Indirect Efficiency Calculation & BS845
The method most standards (including IS8753, BS845 etc.) 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 %.
So, % Efficiency = 100 - (sum of all losses)
This method has one big advantage - since each loss is individually measured, we have quantitative data which we can use to actually reduce an individual loss, thereby increasing efficiency. So this method tells us where we are, and how to get where we want to be.
In a typical oil fired boiler, there are three losses to consider :
1. Loss due to water and hydrogen in fuel:
This is the difference between GCV and NCV of a fuel, and needs to be considered if efficiency is calculated on GCV. Not much can be done to reduce this loss, as it is a function of fuel constituents alone.
2. Stack loss:
Improper combustion is responsible for this loss. In most burners, the manufacturer specifies a minimum level of excess air required to ensure that complete combustion of the fuel takes place. However, typically,excess air levels are higher than this specification, so fuel is being spent to heat air from ambient to flue gas temperature. Further, since the amount of air required depends on amount of fuel (which in turn depends on load on the boiler), it varies continuously, making it that much more difficult to ensure that the excess air levels are kept within specified levels. This loss presents the greatest opportunity for energy conservation schemes, whether manual or through automation. Stack loss can increase if the damper is not correctly positioned, or if the burner nozzles need cleaning, or in the case of oil, even if oil temperature is not controlled.
3. Radiation loss:
This is a function of temperature gradient between the boiler water and the ambient, quality of insulation and surface area of the boiler. It is typically specified by the boiler manufacturer at full load conditions say 1% for a packaged boiler). However, since it is a constant loss, at half load it will be double as a percentage. Accordingly, if steam flow is known, we can work out the instantaneous radiation loss.
In alternative fuels, other losses need to be considered such as ash bed losses in coal fired boilers or combustibles losses in husk fired boilers.
Boiler efficiency and blowdown
Most standards for computation of boiler efficiency, including BS845 and IS8753 are designed for a spot measure of boiler efficiency. Invariably, they ask that the blowdown valve be kept shut throughout the efficiency determination process, and therefore remove blowdown from the perspective. However, depending on feed water quality, boiler blowdown can be between 2 and 5 % of steam generation, and is a huge loss by itself. As utility managers, we are not really as interested in the absolute value of efficiency as per some specified method, but more in the steam/unit fuel figure. Accordingly, the blowdown loss is of utmost importance in reduction of a boiler’s operating cost.