HX Evaluation

Chapter from the Heat Exchangers Design Guide.

HEAT EXCHANGER EVALUATION — REVAMP & DEBOTTLENECK

Evaluation Methodology

Step 1: Collect current operating data (flows, temperatures, pressures)
Step 2: Calculate current duty: Q = m × Cp × ΔT (or enthalpy-based for phase change)
Step 3: Calculate LMTD and correction factor F (for multi-pass)
Step 4: Determine current U (overall HTC): U = Q / (A × F × LMTD)
Step 5: Compare current U to clean U and design U
Step 6: Determine fouling resistance: Rf = 1/U_dirty - 1/U_clean
Step 7: Assess available margin: % excess area = (U_clean/U_required - 1) × 100%

Key Metrics:
• Clean U / Design U ratio — if >1.0, exchanger is over-performing vs. design
• Fouling resistance trend — increasing Rf indicates fouling growth
• ΔP trend — increasing shell or tube ΔP indicates fouling or plugging

Typical Overall Heat Transfer Coefficients

Service	Hot Side	Cold Side	U Clean (BTU/hr·ft²·°F)	U Design (BTU/hr·ft²·°F)	Fouling (hr·ft²·°F/BTU)	Notes
HC liquid–HC liquid	Process HC	Process HC	50-80	35-60	0.002-0.003	Standard feed/effluent
HC liquid–cooling water	Hot HC	Cooling water	80-120	60-90	0.003-0.004	Trim coolers
HC condensing–CW	Condensing HC	Cooling water	100-150	70-110	0.002-0.003	Overhead condensers
Gas–gas	Light HC gas	Light HC gas	15-30	10-25	0.001-0.002	Recycle gas exchangers
Steam–HC liquid	Condensing steam	Process HC	100-200	70-150	0.002-0.003	Steam reboilers
HC–HC (with phase change)	Vaporizing	Condensing	80-150	60-120	0.002-0.003	Feed/effluent with phase change
Air cooled–HC liquid	Process HC	Ambient air	5-12	4-10	0.001-0.002	Product coolers (air side controls)
Air cooled–HC condensing	Condensing HC	Ambient air	10-20	8-15	0.001-0.002	Overhead air coolers

Source: Heat_Exchangers_Design_Guide_v1.xlsx · sheet “HX Evaluation”

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