Heat Transfer and Power Plants MCQ With Answers for JKSSB Junior Engineer (Mechanical)

Technical · 2026-10-21 · 6 min read

Heat transfer and power plant questions cover the three modes of heat transfer, dimensionless numbers, boilers, steam cycles and gas turbines. This page gives a revision list and then solved MCQs with explanations.

Quick revision list

  • Heat transfer: conduction, convection and radiation, with Fourier's law, Newton's law of cooling and the Stefan–Boltzmann law
  • Numbers: Biot number (internal conduction to external convection resistance) and Prandtl number (momentum to thermal diffusivity)
  • Boilers: Babcock and Wilcox is water-tube, locomotive is fire-tube, and an economiser heats feed water with flue gas
  • Cycles: Rankine for steam, Brayton for gas turbines, with reheating and regeneration to improve performance
  • Nuclear: fission of uranium-235 with control rods that absorb neutrons

Worked numerical problems

  • Conduction: a wall with k = 0.8 W/m·K, an area of 10 m² and a thickness of 0.2 m, with a temperature difference of 20 K, transmits kAΔT ÷ L = 0.8 × 10 × 20 ÷ 0.2 = 800 W
  • Convection: h = 25 W/m²·K, an area of 2 m² and a temperature difference of 40 K give hAΔT = 2,000 W
  • Radiation: a black surface of 1 m² at 500 K emits σAT⁴ = 5.67 × 10⁻⁸ × 6.25 × 10¹⁰ = 3,544 W
  • Composite wall: thermal resistances add in series, with R = L ÷ kA for each layer. The heat flow is ΔT ÷ total resistance
  • Thermal efficiency: a plant that burns coal at 100 MW of heat and generates 35 MW of electricity has an efficiency of 35%

Boilers and plant efficiency

The efficiency of a boiler is the heat given to the steam divided by the heat in the fuel. A modern pulverised-coal plant has an overall efficiency of about 35% to 40%. The condenser rejects the largest share of the heat, and the cooling water carries it away to a cooling tower or river. Superheating raises the average temperature of heat addition, and so does a higher boiler pressure, while a lower condenser pressure raises the work output.

Gas turbine and combined cycle

The Brayton cycle has compression, constant-pressure heat addition, expansion and constant-pressure heat rejection. Its efficiency is 1 − 1/r_p^((γ − 1)/γ), where r_p is the pressure ratio. A combined-cycle plant uses the hot exhaust of a gas turbine to raise steam for a steam turbine, which pushes the overall efficiency above 55%.

Types of power plants

  • Thermal: coal, oil or gas heats a boiler
  • Hydroelectric: falling water drives a turbine, and it is the main source in J&K
  • Nuclear: fission heat produces steam, with control rods to regulate the reaction
  • Diesel and gas: used for stand-by and peak loads
  • Renewable: solar, wind and biomass

Heat exchangers

In a parallel-flow exchanger, both fluids move in the same direction. In counter-flow, they move in opposite directions, and for the same terminal temperatures, counter-flow gives a larger log mean temperature difference and a smaller area. Fins increase the surface area for convection, and the critical radius of insulation is k ÷ h for a cylinder.

Questions candidates ask

What is thermal conductivity? The rate of heat flow through a unit area for a unit temperature gradient. Metals have high values and insulators have low ones.

Why are fins used? They increase the surface area, so more heat is transferred by convection.

What does a condenser do in a power plant? It turns exhaust steam into water, which lowers the back pressure and increases the work from the turbine.

What is the role of a cooling tower? It rejects heat from the condenser cooling water to the atmosphere.

Which has higher efficiency, a Carnot cycle or any real engine? The Carnot cycle, between the same two temperatures, gives the maximum possible efficiency.

Revision tip

Make a one-page sheet with the three laws of heat transfer, the main dimensionless numbers and the four cycles used in power plants. Read it before every practice set until you can write it from memory.

Practise

Solve the MCQs below, then read the exam guide. Next: engineering materials and metrology.

By Shivam · Written with AI assistance and fact-checked · Questions on this page are original · Editorial policy · Report an error

Practice questions

Q1. The three modes of heat transfer are:

  1. Absorption, reflection and transmission
  2. Conduction, convection and radiation
  3. Fusion, evaporation and sublimation
  4. Conduction, compression and expansion
Show answer

Conduction, convection and radiation — Heat moves by contact, by fluid motion, or as electromagnetic waves.

Q2. Fourier's law relates heat flow to:

  1. The temperature gradient
  2. The velocity gradient
  3. The pressure gradient
  4. The concentration gradient
Show answer

The temperature gradient — q = −k (dT/dx).

Q3. The SI unit of thermal conductivity is:

  1. W/m²
  2. W/m²·K
  3. J/kg·K
  4. W/m·K
Show answer

W/m·K — It is the heat flow per metre length for a one-kelvin temperature difference.

Q4. Which of these materials has the highest thermal conductivity?

  1. Copper
  2. Wood
  3. Air
  4. Glass wool
Show answer

Copper — Metals conduct heat much better than insulators.

Q5. Newton's law of cooling is the basic equation for:

  1. Conduction
  2. Convection
  3. Diffusion
  4. Radiation
Show answer

Convection — Q = h·A·(Ts − T∞).

Q6. The Stefan–Boltzmann law says the total radiation emitted by a black body is proportional to:

  1. The square of its temperature
  2. The fourth power of its absolute temperature
  3. Its temperature
  4. The cube of its temperature
Show answer

The fourth power of its absolute temperature — E = σT⁴.

Q7. A black body is one that:

  1. Transmits all radiation
  2. Reflects all radiation
  3. Emits no radiation
  4. Absorbs all incident radiation
Show answer

Absorbs all incident radiation — Its absorptivity is 1.

Q8. In a counter-flow heat exchanger, compared with parallel flow, the log mean temperature difference is:

  1. Larger for the same terminal temperatures
  2. Zero
  3. Smaller
  4. The same
Show answer

Larger for the same terminal temperatures — Counter flow is more effective.

Q9. The Biot number is the ratio of:

  1. Convection to conduction
  2. Inertia to viscous force
  3. Internal conduction resistance to external convection resistance
  4. Buoyancy to viscous force
Show answer

Internal conduction resistance to external convection resistance — A small Biot number means the temperature inside the body is nearly uniform.

Q10. The purpose of fins on a surface is to:

  1. Increase insulation
  2. Reduce conduction
  3. Reduce the surface temperature of the fluid
  4. Increase the heat transfer area
Show answer

Increase the heat transfer area — More surface area increases the rate of heat dissipation.

Q11. The Prandtl number is the ratio of:

  1. Convection to conduction
  2. Inertia force to viscous force
  3. Momentum diffusivity to thermal diffusivity
  4. Thermal diffusivity to momentum diffusivity
Show answer

Momentum diffusivity to thermal diffusivity — Pr = μ·Cp / k.

Q12. The function of a boiler in a steam power plant is to:

  1. Generate steam at high pressure
  2. Condense steam
  3. Pump the water
  4. Drive the generator
Show answer

Generate steam at high pressure — It converts feed water to steam by burning fuel.

Q13. The function of a condenser is to:

  1. Increase boiler pressure
  2. Superheat steam
  3. Condense exhaust steam into water
  4. Remove ash
Show answer

Condense exhaust steam into water — A lower condenser pressure increases turbine work.

Q14. Reheating of steam in a Rankine cycle is done to:

  1. Reduce boiler pressure
  2. Condense the steam
  3. Reduce the feed pump work
  4. Reduce wetness at turbine exit and increase work
Show answer

Reduce wetness at turbine exit and increase work — It also slightly improves the cycle efficiency.

Q15. Regenerative feed heating improves cycle efficiency because:

  1. The condenser is removed
  2. The turbine speed rises
  3. Heat is added to feed water at a higher mean temperature using bled steam
  4. The boiler pressure is lowered
Show answer

Heat is added to feed water at a higher mean temperature using bled steam — Less heat is taken from the fuel to heat the feed water.

Q16. Which of these is a fire-tube boiler?

  1. Locomotive boiler
  2. Babcock and Wilcox boiler
  3. Benson boiler
  4. La Mont boiler
Show answer

Locomotive boiler — In fire-tube boilers hot gases pass through tubes surrounded by water.

Q17. Which of these is a water-tube boiler?

  1. Locomotive boiler
  2. Lancashire boiler
  3. Cochran boiler
  4. Babcock and Wilcox boiler
Show answer

Babcock and Wilcox boiler — Water flows inside the tubes and hot gases pass outside.

Q18. A safety valve on a boiler is used to:

  1. Supply feed water
  2. Release steam when the pressure exceeds a safe limit
  3. Measure pressure
  4. Control the water level
Show answer

Release steam when the pressure exceeds a safe limit — A boiler has at least two safety valves.

Q19. An economiser in a boiler heats:

  1. The fuel using air
  2. The feed water using the flue gases
  3. The air using steam
  4. The steam using water
Show answer

The feed water using the flue gases — It recovers heat from the flue gases.

Q20. The Brayton cycle is the ideal cycle for a:

  1. Petrol engine
  2. Gas turbine
  3. Diesel engine
  4. Refrigerator
Show answer

Gas turbine — It has two isentropic and two constant-pressure processes.

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