BSEE BOILERS & HOT WATER
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HEAT EXCHANGERS Should materials govern choice?
There is an ongoing debate in the commercial boiler industry over the choice between stainless steel or aluminium for heat exchangers. Most manufacturers opt for a single material because they believe that one has advantages over the other. However, Potterton Commercial sees merits in both. National Sales Manager, Steven Evans, explains more.
properties and how these impact on the performance and longevity of a boiler’s heat exchanger, there is little evidence to suggest that one is more reliable or long lasting than the other. In fact, heat exchanger material shouldn’t govern choice of boiler as both materials offer excellent performance.
M Stainless steel
As the British Stainless Steel Association (BSSA) points out, “‘stainless’ 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.
The heat exchangers used in Potterton Commercial’s Sirius boilers are the low-carbon stainless steel grade 316L, which 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. From the manufacturing point of view, the choice of the low carbon (L) grade offers better weldability.
Aluminium ‘ Aluminium
heat exchangers have been in development for over 35 years with this being the material of choice for the first condensing boilers ever produced. This is the material that Potterton Commercial uses in its Paramount four and Eurocondense four ranges.
’
Aluminium heat exchangers have been in development for over 35 years with this being the material of choice for the first condensing boilers ever produced. This is the material that Potterton Commercial uses in its Paramount four and Eurocondense four ranges. It is widely recognised as a versatile material with the added advantage that it can be cast into quite complex shapes. This enables a heat exchanger design that maximises the exchange surfaces and improves the hydraulic flow in a compact structure. As with stainless steel, the appropriate alloys play their part in the suitability of aluminium for specific applications. 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.
Water quality
No matter what material the heat exchanger is made from, maintaining the quality of the water circulating through a heating system is essential.
A corroded system will become blocked with sludge and debris, causing the boiler to work harder for longer because the circulating water is unable to efficiently transfer the heat throughout the system. Limescale deposits on heat transfer surfaces also reduce efficiency, because the boiler will need to run at a higher temperature for longer.
18 BUILDING SERVICES & ENVIRONMENTAL ENGINEER SEPTEMBER 2017
uChoosing the correct grade of stainless steel for a demanding application such as a heat exchanger is critical.
uThe heat exchangers in Potterton Commercial’s Sirius boilers are low‐carbon stainless steel grade 316L – a high‐alloyed steel with a chromium content of over 18 per cent.
any boiler manufacturers take an ‘either/or’ approach to their choice of material for heat exchangers. Although many comparisons are made between stainless steel and aluminium, their corrosion-resistant
The design principles of condensing boilers also have to be borne in mind: exhaust gases normally dispersed by the flue into the atmosphere are sent through the heat exchanger, which cools and condenses them back into a liquid form, or condensate, which is mildly acidic and, therefore, corrosive.
It is not enough to simply rely on the corrosion-resistant qualities of the heat exchanger material to combat corrosion. Every boiler manufacturer specifies the water chemistry best suited to an individual boiler model. Typically, whether the heat exchanger is made from stainless steel or aluminium, water with a pH range of between 7.0 and 8.5 is required and it is important that this pH level is measured and maintained. Chemical water treatment should be incorporated at the commissioning stage and included in ongoing planned maintenance. The water treatment programme should comprise a system clean/flush and regular doses of manufacturer- recommended inhibitor.
More than a materials choice
While the debate surrounding materials will undoubtedly continue, it should be remembered that most manufacturers’ heat exchangers are very similar, for the simple fact that many use common components, including heat exchangers, sourced from third-party manufacturers. Potterton Commercial does, however, differ in that we use cast aluminium heat exchangers produced in our own factory in Preston, Lancashire. This means that we control the core design of our own boiler parts. Our design team can also integrate heat exchanger with boiler components in a way that optimises performance and space efficiency at a more competitive cost.
Stainless steel or aluminium, it is not the heat exchanger material that is at the real root of product differentiation. What matters, and what should govern choice of boiler, is how those components are applied and how they work together to achieve performance and efficiency.
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