Performance
The net efficiency of a boiler is defined as the ratio between the useful energy generated and the total energy supplied:
Performance and losses
In other words, it is the ratio between the energy contributed to the system by the fuel and the energy extracted from it in the form of steam. The difference will therefore be all the losses present in the whole. This way of defining efficiency is called the direct method of calculating efficiency.
Another way of defining or calculating the efficiency is by the indirect method, which defines the net boiler efficiency as the theoretical efficiency at 100% minus the losses produced through the flue gases(lG), the energy losses to the surrounding air by radiation and convection(lRC) and the losses due to gaseous unburned fuels in the case of solid fuel boilers (lSF).
This equation for calculating efficiency corresponds to h(N)B in equation 8.6-7 N of UNE-EN 12953 part 11
Losses due to incomplete combustion, such as carbon monoxide (CO), are negligible under normal conditions (usual fuels and usual excess air)
Losses due to water treatment, leaks and electrical losses are not taken into account according to UNE-EN 12953 part 11
Finally, losses due to heat retention and smoldering must be taken into account according to article 8.5.5 of the UNE-EN 12953 part 11 standard.
Radiation and convection losses
Radiation and convection losses(qL) are located at the front door, especially in the interstice between the burner and the cover; at the rear door, through the rear smoke box and at the boiler ferrule through the insulation.
Non-binding diagram for information purposes
It is possible to minimize losses through proper design of the equipment and its insulation. SINCAL boilers offer very low radiation and convection losses, allowing a high efficiency of the steam generation process.
Radiation and convection losses cannot be measured and empirical values are used.
Radiation and convection losses refer to the maximum thermal power.
Losses through gases
In steam and hot water boilers, the main losses occur through the flue gas.
It is possible to quantify these losses through the following equation:
Being:
qA
total losses through flue gas in percentage
A2
specific fuel factor
B
specific fuel factor
KK
Variable that presents qA as a negative value if the dew point is not reached. Necessary for measurements under condensing conditions.
O2
the oxygen content measured in the combustion gases
tA
the flue gas temperature
tL
the reference temperature of the combustion air (25ºC)
If only the carbon dioxide content can be obtained by direct measurement, the oxygen content can be obtained as follows
Being:
O
2
the calculated oxygen content
CO2
the carbon dioxide content measured in the flue gas.
CO2max
the maximum carbon dioxide content depending on the fuel according to the table below
For solid fuels, A2 and B are equal to zero. The formula is simplified by using the factor f for what is called the Siegert formula:
Table to obtain values of A2, B and f according to fuels
