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 utilized to generate steam. There are two methods of assessing boiler efficiency.
- a)The Direct Method: Where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel.
- b)The Indirect Method: Where the efficiency is the difference between the losses and the energy input.
- a)Direct Method
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
Boiler Efficiency = (Heat Output/Heat Input) * 100
Parameters to be monitored for the calculation of boiler efficiency by direct method are :
- Quantity of steam generated per hour (Q) in kg/hr.
- Quantity of fuel used per hour (q) in kg/hr.
- The working gauge pressure (in kg/cm2) and superheat temperature (°C), if any
- The temperature of feed water (°C)
- Type of fuel and gross calorific value of the fuel (GCV) in kCal/kg of fuel
Boiler efficiency = Q x (hg-hf) ×100 /(q×GCV)
hg – Enthalpy of saturated steam in kCal/kg of steam
hf – Enthalpy of feed water in kCal/kg of water
It should be noted that boiler may not generate 100% saturated dry steam, and there may be some amount of wetness in the steam.
Advantages of Direct Method:
- Plant people can evaluate quickly the efficiency of boilers
- Requires few parameters for computation
- Needs few instruments for monitoring
Disadvantages of Direct Method:
- Does not give clues to the operator as to why efficiency of system is lower
- Does not calculate various losses accountable for various efficiency levels
- b)Indirect Method:
There are reference standards for Boiler Testing at Site using indirect method namely 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 brief procedure for calculating boiler efficiency by indirect method is given below.
The principle losses that occur in a boiler are:
- Loss of heat due to dry flue gas
- Loss of heat due to moisture in fuel and combustion air
- Loss of heat due to combustion of hydrogen
- Loss of heat due to radiation
- Loss of heat due to unburnt
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:
- Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content)
- Percentage of Oxygen or CO2in 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
- Percentage combustible in ash (in case of solid fuels)
- GCV of ash in kCal/kg (in case of solid fuels)
With the help of these parameters the boiler engineers find the losses using standard approaches as specified by ASME and other boiler OEMs. Finally losses can be subtracted from the heat added and hence efficiency can be found.