Βiomass boilers in accordance with EN 303-5

Τhe European standard EN 303-5 is designated to determine the design, construction, environmental and operational requirements on heating boilers with solid fuel. Among other things, in the category "solid fuel" includes biomass.
The model, originally adopted in 1998 apply where solid fuel boilers (including biomass boilers) with a nominal heat output up to 300 kW (approximately equal to 1.000.000 BTU / h), with a maximum allowable operating pressure up to 6 bar and maximum temperature for water to 100 ° C. Facilities such as fireplaces or pellet stoves are not included in the scope of the standard EN 303-5. It should also be noted that a revised version of EN303-5 is expected, logically, by the end of 2012.
In accordance with EN303-5 two types of construction materials are suitable for the manufacture of heating boilers: steel and cast iron. According to the type of fuel used in the boiler sets a minimum wall thickness of each part of the boiler (ie, the metal sheet). The nominal heat output also affects the thickness of the wall. Additionally, the wall thickness varies from one part of the boiler from the other. Consequently, different thickness required for walls which come into contact with the water in relation to the walls of the feed and the combustion chamber in contact with both water and fire.

The boiler efficiency is defined as the ratio of useful heat given to the water through the boiler to the thermal power. Apparently the reason he always takes values ​​between 0 and 1.
The minimum performance values ​​of a boiler in relation to the class defined by the following equations:For Class 1, the = 47 + 6 · logQ,For Class 2, the = 57 + 6 · logQ,For Class 3, the = 67 + 6 · logQ,
where is the boiler efficiency (in%) and Q is the nominal heat output of the boiler (in kW).
The interpretation of these equations is that the performance of a boiler output of 20 kW must be at least 74.8% in order to be classified in the third class. It is interesting that, in general, the minimum boiler efficiency as defined by the European Standard EN303-5 is higher than that specified in the U.S. by ASME.
In practice, the individual laws and regulations for the performance of biomass boilers are stricter than those specified in accordance with EN 303-5. For example, in Germany only category 3 boilers permitted while in Austria the only acceptable performance range is between 73 and 83% for non-automatic boilers and 76-86% for automatic feeding. Similarly, in Switzerland may only use Class 3 boilers.