Direct Method for obtaining 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 :
• Quantity of steam generated per hour (Q) in kg/hr.
• Quantity of fuel used per hour (q) in kg/hr.
• The working pressure (in kg/cm2(g)) 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
Where, hg – Enthalpy of saturated steam in kCal/kg of steam
hf – Enthalpy of feed water in kCal/kg of water
Find out the efficiency of the boiler by direct method with the data given below:
– Type of boiler : Coal fired
– Quantity of steam (dry) generated : 8 TPH
– Steam pressure (gauge) / temp : 10 kg/cm2(g)/ 180°C
– Quantity of coal consumed : 1.8 TPH
– Feed water temperature : 85°C
– GCV of coal : 3200 kCal/kg
– Enthalpy of steam at 10 kg/cm2 pressure : 665 kCal/kg (saturated)
– Enthalpy of feed water : 85 kCal/kg
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