Heat Exchanger Selection & Sizing

Q: Which standards or codes apply?

TEMA (shell & tube classes R/C/B), API 660 (S&T), API 661 (air-cooled), ASME VIII (pressure), ISO 16812.

Quick pick (TL;DR)

High pressure/temperature, fouling, large duty → shell & tube (TEMA).
Clean fluids, close temperature approach, compact → plate & frame.
No cooling water available → air-cooled.
Small duty → double-pipe.
Size with Q = U·A·LMTD: get the duty Q, the log-mean temperature difference (LMTD) with its correction factor F, and a realistic overall U, then add ~20% for fouling/margin.
Always use counter-current where possible for the biggest LMTD.

Step 1 — Choose the type

Type	Best for	Limit
Shell & tube	High P/T, fouling	Bulky
Plate & frame	Clean, close approach	P/T limited
Air-cooled	No water	Ambient-limited
Double pipe	Small duty	Area-limited

Step 2 — Worked sizing

Oil cooler, 1200 kW

Given hot oil : 120 → 60 °C cold water: 32 → 42 °C Q = 1200 kW , U = 450 W/m²K Step 1 — LMTD ΔT1 = 120 − 42 = 78 °C ΔT2 = 60 − 32 = 28 °C LMTD = (78 − 28) / ln(78/28) LMTD = 50 / 1.0245 = 48.8 °C Step 2 — Correction F 1-2 shell & tube → F = 0.90 Step 3 — Required area A = Q / (U · LMTD · F) A = 1,200,000 / (450 × 48.8 × 0.90) A = 1,200,000 / 19,764 = 60.7 m² Step 4 — Add margin ×1.20 fouling/margin → ~73 m² design

LMTD × F is the driving force; A = Q ÷ (U·LMTD·F). A realistic U and fouling margin matter most.

Heat exchanger area calculator

LMTD & surface area

Counter-current LMTD × F + fouling margin. Use a realistic overall U.

Hot in (°C)

Hot out (°C)

Cold in (°C)

Cold out (°C)

Heat duty Q (kW)

Overall U (W/m²K)

Correction F

Margin (%)

Screening — rigorous rating (TEMA/HTRI) computes U, true F & tube count.

Reference — typical overall U (W/m²K)

Service	U (W/m²K)
Water ↔ water	800–1,500
Steam ↔ water	1,000–2,000
Water ↔ light oil	300–500
Water ↔ heavy oil	100–300
HC condenser	400–800
Gas ↔ gas	10–50

Indicative — true U from a TEMA/HTRI rating.

Decision flow

1. LMTD from terminal temps

2. A = Q ÷ (U·LMTD·F) + margin

F ≥ 0.8One shell OK

F < 0.8 / crossAdd shells in series

TypeS&T / plate / air-cooled

Counter-current gives the biggest LMTD; an optimistic U is the No.1 failure cause.

Step 3 — Flow & construction

Counter-current for the largest LMTD.
TEMA type by maintenance & fouling.
Fouling fluid on the cleanable side.

Common mistakes

Optimistic U, no fouling margin.
Forgetting F on multipass.
Temperature cross (F < 0.8).

Requisition & datasheet

Service, fluids, temperatures, Q.
Type, LMTD, U, area, TEMA, materials.

Frequently asked questions

What should I know about Heat Exchanger Selection & Sizing?

High pressure/temperature, fouling, large duty → shell & tube (TEMA). Clean fluids, close temperature approach, compact → plate & frame. No cooling water available → air-cooled. Small duty → double-pipe. Size with Q = U·A·LMTD: get the duty Q, the log-mean temperature difference (LMTD) with its correction factor F, and a realistic overall U, then add ~20% for fouling/margin. Always use counter-current where possible for the biggest LMTD.

Which standards or codes apply?