According to the type of fuel they can be:
In each case, the type of boiler to be used must be analyzed according to the application and needs.
One of the fundamental parameters when selecting the type of boiler to be used is its useful performance, which is defined as the relationship in the useful power transmitted to the heat transfer fluid and the available thermal power obtained from burning the fuel under nominal operating conditions:
Useful performance (%) = 100 * Pu/Pc
The useful poweris given by the amount of heat in kW transmitted to the heat transfer fluid, it is given by the following expression:
Pu=Q*(T s-Te)*Ce*Pe
Where:
Q: is the flow rate of the heat transfer fluid in l/h.
Ts: is the temperature of the heat transfer fluid at the outlet of the boiler.
Te: is the temperature of the heat transfer fluid at the inlet of the boiler.
Ce: is the specific heat of the heat transfer fluid.
Pe: is the specific weight of the heat transfer fluid.
The useful thermal poweris given by the type of fuel used by the boiler and its consumption, according to the following expression:
Pc=PCI*C
Where:
PCI: is the lower calorific value of the fuel used.
C: is the amount of fuel consumed by the boiler.
In standard boilersthis nominal useful efficiency is reduced when operating at partial loads other than the nominal power of the boiler. This drop in performance is due to the fact that despite being reduced the thermal power transmitted to the heat transfer fluid, the losses through the envelope remain constant, because the interior temperature of the boiler must be maintained even when there is no demand.
Low temperature boilers manage to improve the performance at partial load of diesel boilers. This improvement is achieved because its technology allows an adaptation of the temperature of water production according to demand, without the need to maintain a minimum temperature inside. In this way, the number of starts and stops of the burners is reduced and the consumption of diesel is considerably reduced.
Condensing boilersare the ones with the best performance. Its main difference with the other types of boilers lies in the use of the energy contained in the gases produced by combustion, in particular it uses the condensation energy of water vapor. The temperature of the flue gases in standard boilers is about 150 ºC, condensing boilers reduce this temperature to 65 ºC by taking advantage of the latent heat contained in the combustion gases.
The performance of combustion boilerscan be higher than 100% since, as we have indicated above, the useful performance is referred to the PCI (Lower Calorific Value). PCI is defined as the total energy contained in the combustion products of a fuel.
The following diagram compares the distribution of energy losses in a low-temperature boiler and a condensing boiler in relation to PCI.
When making an optimal choice from the point of view of energy efficiency, both the useful performance and a correct sizing of the nominal operating power must be considered in order to optimize the performance at partial loads thus improving the average seasonal performance.
In centralized installations it is also of vital importance to carry out a correct programming of the control system of the installation.
The IDAE (Institute for Diversification and Energy Saving) provides a tool for access to a database with different types of boilers on the market: http://www.idae.es/Calderas/Consulta.aspx
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