High
pressure Boilers will always have higher efficiency and lower fuel consumption
as compared to low pressure Boilers.
In high pressure Boilers, heat transfer is more
& losses are minimum, which has been proved theoretically in subsequent
steps. Working of steam at higher pressure and higher temperature does more
work than low pressure and low temperature steam.
Following are the major justified reasons for why high-pressure Boilers have higher
efficiency??
Read more>>>>>Boiler and calculations
1-At higher pressure,
steam can be generated at higher temperature.
Higher operating pressure of the Boiler
facilitates higher steam temperature, which does more work than low pressure
Boilers
2-More heat recovery
options in High pressure Boilers.
High-pressure boilers often
incorporate regenerative feedwater heating like HP and LP heaters more
effectively, improving overall efficiency.
3-High pressure
boilers have lower fuel consumption:
Because of high team generation is at higher
pressure, feed water at economiser also enters at higher temperature leading
into lower fuel consumption.
That is sensible heat addition is lesser in
high pressure Boilers
4-High pressure and temperature boilers will
have more enthalpy
5-Stam generated at higher pressure will have
lower specific volume, hence Boiler will be compact in size.
6-Turbine consuming high pressure steam will
generate more power at lesser quantity of steam input, that is lesser specific
steam consumption & fuel consumption. High
enthalpy steam means
more energy per kg, so less steam is needed to produce
the same power output.
7-Heat losses in High pressure Boilers are
generally less as compared to low pressure Boilers
8-Operate at higher temperatures and
pressures, improving the Rankine cycle efficiency, that
is More work output from the same
amount of fuel (better fuel economy).
9-High pressure leads to faster heat transfer
and quicker steam production,
improving responsiveness.
10-Advanced high-pressure systems allow better instrumentation and
automation, improving safety, monitoring, and control.
Let us have discussions through calculation.
Let us discuss the heat content in 1 kg of
steam at pressure 65 Kg/cm2 and 125 Kg/cm2.
For 65 kg/cm2 pressure Boiler, steam
temperature will be 490 deg C (Tsup)
And feed water temperature at economiser inlet
will be around 130 deg C
Therefore,
Steam enthalpy = 812 kcal/kg
Enthalpy of feed water Hfw= 132 kcal/kg
Enthalpy at operating pressure Hf =296 kcal/kg
Saturation temperature of water at steam drum, Ts=
284 deg C
Enthalpy of evaporation Hfg =367 kcal/kg
Therefore heat content in 1 kg of steam Hg = Hf
+ Hfg + 0.5 X (Tsup-Ts) –(1 X Hfw)
Hg = (296 +
367 + 0.5 X (490-284))-(1 X 132) = 634 kcal/kg
For 125 kg/cm2
pressure Boiler, steam temperature will be 550 deg C
And feed water temperature at economiser inlet
will be around 210 deg C (Tsup)
Therefore,
Steam enthalpy = 832 kcal/kg
Enthalpy of feed water Hfw= 212 kcal/kg d
Enthalpy at operating pressure Hf =364 kcal/kg
Saturation temperature of water at steam drum, Ts=
327 deg C
Enthalpy of evaporation Hfg =271 kcal/kg
Therefore heat content in 1 kg of steam Hg = Hf
+ Hfg + 0.5 X (Tsup-Ts) –(1 X Hfw)
Hg =( 364+ 271 + 0.5 X (550-327))-(1 X 212) = 534 kcal/kg
From the above two calculation, it shows that steam generated at higher
pressure needs lesser (534 kcal/kg) heat addition than steam generated at lower
pressure (634 kcal/kg)
Following chart gives the heat required to
generate 1 kg of steam at various pressure and feed water temperature.
|
SL NO. |
BOILER DESCRIPTION |
UOM |
1 |
2 |
3 |
4 |
5 |
6 |
|
1 |
Operating pressure (P) |
Kg/cm2 |
21 |
45 |
67 |
87 |
110 |
121 |
|
2 |
Operating temperature (T) |
Deg C |
350 |
450 |
490 |
515 |
540 |
550 |
|
3 |
Eco. Inlet feed water temperature |
Deg C |
105 |
125 |
125 |
125 |
125 |
125 |
|
4 |
Enthalpy at operating pressure (Hf) |
Kcal/kg |
223 |
271 |
302 |
325 |
349 |
360 |
|
5 |
Enthalpy of evaporation at operating
pressure (Hfg) |
Kcal/kg |
445.3 |
397 |
361 |
331 |
296.9 |
280 |
|
6 |
Saturation temperature (Ts) |
Deg C |
214.07 |
259.00 |
286.10 |
305.41 |
323.85 |
331.66 |
|
7 |
Heat supplied to generate 1 kg of
steam at rated parameters |
Kcal/kg |
631.27 |
638.50 |
639.95 |
635.80 |
628.97 |
624.17 |
