Where are regenerators used? Identify the furnaces that commonly employ regenerators for cyclic heat storage and high air-preheat.

Introduction / Context:
Regenerators are checker-brick heat storage beds that alternately store heat from flue gas and give it back to incoming combustion air, enabling very high air-preheats and fuel savings in large furnaces.

Given Data / Assumptions:

• Coke oven batteries and historic open-hearth steel furnaces are classic regenerator applications.
• Operation is cyclic with periodic valve reversals.

Concept / Approach:
Both furnace types feature large flues and checker chambers, making them well-known examples of regenerator use for energy recovery and flame temperature enhancement.

Step-by-Step Solution:
Check each furnace for regenerator integration.Coke ovens: yes; Open-hearth: yes.Therefore, select both.

Verification / Alternative check:
Process diagrams show twin regenerator chambers per side with periodic reversals of flow.

Why Other Options Are Wrong:
Single selections omit one valid case; neither is incorrect.

Common Pitfalls:
Confusing regenerators with recuperators; forgetting the cyclic valve-reversal characteristic of regenerators.

Final Answer:
Both (a) and (b)