## Direct Method for obtaining boiler efficiency

**Introduction:**

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.

**Description:**

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

**Example**

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