Boiler design basics: Very high-pressure steam generation is usually accomplished by which type of boiler construction?

Introduction / Context:
Boiler geometry determines allowable pressures and heat-transfer performance. At very high pressures, safety and mechanical constraints favor designs that keep only small volumes of water/steam under high pressure inside strong small-diameter tubes, rather than in large shells.

Given Data / Assumptions:

• “Very high pressure” implies conditions well above typical shell/fire-tube ratings.
• Water-tube boilers circulate water inside tubes with hot gases outside.
• Fire-tube/shell boilers place hot gases in tubes and water/steam in a large shell, limiting safe pressures.

Concept / Approach:
Thin-walled small tubes withstand higher internal pressures with lower hoop stress (σ = P * D / (2 * t)). This makes water-tube configurations the standard for high-pressure power boilers and utility units.

Step-by-Step Solution:
Compare mechanical limits: small tube diameters (water-tube) give lower stress for given wall thickness.Assess safety: lower water inventory reduces stored energy risk.Select water-tube as the preferred high-pressure design.

Verification / Alternative check:
Power stations and high-pressure process boilers are almost universally water-tube, confirming the choice.

Why Other Options Are Wrong:
Fire-tube/shell and natural-circulation shell: limited pressure capability due to large shells.Waste-heat shell boilers: often lower/medium pressure unless specially designed water-tube HRSGs.Once-through electric boiler: niche; not the general industrial answer sought.

Common Pitfalls:
Assuming larger shells mean stronger boilers; pressure containment works the opposite way for cylindrical vessels.

Final Answer:
Water-tube boiler