Boiler comparison: water-tube versus fire-tube steam pressure Fill in the blank: Water-tube boilers generally produce steam at a __________ pressure than fire-tube boilers (for comparable size and technology).

Introduction / Context:
Boilers are broadly classified as fire-tube (hot gases flow through tubes surrounded by water) and water-tube (water flows through tubes surrounded by hot gases). A key practical difference is the pressure level at which each type commonly operates safely and efficiently.

Given Data / Assumptions:

• Comparison is between typical industrial designs of similar thermal capacity.
• Materials and codes are appropriate for the intended pressure range.
• Focus is on achievable/typical working pressure rather than absolute extremes.

Concept / Approach:
Water-tube boilers contain water in small-diameter tubes and have relatively low water inventory with strong tube geometry, allowing higher allowable stress at temperature and better heat transfer. Fire-tube boilers store a larger water volume in a shell and place hot gas inside tubes; shell thickness and stress limitations typically cap the safe working pressure at lower values compared with water-tube units.

Step-by-Step Solution:
Identify structural constraint: shell thickness versus tube strength.Recognize that small tube diameters in water-tube units resist hoop stress more effectively for a given wall thickness.Note improved circulation and faster response reduce local overheating risk in water-tube designs.Conclude: water-tube boilers are generally selected for higher-pressure steam service than fire-tube boilers.

Verification / Alternative check:
Industry practice: high-pressure utility and process steam (often well above 40 bar and into supercritical regimes) uses water-tube configurations, while many low-to-medium-pressure heating or process duties commonly use fire-tube shells in the 10–25 bar range (exact numbers depend on standards). This trend validates the statement that water-tube boilers operate at higher pressure.

Why Other Options Are Wrong:
“Lower” contradicts common design selection. “Approximately the same” ignores structural differences. “Variable and cannot be compared” is too vague—although applications vary, the prevailing design choice is clear. “Identical designs” is factually incorrect because the internal flow arrangements differ fundamentally.

Common Pitfalls:
Assuming capacity alone dictates pressure; overlooking code limitations on shell thickness; confusing superheater temperatures with saturation pressure capability.

Final Answer:
higher