total volume: V = m vf = (50 )(0.001036) = 5 0.0518 m3 EXAMPLE 3–3 Volume and Energy Change during Evaporation A mass of 200 g of saturated liquid water is completely vaporized at a constant pressure of 100 kPa. Determine (a) the volume change and (b) the amount of energy transferred to the water. EXAMPLE 3–8 Approximating Compressed Liquid as Saturated Liquid Determine the internal energy of compressed liquid water at 80 C and 5 MPa, using (a) data from the compressed liquid table and (b) saturated liquid data. What is the error involved in the second case? Problem 1 • Find the state of the following substances under the given conditions along with the region in which they lie (compressed liquid/sat liquid-vapor mixture/superheated vapor) Material Sl No. T P (MPa) P (MPa) State/Region 1 Water 2 3 Material Water Sl No. T 1 Pt. 3, Compressed or subcooled liquid 2 Any point on the 2-phase line, state not fixed 3 Pt. 4, superheated vapor Problem 2 A 1.8 m3 rigid tank contains steam at 220 °C. One-third of the volume is in the liquid phase and the rest is in the vapor form. Determine: (a) Pressure of the steam (b) Mass of liquid (c) Mass of vapor (d) Quality of the mixture (e) Specific volume of the mixture (f) Enthalpy of the mixture Linear Interpolation ► When a state does not fall exactly on the grid of values provided by property tables, linear interpolation between adjacent entries is used. ► Example: Specific volume (v) associated with superheated water vapor at 10 bar and 215oC is found by linear interpolation between adjacent entries in Table A-4. (0.2275 – 0.2060) m3/kg (v – 0.2060) m3/kg → slope = = (240 – 200)oC (215 – 200)oC Table A-4 v = 0.2141 m3/kg Compressed liquid • Superheated vapor (interpolation) • Problem A piston cylinder contains 50 kg of saturated liquid water at 90°C. Determine: – – – – Pressure in the cylinder Volume of the cylinder (m3) Enthalpy (kJ) Internal energy (kJ) – – – – – – Pressure in the cylinder Volume of the cylinder Enthalpy (kJ) Internal energy (kJ) The amount of energy transferred to the water Draw the process in the T-v and P-v diagram. Now the liquid is heated at the constant temperature in order to get the saturated vapor. Find: Problem For water at a state of P=0.5 MPa and h =2890 kJ/kg, determine: a) State of the substance (saturated/superheated/subcooled) b) Temperature Problem Determine the internal energy of compressed liquid water at 80°C and 5 MPa, using: (a) data from the compressed liquid table (b) saturated liquid data. (c) What is the error involved in the second case? Give the missing property of P and phase description (x, if required) for CH 4 at: T = 350 K, v = 0.25 m3/kg Problem A 10-kg mass of superheated refrigerant-134a at 1.2 MPa and 70°C is cooled at constant pressure until it exists as a compressed liquid at 20°C. a) Determine the phase at the start and end of the process b) Show the process on a T-v diagram with respect to saturation lines c) Determine the change in volume. d) Find the change in total internal energy Dr. Naveen Shrivastava Triple point