Hello everyone,

I am writing this post to suggests some questions which are very common for Govt. Job EXAM Preparation for Mechanical Engineering sector. I hope it will be really helpful for current students and job seekers. Those who are currently preparing for Govt. Job/BCS or any Power Plant jobs these questions will be always useful for you. Hope you will find the contents helpful. So Lets starts:

Laws of Thermodynamics: This topic is a very common topic for Question.

1. Explain the three laws of Thermodynamics. (30th BCS, 35th BCS)
   • • What is Zeroth Law? Explain the difference between Thermal Equilibrium and Thermodynamic Equilibrium. (41st BCS),
     • State 1st Law of Thermodynamics. Prove it is the Law of Conservation of Energy.(36th BCS, 28th BCS)
     • State 2nd Law of Thermodynamics. What is Perpetual motion of Second kind and why it is impossible./ Show that no heat engine can be more efficient than a reversible heat engine working between same temperature difference. (31st BCS, 29 BCS)

Answer to 1: 

• Zeroth Law: This Law States that when 2 bodies are in thermal Equilibrium with a Third body, then those 2 bodies are also in thermal equilibrium with each other. For example if a Body “A” and “B” are in thermal equilibrium with another 3rd body “C”, then “A” and “B” are also in mutual thermal equilibrium. The Thermal Equilibrium is a state when two bodies are in contact but no transfer of heat or changes in temperature occurs between those two bodies. The Thermodynamic Equilibrium is state of a Thermodynamic system which has no changes of total heat, mass or energy. Which means the system is at an Equilibrium state.
• 1st Law: This Law States that the heat and mechanical work are mutually convertible. It also states that the energy can neither be created nor destroyed, though it can be converted from one form to another.
• 2nd Law: This Law States that there is a definite limit to the amount of Mechanical energy, which can be obtained from a given quantity of heat energy. This Law has two statements. According to the Claussius this law may be stated as “It is impossible for a self-acting machine working in a cyclic process, to transfer heat from a body at a lower temperature to a body at a higher temperature without the aid of an external agency”. According to the Kelvin-Planck this law may be stated as “It is impossible to construct a machine working in a cyclic process, whose sole purpose is to convert all heat energy supplied into equivalent amount of work.” This statement also call as law of degradation of Energy. Perpetual motion of Second kind is an imaginary condition where Only one heat source is given to make perpetual motion, but its impossible because to obtain perpetual motion 1 heat source and 1 heat sink will be required along with an external agency to input some work/heat. Otherwise according to the 2nd Law of thermodynamics no perpetual motion can be obtained. Reversible Heat Engine efficiency is dependent on only input heat and Output heat, whereas in other heat engine the efficiency dependent not only on the input output temperature but also on different parameters (i.e compression ratio, adiabatic index, cut-off ratio and expansion ratio). So its impossible to achieve more efficiency than a reversible heat engine working between same temperature difference.

2. Explain What is Boiler/Steam Generator? What are the Characteristics of a Good Boiler? What are the Mountings ? Explain with Example. What are the Accessories? Explain with Example.

Answer to 2: Boiler or Steam Generator may be defined as the vessel for production of steam. Its a shell which normally made of Steel and contains steel tubes, the sole purpose of the shell is production of steam at a desired temperature and pressure.

Characteristics of a good boiler:

1. The Boiler should have maximum steam generation rate with minimum fuel consumption that means the efficiency should be high.
2. Starting and stoppage should be done quickly.
3. Initial cost, maintenance cost and operating cost should be low.
4. Should be installed with minimum time and parts should be easily dismountable.
5. All Parts of boiler should be accessible for cleaning and inspection.
6. It should occupy less floor space, free from manufacturing defects.
7. Boiler should have reliable controls and safety apparatus.
8. Mud should not get deposited on the heating surface.
9. It should be able to accommodate the load variation.

Boiler Mountings: Mounting’s are external important parts of a boiler for proper operation and safe functioning. The Main boiler mountings are following:

1. Water level Indicator: It indicates the level of water inside the boiler,
2. Pressure Gauge: It indicates the pressure inside the boiler.
3. Safety Valve: Safety valve is a pressure release valve to release during over pressure situation.
4. Stream Stop Valve: It controls the steam flow in the main steam pipe.
5. Fusible Plug: It extinguish the fire in the boiler if the water level falls below a certain level.
6. Feed Check valve:  It controls the feed water supply into the boiler.

Boiler Accessories: Accessories are not necessary for safely operation but required for improved performance of the boiler.

1. Superheater: It improves the quality of the steam by using heat of the exhaust gas, as a result it also increases the efficiency of the Boiler as well.
2. Economizer: Economizer uses the heat of the exhaust gas to heat of the feed water, as a result the boiler efficiency increases too.
3. Air Preheater: It preheats the incoming air into the furnace thus making the combustion quicker and efficient.