undefined | unit 3 definition

Thermodynamics

Unit 3

Definition

Q1. Explain the various steps and heats involved in formation of steam.

A1. Formation of steam

The above diagram shows the phase change occurring when water from and at 0°C is heated till is converted to steam and further to superheat it.

Temperature is plotted as ordinate and Enthalpy as abscissa.

Phase Change

During part A, water at 0°C is heated till it is converted into saturated liquid at 100°C, i.e., the boiling point of water. During this process, water does not undergo any phase change. Hence, the heat supplied is used only for sensible heating of water. It is denoted by hf and is called as enthalpy of liquid.

During part B, water at 100°C is being heated to convert it into steam. Complete conversion of water into steam with no water droplets presence takes time. The heat transfer taking place during this process is utilized to change the phase of water from liquid to gaseous state. This is denoted by hfg and is called as latent heat of vaporisation.

During parts A and B, the total heat transferred is denoted by hg and it is given by

hg = hf + hfg

During part C, the steam is already dry saturated. Any further heating results in superheating of steam. The heat supplied during this process is denoted by hsup and it is given by

hsup = hg + Cp (tsup - tsat)

tsup = temperature of superheated steam

tsat = temperature of saturation of steam

Cp = Specific heat of water at constant pressure.

Q2. Explain the critical point of water with the help of T-v diagram.

A2.

T-v diagram for pure substance:

Critical Point

As we go on increasing the pressure of the system, the saturation line continues to become smaller and smaller, and it becomes a point when the pressure is 22.06 MPa (220.6 bar) for water.

This point is called the critical point, and it is defined as the point where the saturated liquid and saturated vapour states are same or latent heat of vaporisation becomes 0.

The temperature, pressure, and specific volume of a substance at the critical point are called, respectively, the critical temperature Tcr, critical pressure Pcr, and critical specific volume vcr.

The properties of water at critical-point are Pcr = 22.06 MPa, Tcr = 373.95°C, and vcr = 0.003106 m3/kg.

Q3. Explain triple point of water.

A3.

Triple point

Under some special conditions, all 3 phases of a pure substance can co-exist in equilibrium

On P-v or T-v diagrams, these triple-phase states form a line called the triple line.

The states on the triple line of a substance have the same pressure and temperature but different specific volumes.

The triple line appears as a point on the P-T diagrams and, therefore, is often called the triple point.

For water, the triple-point temperature and pressure are 0.01°C and 0.6117 kPa, respectively.

Q4. Define dryness fraction.

A4. Dryness fraction is defined ratio of mass of dry steam to total mass of mixture of wet steam.

It is denoted by ‘x’.

For dry saturated, it is 1.

For wet steam, it is between 0 and 1.

It is very important property of steam and needs to be determined before the steam is fed to the boiler.

Q5. 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.

A5. Find Properties from Steam Table:

Psat = 100 kPa

a) The volume change:

vfg = vf – vg

=1.6941 - 0.001043

= 1.6931 m3/kg

b) the amount of energy transferred to the water.

The amount of energy needed to vaporize a unit mass of a substance at a given pressure is the enthalpy of vaporization at that pressure, which is

hfg= 2257.5 kJ/kg for water at 100 kPa.

Thus, the amount of energy transferred is

mhfg = (0.2 kg) (2257.5 kJ/kg) = 451.5 Kj

Q6. Calculate the internal energy per kg of superheated steam at a pressure of 10 bar and a temperature of 300°C. Also find the change of internal energy if this steam is expanded to 1.4 bar and dryness fraction 0.8.

A6.

Q7. In a steam power cycle, the steam supply is at 15 bar and dry and saturated. The condenser pressure is 0.4 bar. Calculate the Carnot and Rankine efficiencies of the cycle. Neglect pump work.

A7.

Q8. 1000 kg of steam at a pressure of 16 bar and 0.9 dry is generated by a boiler per hour. The steam passes through a superheater via boiler stop valve where its temperature is raised to 380°C. If the temperature of feed water is 30°C,

Determine:

(i) The total heat supplied to feed water per hour to produce wet steam.

(ii) The total heat absorbed per hour in the superheater.

Take specific heat for superheated steam as 2.2 kJ/kg K.

A8.

Q9. A vessel having a capacity of 0.05 m3 contains a mixture of saturated water and saturated steam at a temperature of 245°C. The mass of the liquid present is 10 kg.

Find the following:

(i) The pressure,

(ii) The mass,

(iii) The specific volume,

(iv) The specific enthalpy,

(v) The specific entropy, and

(vi) The specific internal energy.

A9.