Aluminium heat exchangers are lightweight and have high thermal conductivity, but stainless steel equivalents have higher corrosion resistance. This article weighs up the benefits of each material.
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 an 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 – particularly in aluminium heat exchangers, which are less resistant to corrosion than those built from stainless steel.
Ideal Commercial Boilers manufactures boilers containing steel and aluminium heat exchangers. We believe the best way to avoid corrosion in aluminium heat exchangers is to design out the risk in the first place. Our heat exchangers allow condensate to flow over the surfaces without pooling, and drain away via the condensate trap. Critical design features that ensure this occurs include the heat-transfer pin profile on the heat exchanger and the inclined surfaces in the sump.
The pins, positioned in the flue ways, are 6.5mm diameter cylindrical forms of varying length that maximise the surface area exposed to hot combustion gases. The profile of these pins, and the spacing between them, ensures good drainage of condensate through the heat exchanger to the sump.
Aluminium has the benefit of being lightweight, having a high conductivity (237 W·m-1·K-1 compared with 14.9 W·m-1·K-1 for stainless steel). So the use of aluminium means the exchange surface can be reduced significantly to achieve the same output to a heating circuit when compared to stainless steel. This allows for lightweight and compact boiler designs.
Corrosion from aluminium heat exchangers – and often from aluminium flues, mistaken for heat exchanger corrosion – can occur when acidic condensate is not draining freely from the heat exchanger or is pooling in the flue tube.
On the heat exchanger itself, a chemical reaction contributes to the aluminium being highly resistant to the corrosive effects of condensate during operation. On contact with water or oxygen, a non-porous, protective layer of aluminium oxide is formed naturally; this is alumina, or the passivation layer. It is only a few nanometres thick and the heat output of the appliance is determined after this layer has been created, so no measureable difference in heat transfer is present. The light coating does not create a difference in the surface texture of the pins.
Aluminium designs are more tolerant to variable system quality – for example, flow rate settings, air and dirt – because they allow for bigger waterways. The larger waterways are facilitated by the 4mm thickness of the waterway wall in the aluminium heat exchanger. If the stainless steel tube was enlarged, then the wall thickness would have to increase – with a cost and weight penalty on the appliance. The stainless steel tube thickness is commonly kept to a minimum (often 1.2mm to 2.0mm) to reduce both cost and weight.
The larger waterways – at least double that of typical stainless steel heat exchangers in some cases – means aluminium designs are better suited to boiler replacement on older systems, where there is likely to be more contamination in the system fluid.
This is because the aluminium heat exchanger allows for a wider range of temperature differentials of, typically, 11K to 25K. So, aluminium operates well in older systems – which may be designed to operate at, for example, 82°C flow and 71°C return – where a differential of 20K, typically required for correct operation of modern boilers, may not be available without system modifications. Heat exchangers with higher water content are able to deal with lower water flow rates through the heat exchanger and, consequently, higher temperature differentials without any localised spot boiling of the system fluid.
Stainless steel demands a higher level of commissioning, requiring precise system balancing and pump set up to prevent noisy operation. Noise is possible, particularly if the system pump is not correctly commissioned. This can result where the flow rate through the heat exchanger is lower than that required to prevent localised boiling on the heat exchanger wall.
Stainless steel heat exchangers tending to have narrower waterways means they depend on water velocity to clear air and debris. As well as noise, this will reduce the heat output of the boiler, cutting efficiency.
The lifetime of both types of heat exchanger is designed to be a minimum of 10 years if commissioning and servicing is performed correctly. Water treatment and flushing requirements are similar for both, although care should be taken when flushing old systems when small waterways are present in the heat exchanger design. Aggressive fluxes, that might otherwise cause corrosion, are less common in modern installations, and would be removed with correct flushing.
This article first appeared in CIBSE Journal, April 2017.