Calculation of boiler efficiency
Definition of Boiler Efficiency is “The percentage of the total absorption heating value of outlet steam in the total supply heating value.” In other word, it is a rate how the boiler runs efficiently. The actual calculation for the boiler efficiency is the followings;
Boiler Efficiency = (Steam value per hour :kg) × (h2－h1) × 100/(Fuel consumption per hour :kg) × (Fuel low calorific heating value : kJ/kg)(%)
h2: The ratio enthalpy of feed water (kJ/kg)
h1: The ratio enthalpy of steam (kJ/kg)
Boiler efficiency is a combined result of efficiencies of different components of a boiler. A boiler has many sub systems whose efficiency affects the overall boiler efficiency. Couple of efficiencies which finally decide the boiler efficiency are-
1. Combustion efficiency
2. Thermal efficiency
Apart from these efficiencies, there are some other losses which also play a role while deciding the boiler efficiency and hence need to be considered while calculating the boiler efficiency.
The combustion efficiency of a boiler is the indication of burner’s ability to burn fuel. The two parameters which determine the burner efficiency are unburnt fuel quantities in exhaust and excess oxygen levels in the exhaust. As the amount of excess air is increased, the quantity of unburnt fuel in the exhaust decreases. This results in lowering the unburnt fuel losses but elevating the enthalpy losses. Hence, it is quite important to maintain a balance between enthalpy losses and un burnt losses. Combustion efficiency also varies with the fuel being burnt. Combustion efficiency is higher for liquid and gaseous fuels than for solid fuels.
The thermal efficiency of a boiler specifies the effectiveness of the heat exchanger of the boiler which actually transfers the heat energy from fireside to water side. Thermal efficiency is badly affected by scale formation/soot formation on the boiler tubes.
The initial value of boiler efficiency for Boiler is depending on the boiler type, but it is normally 80-88%. In short, 80-88% in the generated heating value after the fuel is burnt by the burner regenerates into the steam heating value, the remain of 12-20% is loss.
Generally, the main factor of the loss is the following.
1) Loss by fuel incomplete combustion (In case of heavy oil, the loss is a little.)
2) Loss by defective combustion gas
3) Loss by Exh. gas heat
4) Radiating loss from boiler surrounding wall
5) The other loss
Direct and Indirect Boiler Efficiency
The overall boiler efficiency depends on many more parameters apart from combustion and thermal efficiencies. These other parameters include ON-OFF losses, radiation losses, convection losses, blow down losses etc. In actual practice, two methods are commonly used to find out boiler efficiency, namely direct method and indirect method of efficiency calculation.
This method calculates boiler efficiency by using the basic efficiency formula-
η=(Energy output)/(Energy input) X 100
In order to calculate boiler efficiency by this method, we divide the total energy output of a boiler by total energy input given to the boiler, multiplied by hundred.
Calculation of direct efficiency-
E= [Q (H-h)/q*GCV]*100
Q= Quantity of steam generated (kg/hr)
H= Enthalpy of steam (Kcal/kg)
h= Enthalpy of water (kcal/kg)
GCV= Gross calorific value of the fuel.
The indirect efficiency of a boiler is calculated by finding out the individual losses taking place in a boiler and then subtracting the sum from 100%. This method involves finding out the magnitudes of all the measurable losses taking place in a boiler by separate measurements. All these losses are added and subtracted from 100% to find out the final efficiency. Blow down valve is kept closed during the procedure. This method should be implemented as per the norms provided in BS845 standards. The losses calculated include stack losses, radiation losses, blowdown losses etc.
Boiler efficiency does not remain fixed and large variations from ideal values take place during the course of operation. Shifting to real time efficiency monitoring can improve the boiler efficiency significantly depending upon the boiler type and actual conditions at site. In nutshell, monitoring and maintaining the boiler efficiency for the overall operational life of the boiler is a must to cut fuel bills and reduce carbon footprint.