1. The Furnace
In a firetube boiler, the combustion takes place in the furnace. This furnace, usually cylindrical, can either be covered with refractory material like ceramic (dryback furnace) or be in contact with the boiler’s water (wetback furnace) which significantly increases the heat-exchange surface. The end of the furnace is called the reversal chamber since the hot gases make a U-turn and are fed into the first tube-pass. At the end of the reversal chamber, the gas’ temperature must be sufficiently low to avoid excessive thermal stress on the tubes. The reversal chamber may be equipped with a drain to collect the water condensing on its sides (the hot combustion gas usually contain a fraction of water vapour). Even though the furnace and the reversal chamber only represent 10% of the exchange surface, they account for 40-50% of the heat exchange (mainly through radiation) given the very high gas temperatures. Some biomass boilers include two furnaces to make sure that the combustion is as complete as possible. In this case, a secondary air supply must be included in the design of the boiler.
2. The Tube Passes
The first tube-pass is entirely immersed in the water and goes from one end of the boiler to the other. Depending on the boiler’s design there might be a second tube-pass (also fully immersed) in which case the boiler is called a “three pass” model (since the furnace is counted as the first pass). The diameter of the tubes has an important impact on the heat recovery performance. Clearly, for the same overall cross-section, a number of narrow tubes will be more efficient than one large tube since the heat exchange surface will be much greater. However, this multi-tube layout will be more expensive and small tubes are more likely to be blocked so maintenance costs will also be greater. The heat-exchange takes place mainly through a convective process with a limiting heat resistance on the dry side of the tubes.
3. Combustion Gas Circulation
Firetube boilers represent a significant resistance to gas flow: the combustion gas circulation is made possible by the use of a fan. Typically, pressure losses are low in the furnace given its large cross section but are significant in the reversal chambers (the gas undergoes a U-turn) and in the tube passes (large gas velocity). The so-called “draught loss” must be calculated (many correlations are available in the literature) and the fan power deduced.