In boiler selection for small process plants and industrial services, the traditional Lancashire fire-tube boiler is preferred where the required working pressure and power (steam generation rate) are not extreme. For which duty level is a Lancashire boiler typically used?

Introduction / Context:
Lancashire boilers are classic horizontal, internally fired, fire-tube boilers with two furnace flues. They served mills and small powerhouses for decades. This question tests the basic selection guideline: at what pressure and capacity range is a Lancashire boiler most appropriate compared with Cornish (single flue) or modern water-tube designs.

Given Data / Assumptions:

• Boiler type: Lancashire, fire-tube, two internal furnace flues.
• Application: steady industrial duties rather than large utility generation.
• Typical pressure capability: low to moderate compared with water-tube units.
• Power/steam rate: moderate, not extremely high.

Concept / Approach:
Fire-tube boilers hold a large water volume and rely on hot gases passing through tubes within a common shell. Their allowable working pressure is limited by shell thickness and diameter. As pressure or steaming rate requirements rise substantially, the safer and more efficient solution shifts to water-tube types (e.g., Babcock–Wilcox). Therefore, Lancashire is best suited to moderate pressure and power needs where simplicity and stable steam are desirable.

Step-by-Step Solution:
Identify operating range: Lancashire can meet moderate pressures and loads reliably.Compare alternatives: Cornish (single flue) is usually for lower duties; water-tube handles higher pressures and capacities.Infer selection: for neither very low nor very high requirements, “moderate” fits.Conclude answer: choose the option that states “moderate”.

Verification / Alternative check:
Historical data and handbooks place Lancashire in workshops, textile mills, and small process plants where pressures and steam rates are moderate and steady.

Why Other Options Are Wrong:
Low: better matched to smaller Cornish units.

High/very high: typically water-tube domain due to pressure/heat-transfer limits.

None of these: incorrect because “moderate” succinctly captures the customary duty.

Common Pitfalls:
Assuming all boilers can be pushed to any pressure; ignoring shell-thickness and code limits of fire-tube shells.

Final Answer:
moderate