High-temperature heating media — Which of the following is NOT commonly used as a circulating heat-transfer medium for high-temperature service in process plants?

Difficulty: Easy

Correct Answer: mercury

Explanation:


Introduction:
Choosing a heat-transfer medium for high-temperature service involves safety, corrosion, vapor pressure, stability, and pumpability. Common industrial choices include synthetic organics, molten salts, and selected liquid metals. Some substances, although thermally capable, are avoided due to toxicity or operational hazards.


Given Data / Assumptions:

  • Continuous circulation in closed loops for heating.
  • Temperatures ranging from roughly 200°C to 600°C depending on medium.
  • Practical plant considerations (toxicity, vapor pressure, materials compatibility).


Concept / Approach:

Organic fluids (Dowtherm/Therminol) serve up to ~350–400°C. Molten nitrate/nitrite salt eutectics operate ~250–565°C with good thermal stability. Molten sodium or NaK alloys are used in specialized high-temperature systems with proper alloy/containment. Mercury, while a liquid metal and thermally conductive, is toxic, has significant vapor pressure, amalgamation/corrosion issues, and is not a standard circulating heat medium in process plants; its use is limited to niche devices (some heat pipes or laboratory apparatus), not general plant heating.


Step-by-Step Solution:

List common industrial media: organics, molten salts, alkali metal coolants.Assess mercury against safety and compatibility criteria.Conclude mercury is not commonly used as a plant heat-transfer fluid.Select 'mercury' as the exception.


Verification / Alternative check:

Industrial standards and supplier literature emphasize organic fluids, molten salts, and sodium loops; mercury is largely absent due to occupational and environmental hazards.


Why Other Options Are Wrong:

A/D/E are established media; C is used in specialized high-temperature systems (e.g., some reactors or test loops) with appropriate design.


Common Pitfalls:

Assuming high thermal conductivity alone makes a substance suitable; overall safety and materials issues dominate selection.


Final Answer:

mercury

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