Steam Boilers, Turbines, Rankine Cycle and Power Plants for SSC JE Mechanical: Mountings, Accessories, Gas Turbine and Nuclear Basics

Power plant engineering is a standard topic in the SSC JE Mechanical paper, with questions on boilers, their mountings and accessories, steam turbines, the Rankine cycle and gas turbines. This guide on steam boilers, turbines and power plants for SSC JE gives all of them in tables, with formulas, solved examples, practice questions and FAQs.

Rankine Cycle

The Rankine cycle is the basic ideal cycle of the steam power plant.

• Thermal efficiency η = net work / heat supplied = (W_turbine − W_pump) / Q_in.
• The Carnot efficiency between temperatures T₁ and T₂ is 1 − T₂/T₁; no practical cycle can exceed it.
• The Rankine efficiency increases with a higher boiler pressure, a higher steam temperature (superheating) and a lower condenser pressure.
• Reheating improves the dryness of the steam at the turbine exit; regeneration (feed heating by bleeding steam) improves the efficiency.
• The back work ratio (pump work / turbine work) of the Rankine cycle is very small.

Types of Boilers

• Lancashire and Cornish boilers are horizontal and use two internal furnace tubes (Lancashire) or one (Cornish).
• The Benson boiler works at the critical pressure (221 bar) and has no drum.
• Equivalent evaporation is the evaporation from and at 100°C of water to dry saturated steam; boiler efficiency = heat in steam / heat in fuel.
• Boiler capacity is expressed as kg of steam per hour; a boiler horsepower is also used.

Boiler Mountings (Required for Safety and Control)

Boiler Accessories (Improve Efficiency)

Steam Turbines

• Compounding is used to reduce the speed of the rotor: velocity compounding (Curtis), pressure compounding (Rateau) and pressure–velocity compounding.
• Governing controls the speed: throttle, nozzle control and bypass governing.
• A steam turbine has a higher efficiency and gives a uniform torque compared with a reciprocating engine.
• The maximum blade efficiency of a single-stage impulse turbine occurs when the blade speed to steam speed ratio is 0.5 (cosα/2).

Condensers and Cooling Towers

• The condenser converts the exhaust steam into water at a pressure below the atmosphere (vacuum), increasing the turbine work.
• Jet (direct contact) condensers mix the cooling water with steam; surface condensers keep them separate and recover the condensate; the surface condenser is used in large plants.
• Condenser efficiency = actual rise in the cooling water temperature / maximum possible rise.
• Cooling towers cool the warm water from the condenser by evaporation: natural draught and forced draught towers.

Gas Turbine (Brayton Cycle)

• The Brayton cycle consists of isentropic compression, constant-pressure heat addition, isentropic expansion and constant-pressure heat rejection.
• Thermal efficiency η = 1 − 1/r_p^(γ−1)/γ, where r_p is the pressure ratio.
• Efficiency can be increased by intercooling, reheating and regeneration.
• A gas turbine plant has a high power-to-weight ratio, is quick to start, but uses a large part of the turbine work to drive the compressor (back work ratio up to 40–80%).
• Used in aircraft (the jet engine) and as peak-load power plants; combined cycle plants (gas and steam turbines) give efficiency over 55%.

Nuclear Power Plant (Basics)

Solved Examples

Example 1. Find the Carnot efficiency between 600 K and 300 K.
1 − 300/600 = 50%.

Example 2. Find the efficiency of a Brayton cycle with a pressure ratio of 8 (γ = 1.4).
η = 1 − 1/8^0.2857 = 1 − 1/1.811 = 44.8%.

Example 3. Which boiler accessory uses the flue gases to heat the feed water?
The economiser.

Example 4. Which device protects the boiler if the water level drops too low?
The fusible plug.

Quick Tips

• Learn the four processes of the Rankine cycle with the equipment.
• Learn the mountings and accessories as two separate lists.
• Remember the difference between impulse and reaction turbines.
• Remember that the Benson boiler works at the critical pressure.

Quick Revision Points

• The Rankine cycle has a pump, boiler, turbine and condenser.
• Superheating and a lower condenser pressure increase the Rankine efficiency.
• The economiser heats the feed water; the air preheater heats the air.
• The Parsons turbine has 50% reaction.
• The Brayton cycle is the gas turbine cycle.

Practice Questions With Answers

1. Which cycle is the ideal cycle for steam power plants?
   Answer: Rankine cycle
2. Which boiler accessory heats the feed water?
   Answer: Economiser
3. Which device acts as a safety device when the water level is low?
   Answer: Fusible plug
4. Which turbine is a reaction turbine?
   Answer: Parsons turbine
5. Which boiler works at the critical pressure and has no drum?
   Answer: Benson boiler
6. What is the moderator in a nuclear reactor used for?
   Answer: To slow down the neutrons
7. What is the Brayton cycle used in?
   Answer: Gas turbines
8. Which type of condenser keeps steam and cooling water separate?
   Answer: Surface condenser
9. What is the degree of reaction of an impulse turbine?
   Answer: Zero
10. Which valve prevents a back flow of water in the boiler?
    Answer: Feed check valve

How to Practise This Chapter

Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Mechanical Engineering notes page, read the previous topic Machine Design and the next topic Material Science and Heat Treatment.

Frequently Asked Questions

What is the Rankine cycle?

The ideal cycle for the steam power plant, with a boiler, turbine, condenser and pump.

What is a boiler?

A closed vessel in which water is heated to generate steam under pressure.

What are boiler mountings?

The safety and control fittings on a boiler, such as the safety valve and the water level gauge.

What are boiler accessories?

The devices that improve the efficiency, such as the economiser and the air preheater.

What is an economiser?

A device that preheats the feed water using the waste heat of the flue gases.

What is the difference between impulse and reaction turbines?

In an impulse turbine the pressure drop is only in the nozzle; in a reaction turbine it is in both fixed and moving blades.

What is the purpose of a condenser?

To condense the exhaust steam and to maintain a vacuum.

What is a cooling tower?

A tower that cools the hot water from the condenser by evaporation.

What is a moderator?

A material that slows down the fast neutrons in a nuclear reactor.

What is a combined cycle plant?

A power plant using both a gas turbine and a steam turbine for a higher efficiency.

Is this topic asked in SSC JE?

Yes. Power plant engineering is asked in the mechanical paper.

What is superheated steam?

Steam heated above its saturation temperature at the same pressure.