Commercial heating
Heat exchangers: the materials debate
We’ve already heard on pg 24 and 25 about the differences between stainless steel and aluminium boilers, but here Chris Caton, product manager at Ideal Commercial talks more about application and requirements when it comes to the materials debate
alloys, particularly for high efficiency condensing boilers. Installers will usually have a material preference and this can come down to habit or even customer requests. However, the decision should really come down to the application and requirement.
C Minimising the risk of corrosion
While conventional boilers were designed to prevent the condensation of hot gases inside the boiler, modern condensing boilers work in the opposite way. This has led to a huge increase in boiler performance and efficiency, but has created the challenge of managing the acidic condensate produced during low temperature operation. This drips from the flue and if allowed to stagnate will lead to corrosion. Stainless steel has a greater resistance to corrosion than aluminium, but is a poorer conductor of heat so the solution is to engineer the problem out altogether. The aluminium heat exchangers used by Ideal Commercial Boilers have been designed to allow condensate to flow over the surfaces without pooling and drain away via the condensate trap. Critical design features which ensure this occurs include the heat transfer pin profile on the heat exchanger and the inclined surfaces in the sump.
Aluminium is naturally corrosion resistant
On contact with water or oxygen, a non-porous protective layer of aluminium oxide is formed naturally: this is alumina, or the passivation layer. This chemical reaction contributes to the heat exchangers in Ideal Commercial’s Evomax, Imax Xtra and Imax Xtra EL boilers being highly resistant to the corrosion effects of condensate during operation.
Aluminium has greater thermal conductivity
Aluminium has significantly greater thermal conductivity compared to stainless steel (237 W/m2/K compared to 14.9 W/m2/K). The use of aluminium for the heat exchanger therefore means the exchange surface can be significantly reduced to achieve the same output to a heating circuit when compared to stainless steel. This allows for lightweight and compact boiler designs.
Ideal Commercial Boilers’ Evomax wall hung boilers installed at Adam’s Gram- mar school in Shropshire. The Evomax operates an aluminium heat exchanger
New materials in older systems
Aluminium designs are more tolerant to variable system quality (e.g. flow rate settings, air and dirt) as it allows for bigger waterways. This means it’s better suited to boiler replacement on older systems than stainless steel. For example, the waterways used in the Evomax range are 41mm x 12mm, at least double that of a typical stainless steel heat exchanger. The aluminium heat exchanger developed by Ideal Commercial also allows for a wider range of temperature differentials for system operation of typically 11 – 25˚C. Therefore, aluminium operates well in older systems where a 20˚C differential may not be available without system modifications.
Keeping noise levels low
Stainless steel demands a higher level of commissioning, requiring precise system balancing and pump set up to prevent noisy operation. This is possibly due to localised boiling and incorrect heat transfer rates. Stainless steel heat exchangers tend to have narrower waterways that depend on water velocity to clear air and debris (which is inevitable in older systems, even with water treatment). As well as noise, this will also reduce the heat output of the boiler, reducing the efficiency of the system. The Ideal Commercial condensing product range features heat engines made of both stainless steel and aluminium. In the commercial boiler market, aluminium is by far the most popular choice for a heat exchanger.
30 November 2017
ast iron and copper were the materials typically used to manufacture the heat exchanger before the industry began to recognise the benefits of more efficient combustion systems utilising stainless steel and aluminium
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