Select the correct statement regarding coal-fired boilers.

Question

Examoogle · Accepted Answer

The correct answer is Option A: Water-tube boilers are more efficient than fire-tube boilers.. Water-tube boilers are superior in efficiency because they allow for larger heating surfaces, higher pressure operation, and better steam separation compared to fire-tube boilers. The arrangement of tubes allows the hot gases to circulate more effectively around the water-carrying tubes, leading to a faster rate of evaporation.

Step-by-Step Solution & Concepts:
Water-tube boilers are superior in efficiency because they allow for larger heating surfaces, higher pressure operation, and better steam separation compared to fire-tube boilers. The arrangement of tubes allows the hot gases to circulate more effectively around the water-carrying tubes, leading to a faster rate of evaporation.

Working Principle: In water-tube boilers, water circulates inside the tubes while hot combustion gases flow externally. This design facilitates higher heat transfer coefficients, improved circulation through convection, and the ability to withstand much higher steam pressures by utilizing smaller diameter tube geometries that reduce tensile stress in the tube walls.

Key Formulas: $\eta = \frac{\dot{m}_s (h_s - h_f)}{\dot{m}_f 	imes CV}$ — where $\eta$ is boiler efficiency, $\dot{m}_s$ is steam mass flow rate, $h_s$ is steam enthalpy, $h_f$ is feedwater enthalpy, $\dot{m}_f$ is fuel mass flow rate, and $CV$ is calorific value of fuel.

Key Points: Water-tube boilers utilize a larger heating surface area to volume ratio.; They are better suited for fluctuating loads and high-pressure power generation.; Fire-tube boilers are generally limited to pressures below $20 	ext{ bar}$ due to shell design limitations.

Advantages: Capable of producing very high pressure and high steam capacity; Rapid steam generation response to load changes; Smaller diameter tubes are safer under high pressure

Disadvantages: Require high-quality feed water to prevent tube scaling; More complex construction and maintenance compared to fire-tube units

Applications: Thermal power plants; Large-scale industrial steam requirements

Additional Info: Option B is incorrect as fire-tube boilers have higher heat loss and lower evaporation rates.. Option C is incorrect because water-tube boilers involve more sophisticated construction and material usage, making them more expensive.. Option D is incorrect because fire-tube boilers are limited to low-to-medium pressure applications.

Answer: A is correct — Water-tube boilers are more efficient than fire-tube boilers due to better heat transfer surface configuration and ability to handle higher thermal loads.

Exam Tip: Always remember that water-tube boilers are for high pressure/large scale, whereas fire-tube boilers are for low pressure/small scale; this distinction is a frequent examiner favorite.

Examoogle · Answer

Option B: Fire-tube boilers have higher thermal efficiency compared to water-tube boilers.

Examoogle · Answer

Option C: Water-tube boilers are less expensive compared to fire-tube boilers.

Examoogle · Answer

Option D: Fire-tube boilers are typically used for high-pressure applications.

Feature	Water-tube	Fire-tube
Pressure Range	Up to $150-200 \text{ bar}$	Generally up to $20 \text{ bar}$
Safety	Higher (less water stored)	Lower (large volume of pressurized water)

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