Thermal & Fluids - Combined Cycles

Energy/Power System Applications - Combined Cycles - 5 of 80 Problems

“Energy/Power System Applications” accounts for approximately 24 questions on the Thermal & Fluids Mechanical PE exam. Energy/Power System Applications is broken up between four different parts:

  • Energy/Power Equipment (turbines, boilers/steam generators, internal combustion engines, heat exchangers, cooling towers and condensers) 8 questions
  • Cooling/Heating (capacity, loads, cycles) 6 questions
  • Energy/Recovery (waste heat, storage) 5 questions
  • Combined Cycles (components, efficiency) 5 questions

This page covers Combined Cycles - 5 questions.

The information shown on this website is a sample of the material provided in the technical study guide and sample exam. See the STORE to purchase these items.

Combined Cycles

In the Thermodynamics sections, the following cycles were introduced (only portions of the cycles were introduced on the website. The paid version has the full text):

  • Open gas turbine cycle
  • Closed gas turbine cycle
  • Ideal Brayton cycle
  • Actual Brayton cycle
  • Rankine cycle

In this section, combined cycles, multiple variations on these cycles will be covered. These variations are important because in practice, these cycles are used with variations and are not practiced in an ideal case.

The following variations will be applied to the above cycles:

  • Brayton cycle with regeneration (included on website for free)
  • Brayton cycle with reheat and Intercooling (only included in paid version)
  • Rankine cycle with reheat (only included in paid version)
  • Rankine cycle with regeneration (only included in paid version)

Brayton Cycle with Regeneration

As a starting point, you should be familiar with the Brayton cycle without regeneration. Although this is covered in the Thermodynamics section, the Brayton cycle is also provided here for ease.

The basic Brayton cycle can be either open or closed. The figure below shows an open cycle. In an open cycle the low pressure warm air and combustion products are exhausted to the atmosphere. Fuel enters the combustor and travels with the air. In a closed cycle, the air is kept within a closed system and only heat is transferred from the combustor to the air. The combustion products are kept separate from the air.

The basic Brayton cycle starts with low pressure cool air entering a compressor. Work is provided to the compressor, which produces high pressure warm air. Then this air enters the combustor, where it gains heat and increases in temperature. Finally, the high pressure, hot air, enters a turbine where it produces work. Some of this work is used to drive the compressor and the remaining is provided as useful work. The low pressure warm air is then exhausted to the atmosphere.

Each step of the basic Brayton cycle is governed by a thermodynamic transition. In the ideal Brayton cycle, the compressor and turbine are isentropic. The combustor is assumed to occur at a constant pressure or isobaric. The following figure describes each of these transitions graphically.

  • Step 1 to 2: Compressor - The compression process is isentropic, meaning that there is no change in entropy.

  • Step 2 to 3: Combustion chamber and heat exchanger - Heat is transferred from combustion to the air.

  • Step 3 to 4: Turbine - This hot pressurized air then enters the turbine and as the gas expands (loses pressure and energy). The energy is converted to work to turn the turbine. Following the turbine, an electric generator is turned to produce electricity. Some of the work is used to power the compressor.

  • Step 4 to 1: Exhaust - The warm, low pressure air is then exhausted to the atmosphere.

The Brayton cycle with regeneration uses the waste heat from the warm, low pressure air to pre-heat the air before it enters the combustor. As you can see in the figure below, the air leaves the turbine at point 5 and instead of being exhausted; the air enters a regenerative heat exchanger. At the heat exchanger the warm air transfers heat to the entering air.

At point 1, air enters the compressor and the air increases in pressure and temperature to point 2, while the entropy is constant. This is based on the assumption that the compressor is isentropic.

Next, the air enters the regenerative heat exchanger and the air increases slightly in temperature to point 3. This process is assumed to be isobaric, so the pressure does not change. The majority of the heat input and increase in temperature occurs at the combustor, which increases the temperature from point 3 to point 4.

At point 4, air enters the turbine at a high pressure and temperature and then the air leaves at a low pressure and temperature at point 5. The energy loss is transferred to work at the turbine. The work is used to drive the compressor and also to useful work.

The low pressure, warm air then transfers its heat to the incoming air at the regenerative heat exchanger. The air then leaves the regenerative heat exchanger at a lower temperature at point 6. If the regenerative heat exchanger was perfect, then the temperature at point 6 would equal the temperature at point 2. Also the temperature at point 3 would equal the temperature at point 5.

The book also includes similar information as above on the following combination cycles, (2) Brayton Cycle with Intercooling, (3) Rankine Cycle with Reheat and (4) Rankine Cycle with Regeneration. See the technical study guide to get more information.

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