2. Introduction
The Rankine cycle is a model that is used to predict the performance of steam
turbine systems. The Rankine cycle is an idealized thermodynamic cycle of a
heat engine that converts heat into mechanical work. The heat is supplied
externally to a closed loop, which usually uses water as the working fluid.
3. Rankine Cycle And Carnot Cycle
The Rankine cycle is sometimes referred to as a practical Carnot cycle as,
when an efficient turbine is used, the TS diagram begins to resemble the
Carnot cycle. The main difference is that heat addition (in the boiler) and
rejection (in the condenser) are isobaric in the Rankine cycle and isothermal in
the Carnot cycle. A pump is used to pressurize the working fluid received from
the condenser as a liquid instead of as a gas.
All of the energy of vaporization of the working fluid,is supplied in the
boiler.(Latent Heat ) All the vaporization energy is rejected from the cycle
through the condenser. But pumping the working fluid through the cycle as a
liquid requires a very small fraction of the energy needed to transport it - -as
compared to compressing the working fluid as a gas in a compressor (as in the
Carnot cycle)
4. Rankine Cycle Process
•Process 1-2: The working fluid is pumped from low to high pressure, being
liquid, the pump requires little input energy.
•Process 2-3: The high pressure liquid enters boiler where it is heated at
constant pressure to become a dry saturated vapor.
•Process 3-4: The dry saturated vapor expands through a turbine, generating
power. • This decreases the temperature and pressure of the vapor, and some
condensation may occur.
•Process 4-1: The wet vapor then enters a condenser where it is condensed at
a constant pressure to become a saturated liquid
5. Advantages And Limitations Of Rankine Cycle
Limitations of Rankine Cycle : The efficiency of a Rankine cycle is limited by the
working fluid. Without the super critical pressure levels for the working fluid, the
temperature range of the cycle is quite small. Turbine entry temperatures are
typically 565°C (the creep limit of stainless steel) and condenser temperatures are
around 30°C. This gives a theoretical Carnot efficiency of about 63% compared with
an actual efficiency of 42% for a modern coal- fired power station. This low turbine
entry temperature (compared with a gas turbine) is why the Rankine cycle is often
used as a bottoming cycle in combined cycle gas turbine power stations.
Rankine Advantage : Advantages of the Rankine cycle over others cycles is during
the compression stage. Relatively little work is required to drive the pump, As the
working fluid is in liquid phase at this point. By condensing the fluid to liquid, the
work required by the pump is only 1% to 3% of the turbine power and contributes to
a much higher efficiency for a real cycle.
6. Real Rankine Cycle
In a real Rankine cycle, the compression by the pump and the expansion in the
turbine are not isentropic. These processes are non-reversible. and entropy
increases during the two processes. (i.e.Non-Isentropic) This increases the power
required by the pump and decreases the power generated by the turbine.
7. Rankine Cycle With Reheat
In this variation, two turbines work in series. The first accepts steam from the boiler
at high pressure. After the steam has passed through the first turbine, it re- enters
the boiler and is reheated before passing through a second, lower pressure turbine.
Among other advantages, this prevents the steam from condensing during its
expansion which can seriously damage the turbine blades, and improves the
efficiency of the cycle, as more of the heat flow into the cycle occurs at higher
temperature.
8. Regenerative Rankine Cycle
The regenerative Rankine cycle is so named because after emerging from the
condenser the working fluid is heated by steam tapped from the hot portion of the
cycle. On the diagram shown, the fluid at 2 is mixed with the fluid at 4 (both at the
same pressure) to end up with the saturated liquid at 7. This is called "direct contact
heating". The Regenerative Rankine cycle (with minor variants) is commonly used in
real power stations. Another variation is where 'bleed steam' from turbine stages is
sent to feed-water heaters to preheat the water on its way from the condenser to the
boiler. These heaters do not mix the input steam and condensate, function as an
ordinary tubular heat exchanger, and are named "closed feed-water heaters". The
regenerative features here effectively raise the nominal cycle heat input temperature,
by reducing the addition of heat from the boiler/fuel source at the relatively low feed-
water temperatures that would exist without regenerative feed-water heating. This
improves the efficiency of the cycle, as more of the heat flow into the cycle occurs at
higher temperature.
9.
10. Heat Rate And Plant Heat Rate
Heat rate is the common measure of system efficiency in a steam power plant. It is defined as
"the energy input to a system, typically in Btu/kWh, divided by the electricity generated, in
kW." Mathematically:
Plant heat rate is a measure of the combined performance of the gas turbine cycle, the steam
turbine cycle, and any other associated auxiliaries.When calculating plant heat rate, the energy
input to the system is the chemical energy in the fuel.
Chemical Energy of Fuel = Total Fuel Used (scf/hr) x Higher Heating Value
(HHV)(BTU/scf)
The power generated is simply the gross or net generation in kW.
If gross generation is used, then the resultant heat rate is the gross unit heat rate.
If net generation is used, then the resultant heat rate is net unit heat rate.
By substituting from the previous equations we get:
11. Heat Exchanger
A heat exchanger is a device used to transfer heat between one or more fluids. The
fluids may be separated by a solid wall to prevent mixing or they may be in direct
contact.
There are three primary classifications of heat exchangers according to their flow
arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at
the same end, and travel in parallel to one another to the other side. In counter-flow
heat exchangers the fluids enter the exchanger from opposite ends. The counter
current design is the most efficient, in that it can transfer the most heat from the heat
(transfer) medium per unit mass due to the fact that the average temperature
difference along any unit length is higher. In a cross-flow heat exchanger, the fluids
travel roughly perpendicular to one another through the exchanger.
12.
13. Logarithmic Mean Temperature Difference
The logarithmic mean temperature difference (also known as log mean
temperature difference or simply by its initialism LMTD) is used to determine the
temperature driving force for heat transfer in flow systems, most notably in heat
exchangers.
We assume that a generic heat exchanger has two ends (which we call "A" and "B")
at which the hot and cold streams enter or exit on either side; then, the LMTD is
defined by the logarithmic mean as follows:
where ΔTA is the temperature difference between the two streams at end A, and ΔTB
is the temperature difference between the two streams at end B. With this definition,
the LMTD can be used to find the exchanged heat in a heat exchanger:
14. HIGH PRESSURE HEATER
High pressure heater Design features ensure high reliability and availability
associated with long term life and low maintenance.
Independent desuperheating zone closures.
Baffle configuration and spacing based on conservative mass and linear velocity
criteria.
Fully enclosed self-venting drains sub-cooling zone.
Liberal sub-cooling zone entrance areas to permit low approach velocities which
prevent flashing of saturated drains.
Internal, centrally located venting arrangement to provide means of continuously
venting condensing zone.
Channel cover configurations for all nozzle layouts.
Stainless steel impingement plates
15.
16. Process in HPH
The feed-water heating process actually takes place in three distinct steps.
First, the de-superheating zone cools the superheated steam to the point that the
steam is saturated.
Next, the condensing zone extracts the Latent Heat from the steam/water mixture to
preheat the boiler feed-water passing through the tube side.
Finally, a drain cooler is incorporated to capture additional energy from the liquid.
The three heating processes all occur within a single heater.
17. High Pressure Turbine
High pressure turbine: The high pressure (HP) turbine is the first main engine
turbine to receive steam from the main steam system. It is designed to efficiently
extract work out of high pressure steam. The HP turbine is a pressure-velocity
compounded, single axial flow, non-condensing impulse turbine.
18. Low Pressure Turbine
The LP turbine is located next to the HP turbine. The LP turbine is a pressure
compounded, either single or dual axial flow, condensing reaction turbine.
19. Steam Condenser
Definition Condenser is a device in which steam is condensed to water at a pressure
less than atmosphere. Condensation can be done by removing heat from exhaust
steam using circulating cooling water A condenser is basically steam to water
exchanger in which heat from exhaust steam is transferred to circulating cooling
water
