What is the difference between Rankine cycle and ideal reheat cycle?
The difference between the simple ideal Rankine cycle and the ideal reheat cycle is that the expansion process occurs in two stages. In the first stage, the high-pressure stage, the steam expands isentropically to an intermediate pressure and is sent back to the boiler, where it is reheated at constant pressure, to the first inlet temperature.
What is a modified Rankine cycle?
A modified Rankine cycle is used to increase the efficiency of the same Rankine cycle by either reheat or using regenerative cycle. In the reheat cycle the exha If the initial pressure of steam is raised beyond 42 bar the resulting condition of steam after expansion would exceed the safe limit of 12 percentage.
How does a Rankine cycle work in a boiler?
The exhaust from the turbine is converted to liquid in the condenser and is pumped back to the boiler for heating up again. A modified Rankine cycle is used to increase the efficiency of the same Rankine cycle by either reheat or using regenerative cycle. In the reheat cycle the exha
How does superheating the steam affect the Rankine cycle efficiency?
The effect of superheating the steam to a high temperature on the Rankine cycle efficiency is illustrated on a T-s diagram on the left. By superheating the stream to a high temperature (from state 3 to state 3'), the average steam temperature during heat addition can be increased.
What is the purpose of reheat cycle?
The purpose of a reheating cycle is to remove the moisture carried by the steam at the final stages of the expansion process. In this variation, two turbines work in series.
What is reheat and regeneration in Rankine cycle?
The Rankine cycle performance and efficiency can be improved with the reheat cycle as well as by regenerative cycle. It can be improved by increasing the thermodynamic properties of the steam entering into the turbine. At the initial stage, increased pressure results in increased expansion ratio.
What is the main advantage of reheating in a Rankine cycle?
The main practical advantage of reheat (and of superheating) is the decrease in moisture content in the turbine because most of the heat addition in the cycle occurs in the vaporization part of the heat addition process.
What is the process of reheating?
Reheating must be done rapidly, within 2 hours after being removed from refrigeration. Foods reheated in a microwave oven must be reheated so that all parts of the food reach a temperature of at least 165°F. The food must be stirred, covered, and allowed to stand covered for 2 minutes after reheating.
What is reheat and regeneration?
The topic of our discussion, that is regeneration and reheating is a method of increasing efficiency of Rankine cycle. Note that reheat need not give higher efficiency and it is primarily attached to the power plant in order to improve the dryness fraction of the steam obtainable at turbine outlet.
What is regeneration cycle?
Definition of regenerative cycle : a cycle in a steam engine using heat that would ordinarily be lost: such as. a : a multiple-expansion steam-engine cycle in which the receivers are used as successive feed-water heaters.
What is the use of reheat cycle in steam turbine?
The reheat cycle has been developed to take advantage of the increased efficiency with higher pressures and avoid excessive moisture in the low-pressure stages of the turbine. With reheat, the mean temperature of heat addition Tm increases, so efficiency and work output increase, but the steam rate decreases.
What is the unit of heat rate?
The heat rate is the quantity of heat required to produce 1 kWh (also known as Unit) of electricity. Its unit is kCal/kWh (but in certain cases it is kJ/kWh).
What is reheating in gas turbine?
The term "reheat" is used to mean utilizing oxygen remaining in a turbine expansion gas to combust additional fuel, thereby increasing the expansion gas temperature and permitting further, efficient power extraction from that gas.
What is reheating and regeneration of gas turbine?
Reheating is a process utilized to increase the thermal efficiency of the gas and the steam turbine cycles. Regeneration is also a procedure utilized to increase the thermal efficiencies of both the simple gas turbine and the steam turbine cycles.
What are cooling and reheating procedures?
Cooling and reheating are important steps in the preparation of food. During cooling, you must minimize the time food spends in the temperature danger zone. When reheating food, you must make sure it quickly reaches the proper temperature for the right amount of time.
Which processes do the Rankine cycle contain?
What are the Four Process of Rankine CycleProcess 1-2: Isentropic compression in a pump. Here the working fluid is pumped from low to high pressure.2-3: Constant pressure heat addition in the boiler. ... 3-4: Isentropic expansion in a turbine. ... 4-1: Constant pressure heat rejection in a condenser.
What is the effect of reheat and regeneration on efficiency?
Abstract. Reheating in a reheat regenerative steam power cycle increases efficiency by increasing the average temperature of heat reception, but also increases the irreversibility of feed water heaters by raising the temperature of the superheated steam used for the regenerative process.
What is reheating in thermodynamics?
THERMODYNAMICS - CASE STUDY SOLUTION. A reheat process is considered for a simple Rankine cycle to reduce the moisture content at the turbine exit. The pressure at which the steam should be reheated and the net work output are to be determined.
What is regenerative cycle efficiency?
The efficiency of the regenerative cycle may be defined as the ratio of the heat converted into work (in British Thermal Units per pound of steam) to the heat supplied to 1 lb. of steam in the boiler plant.
What are different processes in regenerative Rankine cycle?
In an ideal regenerative Rankine cycle, steam enters the turbine at the boiler pressure and expands isentropically to an intermediate pressure. Some steam is extracted at this state and routed to the feedwater heater, while the remaining steam continues to expand isentropically to the condenser pressure.
Why is a reheat cycle used in steam turbines?
The sole purpose of the reheat cycle is to reduce the moisture content of the steam at the final stages of the expansion process.
How does reheating improve power plant efficiency?
A single reheat process in a modern power plant improves the cycle efficiency by 4 to 5 percent by increasing the average temperature at which heat is transferred to the steam. More reheat processes can be used but, due to the added cost and complexity to modify the cycle, it not practical to use more than two reheat processes.
What is the purpose of reheating steam?
The sole purpose of the reheat cycle is to reduce the moisture content of the steam at the final stages of the expansion process. The difference between the simple ideal Rankine cycle and the ideal reheat cycle is that the expansion process occurs in two stages. In the first stage, the high-pressure stage, the steam expands isentropically ...
How does a steam power plant work?
A steam power plant operates on the ideal reheat Rankine cycle. Steam enters the high-pressure turbine at 8 MPa and 500°C and leaves at 3 MPa. Steam is then reheated at constant pressure to 500°C before it expands to 20 kPa in the low-pressure turbine. Determine the turbine work output, in kJ/kg, and the thermal efficiency of the cycle.
What is the rankine cycle?
In general, the Rankine cycle is an idealized thermodynamic cycle of a constant pressure heat engine that converts part of heat into mechanical work. In this cycle the heat is supplied externally to a closed loop, which usually uses water (in a liquid and vapor phase) as the working fluid.
What are the advantages of the Rankine cycle?
One of the major advantages of the Rankine cycle is that the compression process in the pump takes place on a liquid.
How many processes are involved in the rankine cycle?
In an ideal Rankine cycle, the system executing the cycle undergoes a series of four processes: two isentropic (reversible adiabatic) processes alternated with two isobaric processes:
Why is the Rankine cycle not isothermal?
In contrast to Carnot cycle, the Rankine cycle does not execute isothermal processes, because these must be performed very slowly.
How does isobaric heat rejection work?
Isobaric heat rejection (in a heat exchanger) – In this phase the cycle completes by a constant-pressure process in which heat is rejected from the partially condensed steam. There is heat transfer from the vapor to cooling water flowing in a cooling circuit. The vapor condenses and the temperature of the cooling water increases. The net heat rejected is given by Qre = H4 – H1
What is the common feature of all thermodynamic cycles?
This is the common feature of all thermodynamic cycles. Condenser Pressure.
What is superheated steam?
Superheated vapor or superheated steam is a vapor at a temperature higher than its boiling point at the absolute pressure where the temperature is measured. Reheat allows to deliver more of the heat at a temperature close to the peak of the cycle.
How does the Rankine cycle work?
In the thermal power plant, the Rankine cycle is used to produce electricity. Here, water is pressurized by the pump, takes up the heat and produces vapor in the boiler, and later on, expands in the turbine to generate electricity. Rankine cycle examples thermal power plant. The details of the thermal power plant are already captured in ...
How many processes are in the rankine cycle?
In the ideal Rankine cycle, there are four processes running, we will check each of them and what will happen in each process.
Why is regenerative fluid called regenerative?
Regenerative is named because the fluid emerging from the condenser usually a subcooled liquid, is heating by the steam tapped from the hot portion of the cycle. Rankine cycle with generation example. As you can see in the image above.
Why is reheating important in turbine expansion?
The reheating prevents the vapor from condensing during the expansion, thus reduced the damage in the turbine blades and improves the efficiency of the cycle because there will be more heat flow into the cycle at higher temperatures.
Why is the Rankine cycle irreversible?
The irreversibility’s due to the fluid friction, heat loss to the surroundings the Rankine cycle differs from the ideal one.
How is heat energy supplied to a boiler?
It is defined as a cycle, where heat energy is supplied to the system via the boiler where the fluid typically water is converted to high pressurized steam, and the steam is passed over the turbine and the work is generated.
Why are friction losses neglected in the ideal rankine cycle?
The friction losses and others are neglected in the ideal Rankine cycle to for simplification of the calculation.
How to increase the peak temperature of a Rankine cycle?
The process of superheating is the only way to increase the peak temperatureof the Rankine cycle (and to increase efficiency) without increasing the boiler pressure. This requires the addition of another type of heat exchanger called a superheater, which produces the superheated steam.
Why is reheating necessary?
Reheat allows to deliver more of the heat at a temperature close to the peak of the cycle. This requires the addition of another type of heat exchanger called a reheater. The use of the reheater involves splitting the turbine, i.e. use of a multistage turbine with a reheater. It was observed that more than two stages of reheating are unnecessary, since the next stage increases the cycle efficiency only half as much as the preceding stage.
How does superheating work?
As can be seen also wet steam turbines (e.g. used in nuclear power plants) use superheated steam especially at the inlet of low-pressure stages. Typically most of nuclear power plants operates multi-stage condensing wet steam turbines (the high pressure stage runs on saturated steam). In these turbines the high-pressure stage receives steam (this steam is nearly saturated steam – x = 0.995 – point C at the figure) from a steam generator and exhaust it to moisture separator-reheater (point D). The steam must be reheated or superheated in order to avoid damages that could be caused to blades of steam turbine by low quality steam. High content of water droplets can cause the rapid impingement and erosion of the blades which occurs when condensed water is blasted onto the blades. To prevent this, condensate drains are installed in the steam piping leading to the turbine. The reheater heats the steam (point D) and then the steam is directed to the low-pressure stage of steam turbine, where expands (point E to F). The exhausted steam is at a pressure well below atmospheric, and, as can be seen from the picture, the steam is in a partially condensed state (point F), typically of a quality near 90%, but it is much higher vapor quality, than that it would be without reheat. Accordingly, superheating also tends to alleviate the problem of low vapor quality at the turbine exhaust.
Why do steam turbine blades need to be reheated?
The steam must be reheated or superheatedin order to avoid damages that could be caused to blades of steam turbine by low quality steam. High content of water droplets can cause the rapid impingement and erosion of the blades which occurs when condensed water is blasted onto the blades.
How does a reheater work?
With a reheater, the flow is extracted after a partial expansion (point D), run back through the heat exchanger to heat it back up to the peak temperature (point E), and then passed to the low-pressure turbine. The expansion is then completed in the low-pressure turbine from point E to point F.
What increases steam temperature above saturation temperature?
Superheater – increases the steam temperature above the saturation temperature
What happens to thermal efficiency in a Carnot cycle?
As for the Carnot cycle, the thermal efficiency tends to increase as the average temperature at which energy is added by heat transfer increases. This is the common feature of all thermodynamic cycles.
How does superheating the steam to a high temperature affect the rankine cycle?
By superheating the stream to a high temperature (from state 3 to state 3'), the average steam temperature during heat addition can be increased. The blue area is the net work increased due to superheating the steam to a high temperature.
What is the rankine cycle?
Rankine Cycle. Steam power plants generate most of the electric power in the world. To save fuel, extensive efforts are made to improve the efficiency of the cycle on which steam power plants operate. The general idea is to increase the fluid average temperature during heat addition or decrease the fluid temperature during heat rejection.
How does increasing boiler pressure affect rankine cycle efficiency?
If the operating pressure of the boiler is increased , (process 2-3 to process 2'-3'), then the boiling temperature of the steam raises automatically. For a fixed inlet turbine temperature, the blue area is the net work increased and the gray area is the net work decreased. Also, the moisture content of the steam increases from state 4 to state 4', which is an undesirable side effect. This side effect can be corrected by reheating the steam, and results in the reheat Rankine cycle.
How does lowering the pressure affect the rankine cycle?
So lower the pressure in the condenser, lower the temperature of the steam, which is the heat rejection temperature. The blue area is the net work increases due to the decreasing of the condenser pressure.
What is the blue area of the rankine cycle?
The blue area is the net work increases due to the decreasing of the condenser pressure. 2. Superheating the steam to a high temperature. The effect of superheating the steam to a high temperature on the Rankine cycle efficiency is illustrated on a T-s diagram on the left.
When the number of the reheat stages increases, the expansion and reheat processes approach an isothermal process at the?
When the number of the reheat stages increases, the expansion and reheat processes approach an isothermal process at the maximum temperature. But using more than two stages is not practical.
Does increasing boiler pressure increase the thermal efficiency of the Rankine cycle?
The above section states that increasing the boiler pressure can increase the thermal efficiency of the Rankine cycle, but it also increases the moisture content at the exit of the turbine to an unacceptable level. To correct this side effect, the simple Rankine cycle is modified with a reheat process.
Rankine Cycle Diagram & Components
Ideal Rankine Cycle Processes with T-S & P-V
Rankine Cycle with P-V Diagram
Rankine Cycle Equations Or Formula & Thermal Efficiency
Actual Rankine Cycle
Rankine Cycle with Reheat
- The reheating cycle work is to remove the moisture carried by the steam at the final stages of the expansion process. 1. In case of the reheat cycle, the two turbines work in the series arrangement. 2. The first turbines accept the vapor coming from the boiler at high pressure. 3. After vapor is passed through the first turbine it renters the boile...
Rankine Cycle with Regeneration
Rankine Cycle Examples
Conclusion