Design: pressure, thickness, pattern
Keppels guidelines: water/glycol/oil 1–5 bar (15–70 psi); NH3/Freon about 19 bar (280 psi) at 20 °C / 68 °F standstill; CO2 about 120 bar (1,740 psi); steam 0.5 to under 30 bar (10–440 psi). Top sheet often 0.8–2.0 mm (0.031–0.079 in). Burst example about 180 bar (2,610 psi) at 1.0 mm (0.039 in). Prove on a burst panel.
Figure: 4 steps: sketch, CAD drawing, laser welded pattern, inflated pillow plate
Thickness guidelines
Published Keppels starting values for the top sheet: about 0.6 mm (0.024 in) solar collectors; about 0.8 mm (0.031 in) milk tanks (BMC) and registers; 1.0–2.0 mm (0.039–0.079 in) general tanks, wine plates, falling-film chillers and ice banks; 1.2–1.5 mm (0.047–0.059 in) steam and thermal oil. A 1.0 mm (0.039 in) top has a published burst example around 180 bar (2,610 psi); 2.5 mm (0.098 in) toward 400–500 bar (5,800–7,250 psi) burst. Combined base + top on Keppels machines up to about 30 mm (1.18 in). Width 1500 or 2000 mm (59 or 79 in); wider jackets are split into panels.
Burst estimate
First estimate, assuming the parent sheet fails and the weld holds: P = (k · π · D · S · σ) / A. D circle diameter, S rupturing sheet thickness, σ tensile strength (about 550 N/mm² for 304), A pattern unit area. Convert consistently to bar (or psi). Factor k is typically about 6.5 for CO2 welds and about 7.5–8 for fiber welds. If the panel fails at a border or header, you tested the design, not the pattern.
Choosing a pattern
Tighter pitch and larger weld circles raise pressure and lower pillow height. Diamond/staggered layouts are generally stiffer than square. A geometrically perfect diamond can channel during inflation. Oval pitches are used when a water film must stay even. There is no single standard pattern for every duty.
Heat transfer
Q = A × k × LMTD. The k-value is not a material constant. Keppels performs the calculation for tank jackets, falling-film chillers, ice banks and bulk-solid exchangers. Textbook flat-plate k-values are only a start.
