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Heat Transfer

Air-Cooled Heat Exchanger Sizing Calculator

Air-cooled heat exchangers (fin-fan coolers) reject process heat to ambient air rather than cooling water. This calculator estimates preliminary heat-transfer area, air mass and volumetric flow, face area across bundles and bays, face velocity, and an indicative fan shaft and motor power for early-stage process checks. It is particularly relevant where cooling water is unavailable or where high-ambient conditions (e.g., Pilbara-style locations) dominate sizing. The result is a screening estimate only — it is not a vendor rating tool, not a fan selection tool, and not a TEMA/AS 1210 compliance calculator.

TypeInteractive calculator — separate from unit conversions

Calculator

Process side
kg/h
kJ/(kg·K)
°C
°C
Air side
°C
°C
kJ/(kg·K)
kg/m³
Thermal sizing
W/(m²·K)
%
Bundle / bay estimate
m
m
m/s
Fan estimate
Pa
%
%
Process duty870.833 kW
Air mass flow86.6501 kg/s
Air volumetric flow78.7728 m³/s
Air volumetric flow283582 m³/h
ΔT₁ (hot end)35 °C
ΔT₂ (cold end)15 °C
LMTD23.6045 K
Corrected LMTD21.244 K
Required area512.399 m²
Design area (with margin)589.259 m²
Total face area24 m²
Face velocity3.2822 m/s
Fan shaft power18.1783 kW
Motor power (with margin)19.9962 kW
Preliminary air-cooled screening: required area ≈ 512.399 m² before margin and 589.259 m² with margin, across 24 m² of face area. Bay count is a rough screening estimate only.

Preliminary screening estimate only. Not a vendor rating, fan curve, fan selection, air-side pressure drop design, noise, vibration, structural, AS 1210, or TEMA/API compliance tool. Final design requires vendor rating, equipment-specific geometry, process guarantees, project/client requirements, applicable standards, and qualified engineering review.

Related calculators: Heat Duty · LMTD · Fouling Factor · Heat Exchanger Area

Guides: Heat Exchanger Sizing · Cooling Water Sizing · LMTD vs NTU

References: U-Values · Fouling Factors · Min Approach · Design Margin · AS 1210

Formulas

Process duty
Q = ṁ_process × Cp_process × |T_in − T_out|
Air mass flow
ṁ_air = Q / (Cp_air × (T_air,out − T_air,in))
Air volumetric flow
V_air = ṁ_air / ρ_air
Terminal ΔT (hot end)
ΔT₁ = T_process,in − T_air,out
Terminal ΔT (cold end)
ΔT₂ = T_process,out − T_air,in
LMTD
LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂)
Corrected LMTD (crossflow)
ΔT_corr = F × LMTD
Required area
A = Q / (U × ΔT_corr)
Design area
A_design = A × (1 + margin / 100)
Total face area
A_face = w × h × bundles × bays
Face velocity
v_face = V_air / A_face
Fan shaft power
P_fan = (V_air × ΔP) / η_fan
Motor power
P_motor = P_fan × (1 + motor_margin / 100)

Diagram

Process in (hot)Process outAmbient air ↑FanFinned tube bundleA = Q / (U × F × LMTD) · v_face = V_air / A_face

Worked example

A process stream is cooled by air. Process: 25,000 kg/h, Cp = 4.18 kJ/(kg·K), 90 → 60 °C. Air: ambient 45 °C, outlet 55 °C, Cp = 1.005 kJ/(kg·K), ρ = 1.1 kg/m³. U = 80 W/(m²·K), F = 0.9 (crossflow), 15% design margin. Bundles 3 m × 2 m, 2 per bay, 2 bays. ΔP = 150 Pa, fan efficiency 65%, motor margin 10%.

  1. 01Q = (25,000 / 3600) × 4180 × 30 ≈ 870,800 W ≈ 870.8 kW
  2. 02ṁ_air = 870,800 / (1005 × 10) ≈ 86.65 kg/s
  3. 03V_air = 86.65 / 1.1 ≈ 78.77 m³/s (≈ 283,570 m³/h)
  4. 04ΔT₁ = 90 − 55 = 35 °C; ΔT₂ = 60 − 45 = 15 °C
  5. 05LMTD = (35 − 15) / ln(35/15) ≈ 23.61 K
  6. 06Corrected LMTD = 0.9 × 23.61 ≈ 21.25 K
  7. 07A = 870,800 / (80 × 21.25) ≈ 512.2 m²
  8. 08A_design = 512.2 × 1.15 ≈ 589.0 m²
  9. 09Face area = 3 × 2 × 2 × 2 = 24 m²
  10. 10v_face = 78.77 / 24 ≈ 3.28 m/s
  11. 11P_fan = 78.77 × 150 / 0.65 ≈ 18.18 kW
  12. 12P_motor = 18.18 × 1.10 ≈ 20.00 kW
Result

Preliminary screening: duty ≈ 870.8 kW, design area ≈ 589 m² across 2 bays × 2 bundles (24 m² face area, face velocity ≈ 3.28 m/s), motor power ≈ 20 kW including 10% motor margin.

FAQ

Why are air-cooled U-values so much lower than water-cooled?
Air has a much lower heat capacity and thermal conductivity than water, so the air-side film coefficient is the dominant resistance. Typical air-cooled U-values fall well below water-cooled values, which is why air-cooled exchangers require much larger surface area for the same duty. Confirm against the Typical U-Values Reference.
How sensitive is the size to ambient temperature?
Very sensitive. The corrected LMTD depends on the terminal temperature differences, so a small rise in ambient (e.g., from 35 °C to 45 °C) can significantly reduce LMTD and increase required area. For Australian or Pilbara-style sites, use the design summer ambient, not the annual average.
Is this a fan selection tool?
No. The fan power result is an indicative shaft power from V_air × ΔP / η_fan with a user-supplied air-side ΔP and fan efficiency. It does not select a fan from a vendor curve, account for system effects, or assess noise or vibration.
What face velocity range is sensible?
Typical face velocities for forced-draft air-cooled exchangers fall in the 2.5–4 m/s range. Very low face velocities suggest oversized face area; very high face velocities (above ~5 m/s) flag likely high pressure drop, noise, and fan power. The calculator flags these cases as warnings.
Does this replace a vendor rating?
No. Air-cooled exchanger rating depends on tube/fin geometry, row count, bundle pressure drop, fan and louvre performance, and seasonal ambient profiles. Final design requires vendor rating software (e.g., HTRI Xace or equivalent), project ambient design data, and qualified engineering review.

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