Why is saturated steam widely preferred as a heating medium in heat exchangers and reboilers?

Introduction / Context:
Saturated steam is a workhorse utility for process heating. When steam condenses on a heat-transfer surface, it releases latent heat at an almost constant saturation temperature corresponding to the steam pressure. This makes control straightforward and provides high heat flux for compact equipment.

Given Data / Assumptions:

• Standard saturated steam supply with proper condensate removal.
• Heat exchangers or reboilers designed for condensing service.
• Materials selected appropriately for condensate chemistry.

Concept / Approach:
Compared with single-phase heating media, condensing steam delivers much more heat per unit mass because of its high latent heat. Moreover, since condensation occurs at nearly constant temperature at a given pressure, the temperature driving force is stable and easy to control by adjusting steam pressure/flow. These traits, not absolute low cost or universal non-corrosiveness, are the main reasons steam is preferred.

Step-by-Step Reasoning:

Verification / Alternative check:
Utility cost curves vary by site and time; steam is not always the cheapest, reinforcing that the key advantage is thermodynamic (latent heat and isothermal behavior), not universal cost.

Why Other Options Are Wrong:

• (a) Cost depends on fuel/power; not universally lowest.
• (c) Condensate can be corrosive depending on oxygen/carbonic acid; metallurgy and treatment are needed.
• (d) Film coefficients are high for condensing, but not always “highest possible” under all conditions.
• (e) Temperature control is still required; steam just makes it easier.

Common Pitfalls:
Poor condensate drainage causing flooding; ignoring non-condensables which hamper heat transfer and temperature control.

Final Answer:
It has a high latent heat of condensation, delivering large heat duty at nearly constant temperature