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Chapter: Mechanical - Engineering Thermodynamics - Properties of a Pure Substance and Steam Power Cycle

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Important Questions and Answers: Pure Substance and Steam Power Cycle

Mechanical - Engineering Thermodynamics - Properties of a Pure Substance and Steam Power Cycle

PROPERTIES OF PURE SUBSTANCE AND STEAM POWER CYCLES

 

1) What is pure substance?

 

Pure substance is a substance which has a fixed chemical composition throughout its mass. A pure substance does not have to be of a single chemical element or compound. A mixture of various chemical elements or compounds is also called as pure substance as long as the mixture is homogeneous.

 

Example: Water, Nitrogen, Carbon di oxide and helium.

 

 

 

2) Define Dryness fraction of steam (or) what is quality of steam?

 

It is defined as the ratio of the mass of the total steam actually present to the mass of the total system.

 

Dryness fraction of the steam =        mass of the dry steam    / mass of the total mixture

3) Explain the terms: 1) Degree of superheat 2) Degree of sub cooling.

 

1) Degree of superheat:

 

It is the difference between superheated temperature and saturated temperature at the same pressure.

 

2) Degree of sub cooling:

 

It is the amount by which the water is cooled beyond the saturated temperature at the same pressure.

 

 

 

4) What are the wet and dry steams?

 

The steam which partially evaporated and having water particles in suspension is called wet steam.

 

The steam which fully in evaporated state and is not having any water particles is called dry steam.

 

5) What is meant by superheated steam and states the uses.

 

The dry steam is further heated, then the process is called superheating and steam obtained is known as superheated steam.

 

Uses:

 

1.     It has the more heat energy and more work can be obtained using it.

2.     Thermal efficiency increases as the temperature of superheated steam is high.

 

3.     Heat losses to be condensation of steam an cylinder wall is reduced.

 

6) State phase rule of pure substance.

 

The number of independent variables associated with a multicomponent, multiphase system is given by the phase rule. It is also called Gibbs Phase rule. It is expressed by the equation as

 

n = C -   + 2

 

Where, n = number of independent variables

 

C = number of components

 

=  number of phase present in equilibrium.

 

7) What is meant by work ratio?

 

It is defined as the ratio of network transfer to the positive work transfer.

 

8) Name the different process of Rankine cycle?

 

1-2: Isentropic expansion

 

2-3: Constant pressure and temperature heat rejection

 

3-4: Water is pumped to boiler pressure.

 

4-5: Constant pressure heat addition in boiler up to saturation temperature. 5-1: Constant pressure and temperature in boiler.

 

9) Mention the improvements made to increase the ideal efficiency of rankine cycle. [May/June 2014]

 

1. Lowering the condenser pressure.

 

2. Superheated steam is supplied to the turbine.

 

3. Increasing the boiler pressure.

 

4. Implementing the reheat and regeneration in the cycle.

 

10) What are the different types of steam available?

 

1) Wet steam

 

2) Dry saturated steam

 

3) Superheated steam

 

11) Define efficiency of a Rankine cycle.

 

The efficiency of a Rankine cycle is defined as the ratio of net power developed to the heat supplied

 

.

 

12) Define enthalpy of steam.

 

Enthalpy of steam is the sum of heat added to the water from the freezing point to saturation temperature and the heat absorbed during evaporation.

 

 

 

13)           What is Sensible heat and Latent heat?

The heat supplied to increase the temperature of the substance is known as sensible heat. The

 

heat supplied for the phase change of a substance is known as latent heat.

 

14)           Define P-V-T surface.

P-V-T surface is defined as a three dimensional surface which gives the relationship between

 

pressure, volume and temperature of a pure substance.

 

15)           List the advantages of reheat cycle.

 

a) Increase in work done per kg of steam which results in reduced size of boiler and auxiliaries for the same output.

 

b) We can prevent the turbine from the erosion.

 


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