High-performance ASME pressure tank for the North American market

We knew from the start that the performance of the tank would be defined primarily by the efficiency of its internal heat exchanger. In conventional tanks, the heat exchanger is typically made from a single long tube, usually with a diameter of 25–32 mm for tanks of this size. We decided to completely rethink this approach.

Our goal was to reduce the tube diameter, as a smaller hydraulic diameter improves heat transfer efficiency across the entire tube cross-section. However, using a 16 mm tube, which we selected for this project, would normally limit the achievable heat transfer area. To overcome this constraint, we changed the exchanger concept entirely.

Instead of a single coil, we designed multiple smaller coils connected into a single, highly efficient heat exchanger. This configuration allows the coils to fill almost the entire volume of the tank, forcing the fluid in the shell side to flow between the tubes. As a result, we achieved a very large heat transfer area while using small-diameter tubes.

To ensure proper fluid distribution inside the shell, we introduced a dedicated internal baffle. Its geometry was validated before prototype manufacturing using CFD flow analysis.

Laboratory testing confirmed that the heat exchanger was efficient enough to support continuous water heating. This allowed us to reverse the conventional tank operating principle. Instead of storing domestic hot water in the shell, we used the shell volume as a buffer for heating water, while domestic water was heated instantaneously as it flowed through the coil, only when demand occurred.

This configuration completely eliminates domestic water stagnation, significantly reducing the risk of Legionella growth and removing the need for periodic thermal disinfection cycles.

The tank was designed in accordance with ASME Section VIII, enabling the customer to market the product across North America. The final tank volume was 115 gallons (435 liters), while the total heat transfer area of the coil reached 62.4 sq ft (5.8 m²), resulting in an excellent surface-area-to-volume ratio.

For materials, we selected austenitic stainless steel 304L (low carbon) for the coils and duplex stainless steel S32101 for the pressure vessel. These materials were chosen for their proven corrosion resistance, mechanical strength, and long service life.

Fire resistance was one of the key customer requirements for the external components of the tank. Among all insulation materials, mineral wool offers the best fire performance. We selected mineral wool insulation with reaction to fire class A1 according to EN 14303, suitable for continuous operation at temperatures up to 500°C.

Standard plastic jackets were intentionally avoided, as they would contradict the fire-resistance concept. Instead, the insulation jacket and top and bottom covers were manufactured from galvanized carbon steel with powder coating. This solution provides excellent fire resistance, high mechanical durability, and a premium visual finish.

The tank design also included additional nozzles for essential instrumentation, such as temperature sensors and a safety valve, ensuring correct and safe operation of the system.