Dr. RIYADH S. AL-TURAIHI
POWER PLANT
LEC.NO.(5)
____________________________________________________________________
BOILER ACCESSORIES:
Boiler can fail by explosion as a results of either faulty construction or
improper 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
5-Attemperator
2-Air preheater
3-Reheater
4-Economizer
BOILER CALCULATION:
a-Heat transfer required to form steam (kJ/kg)
h2=specific enthalpy of steam formed
h1= specific enthalpy of liquid (water) of F.W.H (kJ/kg)
Heat requried (Qd)= h2- h1
kJ/kg
of steam
b- energy recived from fuel=( ̇ *Cv)
̇ : mass of fuel
Cv:calorific value of fuel ( kJ/kg)
kJ
c-Boiler thermal efficiency (ηth)=
ηth=
̇
̇
̇ : mass of steam (kg/time)
̇ : mass of fuel
(kg/time)
d-Equivalent evaporation of a boiler: this is used for comparison
total energy received by steam = ̇ (h2-h1)
Water enthalpy of evaporation ᴝ2257 kJ/kg at 100oc
let
̇
̇
̇
̇
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Dr. RIYADH S. AL-TURAIHI
POWER PLANT
LEC.NO.(5)
____________________________________________________________________
E.E at 100oc =
=
̇
̇
̇
For superheated steam
E.E =
̇
̇
where
hg:dry sat. at P inside boiler
cps: specific heat of steam leaving the boiler
tsup: superheated temperature
ts: saturated temperature at P inside boiler
hw: 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-economizer
3- 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 -Hlosses)
HEAD LOSSES AND BOILE EFFICIENCY:
Conversion of energy from fuel to potential energy of steam is
accompanied by heat losses.
ηf =
ηf =
HL > Q f
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Dr. RIYADH S. AL-TURAIHI
POWER PLANT
LEC.NO.(5)
____________________________________________________________________
(η) include the following losses:
1- Due to λ<1 (loss O2 available)
2- Due to incomplete combustion
a- Subsidence through grate
b- Coal grains remaining in
c- Coal dust in slag fuel
3- Boiler heat loss
4- 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- 087
ηe=total boiler efficiency
ηe= ηh * 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,O2 ,ammonia ,…
The most impurities are the scale forming ,they are normally dissolved
in 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 .
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Dr. RIYADH S. AL-TURAIHI
POWER PLANT
LEC.NO.(5)
____________________________________________________________________
TWO
GROBECE
IMPROTUTING:
OF
SCALE
FORMING
1- In suspension med ,silica, clay ,ect ..they must be removed outside the
boiler (before entering) by filter and dram.
2- Dissolved impurity:
Salt: a-Calcium
b-magnesium
they must be removed ,two way of water treatment :
1-External cleaning ( by filtration, evaporation,….)
2-Internal cleaning :is needed because of the continuous evaporation of
water hence construction of impurities increase .
METHOD OF TREATEMENT:
1- Evaporation
2- Lame, soda treatment.
3-Zeol treatment (grean sand)
REQUIREMENTS OF A GOOD BOILER:
A good boiler must possess the following qualities:
1. The boiler should be capable to generate steam at the required pressure
and quantity as quickly as possible with minimum fuel consumption.
2. The initial cost, installation cost and the maintenance cost should be as
low as possible.
3. The boiler should be light in weight, and should occupy small floor
area.
4. The boiler must be able to meet the fluctuating demands without
pressure fluctuations.
5. All the parts of the boiler should be easily approachable for cleaning
and inspection.
6. The boiler should have a minimum of joints to avoid leaks which may
occur due to expansion and contraction.
7. The water and flue gas velocities should be high for high heat transfer
rates with minimum pressure drop through the system.
8. There should be no deposition of mud and foreign materials on the
inside surface and soot deposition on the outer surface of the heat
transferring parts.
9. The boiler should conform to the safety regulations as laid down in the
Boiler Act.
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