CMI IBO Unit 1 Lecture 2

8/13/2019 CMI IBO Unit 1 Lecture 2

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Caribbean Maritime Institute

School of Academic Studies

Industrial Boiler Operation

Unit 2Steam Boilers

Eng. Earl S. Green, Ph.D.


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Properties of Steam (T-h dia)


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Entropy

For a closed thermodynamic system, aquantitative measure of the amount ofthermal energy not available to do work.

A measure of the disorder or

randomness in a closed system


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Enthalphy

Enthalpy is a measure of the total energy of athermodynamic system. It includes the internalenergy, which is the energy required to create

a system, and the amount of energy requiredto make room for it by displacing itsenvironment and establishing its volume andpressure (SIJoule). The total enthalpy, H, ofa system cannot be measured directly. Thus,change in enthalpy, H, is a more usefulquantity than its absolute value.


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Enthalphy(H)

H is the sum of the Eint of the system + theproduct of the P the gas in the system and its V

After a series of rearrangements, and ifpressure if kept constant, we can arrive at the

following equation:

where H is the Hfinalminus Hinitialand q is heat


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Properties & Uses of Superheated Steam(The Mollier H-S Chart)

An enthalpyentropy chart, also known as the HSchartor Mollier diagram, plots the total heat againstentropy, describing the enthalpy of a thermodynamicsystem. A typical chart covers a pressure range of 0.011000 bar, and temperatures up to 800 C. It showsenthalpy H in terms of internal energy U , pressure P,

and volume V using the relationship:

H = U + PV
http://upload.wikimedia.org/wikipedia/commons/2/2e/HS-Wasserdampf_engl.png


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http://upload.wikimedia.org/wikipedia/commons/2/2e/HS-Wasserdampf_engl.png


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Properties of Steam, contd.Design of an Industrial Boiler

- can cope with much higher pressures thanpressure cookers

- welded from thick steel plates(up to 35 mmthick, making P of > 30 bar more possible

- stable, robust design absolutely essential (if aboiler of this type were to collapse, explosive forcescomparable to the explosive power of a ton of

gelignite would be released)


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Sectional drawing of a three pass shell boiler

1

2

3

45

1. Waste gas connectionto chimney

2. Smoke tube pass (2nd pass)

3. 1st Flame tube

4. Burner

5. Smoke tube pass (3rd pass)


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Process control system

Universal Steam Boiler

System

control

Water treatmentmodule

Water servicemodule

Condensateservice module


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- Thermal output of up to 38 MW is possiblefrom a single boiler (corresponds ~ to thepower of 500 average VW Golf cars)

- Up to five boilers can be combinedeconomically

- A boiler filled with water and ready forfunction, can weigh as much as 165 tons (the

~ weight of 120 VW Golfs)


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At full capacity a boiler of this sizeconverts 3 000 litres of fuel oil or a

corresponding amount of natural gas tothermal or process heat every

hour. This would be sufficient to heatmore than 2 000 houses.


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Properties of Steam, contd.

The heart of a boiler system is hotwater or steam, operated with acertain kind of fuel. The boiler heats

up or evaporates the water inside it,which is then transported for end use

via pipes. In the case of hot water,the transported E is aided by pumps.In the case of steam, the transport is

based on inherent pressure.


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Properties of Steam, contd.

The cooled water or the condensedsteam returns to the boiler where it canbe heated again. Loss of water must be

compensated by treated fresh water toavoid corrosion. Flue gases created bycombustion are discharged into theatmosphere through a chimney.Particularly efficient systems additionally

use the residual heat in the flue gases.


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Properties of Steam, contd.Steam Tables

It is the compilation of experimental results of thethermodynamic properties (viz specific volume, internalenergy, sensible heat, latent heat, saturation temperatureetc.) of 1 kg of steam in a tabular column, on a T or P basis.

These tables are useful for steam engineering calculations,as vapours do not obey gas laws. The pressures in the steamtables are in bar (absolute). In case of gauge pressures, theymust be converted in to absolute pressure by adding

atmospheric pressure to them. All the values given in thesteam tables are assumed to be above 0C. If the initialtemperature of water is other than 0C, the enthalpy of steamwill be calculated from the steam tables by deducting the

amount of heat contained initially by the water.


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Properties of Steam, contd.Steam Tables

P ti f St td


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Properties of Steam, contd.(Dryness Fraction)

A measure of quality of wet steam. It is theratio of the mass of dry steam (mg) to the

mass of total wet steam (mg + mf), where mfis the mass of water vapor.

X= mg/(mg + mf)

P ti f St td


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Properties of Steam, contd.(Flash Steam)

Flash steam is a name given to the steam formedfrom hot condensate when the pressure isreduced.

Flash steam is no different from normal steam, itis just a convenient name used to explain howthe steam is formed. Normal or live steam isproduced at a boiler, steam generator, or wasteheat recovery generatorwhereas flash steamoccurs when high pressure / high temperaturecondensate is exposed to a large pressure drop

such as when exiting a steam trap.

P ti f St td


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Properties of Steam, contd.(Flash Steam)

High temperature condensate contains highenergy that cannot remain in liquid form at a

lower pressure because there is more energythan that required to achieve Saturated water atthe lower pressure. The result is that some of the

excess energy causes a % of the condensate toFlash.

P ti & U f S h t d St


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Properties & Uses of Superheated SteamThe Rankine Cycle



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Properties & Uses of Superheated SteamThe Rankine Cycle

The Rankine cycle is the fundamental operating cycle ofall power plants where an operating fluid is continuouslyevaporated and condensed. The selection of operatingfluid depends mainly on the available temperaturerange. The RC is used to predict the performance ofsteam engines. The RC is an idealised thermodynamiccycle of a heat engine that converts heat into

mechanical work. The heat is supplied externally to aclosed loop, which usually uses water as the workingfluid.

The four processes in the Rankine cycle


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Ts diagram of a typical Rankine cycle operating between pressures of 0.06bar and 50bar

Th f i th R ki l
http://en.wikipedia.org/wiki/File:Rankine_cycle_Ts.png


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Process 1-2: The working fluid is pumped from low to highpressure. As the fluid is a liquid at this stage the pump requires

little input energy.

Process 2-3: The high pressure liquid enters a boiler where it isheated at constant pressure by an external heat source to becomea dry saturated vapour. The input energy required can be easily

calculated using Mollier dia or h-s chart or enthalpy - entropychart also known as steam tables.

Process 3-4: The dry saturated vapor expands through aturbine, generating power. This decreases the temperature and

pressure of the vapour, and some condensation may occur. Theoutput in this process can be easily calculated using the Enthalpyentropy chart or the steam tables.

Process 4-1: The wet vapour then enters a condenser where it is

condensed at a constant pressure to become a saturated liquid.


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In an ideal Rankine cycle the pump and turbinewould be isentropic, i.e., the pump and turbinewould generate no entropy and hence maximize thenet work output. Processes 1-2 and 3-4 would be

represented by vertical lines on the T-S diagram andmore closely resemble that of the Carnot cycle.

The Rankine cycle shown here prevents the vapourending up in the superheat region after theexpansion in the turbine, which reduces the energyremoved by the condensers.



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Properties & Uses of Superheated Steam(The Carnot Cycle)

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Properties & Uses of Superheated Steam(Superheated Steam Tables)

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What is ?

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Questions?

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CMI IBO Unit 1 Lecture 2