Heat Exchanger World Magazine October 2026 | Page 38

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Shell & Tube HEX

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Polished Billet Head. Photo Credit: Jonathan Wallen

Seal Welded & Polished Double Tubesheet. Photo Credit: Jonathan Wallen

this gap directly into the secondary tubesheet to maximize efficiency. This design has become a cornerstone of high-purity applications, providing a physical fail-safe that maintains the integrity of pharmaceutical batches.

Eliminating dead legs: Billet head fabrication As the industry moved toward Continuous Manufacturing and Batch Processing with zero-tolerance for microbial growth, the focus shifted to the“ geometry of cleanliness.” Traditional cast or welded heads often contained internal areas of low flow where bacteria could proliferate. The engineering response was the development of the unique milled-out billet head. Unlike traditional designs where pass partitions are welded into a head, the modern sanitary approach involves machining the entire head from a single solid billet of stainless steel. This allows for integral pass partitions and the inclusion of specialized drainage slots. These slots ensure that the unit is“ free draining,” meaning no product is trapped when the system is evacuated. Furthermore, modern design philosophy emphasizes that only the sanitary fittings are

welded to the head, reducing internal weld seams that could serve as sites for contamination. By utilizing integral pass partitions, the risk of“ bypass” or crevice-related corrosion is virtually eliminated, aligning perfectly with the self-draining requirements of ASME BPE.

Joinery and integrity: Seal welding of tubes The methods used to secure tubes to the tubesheet have also advanced significantly. Early exchangers relied heavily on mechanical expansion, which, while effective for pressure retention, could leave microscopic gaps at the joint interface. In a sanitary context, those crevices are unacceptable. Modern standards now favor the seal welding of tube ends to the forward tubesheet in addition to mechanical expansion. This process creates a seamless, crevice-free transition between the tube and the tubesheet surface. When combined with precision machining, this technique ensures that there are no“ shadow areas” that could escape Cleaning-in- Place( CIP) or Sterilization-in-Place( SIP) cycles. Engineering teams often focus on these specific attachment details to ensure that the thermal and mechanical design of the exchanger is robust enough for numerous thermal cycles, without compromising the sanitary seal.

Surface science: Advanced polishing techniques Perhaps the most visible evolution in sanitary heat exchangers is the refinement of surface finishes. The industry has progressed from basic mechanical buffing to sophisticated, multi-step mechanical and sometimes electropolishing( EP) regimes. The goal is to achieve a surface roughness( Ra) value that minimizes the surface area available for microbial adhesion. Today, a finish of 25 Ra or better is a common requirement for product-contact parts. Surface finishes are typically specified using ASME BPE SF levels( Table SF 2.4-1.1). Achieving this level of precision requires a nuanced approach:

• Multi-step mechanical polishing: Surfaces are polished in progressive stages to remove any pits, scratches, or inclusions in the base metal.

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Table 1. Key technical specifications for modern sanitary units. Feature Evolution Modern Standard Benefit

Head Construction Welded / Cast to Billet Integral partitions; free-draining.

38 Heat Exchanger World October 2026 www. heat-exchanger-world. com