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Engineering Reference

Minimum Approach Temperature Reference for Heat Exchangers

Typical minimum approach temperature values for common heat exchanger services. The approach temperature controls the trade-off between heat transfer area and energy recovery.

TypeEngineering reference — data table with context

Purpose

This reference provides typical minimum approach temperature values for common heat exchanger services. The minimum approach temperature (or “approach”) is the smallest temperature difference between the hot and cold streams at any point in the exchanger. It directly controls the required heat transfer area and the capital cost of the exchanger.

Caution

These are typical values for preliminary sizing only.

The optimal approach temperature for a specific project depends on energy costs, exchanger cost, plot space, cooling utility availability, and process constraints. Pinch analysis or techno-economic optimisation should be used for integrated heat recovery networks.

Why approach temperature matters

A smaller approach temperature means more of the available thermal driving force is used, which recovers more energy — but requires more heat transfer area (and therefore more capital cost). A larger approach temperature reduces exchanger size and cost but wastes more energy. The “right” approach is a trade-off between capital cost (CAPEX) and operating cost (energy, cooling water, refrigeration).

Key relationship
Smaller approach → larger area → higher CAPEX, lower OPEX

Typical minimum approach temperatures

ServiceTypical rangeNotes
Liquid–liquid (S&T)5–10 °C (9–18 °F)Standard shell-and-tube in refining/chemical service
Liquid–liquid (PHE)2–5 °C (4–9 °F)Plate heat exchangers achieve closer approach due to higher U-values
Gas–gas10–30 °C (18–54 °F)Low U-values require large area; approach is a cost trade-off
Gas–liquid10–20 °C (18–36 °F)Gas-side resistance dominates
Condensing steam–liquid3–10 °C (5–18 °F)High condensing-side coefficient allows close approach
Cooling water service5–10 °C (9–18 °F)Constrained by cooling water return temperature limits
Refrigeration / cryogenic1–3 °C (2–5 °F)Tight approach justified by high energy cost; requires premium exchanger types
Waste heat recovery15–30 °C (27–54 °F)Wider approach accepted due to low-grade heat economics

Units

  • •SI: °C or K (temperature difference — numerically identical for intervals)
  • •Imperial: °F (multiply °C by 1.8 for the equivalent °F interval)
  • •Use the °C to °F converter for absolute temperature conversions.

Assumptions

  • •Values assume single-phase (sensible heat) service unless noted otherwise.
  • •Approach temperatures are at the closest end of the exchanger (minimum ΔT), not the average.
  • •Ranges assume counter-current flow arrangement (or equivalent thermally).
  • •Economic optimum varies by region, energy cost, and cooling utility availability.

Boundaries and exclusions

  • •No pinch analysis methodology — this reference provides heuristic starting points, not optimised targets.
  • •No heat integration or network design — each value applies to an individual exchanger.
  • •No multi-zone exchangers (condensers with desuperheat/subcool zones have different approaches at each zone boundary).

How to use this in calculations

  1. 01Define the hot-side and cold-side inlet/outlet temperatures for your exchanger.
  2. 02Check the minimum ΔT at both ends of the exchanger. The smaller value is the approach temperature.
  3. 03Compare against the typical range in the table above. If the approach is tighter than typical, the exchanger may be uneconomically large. If wider, you may be leaving recoverable energy unused.
  4. 04Use the LMTD Calculator to compute the log mean temperature difference for your chosen temperatures.
  5. 05Use the Heat Exchanger Area Calculator to estimate the required heat-transfer area from duty, U-value, and LMTD.

Sources

  • •Linnhoff, B. et al., “A User Guide on Process Integration for the Efficient Use of Energy” (IChemE, 1982)
  • •Coulson & Richardson's Chemical Engineering, Volume 6: Chemical Engineering Design
  • •Sinnott, R.K., “Chemical Engineering Design”, 5th Edition

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