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Reasons for Gaps Between Direct and Indirect Efficiency

As we are aware, boiler efficiency can be stated in two different ways, one being direct and another, indirect. Each method has its own advantages and disadvantages.

The direct method gives us more realistic efficiency values but in order to understand where the losses are taking place, indirect efficiency will be more helpful.

Constant topic of contemplation among the steam users is the difference between direct and indirect efficiency.  Studies done at a large number of plants show that there exists a huge gap between direct and indirect efficiency depending on the type of the boiler and boiler operating practices followed. If one tries to find out the reasons behind this gap, we can find out where these unaccounted losses are taking place and think of possible solutions.

This article tries to find out the reasons behind this wide gap between direct and indirect efficiency. First of all, we will have a look at both the methods of calculating boiler efficiency.

Direct Efficiency

In the direct method, efficiency is calculated by dividing energy delivered by the boiler by energy input as fuel, using the equation:

% Efficiency = F (hs - hw) / NCV X f

where F = Steam generation in Kg/hr

hs = enthalpy of steam at operatin pressure in Kcal/Kg

hw = enthalpy of feed water in Kcal/Kg

NCV = Net calorific value of fuel in Kcal/Kg

f = actual fuel flow in Kg/hr

Indirect Efficiency

The method most standards follow is the indirect efficiency calculation method. In this method, each loss is individually calculated, and the sum of these losses is then subtracted from 100 to give efficiency %.

This method has one advantage - since each loss is individually measured, we have quantitative data which we can use to actually reduce an individual loss, thereby increasing efficiency. The losses which occur during the operation of the boiler are Stack losses, Radiation losses and losses due to water and hydrogen in fuel.

So, % Efficiency = 100 - (L1 + L2 + L3+L4)


L1= Stack Loss

L2= Losses due to enthalpy in water vapour in flue gases

L3= Radiation Loss

L4= Unburnt Loss

Why does the difference exist between direct and indirect boiler efficiency?

1. Indirect method is snapshot after boiler is tuned

Indirect efficiency gives us the efficiency of the boiler at a particular time. It does not give us the overall picture of the boiler over a period of time. The boiler is tuned to operate under certain specific conditions, but these conditions are never constant. For e.g the boiler is set to operate at a certain ambient temperature. This temperature is never constant and changes during the course of time. Hence the efficiency of the boiler also changes with the change in conditions.

2. Efficiency is at 100% load condition

The indirect efficiency of any boiler is specified at 100% load. But practically no boiler operates at 100% load. Hence in actual the committed efficiency will never be achieved. In reality the efficiency of the boiler falls down at lower loads. Since boilers rarely operate at full load conditions the actual efficiency figures achieved are always less than that specified in indirect method. This adds to the gap between direct and indirect efficiency.

3. Start up and shut down losses

Start up and shut down losses are mandatory in every boiler. Burners are incorporated with pre- purge and post-purge which actually is a safety measure. During start-up, the burner does not start firing immediately. Instead it purges air for a period of 30 seconds before the actual atomization. The purpose of Pre-purge is to blow away the residual exhaust flue gases that exist in the furnace and the boiler tubes since the boiler is shut down. Similarly a post purge cycle is carried out after the shutdown. These purging cycles blow away hot flue gases which actually is a loss. This problem gets aggravated when boiler is operated on lower loads. Reason: if the loads drop below the turn-down ratio, the burner trips and the boiler shuts down. This would get reduced if the loads are higher and do not fluctuate.

4. Blowdown Losses

Blowdown has to be carried out from the boiler to maintain the correct TDS levels in the boiler. Water contains certain level of TDS. When the water is heated it leads to increased concentration of the TDS which is not good for the boiler. Hence after a certain period of time some quantity of water is removed and fresh water is charged. This is actually a loss as usable heat is being drained out. The fresh charge leads to lowering of water temperature and hence higher fuel quantity will be required. Hence blowdown affects the steam fuel ratio.

5. Ambient temp variation

Each boiler is set to operate under certain temperature conditions. But the ambient temperature is not constant and varies during the day. This leads to the change in steam fuel ratio.

In case of solid fuel fired boilers, following additional factors come in to the picture further reducing the gap between direct and indirect efficiency.

1. Ash & Grit Losses:

Ash is generated when any solid fuel is burnt. The quality of solid fuel is not consistent at all the times which leads to higher losses due to unburnt particles. Thus the fuel consumed is higher for generating a specific amount of steam. The quality of fuel leads to the change in the steam fuel ratio.

2. Inconsistency in fuel firing:

In case of manual solid fuel fired boilers the fuel feeding rate is not even. This depends purely on the experience and the assumption of the boiler operator. This leads to inconsistent fuel feeding and will lead to lower steam fuel ratio. In case of indirect efficiency the fuel firing rate is considered constant and hence it does not show the real picture.

3. Consistency of fuel quality in terms of calorific value:

The calorific value of the fuel is not constant at all times. It changes with the change in season. The moisture / impurities change from time to time leading to an inconsistent steam fuel ratio. This is immediately reflected in direct efficiency because the steam generated to the heat input ratio will vary.

4. Fouling nature of fuels

All the solid fuels have fouling tendency. The flue gases generated will foul the tubes during the course of time. This leads to reduces heat transfer area and will increase the fuel consumption.

From the above points it evident that direct efficiency shows the real picture as against indirect efficiency. Any change in external factors and conditions will have a direct impact on the fuel consumed for generating a specific quantity of steam. In reality the fuel consumption is much higher than what is committed due to the above reasons. Hence the direct efficiency is lesser than indirect efficiency. It is always advisable to aim at reducing the gap between direct and indirect boiler efficiency.