skmm 2423 applied thermodynamics

SKMM 2423 Applied Thermodynamics

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Md. Mizanur RahmanPhD, Chartered Energy Engineer, CEng, MEI

School of Mechanical Engineering Universiti Teknologi Malaysia UTMOffice: C23-228 Email: [email protected]

mailto:[email protected]

Course outline

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Chapter 1 Steam Cycle Chapter 2 Gas Turbine Cycle Chapter 3 Positive displacement machinesChapter 4 Reciprocating internal-combustion engines

Chapter 5 Refrigeration and heat pumpsChapter 6 Psychrometry and air-conditioning processes

Test 1 Chapter 1: Steam CycleTest 2 Chapter 2: Gas turbine cycle Test 3 Chapter 3: CompressorFinal exam Chapter 1-6

Teaching Methodology :Lectures, in-class exercise, quizzes, assignments and tutorials

Assessment :

1. Assignments.................... …. 10%2. Project.................................................. 5%3. Test 1.................................................... 15% 4. Test 2.................................................... 15% 5. Test 3………………………………… 15%6. Final examination................................... 40%Total 100%

Textbook:

1) Eastop & McConkey, "Applied Thermodynamics for Engineering

Technologists", 5th Edition, Prentice Hall (Pearson Education), Essex, England, 1993

2) Y.A. Cengel& M.A. Boles, "Thermodynamics: An Engineering Approach", 6th Edition, McGraw-Hill Inc., New York, 2007

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Chapter 1

STEAM CYCLES

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Various type of Steam Power Plant

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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: 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

Basic

Ideal/actualWith superheat

Ideal/actualReheat cycle

Ideal/actualRegenerative

cycle with open-type feedwater

heater

Ideal/actual

Regenerative cycle with

closed-type feedwater

heater

Ideal/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

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The Rankine Cycle

Basic





heater

Ideal/actual



heater

Ideal/actual


*Thermodynamic heat engine ideally working in a Carnot cycle,

any comment ?

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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

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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

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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

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

iv) Isentropic efficiency (is)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

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Example 8.1

A steam power plant operates between a boiler pressure of 42 bar and a condenser pressure of 0.035 bar. Calculate for these limits the thermal efficiency, the work ratio and the specific steam consumption:

a) For a Carnot cycle using wet steam

b) For a Rankine cycle with dry saturated steam at entry to the turbine

c) For a Rankine cycle with the turbine isentropic efficiency of 80%.

Sketch the cycle on a T-s diagram

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The Rankine Cycle

Basic





heater

Ideal/actual



heater

Ideal/actual


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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

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Rankine cycle with & without Superheat

Basic Rankine Cycle Rankine Cycle with superheat

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Example 8.2A steam power plant operates between a boiler pressure of 42 bar and a condenser pressure of 0.035 bar.

(Reconsider the above vapour power cycle of Example 8.1). Calculate it’s

thermal efficiency and s.s.c if the steam exiting the boiler is heated to 500C

before entering the turbine. Assume the pump work is small and can be

neglected.

Sketch the cycle on a T-s diagram

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The Rankine Cycle

Basic





heater

Ideal/actual



heater

Ideal/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

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✓ 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 (Cengel example 10.4)

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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

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The Rankine Cycle

Basic




cycle with open-typefeedwater

heater

Ideal/actual


closed-typefeedwater

heater

Ideal/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.

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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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Types 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

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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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(Mixing chamber calc.)

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---------------------------------------------------------

-------------------------------------------

Note: y is chosen so that the condition of

point 6 is saturated liquid

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Example 4 (Cengel 10.44)

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(Heat exchanger calc.)

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Eastop

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Example 5 (Cengel 10.45)

A steam power plant operates on an ideal regenerative Rankine cycle.Steam enters the turbine at 6 MPa and 450C and is condensed in thecondenser at 20 kPa. Steam is extracted from the turbine at 0.4 MPa toheat the feedwater in a closed feed-water heater. Assume that the feed-water leaves the heater at the condensation temperature of theextracted steam and that the extracted steam leaves the heater as asaturated liquid and is pumped to the line carrying the feed-water.i) Show the schematic system and T-s diagramii) Calculate the net work output per kg of steamiii) Thermal efficiency

Answers: (a) 1006 kJ/kg, (b) 37.3%

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Exercise Cengel 10.49

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A steam power plant operates on an ideal regenerative Rankine cycle with oneopen-type feedwater heater. Steam enters the turbine at a pressure of 8 MPaand a temperature of 450 oC. A small amount of the steam is extracted (bledoff) from the turbine at a pressure of 300 kPa for heating the feedwater in thefeedwater heater. The remaining steam expands to the condenser operating ata pressure of 10 kPa. Water leaves the feedwater heater as a saturated liquid.All expansion and pumping processes are isentropic, and there are no pressuredrops in the boiler, feedwater heater and condenser. Neglect the work input tothe pumps.•Sketch the schematic diagram of the plant and the cycle on a temperature-entropy (T-s) diagram with respect to the saturation lines.•Determine the mass of steam extracted (bled off) from the turbine for each kgof steam flowing through the boiler, in kg.•If the mass flow rate of steam through the boiler is 20 kg/s, determine the netpower output of the plant, in kW.•Determine the thermal efficiency of the plant, in %.•Determine the specific steam consumption, in kg/kW.h.

Test 1

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Cengel 10.34

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Simplified Model for Analysis

A – Energy conversion process occursB – Energy required to vaporize the liquid waterC – Cooling water circuitD – Electric power generation

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A – Energy conversion process occursB – Energy required to vaporize the liquid waterC – Cooling water circuitD – Electric power generation

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1 → 2: Isentropic compression in a pump2 → 3: Constant-pressure heat addition in a boiler3 → 4: Isentropic expansion in a turbine4 → 1: Constant-pressure heat extraction in a condenser

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The component of the Rankine cycle that leads to relatively low cycle efficiency is:1.The pump2.The boiler3.The turbine4.The condenser

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The Regenerative Cycle: 1 open-type FWH

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The Regenerative Cycle: 1 Closed-type FWH


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The Regenerative Cycle: 2 Closed-type FWH

skmm 2423 applied thermodynamics

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