Difficulty: Easy
Correct Answer: Ratio of true temperature difference to the LMTD
Explanation:
Introduction / Context:
Log-mean temperature difference (LMTD) is central to sizing heat exchangers. Real exchangers often have multipass or crossflow arrangements that deviate from ideal countercurrent or cocurrent flow. The LMTD correction factor, FT, adjusts the ideal LMTD to reflect the true effective temperature driving force.
Given Data / Assumptions:
Concept / Approach:
For an ideal countercurrent exchanger, the temperature driving force is exactly the LMTD. For more complex flow patterns, the true mean temperature difference is smaller. The correction factor FT relates these by: true ΔT_mean = FT * LMTD. Rearranging gives the definition of FT as a ratio.
Step-by-Step Solution:
Verification / Alternative check:
Standard FT charts (based on temperature effectiveness parameters R and P) show FT values below unity except for idealized limits; these confirm the ratio definition.
Why Other Options Are Wrong:
Common Pitfalls:
Applying FT above 1; using FT outside recommended ranges (e.g., FT < 0.75 often suggests reconsidering configuration); mixing temperature levels or units inconsistently.
Final Answer:
Ratio of true temperature difference to the LMTD
Discussion & Comments