4 Performance Tools: Basic Parameters, NTU-Effectiveness, LMTD Analysis, Fouling Factors
Input core parameters to analyze heat exchanger performance
Analyze exchanger performance using the NTU-effectiveness method
ε = (1 - exp[-NTU(1 - Cr)]) / (1 - Crexp[-NTU(1 - Cr)]) for counter flow
Where: NTU = UA/Cmin, Cr = Cmin/Cmax
Calculate log mean temperature difference for heat transfer
LMTD = (ΔT1 - ΔT2) / ln(ΔT1/ΔT2)
Correction factors account for deviations from pure countercurrent flow
Assess the impact of fouling on heat transfer performance
1/Udirty = 1/Uclean + Rf,hot + Rf,cold
Typical fouling factors: Cooling water (0.0002), Steam (0.0001), Oil (0.0009)
Actual Heat Transfer
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kW
Effectiveness
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ε
Corrected LMTD
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°C
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The Number of Transfer Units (NTU) method is ideal when inlet and outlet temperatures are unknown. Effectiveness (ε) is the ratio of actual heat transfer to maximum possible heat transfer. The method relates ε to NTU and heat capacity ratio (Cr = Cmin/Cmax).
The Log Mean Temperature Difference (LMTD) method is used when all four terminal temperatures are known. The heat transfer rate is calculated as Q = UA·F·LMTD, where F is the correction factor for complex flow arrangements.
Fouling factors account for thermal resistance due to material buildup on heat transfer surfaces. They reduce the overall heat transfer coefficient (U) by 20-40%, impacting performance. Regular cleaning and material selection can mitigate fouling effects.