David Doherty, Vice-Chairman CIBSE Ireland and Projects Manager, Hevac
Section 11.2 of the revised changes to Technical Guidance Document TGD-030 covers boiler selection specification. The revised issue now reads: “where natural gas supply is available, suitably-sized aluminium or stainless steel modulating boilers shall be provided”. The key wording here is aluminium. This change now allows for more competitive and efficient condensing boiler plant to be considered.
Aluminium has a number of favourable characteristics. The alloy is perfect for casting of boiler bodies with complex shapes which allow increased surface areas for maximum heat transfer with low water volumes.
As aluminium conducts heat better, in choosing this material, we can significantly reduce the exchange surfaces to achieve the same output transmission to the heating circuit with a smaller exchanger. At an equivalent output, aluminium heating bodies are therefore significantly more compact. Aluminium is three times lighter than stainless or copper.
The compactness of aluminium exchangers – combined with its excellent thermal properties – allows mechanical contractors to take advantage of significant weight reductions with the same amount of power.
Aluminium silicium is extremely flexible, which allows considerable temperature differences (up to 30k) between the boiler flow-and-return. There is no risk of metal fatigue caused by repeated thermal shocks throughout the heating season which can lead to breakage of components.
The thermal conductivity of aluminium (99.9% purity) is 237 (W.M-1.K-1 @ 20°C) while stainless is 46 (W.M-1.K-1 @ 20°C). This represents a greater heat transfer by five times that of stainless steel. This in turn allows for smaller exchangers and boiler sizes. The density of steel is more and therefore weighs more. This can become an issue on large boiler plant for installation and building structural loads.
The construction of stainless steel heating bodies involves weld assemblies, folds and pressed parts which are susceptible to the constraints relating to the operation of the boiler. The changes in temperature relating to the operation of the boiler are the root cause of stress in materials. These manufacturer welds and lock seams can be a weakness in the exchanger assembly. An aluminium boiler does not incorporate any folds or welds.
Alloy resistance to acidic conditions is critical, especially during condensing mode. Aluminium can resist these corrosive conditions due to its ability to become passive. On contact with water or oxygen, a non-porous protective layer of aluminium oxide is formed naturally. This is alumina, or the passive layer. It is this layer that makes the alloy suitable to the condensing conditions of modern boilers.
As the boiler is not susceptible to thermal shocks, the boiler can have low return temperatures. thus allowing it to condense and therefore recover heat. During condensing operation, the condensate run-off flows down over the heat exchanger. This acts as a method of self-cleaning by preventing the accumulation of any residues and non-combustible materials on the exchanger and, in effect, continuously washes the exchanger.
In order for any heating system to operate properly clean neutral water is ideal. The addition of an inhibitor to the system at commissioning stage will keep any remaining grit in suspension and prolong the life of the system and the boiler. Proper system flushing to rid the pipes of filings, dirt or grit is recommended.
Water quality parameters are measured by pH, hardness, conductivity and chloride levels. These levels will vary geographically from county to county. The table shows why it is necessary to include a protective inhibitor. Steel and cast iron corrode easily on contact with water, as the pH of the water network is not naturally compatible with these alloys. Conversely, aluminium presents good resistance to neutral or even acidic pH, and is one of the metals most resistant to corrosion due to its broad tolerance range.
In conclusion, aluminium silicium boilers have a number of positive characteristics – including corrosionresistance, longevity, ductility and conductivity – and these are prime considerations when specifying or making boiler selections. ■