Direct and Indirect efficiency in boilers

Boiler efficiency is depended on the proper combustion of fuel, effectiveness of Heat transfer, operation and maintenance. Thermal efficiency of boiler is defined as the percentage of heat input that is effectively utilised to generate steam.

There are two methods for boiler efficiency calculation:-

  1. Direct Method.
  2. Indirect Method.
  1. Direct method:

Direct method is quick method for boiler efficiency calculation, In this method, Percentage of total heat output vs heat input is calculated to establish boiler performance.

Boiler Efficiency = Heat output / Heat Input X 100.

Or Boiler Efficiency = Q X (h2 – h1) / (q X GCV) X 100

Where Q = Qty of steam generated.

h2 = Enthalpy of steam at output of boiler.

h1 = Enthalpy of feed water

Heat Input = q X GCV

q = Fuel Qty

GCV = Gross calorific Value

Below Link can be used to calculate direct boiler efficiency:- 

https://www.tlv.com/global/TI/calculator/boiler-efficiency.html 

2. Indirect method:

Indirect efficiency is also known as heat loss efficiency calculation method. In this method, efficiency calculation is done by subtracting heat loss fraction from 100. Standard is ASME PTC-4-1 isused for calculating boiler efficiency by indirect method. The standards do not include blow down loss in the efficiency determination process.

Boiler indirect Efficiency = 100 – (i +ii + iii + iv + v + vi + vii)

Where,

i. Percentage heat loss due to dry flue gas

ii. Percentage heat loss due to evaporation of water formed due to H2 in fuel

 iii. Percentage heat loss due to evaporation of moisture present in fuel

iv. Percentage heat loss due to moisture present in air.

v. Percentage heat loss due to unburnt in fly ash

vi. Percentage heat loss due to unburnt in bottom ash

vii. Percentage heat loss due to radiation and other unaccounted loss

Stepwise Calculation of boiler indirect efficiency:-

Step-1 Data Collection

Following Data needs to be collected :-

Ultimate analysis of fuel (H2, O2, S, C, moisture content (M), ash content)

Percentage of Oxygen or CO2 in the flue gas

 Flue gas temperature in °C (Tf)

Ambient temperature in °C (Ta) & humidity of air in kg/kg of dry air

GCV of fuel in kCal/kg (GCV)

Percentage combustible in ash (in case of solid fuels)

GCV of ash in kCal/kg (in case of solid fuels)

Step-2 Find Theoretical air qty and Actual mass of air supplied

Theoretical air requirement =[(11.6 × C) + {34.8 × (H2 – O2/8)} + (4.35 × S)]/100 kg/kg of fuel

Excess Air supplied (EA) = O2% / (21-O2%) X 100

Actual mass of air supplied/ kg of fuel (AAS) = {1 + EA/100} × theoretical air

Step-3 Calculate different percentage loss

i Percentage heat loss due to dry flue gas

= m X Cp X (Tf – Ta) X 100 / GCV

Where, m = mass of dry flue gas in kg/kg of fuel, Cp = Specific heat of flue gas (0.23 kCal/kg °C), Tf= Flue gas temp Deg C, Ta = Ambient air Temp Deg C, GCV = GCV of fuel

ii. Percentage heat loss due to evaporation of water formed due to H2 in fuel

= 9 X H2 X [584 + Cp X (Tf-Ta)] X 100 / GCV

Where, H2 – kg of H2 in 1 kg of fuel Cp – Specific heat of superheated steam (0.45 kCal/kg °C), Tf= Flue gas temp Deg C, Ta = Ambient air Temp Deg C, GCV = GCV of fuel

 iii. Percentage heat loss due to evaporation of moisture present in fuel

= M X [584 + Cp X (Tf-Ta)] X 100 / GCV

Where, M – kg of moisture in 1kg of fuel , Cp – Specific heat of superheated steam (0.45 kCal/kg)°C 584 is the latent heat corresponding to the partial pressure of water vapour. Tf= Flue gas temp Deg C, Ta = Ambient air Temp Deg C, GCV = GCV of fuel

iv. Percentage heat loss due to moisture present in air.

= AAS X humidity Factor X Cp X (Tf-Ta) X 100 / GCV

Where, AAS = Actual air supplied, Tf= Flue gas temp Deg C, Ta = Ambient air Temp Deg C, GCV = GCV of fuel

v. Percentage heat loss due to unburnt in fly ash

= Total ash collected per Kg of Fuel Burnt X GCV of Fly Ash X 100 / GCV of Fuel

vi. Percentage heat loss due to unburnt in bottom ash

= Total ash collected per Kg of Fuel Burnt X GCV of bottom Ash X 100 / GCV of fuel

vii. Percentage heat loss due to radiation and other unaccounted loss

The actual radiation and convection losses are difficult to assess because of particular emissivity of various surfaces and shape of boiler, air flow pattern etc. In generally it can be assumed between 1% to 2% for smaller boilers and 0.2% to 1 % for higher size boilers of about 500MW.

Step-4 Calculate indirect boiler efficiency

Boiler indirect Efficiency = 100 – (i +ii + iii + iv + v + vi + vii)

Below Excel sheet may be used to calculate indirect boiler efficiency

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