Why do high pressure Boilers have higher efficiency??

 

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??

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

 

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