New PulsaCoil Replacements - Copper Cylinder Vs Stainless Steel Cylinder

Are you thinking of replacing your PulsaCoil Boiler? Are you unsure which type of cylinder to choose?

An unbiased Copper Cylinder Vs Stainless Steel Cylinder Report

The University of Ulster’s – Centre of Sustainable Technologies commissioned a comparative performance analysis which directly compared the performance characteristics of copper cylinder vs stainless steel cylinder and their heat exchangers.

There is a common misconception among installers and homeowners that Stainless Steel alloys are more cost effective and of a superior quality in the performance of heating domestic hot water storage compared to that of Copper.

An unbiased performance comparison of “off the shelf” 144 litre Copper and Stainless Steel cylinders where tested by V. O’Brien*, Dr M. Smyth, M. Anderson.

The comprehensive test facility was configured to provide a vented, indirect charging circuit and a vented, direct discharge circuit. The data in the test chamber, charging and discharging circuits and the cylinders were autonomously collected at 10 seconds intervals using calibrated T-type thermocouples and inline turbine flow rate sensors connected to a DT85 data logger. The test cylinders were fitted with a series of temperature sensor pockets along their height allowing the profile of the stored water to be investigated throughout the experimental program. During the retrofit of the temperature sensors on the Stainless Steel cylinder a substantial amount of rust was encountered on the inside of the cylinder.

THE REPORT IN CONCLUSION: Copper Cylinder Vs Stainless Steel Cylinder
The perceived cost effectiveness and superiority in the performance of Stainless Steel alloys used in domestic hot water storage is unfounded.

During comparative charging tests the efficiency of the heat exchangers in the Copper cylinders was significantly higher (per metre) than the coil in the Stainless Steel cylinder tested. This increase in performance was attributed to the higher thermal conductivity of Copper.

The significant factors in the suitability of using Stainless Steel alloys in the manufacture of hot water storage cylinders are the lack of official guidelines equivalent to the British Standards describing best practice in the manufacture of Copper hot water storage cylinders and the limitations on use identified by the diameter of the heat exchanger coil and the upper limit on hot water storage temperature.

The key findings from the investigation into the use of Copper and Stainless Steels for manufacturing hot water storage cylinders
The most common Stainless Steel alloys used for hot water storage applications are low carbon austenitic (304L and 316 L) and Duplex (2205 and 2304) Stainless Steels. The corrosion resistance of Stainless Steel alloys is dependent upon the maintenance of the ‘stainless’ passivation layer which requires a minimum Chromium concentration of 11 % and exposure to an oxygenated environment. Low temperature hot water systems are designed to limit oxygen ingress which may affect the maintenance of the passive layer.

The manufacturing process used in the appropriate production of hot water storage cylinders from Stainless Steel alloys is much more involved than that of Copper cylinders. In order to allow the formation of the passive layer the cylinder welds must be purged and pickled during construction and care must be taken when selecting suitable insulating foam. There are, at present, no official guidelines on the manufacturing process for Stainless Steel hot water storage cylinders or the alloys most appropriate for this application.
A best practice manufacturing process for the production of Copper hot water storage cylinders has been presented in a number of relevant British Standards (most notably BS 1566 and BS 12897) and is much less aggressive on the raw materials used. These standards present detailed information in the form of; minimum heat exchanger area, storage capacities, material thickness, top and bottom design etc.

As a non-reactive base metal, Copper is also less prone to the formation of rust and attack by chlorides leached from insulating foam. However, when scaling up storage capacity, the yield strength of Copper makes it less competitive financially than Stainless Steel alternatives.

In order to address the commonly held belief that Stainless Steel cylinders are a cheaper alternative to Copper cylinders and that they have no inherent scrap value, the initial capital cost and the scrap value of both Copper and Stainless Steel cylinders was investigated. In collating the information gathered it was found that Stainless Steel cylinders have a higher capital cost than Copper cylinders ii and that Copper cylinders have a higher scrap value than Stainless Steel cylinders, although there is an intrinsic scrap value for both materials.

In terms of the effect material choice has on the control of bacterium although not directly related to domestic hot water storage, there is an increasing body of work highlighting the benefits of using Copper instead of Stainless Steels for infection control in hospitals. The only information provided for either material in the context of hot water storage and distribution states that the corrosion of Stainless Steel “may lead to conditions which encourage the growth of legionella” (Anon, 2012).

Finally, the practical considerations associated with heat exchanger design and operating temperature are discussed. When combined with an uncontrolled heat source, a minimum heat exchanger pipe diameter of 28 mm is required by BS 1566-2 and in instances where the storage temperature may exceed 65 oC Stainless Steel is an unsuitable material for use based on the manufacturer’s literature which accompanies the cylinders.

For a copy of the full report please visit http://www.pulsacoil-bp.co.uk/copper-cylinder-vs-stainless-steel-cylinder/