Gas Turbines: Key Concepts & Thermodynamics

Introduction to Gas Turbines

  • Essential requirements
    • Ease of installation and maintenance
    • High reliability and efficiency
    • Conformance to environmental standards
    • Flexibility regarding service and fuel requirements
  • Advantages over other prime movers
    • Flexibility in installation and operation
    • Compactness
    • Operational reliability
    • High performance
    • Multiple fuel capability
  • Applications
    • Aircraft propulsion
    • Power generation
    • Mechanical drives
    • Marine propulsion
    • Aircraft propulsion types:
    • Turbojet
    • Turboprop
    • Turbofan

Performance & Design Parameters

  • Factors having maximum impact on efficiency
    • Pressure ratio
    • Firing temperature
    • Increase in these parameters leads to:
    • Increased work output
    • Increase in overall plant efficiency
  • Turbine categories for power generation
    • Small turbines: < 2\,\text{MW}
    • Usually centrifugal compressors and radial turbines
    • Medium turbines: 5-50\,\text{MW}
    • Usually axial flow compressors and turbines
    • Large frame-type turbines: 50-480+\,\text{MW}
    • Firing temperatures approach 1300^{\circ}\mathrm{C}
  • Ground-based turbines are broadly classified as
    • Frame Type Heavy-Duty
    • Industrial Type
    • Aircraft derivative
    • Small units
    • Micro-Turbines

Turbine Architectures & Classifications

  • Heavy-duty frame type gas turbines
    • Efficient and largely operate in a combined gas cycle
    • High firing temperatures around 1450^{\circ}\mathrm{C}
    • High operating reliability and long life
  • Medium size industrial gas turbine
    • Largely used in petrochemical plants
    • Operate on simple and/or combined cycles
  • Industrial Gas Turbines can be categorized as
    • Aero-derivative Industrial turbines
    • Non-aero derivative Industrial turbines
  • Aero-derivative advantages
    • Power range up to ~140\,\text{MW}
    • Fuel flexibility
    • High heat recovery
    • Long inspection intervals (12{,}000–16{,}000 h)
    • Variable speed (control flexibility)
    • Low weight; fast maintenance; fast start-up
  • Non-aero derivative Industrial Turbines
    • Highly reliable; designed for base-load
    • Good for constant speed & large power applications
    • Generally more rugged and easier to repair
  • Examples of aero-derivative engines
    • Rolls-Royce Aero Trent
    • General Electric LM6000
    • LMS100
  • Small and Micro Gas Turbines
    • Generate < 5\,\text{MW}
    • Rugged and simple in construction

Aero-derivative vs Industrial (Power Plant Perspective)

  • Aeroderivative (Aero-derivative Industrial Gas Turbines)
    • Advantages: faster start-up; high power-to-weight; low specific cost; suited to stand-by, marine, emergency applications
    • Typical performance: high efficiency (often >40\%) and high flexibility
    • Common use: peak/standby, modular plants
  • Heavy Industrial (Single Shaft)
    • Advantages: robust for base-load; large-scale outputs
    • Disadvantages: heavier; slower start-up; lower efficiency (roughly 30-34\%); heat recovery often < 15\%; larger, less flexible
  • Perceptions
    • Aero-derivative: high reliability/availability; higher inertia is generally associated with heavier machines

Gas Turbine Thermodynamics — Ideal Cycles

  • Reversible cycles with ideal gas assumptions
    • Working fluid is a perfect gas with constant specific heats
    • Processes are isentropic during compression and expansion
    • No pressure loss in combustor, heat exchanger, inter-cooler, or ducts
    • No variation in mass flow through the cycle
    • Heat transfer in heat exchangers is 100%
  • Brayton (Constant-Pressure) Cycle
    • Consists of two isobaric and two isentropic processes
    • Simple cycle: 1-2 isentropic compression; 2-3 constant-pressure heating; 3-4 isentropic expansion; 4-1 exhaust
  • Heat Exchange/Regeneration Cycles
    • Simple Heat Exchange Cycle (regeneration)
    • Inter-cooled Cycle
    • Inter-cooled with regeneration
    • Multi-stage adiabatic compression with intercooling improves net work
  • Intercooling and Reheat concepts
    • Intercooling reduces compressor work; reheating restores turbine inlet temperature after high-pressure stage
    • Basic inter-cooled/reheat cycles lead to higher work output and/or efficiency
  • Combined Inter-cooling & Reheat Cycle (hybrid)
    • Combines advantages of both to yield greater efficiency and work output
  • Actual gas turbine cycles
    • Include compressor, combustor, and turbine efficiencies and pressure losses
    • Result in lower work output and lower thermal efficiency than ideal cycles
  • Key cycle diagrams
    • T-ɸ and S-T (T-S) diagrams illustrate heat exchange, intercooling, inter-cooling, and reheat effects

Practical Cycle Variants & Takeaways

  • Inter-cooled cycle vs simple/reheat
    • Inter-cooling lowers compressor work and can improve efficiency at a given overall pressure ratio
  • Reheat cycle
    • Splits fuel addition across multiple combustors to improve efficiency and control
  • Split-Shaft, Inter-cooled Regenerative Reheat Cycle
    • Combines multiple techniques to maximize efficiency and flexibility
  • Cogeneration (Combined Heat and Power)
    • Waste heat from Brayton cycle used for steam, heating water/air
    • Widely used in industry and power plants
    • EU example: about 11\% of electricity generated via cogeneration; some countries plan to double capacity by 2020

Quick Reference: Key Points to Recall

  • Essential requirements drive GT adoption: reliability, efficiency, environmental compliance, flexibility
  • Efficiency driven by higher pressure ratio and firing temperature
  • GTs span from small (
  • Main architectures: Aero-derivative vs Non-aero Industrial vs Heavy Industrial frame-type
  • Thermodynamics: Brayton cycle basics; ideal vs actual cycles; regeneration/intercooling/reheat options
  • Cogeneration leverages waste heat to improve overall plant efficiency and reduce fuel use