publication_9_30888_310

8/9/2019 publication_9_30888_310

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Dr. RIYADH S. AL-TURAIHI POWER PLANT LEC.NO.(5)

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BOILER ACCESSORIES:Boiler can fail by explosion as a results of either faulty construction orimproper operation .To minimize the danger of failure the following are

added to the boiler:1-Pressure gage for saturated steam pressure recording.2-Relieve valve as a safety precaution to blow off at elevated pressure.3-Water gage glass to indicate the level inside the steam drum.4-Key valve to change water flow rate.

BOILER AUXILIARIES:1-Superheater 2-Air preheater 3-Reheater 4-Economizer

5-Attemperator

BOILER CALCULATION:a-Heat transfer required to form steam (kJ/kg)h2=specific enthalpy of steam formedh1= specific enthalpy of liquid (water) of F.W.H (kJ/kg)Heat requried (Q d)= h 2- h 1 kJ/kg of steam

b- energy recived from fuel=( *Cv) kJ : mass of fuelCv:calorific value of fuel ( kJ/kg)

c-Boiler thermal efficiency (η th)=

ηth=

: mass of steam (kg/time) : mass of fuel (kg/time)

d-Equivalent evaporation of a boiler: this is used for comparisontotal energy received by steam = (h2-h1)Water enthalpy of evaporation 2257 kJ/kg at 100 oc

let

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E.E at 100 oc =

=

For superheated steam

E.E =

wherehg:dry sat. at P inside boilercp s: specific heat of steam leaving the boilertsup: superheated temperature

ts: saturated temperature at P inside boilerhw: Cwater * tentering the economizer Cwater =4.2

BOILER HEAT BALANCE:

1-Usefuel (heat absorbed by steam) =

2-Heat absorbed in boiler fluids

a-preheater b-superheater c- reheater d-economizer3- Heat loss to dry gases.4-Heat loss to moisture content .5-heat loss due to evaporation of moisture.6- heat loss due to co formation.7- heat loss due to unburned fuel .8- heat loss due to exhaust gases.

9- heat loss by radiation.( Hgained -H losses )

HEAD LOSSES AND BOILE EFFICIENCY:Conversion of energy from fuel to potential energy of steam isaccompanied by heat losses.

ηf =

ηf =

HL > Q f

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(η) include the following losses:1- Due to λ<1 (loss O 2 available)2- Due to incomplete combustion

a- Subsidence through grate b- Coal grains remaining inc- Coal dust in slag fuel

3- Boiler heat loss4- Stack loss (7-10 %)5- Heat needed for

a- Drying up of coal b- Feed water heating

6- Mechanical energy required to drive grate ,pumps, fan ,… ηf ~0.87 -0.95

Heat surface ( ηh):

ηh= < 1

ηh: involves heat losses from the hot gases to the steam

ηh=0.61- 087ηe=total boiler efficiency

ηe= η h * ηf = * = =(60 - 90)%

FEED WATER:It is the boiler water that must be clean i.e (free from impurities):

WATER IMPURITIES:-Mechanical: sand , stone, grass, wood,…. - Chemical: salts , acids,… -Gases :CO2,O 2 ,ammonia ,… The most impurities are the scale forming ,they are normally dissolvedin water ,when the water temperature increase ,the solvability decreases,hence deposed and then scale are form problem for scale forming:1- As scale thickness decreases the temperature difference hence denier

of tube rapture.2-As the scale thickness increase the flow area decrease .

8/9/2019 publication_9_30888_310


publication_9_30888_310

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