Improving furnace thermal efficiency — select the comprehensive strategy Which measures collectively raise the thermal efficiency of industrial furnaces during routine operation?

Introduction / Context:
Furnace thermal efficiency is the fraction of fuel energy delivered to the stock. Operators improve efficiency by enhancing heat recovery, limiting losses, and maintaining stable combustion with correct air flow (draught).

Given Data / Assumptions:

• We compare three common efficiency levers.
• Goal: higher useful heat to the load per unit fuel.

Concept / Approach:
Flue gas heat is a major loss; recuperators/regenerators reclaim it to preheat combustion air or load. Wall and opening losses are reduced via insulation, tight doors, and sealing. Proper draught ensures the intended air-to-fuel ratio, reduces infiltration, and stabilizes flames for high radiant heat transfer.

Step-by-Step Solution:

Verification / Alternative check:
Energy balances routinely show stack losses and wall losses as leading terms; draught control affects both excess air and infiltration, validating the combined improvements.

Why Other Options Are Wrong:

Common Pitfalls:
Over-relying on one measure (e.g., only insulation) while running with high excess air; the best gains are holistic.

Final Answer:
all (a), (b) and (c).