The performance parameters of boiler, like efficiency and evaporation ratio reduces with time due to poor combustion, heat transfer surface fouling and poor operation and maintenance. Even for a new boiler, reasons such as deteriorating fuel quality, water quality etc. can result in poor boiler performance. Boiler efficiency tests help us to find out the deviation of boiler efficiency from the best efficiency and target problem area for corrective action.
Thermal efficiency of boiler is defined as the percentage of heat input that is effectively utilised to generate steam. There are two methods of assessing boiler efficiency.
This is also known as ‘input-output method’ due to the fact that it needs only the useful output (steam) and the heat input (i.e. fuel) for evaluating the efficiency. This efficiency can be evaluated using the formula
Parameters to be monitored for the calculation of boiler efficiency by direct method are :
Where, hg – Enthalpy of saturated steam in kcal/kg of steam
hf - Enthalpy of feed water in kcal/kg of water
Advantages of direct method:
Disadvantages of direct method:
There are reference standards for Boiler Testing at Site using indirect methodnamely British Standard, BS 845: 1987 and USA Standard is ‘ASME PTC-4-1 Power Test Code Steam Generating Units’.
Indirect method is also called as heat loss method. The efficiency can be arrived at, by subtracting the heat loss fractions from 100. The standards do not include blow down loss in the efficiency determination process. A detailed procedure for calculating boiler efficiency by indirect method is given below. However, it may be noted that the practicing energy mangers in industries prefer simpler calculation procedures.
The principle losses that occur in a boiler are:
In the above, loss due to moisture in fuel and the loss due to combustion of hydrogen are dependent on the fuel, and cannot be controlled by design.
The data required for calculation of boiler efficiency using indirect method are:
Solution :
Actual mass of air supplied/ kg of fuel (AAS) = {1 + EA/100} x theoretical air
m = mass of dry flue gas in kg/kg of fuel
m = Combustion products from fuel: CO2+ SO2 + Nitrogen in fuel + Nitrogen in the actual mass of air supplied + O2 in flue gas. (H2O/Water vapour in the flue gas should not be considered)
Cp = Specific heat of flue gas (0.23 kcal/kg0C)
Where, H2 – kg of H2 in 1 kg of fuel
Cp – Specific heat of superheated steam (0.45 kcal/kg0C)
Where, M – kg of moisture in 1kg of fuel
Cp – Specific heat of superheated steam (0.45 kcal/kg)0C
* 584 is the latent heat corresponding to the partial pressure of water vapour.
Cp – Specific heat of superheated steam (0.45 kcal/kg0C)
The actual radiation and convection losses are difficult to assess because of particular emissivity of various surfaces, its inclination, air flow pattern etc. In a relatively small boiler, with a capacity of 10 MW, the radiation and unaccounted losses could amount to between 1% and 2% of the gross calorific value of the fuel, while in a 500 MW boiler, values between 0.2% to 1% are typical. The loss may be assumed appropriately depending on the surface condition.
Efficiency of boiler (η) = 100 – (i + ii + iii + iv + v + vi + vii)