Open Cycle Gas Turbine – working , Diagram ,Advantages

Introduction to thermodynamics Cycles 

Two important area of application of thermodynamics are power generation and refrigeration. Both power generation and refrigeration are usually accomplished by a system that operates on a thermodynamics cycle.

Thermodynamics cycles can be divided into two generation categories :
(a) Power Cycles
(b) Refrigeration Cycles
The devices or systems used to produce a net power output are often called engines and the thermodynamics cycles they operate on are called power cycle.
The devices or systems use to produce refrigeration are called refrigerator, air conditioners or heat pumps and the cycles they operates on are called refrigeration cycles.

Thermodynamic cycles can be categorized as :

(a) Power cycles or Refrigeration cycles.
(b) Gas Cycles or Vapor Cycles : In gas cycles, the working fluid remains in the gaseous phase throughout the entire cycle, where as in vapor cycles the working fluid exists in the vapor phase during one part of the cycle and in the liquid phase during another part.
(c) Closed Cycles or Open Cycles : In closed cycles, the working fluid is returned to the initial state at the end of the cycle and is re-circulated. In open cycle, the working fluid is renewed at the end of each cycle instead of being re-circulated.

Open cycle Gas Turbine Diagram : 

open gas turbine cycle
open gas turbine cycle

Working Principal

  • Fresh air enters the compressor at ambient temperature where its pressure and temperature are increased.
  • The high pressure air enters the combustion chamber where the fuel is burned at constant pressure.
  • The high temperature (and pressure) gas enters the turbine where it expands to ambient pressure and produces work.

Features:

  • Gas-turbine is used in aircraft propulsion and electric power generation.
  • High thermal efficiencies up to 44%.
  • Suitable for combined cycles (with steam power plant)
  • High power to weight ratio, high reliability, long life
  • Fast start up time, about 2 min, compared to 4 hr for steam-propulsion systems
  • High back work ratio (ratio of compressor work to the turbine work), up to 50%, compared to few percent in steam power plants.

Advantages of Open Cycle Gas Turbine : 

  • Warm-up time: Once the turbine is brought up to the rated speed by the starting motor and the fuel is ignited, the gas turbine will be accelerated from cold start to full load without warm-up time.
  • Low weight and size: The weight in kg per kW developed is less.
  • Fuels: Almost any hydrocarbon fuel from high-octane gasoline to heavy diesel oils can be used in the combustion chamber.
    Open cycle plants occupies less space compared to close cycle plants.
  • The stipulation of a quick start and take-up of load frequently are the points in favor of open cycle plant when the plant is used as peak load plant.
    Component or auxiliary refinements can usually be varied in open cycle gas turbine plant to improve the thermal efficiency and can give the most economical overall cost for the plant load factors and other operating conditions envisaged.
  • Open cycle gas turbine power plant, except those having an intercooler, does not need cooling water. Therefore, the plant is independent of cooling medium and becomes self-contained.

Disadvantages of Open Cycle Gas Turbine :

  • The part load efficiency of the open cycle gas turbine plant decreases rapidly as the considerable percentage of power developed by the turbine is used for driving the compressor.
  • The system is sensitive to the component efficiency; particularly that of compressor. The open cycle gas turbine plant is sensitive to changes in the atmospheric air temperature, pressure and humidity.
  • The open cycle plant has high air rate compared to the closed cycle plants, therefore, it results in increased loss of heat in the exhaust gases and large diameter duct work is needed.
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