Sizing a heat exchanger is energy balance first, geometry second. You calculate the duty the process needs, the temperature difference available to drive that duty, and a defensible overall coefficient including fouling. Area follows. Tube count, length, shell diameter, baffles, and passes are then iterated until pressure drop, vibration, and mechanical design close. Ohm does not publish a single area or kW envelope; the arithmetic below is the method, not a shop performance claim.
Step 1 — Duty: Q = m · Cp · ΔT
For a sensible-heat stream, Q = m · Cp · ΔT, using the mass flow and temperature change of that stream. Both sides must agree within heat-loss allowance: the heat leaving the hot stream equals the heat entering the cold stream. Use a consistent Cp at the stream's average temperature, or integrate if Cp changes sharply.
Phase change: Q includes m · λ (latent heat) plus any desuperheat or subcooling. A condenser datasheet that gives only 'inlet vapour at T' without quality, pressure, and coolant rise cannot be sized.
Illustrative process arithmetic — not an Ohm rating: a liquid at 8 000 kg/h, Cp 2.4 kJ/kg·K, cooled 25 K has Q = 8 000 × 2.4 × 25 / 3600 ≈ 133 kW. That number belongs on your datasheet as the required duty. It is not a catalogue capacity.
Step 2 — LMTD and Ft
For counter-current flow, LMTD = (ΔT1 − ΔT2) / ln(ΔT1 / ΔT2). ΔT1 and ΔT2 are the differences at the two ends. If either end difference is very small, LMTD collapses and area grows rapidly — that is a process problem (approach too tight), not a fabrication problem.
Multi-pass 1-2 exchangers use Ft applied to the counter-flow LMTD. Ft comes from P and R parameters (temperature effectiveness and capacity-ratio). If Ft is low, change arrangement (series shells, F-shell, fewer tube passes) before inflating area. Do not 'add 20% area' to hide a temperature cross.
Step 3 — Overall U (clean and dirty)
1/U_dirty = 1/h_o + Rf_o + wall resistance + Rf_i × (area ratio) + 1/h_i × (area ratio), with outside/inside referred to a consistent area basis. Film coefficients h depend on fluid, velocity, viscosity, and whether the service is boiling or condensing. Fouling resistances Rf are design allowances from TEMA tables, plant history, or the licensor — not guesses labelled as 'Ohm standard U'.
Order-of-magnitude industrial ranges (literature, not shop guarantees): water-to-water tubular units often sit in the high hundreds to a few thousand W/m²·K when clean; viscous organics and gases sit much lower; condensing steam on one side can be high on that film and still be limited by the other side or by fouling. A vendor U without stated fouling, velocity, and geometry is not a sizing input.
Ask for U_clean and U_dirty (or overdesign factor Uc/Ud) on the quotation. Stacking TEMA fouling plus '10% extra area' plus a conservative U is how units become oversized, bypass, and vibrate at part load.
Step 4 — Area, then geometry
Required outside area A = Q / (U_dirty · LMTD · Ft). Geometry converts area into N_tubes × π × OD × effective length (plus U-bend allowance if applicable). Then check:
- Tube-side velocity (typically targeted in a band that balances film coefficient, erosion, and fouling — the process engineer owns the limits).
- Tube-side and shell-side ΔP versus allowable.
- Baffle spacing versus TEMA unsupported span and vibration.
- Shell diameter versus plot, nozzle sizes, and (for kettles) disengagement.
- Whether one shell is enough or series/parallel is required.
What data to send — the sizing pack
| Item | Why the designer needs it |
|---|---|
| Fluid names, phase, composition if multi-component | Properties, fouling class, MOC, condensation/boiling curve |
| Mass or volume flow with basis | Duty and velocity |
| T_in and T_out each side, or duty plus the free variable | Q and LMTD |
| Operating and design P and T | Mechanical design and properties |
| Allowable ΔP each side (nozzles in or out) | Baffles, passes, nozzle size |
| Fouling resistances or plant standard | U_dirty and overdesign |
| Preferred tube OD / length / TEMA type if any | Plot fit and plant spare philosophy |
| Materials and named code | Wall thickness, not just area |
What sizing is not
It is not picking a kW from a website. It is not copying an old exchanger's area if the process case changed. It is not 'U = 500, add 30%'. Replacement units should include the existing geometry and current performance so the new rating can attack the reason the old one failed. Ohm sizes from the enquiry data or from an approved customer drawing — not from an unpublished rating chart.
RFQ for a sized unit
Put the duty case on a datasheet (or use /products/heat-exchangers/datasheet as a prompt for fields) and submit it at /rfq?product=heat-exchangers. If you already have an HTRI/EDR file or a marked-up API 660 sheet, attach it. If you only have flows and temperatures, send those — the missing properties will be flagged rather than invented. Mechanical design to TEMA practice and ASME VIII-1 or IS 2825 follows when those documents are named on the order.
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