Temperature measurement in heat exchangers — continuous shell-side monitoring For a liquid–liquid shell-and-tube exchanger, which device is most suitable for continuous measurement of shell temperature in process service?

Introduction / Context:
Reliable temperature measurement on the shell side of heat exchangers is essential for control, energy balance, and protection. Sensors must be accurate, stable, and suitable for continuous service.

Given Data / Assumptions:

• We require continuous measurement in an industrial environment.
• Instrument should be compatible with standard transmitters and control systems.

Concept / Approach:
Resistance thermometers (RTDs), especially Pt100/Pt1000, offer high accuracy, stability, and repeatability in the –200 to 600°C range. They are commonly installed in thermowells on exchanger nozzles or shell taps and connected to 4–20 mA transmitters. Thermocouples are also used but exhibit greater drift and lower absolute accuracy in moderate ranges; mercury-in-glass is not practical for continuous remote measurement; vapor pressure thermometers are legacy and less common in modern instrumented control loops.

Step-by-Step Solution:
Identify need: continuous, accurate, remotely transmitted shell temperature.Select RTD for best accuracy and stability with standard transmitters.Reject alternatives on practicality and accuracy grounds.

Verification / Alternative check:
Industry practice and ISA guidelines favor RTDs over thermocouples for moderate temperatures where precision is important.

Why Other Options Are Wrong:

• Thermocouple: acceptable but typically less accurate/stable than RTD for this service.
• Mercury-in-glass: local indication only; not continuous remote output.
• Vapor pressure thermometer: mechanical and less precise; mostly obsolete for modern DCS/PLC systems.

Common Pitfalls:

• Choosing local-only devices for applications needing continuous remote monitoring.

Final Answer:
resistance thermometer (RTD)