water heater

(at a water boiler – the

water heater only)

STEAM BOILER

evaporato
r

steam super
heater

drum –
steam
separation

blowdown

desalination

supplying
water

superheated
steam

A solubility of some salts in water decreases with temperature
(calcium and magnesium carbonates and sulfates). When the
temperature rises the salts precipitate in a solid form: boiler
scale or sludge . A special chemical water treatment results in
creation of a soft forms of the salts – a sludge. So internal pipe
surface are not being covered with a boiler scale.

boiler blowdown

– removal of salts precipitated in a soft

form of sludge; done with a periodical blow-off of a boiler
water; blowdown from the lower chambers and other spaces
where the sludge can deposit due to a reduced flow velocity;

at water boilers and at steam boilers

Co

mm

en

ts

A solubility of some salts is much higher in a liquid water than in a
steam. If salt does not precipitate during the water heating than it
stays in a liquid water during the water evaporation. As new
amounts of salt are delivered to the boiler with a supplying water,
even if it is very small amount, the salt content in a boiler water
would rise if no action was taken. A high content of salts dissolved
at water results in a foam creation at steam separator.

desalination

– a removal of salts dissolved in a boiler water with

a permanent outflow of a limited amount of the water. As the salts
content in the boiler water is much higher than in supplying water
a desalinating stream is much less then a supplying water flow (1-
2% of it).

at steam boilers

Co

mm

en

ts

an effect of a boiler operation – an increase of an enthalpy of
an agent (mass stream) collected from the boiler (steam or hot
water)

where in – supplying water,

out – steam or out-flowing water (the boiler product)

thermal efficiency of the boiler:

)

(

in

out

out

B

i

i

m

Q





 



LCV

m

i

i

m

LCV

m

Q

fuel

in

out

out

fuel

B

B





















)

(



In the case of boilers operating with vapors condensation at
exhaust gases, when the exhaust gases are cooled down below a
dew-point (saturation-point), an efficiency must be related to HCV

An efficiency of condensing boilers is higher than efficiency of
boilers when exhaust gases are removed with high temperature,
as a heat of condensation of vapors contained in the exhaust
gases is not utilized.

It is possible to operate with a boiler in a condensing mode if it is
supplied with a water of a sufficiently low temperature, to make it
possible to cool down the exhaust gases below a dew-point or
when exhaust gases are deeply cold by cold air supplied to the
boiler.

If efficiency of a condensing mode boiler is related to LCV than
the computed value can be higher than 100%.

HCV

m

Q

fuel

B

B











energy losses in a boiler:

•an outlet loss (chimney loss) – an excess of enthalpy of exhaust
gases being removed from the boiler above the exhaust gases
enthalpy at the ambient temperature

•incomplete combustion loss S

ic

•imperfect combustion loss S

ip

•rest of losses S

r

including:

•heat losses through the casing
•losses at a hot slug (in a case of a boiler fired with a solid
fuels)

• …

•blowdown and desalination loss (if an enthalpy of the blow-
off and the desalination streams is not utilized in e.g.
expanders)







i

i

B

s

1



)

(

_

_

a

eg

out

eg

p

eg

outlet

t

t

c

m

S







 



specific energy losses :

•specific outlet loss

LCV

m

S

s

fuel

i

i









LCV

t

t

c

p

g

l

LCV

m

t

t

c

m

m

m

LCV

m

t

t

c

m

s

a

eg

out

eg

p

Cic

r

fuel

a

eg

out

eg

p

slag

fuel

air

fuel

a

eg

out

eg

p

eg

outlet

)

(

)]

(

1

[

)

(

)

(

)

(

_

_

_

_

_

_





















































mass balance for the boiler (for the heated agent) :

energy balance of the boiler (possible description)

)

&

.

(

)

_

_

_

(

)

_

.

(sup

blowdown

desal

water

hot

or

steam

out

water

pl

in

m

m

m











B

fuel

i

i

fuel

in

out

out

B

LCV

m

S

LCV

m

i

i

m

Q

































)

(

exergy balance of a boiler:

where B

ch

is a chemical exergy of a fuel, S

B

i

– exergy losses.

There is a relation between a LCV and a chemical exergy of the fuel –
empirical relation as below:

exergy efficiency of a boiler:





LCV

b

ch































i

i

B

ch

fuel

a

in

out

in

out

out

in

out

out

B

S

b

m

T

s

s

i

i

m

b

b

m

B











]

)

(

)

[(

)

(

ch

fuel

B

Bb

b

m

B













A steam boiler is supplied with a water of temperature 105°C and
generates a steam with parameters 4.2 MPa, 450°C and a flow 36
t/h. A specific enthalpy of the steam is 3330 kJ/kg, specific entropy
is 6.92 kJ/kgK. Specific entropy of the supplying water is 1.36 kJ/kgK
and an average specific heat is 4.22 kJ/KgK.

The boiler is fired with a hard coal. The hard coal composition is:
g

C

=65%, g

N

=3%, g

H

=2.5%, g

O

=2.5%, g

s

=0,5%, w=14%, p=12.5%.

The slag and fly-ash contains 5% of carbon. An outlet temperature
of exhaust gases is 150°C and an average specific heat of the
exhaust gases is 1.04 kJ/kgK. Rest of losses can be estimated as 3%
(rest of losses apart the outlet loss, imperfect and incomplete
combustion losses). The ambient temperature is 10°C.

An oxigen volumetric content at the exhaust gases is 3%. CO
amount at the exhaust gases in neglectabel

Estimate what are:

1. Excess air coefficent and CO2 share at the exhaust gases

2. specific losses, energy efficiency, and fuel consumption of the

boiler

3. exergy efficiency of the boiler, if β coefficient is 1.06

A boiler is fired with natural gas. The gas volumetric composition is:
r

CH4

=98.14%, r

C2H6

=0.91%, r

N2

=0.84%, r

CO2

=0.11%, and its LHV=35.54

MJ/m

3

st.c.

. A dry exhaust gas examination proved that r

CO(d.e.g.)

≈ 0, and

r

O2(d.e.g.)

=1.1%. The exhaust gas outlet temperature is 130˚C and pressure

1020 hPa. The ambient air temperature is 20˚C, and pressure is 990 hPa.
Relative humidity of the air is 40%.

So called „other” losses from the boiler are estimated to be about 2%. c

p

air

is 1.005 kJ/kg K, and c

p e.g.

=1.04 kJ/kg K.

Estimate:

1. Dew point for the exhaust gas of 1020 hPa preassure

2. The boiler efficiency