The reheat cycle

The reheat cycle

1. Koya University
Faculty of Engineering
Chemical Engineering Department – 2nd stage
Instructor
Mr. Ahmed A. Maaroof
Prepared by
Safeen Yaseen Jafar
Submitted Date
8 Dec 2020
Thermodynamics
The Reheat Cycle

2. Table of Content
1. Abstract..................................................................................................................... 1
2. Introduction................................................................................................................ 2
3. Body........................................................................................................................... 3-9
3.1. What is the Reheat Cycle in Thermodynamics........................................................ 3
3.2. Reheat Cycle’s Method to Increase the Efficiency of The Rankine Cycle............. 4
3.3. The Use of Reheat Cycle in Steam Turbines......................................................... 5-6
3.4. Why We Use Reheat................................................................................................ 6-9
4. Conclution......................................................................................................................... 10
5. Reference............................................................................................................................ 11

3. 1

Abstract
We know that nowadays the Power is the most energy which we used in our life for everyday and
many of things. Development in this type of energy is important and we need to use best and safe method to
convert a varies of energy to power like solar, mechanical heat...etc. So, in this essay we talking about the
one of power production method by the cycle of thermodynamic process that called as The Reheat Cycle. Its
mean that The Reheat Cycle is the cycle of conversion of the heat energy to Work or Power by increase the
work turbines in this cycle. In basic Rankine cycle, after the isentropic development in turbine, steam is
specifically fed into condenser for condensation process. There are two turbines in the reheat system which is
utilized for enhancing effectiveness. Steam, after extension from high pressure turbine, is sent again to boiler
and heated till it comes to superheated condition. It is then left to extend in low pressure turbine to
accomplish condenser pressure.

4. 2

Introduction
In thermodynamics we have the important subject or we can say by other word the reheat cycle is the
subject which related to thermodynamics. Generally, the reheat cycle is the reversible cycle (Carnot cycle)
and one type of Rankine cycle is the (Rankine cycle with reheat/reheating cycle). this cycle exits in steam
power plant and its purpose to convert the energy (especially heat energy) to power or work.
Today, most of the electricity produced throughout the world is from steam power plants. However,
electricity is being produced by some other power generation sources such as hydropower, gas power, bio-
gas power, solar cells, etc. One newly developed method of electricity generation is the Magneto hydro
dynamic power plant. This paper deals with steam cycles used in power plants. Thermodynamic analysis of
the Rankine cycle has been undertaken to enhance the efficiency and reliability of steam power plants. The
thermodynamic deviations resulting in non-ideal or irreversible functioning of various steam power plant
components have been identified. A comparative study between the Carnot cycle and Rankine cycle
efficiency has been analyzed resulting in the introduction of regeneration in the Rankine cycle. Factors
affecting efficiency of the Rankine cycle have been identified and analyzed for improved working of thermal
power plants. (Kapooria, R K; Kumar, S And Kasana, K S., 2008)
Basic steam power plants operate based on Rankine cycle (with reheating) and can be employed where both
electricity and heat are required. Many techniques are being used to increase the efficiency of the steam
cycle. Most outstanding of these techniques are reheating and/or reducing the irreversibilities. Reheating
increases the efficiency by raising the mean temperature of the heat addition process, by increasing the steam
temperature at the turbine inlet and also by increasing the efficiency of the expansion process in the steam
turbine.1 The irreversibility of the steam generator can be decreased by raising the steam temperature at the
turbine inlet. Another technique to improve the efficiency is to enhance the quality of steam at the condenser
inlet. (G Gonca &H K Kayadelen 2016)
Reheating is a method of improving Rankine cycle efficiency which consists of inter-stage heating of
the expanding steam. After the first stage of expansion which typically reduces initial steam pressure by one-
fourth, the steam is heated up (or close) to the maximum heat source temperature. After the second
expansion stage the steam reaches condensation pressure. Although more heat input is required for reheating,
the efficiency of a reheating cycle is higher because the two-stage turbine develops relatively more work.
(Ibrahim Dincer, C. Z., 2014)

5. 3


Figure (1) This figure shows us the Reheat cycle and T-S Diagram.

What is the Reheat Cycle in Thermodynamics
A process in which the gas or steam is reheated after a partial isentropic expansion to reduce moisture
content. Also known as resuperheating.

Figure (2) in this figure we can see The Reheat Cycle process diagram.

6. 4

Reheat Cycle’s Method to Increase the Efficiency of The Rankine Cycle
As we see above in the process diagram of reheat cycle. Increase the average temperature at which heat is
transferred to the working fluid in the boiler, or decrease the average temperature at which heat is rejected
from the working fluid in the condenser. (Bajpai, P., 2020) In shorter we can focus on these points below:
1. Lowering the condenser Pressure
Lowering the operating pressure of the condenser lowers the temperature at which heat is rejected. The
overall effect of lowering the condenser pressure is an increase in the thermal efficiency of the cycle.
(Bajpai, P., 2020)
2. Superheating the steam to high temperatures
The average temperature at which heat is added to the steam can be increased without increasing the boiler
pressure by superheating the steam to high temperatures. Superheating the steam to higher temperatures has
another very desirable effect: It decreases the moisture content of the steam at the turbine exit. (Bajpai, P.,
2020)
3. Increasing the Boiler pressure
Increasing the operating pressure of the boiler, automatically raises the temperature at which boiling takes
place. This raises the average temperature at which heat is added to the steam and thus raises the thermal
efficiency of the cycle. (Bajpai, P., 2020)

7. 5

The Use of Reheat Cycle in Steam Turbines
The fundamentals of steam power plant to generate the power by steam is the reheating cycle as we
see and talk about their info as long. So, the reheat cycle increases the turbine work and consequently the
total work of the cycle. This is accomplished not by changing the compressor work or the turbine inlet
temperature but instead by dividing the turbine expansion into two or more parts with constant pressure
heating before each expansion. This cycle modification, as shown in Figure (1) below, is known as reheating.
By reasoning similar to that used in connection with intercooling, it can be seen that the thermal efficiency of
a simple cycle is lowered by the addition of reheating, whereas the work output is increased. However, a
combination of regenerator and reheater can increase the thermal efficiency. (Boyce, 2012)
The purpose of the Reheat Cycle Reheat cycle is worked with Rankine cycle to remove the moisture
from the steam. We use the reheating method is to avoid excess moisture which is carried by the steam at
final stages of expansion process in a turbine. The blades of the turbine erode due to excessive moisture. The
efficiency of steam turbine can be improved by decreasing condenser pressure, increasing boiler pressure or
by using the reheat cycle and regenerative cycle. In a reheat cycle, the vapor is expanded in multiple stages
in different turbines. After the first stage of expansion, the vapor is at the super-heated state(state 1 in the fig)
or dry saturated state (To avoid excess moisture). After every stage of expansion, the vapor is sent to the
boiler where it is reheated, followed by expansion in the turbine of next stage and is finally condensed in a
condenser. (Wiki, 2020)


Figure (3) this figure understand us the cycle of steam power plant and it show us the cycle of reheating.

8. 6


Figure (4) in this figure we see the steam turbine (reheat turbine) STF-D650 is GE’s highest-
performing combined-cycle steam turbine and delivers the reliability and availability needed in today’s
demanding energy environment.
Why We Use Reheat
Consider the Rankine Cycle shown in Figure 1. While at first glance this appears to be an attractive
cycle (a thermal efficiency of 40% is quite good), it has two problems considered as a practical device. First,
the pressure ratio assumed across the turbine is unrealistically high (about 1000). Second, the steam coming
out of the turbine (at S3) is over 20% wet, which may damage a real turbine. We could address the pressure
ratio problem by splitting the turbine into two sequential turbines, each with a PR of about 32. The low
quality of the steam at the turbine outlet is a more interesting problem, and one that can occur in many vapor
powers cycles. When the steam at the outlet of a turbine becomes wet, the liquid present is in the form of
water droplets. If the steam is not very wet, the amount of water is small and the droplets are not too
troublesome to the operation of the turbine. This is because the liquid drops are formed by condensation
from the steam to form a kind of fog. Like atmospheric fog, this fog contains extremely small drops and the
drops move with almost the same velocity as the surrounding steam.
However, as the quality of the steam decreases, the concentration of these water droplets increases.
The turbine blades move rapidly through the steam and tend to collect the water droplets. This is because the
denser water droplets do not move with quite the same velocity as the steam, and so get "scooped up" by the
blade. Once on the blade, the water forms a film and runs to the back of the blade. Here the water is re-
entrained into the steam. But this droplet formation method is completely different to the original one. The
droplets are a completely different size: they are much larger and now no longer follow the steam flow.
When these large drops impact with the turbine blades they can do much damage and certainly impair the
efficiency of the turbine. It is often considered unwise to allow steam with qualities of less than around 85%
to 90% to remain in the turbine. (Peter B. Whalley, Kenneth D. Forbus, M. E. Brokowski., 1997)

9. 7

Thermodynamic Analysis of Reheat Cycle Steam Power Plants
A thermodynamic analysis of a Rankine cycle reheat steam power plant is conducted, in terms of the
first law of thermodynamic analysis (i.e. energy analysis) and the second law analysis (i.e. exergy analysis),
using a spreadsheet calculation technique. The energy and exergy efficiencies are studied as 120 cases for
different system parameters such as boiler temperature, boiler pressure, mass fraction ratio and work output.
The temperature and pressure values are selected in the range between 400 and 590°C, and 10 and 15 MPa,
being consistent with the actual values. The calculated energy and exergy efficiencies are compared with the
actual data and the literature work, and good agreement is found. The possibilities to further improve the
plant efficiency and hence reduce the inefficiencies are identified and exploited. The results show how
exergy analysis can help to make optimum design decisions in a logical manner. (John Wiley & Sons, 2001)

Figure (5) If we see the figure above we can see Steam Power Plant, Reheat Cycle process and their
connection in the plant.

10. 8

Reheat Cycle in Thermal Power Plant
The efficiency of the Reheat Cycle can be improved by increasing the pressure and temperature of the
steam entering into the turbine. As the initial pressure increases, the expansion ratio of the turbine also
increases and the steam becomes quite wet at the end of expansion. This is not desirable as increase
in moisture content of the steam causes corrosion of the turbine blades and increases the losses. This reduces
the nozzle and blade efficiency. In the reheat cycle, the steam is extracted from the suitable point in the turbine
and is reheated with the help of the flue gases in the boiler furnace. The main purpose of reheating is to increase
the dryness of the steam passing through the lower stages of the turbine. The dryness fraction coming out of
turbine should not be less than 0.88. The increase in the thermal efficiency due to the reheat depends upon the
ratio of the reheat pressure to original pressure of the steam. The reheat pressure is generally kept within 20%
of the initial pressure of the steam. The efficiency of the reheat cycle may be less than the Rankine efficiency
if the reheat is used at low pressure. (Sostenes, 2015)
Advantages:
1. The reheating reduces 4 to 5% fuel consumption with a corresponding reduction in the fuel handling
2. The reheat cycle reduces the steam flow of 15 to 20% with corresponding reduction in the boiler,
turbine and feed handling equipment capacities. This also reduces the pumping power in that
proportion.
3. The wetness of the exhaust steam with the reheat cycle is reduced to 50% of the Rankine cycle with a
corresponding reduction in the exhaust blade erosion.
4. Lower steam pressures and temperatures and less costly materials can be used to obtain the required
thermal performance. (Sostenes, 2015)
Disadvantages:
1. The cost of the extra pipes, equipment and controls make the cycle more expensive than the normal
Rankine cycle.
2. The greater floor space is reduced to accommodate the longer turbine and reheat piping.
3. The complexity of the operation and control increases with the adoption of the reheat cycle in thermal
powerplant.
4. All the lighter loads, the steam passing through the last blade rows to the condenser are seriously
super-heated if the same reheat is maintained. Feed water is sometimes sprayed into the low-pressure
cylinders also steam flows as a precaution against over-heating of blades.
5. increases the size of condenser based on unit mass flow of steam due to improved quality of steam at
exhaust from L.P turbine. (Ijesrt Journal, 2015).

11. 9


Figure (6) Above Diagram show us Thermal Power Plant, Reheat Cycle process and their connection
in the plant.
Use of Reheat Cycle and Regenerative Cycle Together
We examine the performance of regenerative-reheat power plants in terms of irreversibility analysis.
The reduction in the irreversible losses with the addition of backward, cascade-type feedwater heaters and/or
a reheat option are compared with a conventional energy-balance approach. The results indicate that most of
the irreversible losses occur in the boiler and that these losses are significantly reduced by the incorporation
of feedwater heating. The incorporation of feedwater heating results in a reduction of the total irreversibility
rate of the cycle by 18%. The corresponding improvement in efficiency is 12%. These two figures are
augmented to 24 and 14%, respectively, by the incorporation of reheat in addition to regeneration. (Habib
and Zubair, 1992)

12. 10

Conclusion
As we talked about in this essay. The most thermodynamics cycles are useful and they are use in
many of thermodynamics processes, and plants. Also, they have a lot of advantages to increase efficiency of
somethings such as generation of energy like work or power. So, we focused on one of the cycle processes or
irreversible processes in thermodynamics like this cycle (Reheat Cycle). this cycle as we talked about before
has many of other points, advantages and uses in plants or our daily life. We can conclude our words in these
points below:
➢ Reheat cycle is one of the cycling processes in thermodynamics like all reversible processes in
thermodynamics.
➢ One of the Rankine Cycle types is Reheat Cycle.
➢ In reheating process, the efficient of generation of power usually is increased.
➢ The reheat cycle can be used in steam and thermal power plants as the process of thermodynamics.

13. 11

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