Steam Cycles Lecturemazlan/?download=Steam Cycles - Lecture...Reheat cycle Ideal/actual Regenerative...

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FKM UTM FKM UTM APPLIED THERMODYNAMICS SKMM 24231

Chapter 1

STEAM CYCLES

Assoc. Prof. Dr. Mazlan Abdul WahidFaculty of Mechanical Engineering

Universiti Teknologi Malaysiawww.fkm.utm.my/~mazlan


Chapter 1STEAM CYCLES

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Chapter Objectives� To carry out 1st law and 2nd law

thermodynamic analysis on a vapour power plant in which the working fluid is alternatively vaporized and condensed as it completes a thermodynamic cycle

Steam/Vapour Power Plant

� Is a thermodynamic heat engine used to produce mechanical power output from energy sources such as fossil fuel, nuclear.


Various type of Steam Power Plant

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Simplified Model for Analysis

A – Energy conversion process occurs

B – Energy required to vaporize the liquid water

C – Cooling water circuit

D – Electric power generation



A – Energy conversion process occurs

B – Energy required to vaporize the liquid water

C – Cooling water circuit

D – Electric power generation

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Basic Components in a Steam Cycles

1. Boiler: to transform liquid water into vapour (steam) of high pressure and temperature.

2. Turbine-Generator: to transform kinetic energy of the vapour into mechanical power (rotating shaft). The mechanical power is used to drive an electric generator.

3. Condenser: to cool off the wet vapour exiting the turbine and transform it back into the liquid water

4. Feed-water pump: to deliver the water exiting the condenser back into the boiler, thus completing one thermodynamic cycle

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The Rankine Cycle

BasicIdeal/actual

With superheatIdeal/actual

Reheat cycleIdeal/actual

Regenerative cycle with open-type feedwater

heaterIdeal/actual

Regenerative cycle with

closed-type feedwater

heaterIdeal/actual

Cycle for Vapour Power Plant

*Thermodynamic heat engine ideally working in a Carnot cycle, any comment ?

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The Carnot Vapour Cycle

T-s diagram of two Carnot vapor cycles.

The Carnot cycle is the most efficient cycle operating between two specified temperature limits but it is not a suitable model for power cycles. Because:Process 1-2 Limiting the heat transfer processes to two-phase systems severely limits the maximum temperature that can be used in the cycle (374°C for water)Process 2-3 The turbine cannot handle steam with a high moisture content because of the impingement of liquid droplets on the turbine blades causing erosion and wear.Process 4-1 It is not practical to design a compressor that handles two phases.The cycle in (b) is not suitable since it requires isentropic compression to extremely high pressures and isothermal heat transfer at variable pressures.

1-2 isothermal heat addition in a boiler 2-3 isentropic expansion in a turbine 3-4 isothermal heat rejection in a condenser4-1 isentropic compression in a compressor


The Rankine Cycle

BasicIdeal/actual




heaterIdeal/actual



heaterIdeal/actual



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Rankine Cycle: The Ideal Cycle for Vapour Power Cycles� Many of the impracticalities associated with the Carnot cycle can be eliminated by superheating the steam in the boiler and condensing it completely in the condenser.

� The cycle that results is the Rankine cycle, which is the ideal cycle for vapor power plants. The ideal Rankine cycle does not involve any internal irreversibilities.

The simple ideal Rankine cycle

1-2 Isentropic expansion in a turbine

2-3 Constant pressure heat rejection in a condenser

3-4 Isentropic compression in a pump

4-5-1 Constant pressure heat addition in a boiler


Energy Analysis of Basic Rankine Cycle (ideal)

� The steam flows round the cycle and each process is analyzed using steady flow energy equation. Using energy balance for a steady flow system

� For single stream (one-inlet-one-exit) systems, mass flow rate remains constant.

� If kinetic and potential energy are negligible, the energy equation becomes

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1) The cycle analysis

i) BoilerSince there is no work interaction between the working fluid and surrounding, W=0. Thus, heat addition to the working fluid

kJ/kg

ii) TurbineSince the expansion process is assumed to be isentropic (reversible adiabatic), then Q=0. Thus, amount of work produced by turbine

kJ/kg



1) The cycle analysis

iii) CondenserNo work interaction between the working fluid and surrounding, W=0. Heat rejected from working fluid to the cooling water

kJ/kg

ii) Feed-water pumpSince the pumping process is assumed to be isentropic ,then Q=0. Thus, amount of work required by feed-water pump

kJ/kg

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2) Performance of steam plant

i) Specific steam consumption (ssc)Define as the steam flow rate in kg/hr required to develop 1 kW of power output. The lower the ssc the more compact the steam plant

kg/kW.s

kg/kW.hr

ii) Work ratio (wr)Define as the ratio of the net work produced by the plant to the work produced by the turbine




iii) Thermal efficiency (ηth)Defined as the ratio of net work produced by the plant to the amount of heat added to the working fluid

iv) Isentropic efficiency (ηis)The actual expansion and pumping processes are adiabatic but nor reversible. Thus, they are not isentropic.

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v) Back work ratioDefined as the ratio of the work supplied to the feed-water pump to the work produced by turbine

iv) Efficiency ratio


Example 1

A steam power plant operates between a

boiler pressure of 50 bar and a condenser

pressure of 0.03 bar. Calculate for these

limits the thermal efficiency, the work ratio

and the specific steam consumption:

a) For a Rankine cycle with dry saturated

steam at entry to the turbine

b) For a Rankine cycle with the turbine

isentropic efficiency of 85%.

Sketch the cycle on a T-s diagram

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The Rankine Cycle

BasicIdeal/actual




heaterIdeal/actual



heaterIdeal/actual




Rankine cycle with Superheat

Why superheat ?

� Improvement in the basic Rankine cycle

� Steam temperature at inlet to the turbine is increased at boiler pressure, thus increasing the average temperature of heat addition.

� Increase the cycle efficiency

� Steam exits the turbine is more dry

� Specific steam consumption drops

The technique

� The saturated steam exiting the boiler is passed through a second bank of smaller tubes located within the boiler, heated by the hot gas from the furnace

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Rankine cycle with Superheat

Degree of superheat


Rankine cycle with & without Superheat

Basic Rankine Cycle Rankine Cycle with superheat

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Example 2

Reconsider the vapour power cycle of Example 1. Calculate it’s

thermal efficiency and s.s.c if the steam exiting the boiler is heated to

500°C before entering the turbine. Assume the pump work is small

and can be neglected.

Sketch the cycle on a T-s diagram


The Rankine Cycle

BasicIdeal/actual




heaterIdeal/actual



heaterIdeal/actual



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Rankine cycle with Reheating

� Improvement in the superheat Rankine cycle

� The average heat addition is increased in another way

� Usually, steam is reheated to the inlet temperature of the high-pressure turbine

� The dryness fraction of the steam exiting the turbine stages is further increased, which is the desired effect

� Specific steam consumption is improved (decrease)

� The steam is reheated at constant pressure



� Improvement in the superheat Rankine cycle

� The average heat addition is increased in another way

� Usually, steam is reheated to the inlet temperature of the high-pressure turbine

� The dryness fraction of the steam exiting the turbine stages is further increased, which is the desired effect

� Specific steam consumption is improved (decrease)

� The steam is reheated at constant pressure

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The cycle analysis

i) Heat input ……….. ?

ii) Work output ………. ?

iii) Work input ………… ?

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Example 3


The enthalpy-entropy (h-s) chart

� Also known as Mollier diagram or h-s diagram

� The chart contains a series of constant temperature lines, a series of constant pressure lines, a series of constant quality lines and a series of constant superheat lines

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Mollier diagram


The Rankine Cycle

BasicIdeal/actual




heaterIdeal/actual



heaterIdeal/actual



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The Regenerative Cycle

What is regeneration process ?

�In a regenerative cycle, the feed-water is preheated in a feed-water heater (FWH), using some amount of steam bled off the turbine, before it is delivered back into the boiler.

�The preheating process occurs in the FWH at a constant pressure. The steam required for heating the feed-water is bled off the turbine at certain bleeding pressure, Pbleed.



Purpose of regeneration process

�The main purpose of regeneration process is to increase the thermal efficiency

�If the feed-water is preheated before entering the boiler, then less heat will be required to transform the feed-water into steam, in the boiler

�As a result, thermal efficiency of the plant increases

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The Regenerative CycleTypes of Feed-water Heater (FWH)

There are 2 types of feed-water heater; an open-type and a closed-type.

1) Open-type Feed-water heater

� An open-type FWH is basically a “mixing chamber”

� The feed-water is preheated by direct mixing with the steam extracted from the turbine.

� The plant can use more than one open feed-water heater

� Each open-type FWH requires one extra pump

2) Closed-type Feed-water heater

� An closed-type FWH is basically a “heat exchanger”

� The feed-water does not mix freely with the bled off steam, hence both fluids can be at different pressure.

� The condensate exiting the closed-type is throttled back into condenser and mix with the feed-water in the condenser


The Regenerative Cycle: Open-type FWH

Ideal regenerative cycle using open-type FWH

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The Regenerative Cycle: Closed-type FWH

Ideal regenerative cycle using closed-type FWH



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Ideal regenerative cycle using closed-type FWH

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