On your metal
As the steel vs aluminium heat exchanger debate continues, Steven Evans argues that perhaps it’s time to look beyond the materials to focus on design and maintenance
Building services engineers across the country are under increasing pressure to increase the life expectancy of the boilers they install, at the same time as meeting ever-more strict performance and efficiency targets. In this context, the quality and durability of heat exchangers are often put under scrutiny, with the metal used to construct them at the heart of the debate.
The issue centres around the merits of stainless steel and aluminium, their corrosion-resistant properties and how these impact on the performance and longevity of a boiler’s heat exchanger.
With both materials offering excellent performance, the debate needs to look more closely at system design and maintenance, which can have a far bigger impact on performance.
Stainless steel is a generic name for a range of steel types and grades for corrosion or oxidation resistant applications. As a family, they are iron alloys with a minimum chromium content of 10.5 per cent, which is the key element in corrosion resistance. Other elements, including nickel, molybdenum, titanium and copper are also added to enhance their structure and properties.
Choosing the correct grade of stainless steel for a demanding application such as a heat exchanger is critical. While chromium is the key element in resisting the oxidisation process that causes corrosion, this is further enhanced by the addition of the metal molybdenum to create stainless grade 316/316L.
This low-carbon stainless steel is a good option for a heat exchanger because it is a high-alloyed steel with a chromium content of over 18 per cent. When the chromium is oxidised, it forms a thin surface layer which protects the underlying metal. With this thin film covering the metal surface, corrosion resistance is greatly enhanced.
The choice is yours
From the manufacturing point of view, the choice of the low carbon (L) grade offers better weldability. Versatility is also widely seen as major benefit of aluminium heat exchangers, which have traditionally been the material of choice for the first condensing boilers ever produced.
With this metal making it easy to cast into complex shapes, heat exchanger design can maximise the exchange surfaces and improve the hydraulic flow in a compact structure.
As with stainless steel, the appropriate alloys play their part. In the case of heat exchangers, an aluminium-silicon alloy, or AISi, is particularly suitable. AISi alloys can cope with considerable differences in temperature to reduce damage caused by thermal shock, and they have excellent heat transfer properties. Because of this ability to conduct heat extremely efficiently, the heat exchanger can be designed to work in a more compact boiler without reducing its heat exchange capability.
Aluminium is also known for its resistance to water in a wide range of applications and is, in fact, resistant to most environments and a variety of chemical agents. Essentially, like stainless steel, aluminium develops its own protection with an inert and protective aluminium oxide film which forms on the metal surface as soon as it comes into contact with water.
Clearly there are merits to both stainless steel and aluminium, and while the debate surrounding materials will undoubtedly continue, it is important to consider a number of other factors that can impact the life expectancy of a boiler.
For example, regardless of the choice of heat exchanger material, maintaining the quality of the water circulating through a heating system is essential. All boilers will perform more efficiently and reliably with good-quality water circulating in them.
Best practice is to incorporate chemical water treatment into both the commissioning and ongoing maintenance of a heating system, including a system clean/flush and regular doses of inhibitor as required. This is critical to ensure boiler efficiency and longevity in any commercial heating system, large or small.
Even completely new heating systems with new boilers and pipework need to be chemically treated as failing to invest in appropriate water treatment can result in corrosion and the build-up of limescale. Furthermore, failing to undertake adequate water treatment can invalidate manufacturer warranties in some cases.
Systems that are corroded become blocked with sludge and debris, which means the boiler must work harder and for longer, as the circulating water is unable to transfer the heat efficiently throughout the system. Meanwhile, limescale deposits on heat transfer surfaces will cause the boiler to run at higher temperatures for longer which - according to British Standards, BS 8558 - reduces efficiency by up to 30 per cent.
The corrosion-resistant qualities of the heat exchanger material cannot be relied on alone to combat acidity. It is therefore important to keep the water at a pH level specified by the boiler manufacturer. To combat problems caused by scale and corrosion, BSRIA, ICOM and leading boiler manufacturers all recommend flushing, chemical cleaning and the use of appropriate inhibitors.
In terms of the processes involved, the engineer should perform a chemical clean, ideally in conjunction with a power flush, as the first step, even if the boiler and pipework are new. If a new boiler is being installed on an old system, then the engineer should perform a gentle, neutral pH clean four to six weeks prior to the installation, followed by a more aggressive (though still neutral pH) chemical clean over one to two days, along with power flushing.
Another important consideration is correct sizing of the boiler. It has long been recognised that there has been a tendency to oversize boilers, which often comes about due to a natural desire to err on the side of caution to avoid complaints from users about under-performing systems.
It is essential to size a replacement boiler based on the current building requirements, rather than on the size of the boiler originally specified. A good option is a cascade system, which is flexible and modular in nature and can respond to changes in building use.
A scheduled maintenance and service regime must also be implemented from the installation stage of any heating system to safeguard long term energy efficiency and performance. This should involve analysing the flue gas to determine the right mixture of gas and air, and performing a deep clean of the heat exchanger.
When choosing a boiler, both stainless steel and aluminium options can deliver superb performance, reliability and a long life span. However, to get the best out of any heating system, what really matters is the way the system is set-up and maintained.
Steven Evans is sales director at Potterton Commercial