Economiser benefit — is a 15–20% coal saving claim reasonable? Evaluate the statement: “There is about 15 to 20% saving of coal when an economiser is fitted in the boiler flue-gas path.”

Introduction / Context:
An economiser is a feedwater heater that recovers sensible heat from hot flue gases. By preheating feedwater, the boiler needs less fuel to raise the water to saturation and, if applicable, to superheat it. Many textbook problems summarize the benefit as “on the order of 10–20% fuel saving,” depending on site conditions. This item asks you to judge whether a 15–20% saving is plausible.

Given Data / Assumptions:

• Flue gases exit the boiler at a temperature high enough to transfer additional heat to feedwater.
• Feedwater enters below saturation temperature (common in practice).
• Economiser effectiveness is reasonable (not extremely fouled or bypassed).

Concept / Approach:
Fuel saving arises because the enthalpy rise required in the evaporator and superheater is reduced by the economiser’s contribution. The percentage saving depends on stack temperature, feedwater inlet temperature, flow rate, and excess air. In many standard references and solved examples, savings of roughly 10–20% are cited for well-designed retrofits, making the 15–20% figure a reasonable rule-of-thumb for exam purposes.

Step-by-Step Solution:
Let Q_in be fuel heat input and Q_useful the heat delivered to water/steam.With an economiser, a portion Q_econ is recovered from flue gases.Fuel saving ≈ Q_econ / Q_in_baseline, often falling in the mid-teens percent for practical cases.Hence, “about 15–20%” is acceptable as a rounded estimate.

Verification / Alternative check:
Heat-balance worksheets show significant drop in stack loss and fuel rate after raising feedwater, supporting mid-teens percent improvement when the baseline had high stack temperatures and cool feedwater.

Why Other Options Are Wrong:

• “False” denies widely documented savings in appropriate conditions.
• Other qualifiers (superheat, draught, deaeration) are not prerequisites, though they may influence magnitude.

Common Pitfalls:
Expecting exactly 15–20% in every plant. Real savings vary (often 8–18%); the question accepts the typical textbook range.

Final Answer:
True