to treat high salinity feedwater such as seawaters.2 Reliable and rigid system.
3 Require minimal conventional pretreatment for feedwater.
4 Capacity to use low waste thermal heat (low grade) from power plants to save energy.
5 Economical system if a heat source is available.
6 Easy in maintenance.
Disadvantages of thermal distillation:
1 The amount of water production depends on feed thermo‐physical properties (Tf, xf, …).
2 Tube scaling (CaSO4).
3 High value of SEC specially at elevated values of thermal performance.
Example 1.1 Boiling Phenomenon
Steam flows in a nonadiabatic horizontal pipe (control volume) with an inlet conditions of
1 Explain heat transfer phenomenon.
2 Identify the process type.
Solution
1 Since T1 > T2, it is clear that steam is subjected to heat loss.Since , then . Since T1 > T2, then h2 – h1 is negative, which indicate that heat transfer sign is negative, implying that heat is lost from the system (steam) to the surrounding.
2 Isobaric process.
Extra activity:
Student can perform the following:
1 Describe boiling phenomenon in MEE system.
2 Why kinetic and potential energy terms have been neglected from energy balance (first law of thermodynamics)? Explain.
3 Explain the process associated with change of water droplet phase that surrounds the exterior pipe wall. What is the type of evaporation process?
4 Based on real numbers, compare the value of h1 against h2. Explain the physical meaning of such difference.
5 Sketch T–v, P–v, and T–s diagrams showing boiling process.
Example 1.2 Flashing Phenomenon
Adiabatic rigid tank (control mass) is divided into two nonequal volumes: A and B by a flexible membrane. Part A contains H2O liquid at 7 kg, 220 kPa,
If membrane is ruptured by making a hole such that two systems reach a final equilibrium state at 25 °C:
Find:
1 What is the type of water evaporation process?
2 Final specific volume.
3 Final pressure.
4 Equations that represent exergy destruction (irreversibility).
Solution
1 Flashing process, where sudden drop in pressure causes water to evaporate via flashing, it is always associated with lightning and heat release.
2 Hence
3 P2 = 3.17 kPa
4 Entropy generation
Exergy destruction
Extra activity:
Student can perform the following:
1 Describe the flashing process in MSF system.
2 Sketch T–v, P–v, and T–s diagrams showing flashing process.
3 Perform a parametric study to investigate the effect of initial water temperature on final phase. Sketch and explain.
4 Perform a parametric study to investigate the effect of VB on final phase. Sketch and explain.
5 Calculate the amount of heat release from flashing process, and explain physically what is the source and cause of such heat.
6 Perform a parametric study to investigate the effect of ratio on irreversibility.
Example 1.3 Feedwater Heat Exchanger
An adiabatic heat exchanger is used to heat feedwater (H2O) from 40 to 120 °C.
Find:
1 ratio.
2 Heat transfer rate from thermal load (steam) to feedwater.
3 Exergy destruction (irreversibility) per kg of feedwater.
Solution
1 Mass balance: Energy balance (first law of thermodynamics):or .
2
3 Hence
Extra activity:
Student can perform the following:
1 Perform parametric study to investigate the effect of varying Tfeed during summer to winter seasons on heat transfer performance. Plot and explain. Take Tsummer = 35 °C.
2 Calculate exergy flow rate at all four states. Explain.
3 Resolve example using feedwater as seawater with salinity of 40 000 ppm. Refer to Appendix A for thermo‐physical properties.
1.4.2
