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. The approach temperature: the central design variable The approach temperature (or minimum temperature difference, $\Delta T{min}$) is the smallest temperature gap between the two streams — typically at the outlet of the hot side. For a condenser, it is the difference between the condensing temperature and the inlet air temperature. For an evaporator, it is the difference between the return air temperature and the evaporating temperature. Approach temperature is the single most important parameter the designer controls when sizing a heat exchanger: - A smaller approach temperature means the condensing (or evaporating) temperature is closer to the ambient (or room) temperature — which means a lower pressure ratio, better COP, and less compressor work. But it requires a larger heat exchanger area. - A larger approach temperature allows a smaller, cheaper heat exchanger but forces a higher condensing (or lower evaporating) temperature, increasing the compressor's workload and reducing COP. The trade-off is economic and thermodynamic: pay for heat exchanger area upfront (capital cost) to save on compressor power over the system lifetime (operating cost). Optimum approach temperatures vary by application: Application Typical condenser approach ΔT Typical evaporator approach ΔT…