Reheat Rankine Cycle

In this comprehensive exploration of the Reheat Rankine Cycle, you'll gain both a fundamental understanding and deep technical insight into this core concept in Engineering. Delving from its basic meaning and historical evolution up to its diverse applications in everyday life and industry, the Reheat Rankine Cycle comes to life through simple and more complex examples. You'll decipher its formula, compare ideal versus real conditions, and finally measure efficiency. Whether your interest is academic, professional, or simply inquisitive, this thorough guide sheds light on all facets of the Reheat Rankine Cycle.

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Jetzt kostenlos anmeldenIn this comprehensive exploration of the Reheat Rankine Cycle, you'll gain both a fundamental understanding and deep technical insight into this core concept in Engineering. Delving from its basic meaning and historical evolution up to its diverse applications in everyday life and industry, the Reheat Rankine Cycle comes to life through simple and more complex examples. You'll decipher its formula, compare ideal versus real conditions, and finally measure efficiency. Whether your interest is academic, professional, or simply inquisitive, this thorough guide sheds light on all facets of the Reheat Rankine Cycle.

The Reheat Rankine Cycle can be defined as an extended version of the Rankine cycle that involves reheating the working fluid, usually steam after partial expansion and cooling it before sending it back to the steam generator or boiler.

- Boiler (or Steam Generator)
- High-Pressure Turbine
- Reheater
- Low-Pressure Turbine
- Condenser

Over the next few decades, several modifications were made to improve the cycle's thermal efficiency. One significant modification was the introduction of the reheat process.

1859 | Rankine describes the basic cycle |

20th Century | Introduction of the reheat process |

Today | Widely implemented in power stations worldwide |

For instance, assume steam enters the turbine at 15 MPa and 600 degrees Celsius. It expands to a pressure of 1 kPa in the turbine. The steam is then condensed at this low pressure and pumped to the initial high pressure of 15 MPa, thereby completing the basic Rankine cycle.

Let's consider again that steam is entering the turbine at 15 MPa and 600 degrees Celsius in a reheat cycle. However, this time it expands to an intermediate pressure of 2 MPa. It is then reheated to 600 degrees Celsius and expanded to the low pressure of 1 kPa in the second stage of the turbine. Finally, it is condensed and pumped back to the high pressure of 15 MPa.

Consider a steam power plant operating on the Reheat Rankine Cycle where the steam enters the high-pressure turbine at 8 MPa and 480°C and the low-pressure turbine at 2 MPa and 480°C. The steam is then condensed in the condenser at a pressure of 8 kPa. We can solve for the heat and work interactions and the thermal efficiency for this cycle using thermodynamics principles.

- Net power output
- Thermal efficiency
- Heat supplied in the boiler and reheater

In a standard **coal-fired power plant**, coal is burned in a boiler, which heats up the water to produce high-pressure steam. This steam drives a turbine connected to an electric generator, converting the kinetic energy into electric power. The steam is then passed through a condenser and condensed back into water, before being returned to the boiler to start the cycle again.

**Heavy Manufacturing Industries**: High-demand industries, such as steel, rely on vast quantities of energy to operate their machinery. Here, adopting the Reheat Rankine Cycle enables lower energy costs, reduced emissions, and overall increased efficiency.**Marine Industry**: In shipping and shipbuilding industries, efficient power production is key for propulsion and onboard utility functions. Implementing the Reheat Rankine Cycle can make a huge difference to a ship's operating cost and overall carbon footprint.**Geothermal Power Plants**: These facilities utilise the heat generated and stored in the Earth. As a natural resource, it's sustainable and renewable over human timescales. Geothermal power plants apply the principles of the Reheat Rankine Cycle by using geothermal steam to turn turbines and generate electricity.

**Work Done in the High-Pressure Turbine (\(W_{HPT}\))**: This refers to the energy obtained from the steam expansion in the initial, high-pressure stage of the turbine.**Work Done in the Low-Pressure Turbine (\(W_{LPT}\))**: This refers to the energy derived from the steam expansion in the subsequent, lower-pressure stage of the turbine, post-reheat.**Work Required for the Feed Pump (\(W_{FP}\))**: This corresponds to the energy needed to pump the water at the bottom of the cycle back up to the boiler, completing the loop and restarting the process.**Heat Input in the Boiler (\(Q_{Boiler}\))**: This is the energy added to the cycle in the primary stage of heating, transforming water to high-pressure steam.**Heat Input in the Reheater (\(Q_{Reheater}\))**: This represents the additional energy provided to the lower pressure steam post-expansion, regaining its energy level before entering the low-pressure turbine.

In the first step, it's necessary to obtain individual component data. This includes the parameters of the high-pressure turbine, low-pressure turbine, feed pump, boiler, and reheater. Data can be obtained from the technical specifications or operating charts of the plant.

The next step engages the usage of the First Principle of Thermodynamics, also known as the Law of Energy Conservation. This law suggests that energy cannot be created nor destroyed, only transferred. Because of this, the energy input into the process, in the form of \(Q_{Boiler}\) and \(Q_{Reheater}\), equals the overall energy output, in the form of the work done by the high-pressure and low-pressure turbines, and the work needed to power the feed pump. This step essentially sets up the numerator and denominator in the Reheat Rankine Cycle formula.

**Isentropic Expansion**: The steam expansion in the turbine is assumed to be isentropic, meaning there is no change in entropy and it corresponds to a reversible adiabatic process.**Isentropic Compression**: Similarly, the compression in the pump is also isentropic. This means the feedwater is pumped from the condenser pressure to the boiler pressure without a change in entropy.**Perfect Heat Transfer**: Both in the boiler and the reheater, it is assumed that all the heat transferred to steam is effectively converted into work, leaving no energy wasted.**No Mechanical Losses**: There are no mechanical losses in the turbine or the pump, meaning all the input to these devices is effectively turned into useful work.

**Irreversibilities in Expansion and Compression**: In actual plants, the expansion in the turbine and the compression in the pump are not perfectly isentropic. Mechanical inefficiencies, heat losses, and other factors lead to a small increase in entropy, reducing the overall efficiency.**Heat Transfer Losses**: In real cycles, a certain portion of the heat transferred in the boiler and reheater will be lost to the surroundings without contributing to the work output, against the assumption of perfect heat transfer.**Mechanical Losses**: Mechanical elements of the turbine and pump, such as bearings, blades, and seals, can experience wear and tear that leads to energy losses, departing from the assumption of perfect mechanical operation.

**\(W_{HPT}\)**: Work output of the High-Pressure Turbine**\(W_{LPT}\)**: Work output of the Low-Pressure Turbine**\(W_{FP}\)**: Work required by the Feed Pump

**\(Q_{Boiler}\)**: Heat added during the boiling process**\(Q_{Reheater}\)**: Heat added during the reheating process

**Temperature and Pressure Levels**: The levels of temperature and pressure at various stages of the cycle, particularly during the boiler, turbine, and reheater stages, widely affect the overall thermal efficiency. Higher the average temperature at which heat is added to the cycle, higher is the efficiency, as per the Carnot Theorem. However, practical limitations, safety concerns, and material constraints often limit how high these values can go.**Irreversibilities**: The second law of thermodynamics reveals that no process can be entirely reversible, and there will always be some degree of irreversibility involved. This irreversibility, which could be due to factors like friction or heat losses, directly affects the work output from the turbines and the feed pump, thereby influencing the efficiency.**Fuel Type and Quality**: The quality and type of fuel used in the boiler will impact the amount of heat that can be transferred to the water, and therefore, has a direct bearing on the efficiency of the plant.**Condenser Cooling System**: The efficiency of the cooling system in the condenser can impact the heat rejection in the cycle and the specific volume of the feedwater entering the pump, thus affecting the cycle efficiency.

**Reheat Rankine Cycle:**A thermodynamic process where steam enters the high-pressure turbine at high temperature and pressure and low-pressure turbine at high temperature and low pressure. The steam is then condensed in a condenser at a low pressure.**Thermodynamics principles:**Utilized to determine the heat and work interactions and the thermal efficiency for the Reheat Rankine Cycle, involving calculations of net power output, thermal efficiency, and heat supplied in the boiler and reheater.**Reheat Rankine Cycle Applications:**Used extensively in power generation, such as steam power generation in thermal power plants, heavy manufacturing industries, marine industry, and geothermal power plants, enhancing efficiency, reducing costs and minimizing environmental impact.**Reheat Rankine Cycle Formula:**Key to calculate efficiency, includes computation of work done in high-pressure turbine, work done in low-pressure turbine, work required for the feed pump, heat input in the boiler, and heat input in the reheater. Total work output and total heat input are calculated meticulously.**Ideal Reheat Rankine Cycle:**Theoretical version of the actual cycle operated under perfect conditions with no losses due to frictions or heat transfers and perfect behaviours of the working fluid. Real-world conditions, however, introduce several factors that aren't accounted for in the perfect setup, leading to differences in performance and efficiency.

The Reheat Rankine Cycle is a modified version of the Rankine cycle, a model used in thermodynamics. It features an additional reheat process to improve its efficiency by reducing heat loss during condensation, thus maximising the energy extracted from the high-pressure steam.

Reheating in the Rankine cycle is done to improve the efficiency of the cycle and to reduce the moisture content at the final stages of the turbine. This helps prevent damage to the turbine blades caused by high moisture content.

Reheating in a Rankine cycle improves efficiency by reducing the moisture content of the steam at the end of the expansion process, thus limiting turbine blade erosion. It also increases the cycle's output work and thermal efficiency, whilst reducing the condenser's heat rejection.

Reheating in the Rankine cycle offers several benefits: it boosts the cycle's efficiency by reducing moisture content in the steam at the final stages of the turbine. This prevents turbine blade corrosion and erosion, increases power output and considerably reduces condenser load.

The Rankine cycle with reheating and superheating involves multiple stages of heat absorption to boost thermal efficiency. Initially, steam is superheated, raising the temperature beyond the saturation point. Then, reheating is executed after partial expansion, leading to reduced moisture content in the final stages of the turbine and boosted cycle efficiency.

What is the Reheat Rankine Cycle in the context of Engineering?

The Reheat Rankine Cycle is a version of the Rankine cycle that involves reheating the working fluid, usually steam after partial expansion, and cooling it before sending it back to the boiler. It improves thermal efficiency of power plants and reduces carbon emissions.

Who was the Reheat Rankine Cycle named after and when was it first described?

The Rankine cycle was named after William John Macquorn Rankine, a Scottish physicist, who first described it in 1859. The concept of reheating was introduced in the 20th century.

What is the Reheat Rankine Cycle, and how does it work in a steam power plant?

The Reheat Rankine Cycle allows steam to expand to an intermediate pressure before it's reheated and expands again to the lowest pressure, reducing moisture content and turbine damage. It involves heat supplied in the boiler, work output of turbines, and work input of the pump.

How can you calculate the net power output and thermal efficiency in the Reheat Rankine Cycle?

The net power output is calculated as the difference between the work output of the turbines and the work input of the pump. Thermal efficiency is the net power output divided by the total heat input (sum of heat supplied in the boiler and reheater).

What is the key role of the Reheat Rankine Cycle in power generation?

The Reheat Rankine Cycle is used in thermal power plants. Here, fuels are burned to heat water and produce steam, which turns turbines and generators to produce electricity. The steam is then reheated in a secondary boiler to add additional energy before passing through a second, low-pressure turbine. This increases efficiency and output.

What are some industrial applications of the Reheat Rankine Cycle?

The Reheat Rankine Cycle finds application in heavy manufacturing industries like steel for increased efficiency and reduced costs. It's also used in the marine industry for efficient power production and in geothermal power plants where geothermal steam is used to turn turbines and generate electricity.

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