Biomass & HE Condensing Boilers
A condensing boiler is a water heating device designed to recover energy normally discharged to the atmosphere through the flue. It can do this through the use of a secondary heat exchanger which most commonly uses residual heat in the flue gas to heat the cooler returning water stream or by having a primary heat exchanger with sufficient surface for condensing to easily take place. The best term for boilers designed to condense on the primary heat exchanger may be "fully condensing."
In a conventional non-condensing boiler, hot combustion gases from the burning of fuel heat water contained in a heat exchanger. The waste gases are still quite hot (180°C - 200°C) and significant heat is lost to the atmosphere.
In a condensing boiler working at peak efficiency, the water vapour produced by the burning fuel in the boiler is condensed back into liquid water. Provided the returning water is sufficiently cool, the steam condenses to liquid water, hence the name "condensing boiler". Some of the extra efficiency of the condensing boiler is due to the cooling of the exhaust gases, but the majority of the energy recovered is from the condensation of the water vapour in the exhaust gases. This releases the latent heat of vaporisation of the water - 2260 kJ/kg (970 btu/pound) of condensate - into the heat exchanger.
The actual operating efficiency of a condensing boiler depends on the temperature of the return water stream: if it is too warm then little condensation takes place and little extra energy is extracted. Because of this, a newer generation of condensing boilers (so-called 'modulating control' boilers) have microprocessor-controlled combustion that modulates the quantity of gas/air fuel mixture which is supplied to the burner using a configurable embedded algorithm that considers outdoor air temperature, water temperatures supplied and returned to the boiler, and time at a specific temperature.
The most sophisticated algorithms learn the building requirements at specific outdoor air temperatures, more successfully returning cool water that condenses the vented exhaust gases and recovering the heat of vaporisation. 'Mod con' units also minimize on-off cycling to increase efficiency. They attempt to supply only the amount of heat to the building that the building loses at a specific outdoor air temperature.
Source : Wikipedia