Heat Exchanger Sizing

Heat Exchanger Sizing (LMTD)

Overview

LMTD-based duty and area calculation for a known overall heat transfer coefficient U. Counter-current or co-current; multi-pass F-factor via the ht library. Duty can be computed from terminal temperatures or supplied explicitly.

Prerequisites

1. uv available.

When to Use

• Sizing a new exchanger when you have a defensible U value (vendor data,

prior project) and terminal temperatures.

• Sanity-checking an existing exchanger for new conditions.

Don't use for

• Bell-Delaware shell-side design with detailed baffle / bundle

configuration.

• Rate-based two-phase boiling or condensation (use a vendor or

specialized tool).

• Calculating U from first principles when h on either side is unknown

(use convective-heat-transfer-correlations first).

Utility Scripts

• uv run scripts/size_heat_exchanger.py --hot-in-c 150 --hot-out-c 90 --cold-in-c 25 --cold-out-c 90 --arrangement counterflow --u-w-m2-k 500 --duty-w 1e6 --output /tmp/hx.json

Procedure

1. Define hot/cold inlet and outlet temperatures (°C for the bundled script).
2. Decide arrangement: counterflow (preferred), co-current, or

multi-pass with F-factor.

1. Estimate U from either side's h (or vendor data); typical ranges:

liquid-liquid 300-1000 W/m²K, liquid-gas 30-100, condensing steam 2000-5000, boiling 1000-4000.

1. Provide duty from one stream's sensible heat (m × cp × ΔT) or pass

the value if known.

1. Run the script. Inspect area; round up to the nearest commercial

bundle size.

1. Cross-check by computing the other stream's duty and verifying the

energy balance closes.

Pitfalls

• Using bare LMTD without the F-factor for multi-pass exchangers.
• Picking U from a textbook table without flagging the fouling

factor (overstates U).

• Forgetting that LMTD blows up to ±infinity when terminal differences

approach zero (temperature pinch).

• Using sensible-heat duty for a two-phase service (latent heat

dominates).

• Reporting required area without acknowledging the fouling allowance.
• Choosing LMTD when one stream is isothermal (condensing / boiling) and

using F = 1 by default. Even for single-phase the F-factor is < 1 for multi-pass.

• Picking shell-and-tube as default; for liquid-liquid services with low

fouling, a plate-and-frame is often smaller and cheaper.

• Treating the calculated area as final without vendor-validated TEMA

layout and pressure-drop check.

Fallback Strategies

• If ht.LMTD is unavailable, the script falls back to the direct LMTD

formula and flags the substitution.

• If only one stream's enthalpy data is supplied, the script computes the

matching cold/hot side from the energy balance, but only when arrangement allows it.

Verification

• Run the listed script with representative inputs and an --output file when a deterministic calculation is available.
• Confirm the JSON result contains ok: true, expected units, and no unhandled warnings.
• Check result magnitudes against the stated assumptions, references, and a hand calculation or known operating range before reporting them.

References

• references/methods.md — assumptions, formulas, and U ranges.
• TEMA standards (procure separately).
• Sinnott / Coulson & Richardson Vol 6 for U guidance.

Anti-Patterns

• Reporting required area without specifying U and fouling factors.
• Using LMTD with widely different specific heats across the exchanger

(Cp varies strongly with T).

• Sizing without checking the temperature pinch.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: jskherman
• Source: jskherman/engg-skills
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.