Heat exchanger sizing: from LMTD theory to catalog selection
By DigiEntropy Engineering · 2026-01-17 · 10 min read
Condenser and evaporator sizing rests on a small set of heat transfer equations that every engineer should be able to apply by hand before trusting a selection tool. This post works through the LMTD method, explains approach temperature as the central design variable, and shows how the theory maps to real selection results.
About this post — Authored by an AI assistant using DigiEntropy's polynomial corpus, the universal compressor predictor, and the project's chart-generation tooling. Charts are produced by Python scripts that read the same database the live site queries; tables and formulas are pulled from the same engineering modules. Findings reflect the project's current dataset and methodology — send corrections or deeper questions to admin@digientropy.com.
The fundamental heat exchanger equation
Every heat exchanger — condenser, evaporator, gas cooler, liquid subcooler — obeys the same governing equation:
$\dot{Q} = U \cdot A \cdot \Delta T{lm}$
where: - $\dot{Q}$ is the heat transfer rate (kW) - $U$ is the overall heat transfer coefficient (W/m²·K) — a lumped parameter that accounts for convection on both fluid sides and conduction through the wall - $A$ is the effective heat transfer area (m²) - $\Delta T{lm}$ is the log mean temperature difference (K)
For a counterflow heat exchanger with one fluid condensing or evaporating at constant temperature (as in refrigeration), the LMTD simplifies to:
$\Delta T{lm} = \frac{\Delta T1 - \Delta T2}{\ln(\Delta T1 / \Delta T2)}$
where $\Delta T1$ and $\Delta T2$ are the temperature differences between the two fluid streams at each end of the exchanger.
8–12 K - condenser approach, commercial - air-cooled refrigeration
Commercial refrigeration (air-cooled): typical condenser approach ΔT 8–12 K, typical evaporator approach ΔT 6–10 K.
3–6 K - condenser approach, data centre - the tightest in the table
Data centre cooling: typical condenser approach ΔT 3–6 K.
20–100 - air-side h, W/m²·K - the air side dominates UA
Air-side resistance dominates — air heat transfer coefficients are 20–100 W/m²·K and the fin efficiency is typically 0.7–0.9.
0.7–0.9 - typical fin efficiency - air-cooled coils
Air heat transfer coefficients are 20–100 W/m²·K and the fin efficiency is typically 0.7–0.9.
20 % - more UA in the example - COP improves
Suppose the condenser is replaced with a model that has 20 % more UA. … The entire p-h diagram loop shifts and narrows — COP improves.
99th - percentile hot day - the usual design ambient
Define the design operating point — ambient temperature (typically the 99th-percentile hot day for the location), required evaporation temperature, refrigerant, target condensing temperature.
The approach temperature: the central design variable
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