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Qwestrum Engineering360 · Mechanical Engineering · Power Plant Engineering

Gas Turbine Plant

A gas-turbine plant runs the Brayton cycle; efficiency η = (W_turbine − W_compressor)/Q_combustor. Its high power-to-weight and fast start suit peaking duty, per P.K. Nag.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • Open cycle GT: compressor, combustor, turbine
  • Closed cycle: external heat source (nuclear GT concept)
  • Cogeneration: exhaust heat for process steam

Topic details

Introduction

Gas-turbine power plants offer quick startup and high power density, used for peaking and, in combined cycles, base load. P.K. Nag analyses the open-cycle plant on the Brayton cycle.

Scope in B.Tech and GATE syllabus

Air is compressed, fuel burned at constant pressure, and hot gas expanded through the turbine, which drives both the compressor and the generator. The large compressor work (high back-work ratio) is characteristic.

Why this topic matters in practice

Improvements — intercooling, reheating, and regeneration — reduce net work penalty or recover exhaust heat. Gas turbines exhaust hot gas that a combined cycle exploits. Computing net work and efficiency and understanding the plant's role are the exam demands.

Key relations & formulas

Formulas (Indian textbook notation)

  • η=WnetQin=(Wturb−Wcomp)Qcombustor\eta = \frac{W_{net}}{Q_{in}} = \frac{(W_{turb} - W_{comp})}{Q_{combustor}}

Formulas (Indian textbook notation)

  • Wcomp=cp(T2−T1);Wturb=cp(T3−T4)W_{comp} = c_{p}(T_{2} - T_{1}); W_{turb} = c_{p}(T_{3} - T_{4})

Formulas (Indian textbook notation)

  • Specificworkoutput=Wnet/m˙airSpecific work output = W_{net}/ṁ_air
Heatrate=cp(T3−T2)ηHeat rate = c_{p}\frac{(T_{3} - T_{2})}{\eta}
(related to fuel consumption)

Notation and sign conventions

Relation 1 —
η=WnetQin=\eta = \frac{W_{net}}{Q_{in}} =

Formulas (Indian textbook notation)

  • η=WnetQin=(Wturb−Wcomp)Qcombustor\eta = \frac{W_{net}}{Q_{in}} = \frac{(W_{turb} - W_{comp})}{Q_{combustor}}
Write this relation with symbols exactly as in Power Plant Engineering — P.K. Nag before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
Wcomp=cpW_{comp} = c_{p}

Formulas (Indian textbook notation)

  • Wcomp=cp(T2−T1);Wturb=cp(T3−T4)W_{comp} = c_{p}(T_{2} - T_{1}); W_{turb} = c_{p}(T_{3} - T_{4})
Write this relation with symbols exactly as in Power Plant Engineering — P.K. Nag before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
Specificworkoutput=Wnet/m˙airSpecific work output = W_{net}/ṁ_air

Formulas (Indian textbook notation)

  • Specificworkoutput=Wnet/m˙airSpecific work output = W_{net}/ṁ_air
Write this relation with symbols exactly as in Power Plant Engineering — P.K. Nag before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 4 —
Heatrate=cpHeat rate = c_{p}
Heatrate=cp(T3−T2)ηHeat rate = c_{p}\frac{(T_{3} - T_{2})}{\eta}
(related to fuel consumption)
Write this relation with symbols exactly as in Power Plant Engineering — P.K. Nag before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Fundamentals and definitions

The gas-turbine plant compresses air (consuming much of the turbine output), adds heat by burning fuel at constant pressure in the combustor, and expands the hot gas through the turbine. Net output is turbine work minus compressor work.

Governing relations in practice

Because compressing gas is costly, the back-work ratio (compressor/turbine work) is high (~50 %), so component efficiencies strongly affect net output — a small drop in turbine or compressor efficiency sharply cuts net work.

Design and analysis considerations

Efficiency rises with pressure ratio and turbine inlet temperature; the latter is limited by blade materials and cooling. Regeneration recovers exhaust heat to preheat combustor air (when exhaust is hotter than compressor discharge), while intercooling and reheating improve the work ratio.

Advanced theory and extensions

Advantages are compact size, low water needs, and fast startup (ideal for peaking); disadvantages are lower standalone efficiency and sensitivity to inlet temperature. The hot exhaust makes gas turbines the topping cycle of combined-cycle plants — the practical context.

Assumptions and validity limits

State assumptions explicitly before using any relation for gas turbine plant — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Power Plant Engineering viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Power Plant Engineering papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to gas turbine plant.
4. Use equation 1:
η=WnetQin=\eta = \frac{W_{net}}{Q_{in}} =
.
5. Use equation 2:
Wcomp=cpW_{comp} = c_{p}
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Gas Turbine Plant appears in thermal and combined-cycle plants. In Indian mechanical curricula this topic is tested because it connects theory to steam and gas-based power generation.
GATE and semester exams often combine gas turbine plant with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use gas turbine plant?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Ignoring the high back-work ratio and equating net to turbine work
• Forgetting turbine-inlet-temperature limits set by materials/cooling
• Applying regeneration when exhaust is cooler than compressor discharge
• Using volume ratio instead of pressure ratio in Brayton efficiency

Quick revision checklist

Before attempting gas turbine plant problems, confirm you can:
1. Open cycle GT: compressor, combustor, turbine
2. Closed cycle: external heat source (nuclear GT concept)
3. Cogeneration: exhaust heat for process steam
Revise the solved examples in Power Plant Engineering — P.K. Nag and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Gas-turbine net work

Problem

A gas-turbine plant has turbine work 520 kJ/kg and compressor work 300 kJ/kg, with combustor heat 600 kJ/kg. Find net work and efficiency.

Solution

W_net = 520 − 300 = 220 kJ/kg; η = W_net/Q = 220/600 = 0.367, i.e. 36.7 %.

Conceptual check — Gas Turbine Plant

Problem

In a Power Plant Engineering semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of gas turbine plant." What should a complete answer include?

Practice questions

Most-asked interview and GATE questions for this topic — expand any item for a model answer.

  1. 1
    What is Gas Turbine Plant, and why does it appear in B.Tech / GATE syllabi?

    Model answer

    A gas-turbine plant runs the Brayton cycle; efficiency η = (W_turbine − W_compressor)/Q_combustor. Its high power-to-weight and fast start suit peaking duty, per P.K. Nag.
  2. 2
    State the relation η = W_net/Q_in = and name each symbol.

    Model answer

    The governing relation is η=WnetQin=\eta = \frac{W_{net}}{Q_{in}} =. Write every symbol with SI units before substituting numbers.
  3. 3
    State the relation W_comp = c_p and name each symbol.

    Model answer

    The governing relation is Wcomp=cpW_{comp} = c_{p}. Write every symbol with SI units before substituting numbers.
  4. 4
    State the relation Specific work output = W_net/ṁ_air and name each symbol.

    Model answer

    The governing relation is Specificworkoutput=Wnet/m˙airSpecific work output = W_{net}/ṁ_air. Write every symbol with SI units before substituting numbers.
  5. 5
    State the relation Heat rate = c_p and name each symbol.

    Model answer

    The governing relation is Heatrate=cpHeat rate = c_{p}. Write every symbol with SI units before substituting numbers.
  6. 6
    Explain: Open cycle GT: compressor, combustor, turbine

    Model answer

    Open cycle GT: compressor, combustor, turbine — state the assumption range and one exam trap linked to this point.
  7. 7
    Explain: Closed cycle: external heat source (nuclear GT concept)

    Model answer

    Closed cycle: external heat source (nuclear GT concept) — state the assumption range and one exam trap linked to this point.
  8. 8
    Explain: Cogeneration: exhaust heat for process steam

    Model answer

    Cogeneration: exhaust heat for process steam — state the assumption range and one exam trap linked to this point.
  9. 9
    How would you correct this error in a viva: Ignoring the high back-work ratio and equating net to turbine work?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  10. 10
    How would you correct this error in a viva: Forgetting turbine-inlet-temperature limits set by materials/cooling?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  11. 11
    How would you correct this error in a viva: Applying regeneration when exhaust is cooler than compressor discharge?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  12. 12
    How would you correct this error in a viva: Using volume ratio instead of pressure ratio in Brayton efficiency?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.

Exams & GATE

  • 1
    P.K. Nag — regenerative GT when exhaust T > compressor outlet T.
  • 2
    Avoid: Ignoring the high back-work ratio and equating net to turbine work
  • 3
    Avoid: Forgetting turbine-inlet-temperature limits set by materials/cooling
  • 4
    Avoid: Applying regeneration when exhaust is cooler than compressor discharge

📖 Standard books (India)

  • Power Plant Engineering — P.K. Nag

    Read: Syllabus unit

    Steam, gas turbine, and plant economics

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