Heat Exchanger Calculator
Duty over a log mean — Q = U × A × LMTD, with counterflow scored against parallel on the same terminals.
Heat Exchanger Calculator
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What this result does not account for
- No correction factor — the ideal counterflow/parallel LMTD
- Constant cp and U across the exchanger
In short: Hot water falls 80 → 50 °C while cold water rises 20 → 40 °C: the counterflow log-mean difference is 34.760595 °C, and at U = 1,000 W/m²K over 2 m² the duty is 69,521.19 W. Run the same four terminals in parallel and the log mean collapses to 27.905531 °C — about 20% of the duty left on the table, which is why the industry builds counterflow.
Formula
Q = U × A × LMTD; LMTD = (ΔT₁−ΔT₂) ÷ ln(ΔT₁/ΔT₂)
A heat exchanger's driving force changes along its length, so the duty uses the logarithmic-mean of the end differences: counterflow takes ΔT₁ = hot in − cold out and ΔT₂ = hot out − cold in; parallel takes hot-in-vs-cold-in and hot-out-vs-cold-out. The overall coefficient U bundles every film and wall resistance — water-to-water exchangers run 800–1,500 W/m²K. Counterflow's log mean is never smaller than parallel's on the same terminals; that gap is the reason for the plumbing.
Worked Example
- Enter the four terminal temperatures from the duty spec.
- Enter U for the fluid pair and the transfer area.
- Read the duty, the log mean, and what parallel would forfeit.
Defaults: 69,521.19 W over a 34.760595 °C log mean. Drive the area to 10 m² and the duty scales to 347,605.95 W — area is the only linear lever in a nonlinear formula.
Strengths & Limits Of This Model
Where this engine is strong
- Parallel scored live on the same terminals
- The hot-side flow derived — the duty gets a plumbing number
Where it stops
- Multi-pass shells need the F-correction this page omits
Practical Use Cases
Duty checks
will this exchanger carry the load
Terminal design
counterflow vs parallel, priced
Fouling watch
duty falling means U falling
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Heat Exchanger Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why a logarithmic mean instead of a simple average?
Because the temperature difference decays exponentially along the exchanger — the ends do not contribute equally. The log mean is the constant difference that would transfer the same total heat as the varying one; the arithmetic mean overstates it whenever the end ratios are far apart.
Why does counterflow win?
Because it keeps the temperature profiles sliding in opposition: the coldest cold meets the coldest hot, keeping a difference alive the whole length. Parallel flow's streams converge on a middle temperature and the driving force dies early — on these terminals, 27.9 °C against 34.8 °C of log mean.
What is a typical U?
Water-to-water: 800–1,500 W/m²K. Water-to-oil: 100–600. Gas-to-gas: 10–40. The coefficient collapses every film, wall and fouling layer into one number; when duty drifts down over months, U is the number that moved — fouling, not the formula.
How do I get the flow rates from the duty?
From the energy balance: Q = ṁ · cp · ΔT. The hot side losing 30 °C at 69,523 W moves about 0.55 kg/s of water (cp ≈ 4,186 J/kgK) — the flow card prints it.
What if the end differences are equal?
Then ΔT₁ = ΔT₂ and the log mean is that same value — the formula's limit case. Equal end differences are the thermodynamic sweet spot; the log mean equals the arithmetic mean there and nowhere else.
Can the cold outlet exceed the hot outlet?
In counterflow, yes — it can approach the hot INLET. That crossover is exactly what parallel flow cannot do, and it is why process exchangers that need a tight approach are always counterflow.
Where does effectiveness fit?
Effectiveness is duty over the maximum possible duty — what the exchanger did against what infinite area would do. This page prices the actual duty at the given area and U; effectiveness needs the heat-capacity rates on both sides.
What does fouling do, numerically?
It stacks resistances inside U: a 0.0002 m²K/W fouling layer on both sides of a 1,000 U exchanger can pull the effective U down 20–30%. Duty falls with it, linearly — the log mean never moved.