Air Cooled Heat Exchanger Design Calculation

Air Cooled Heat Exchanger Design Calculation

An air cooled heat exchangers removes heat from hot liquid or gas using air.
Instead of cooling water, fans blow air across finned tubes to cool the fluid.
If you are looking for an industrial unit, check our Air Cooled Heat Exchangers Manufacturer.

These heat exchangers are used in:

1. Oil & gas plants
2. HVAC
3. Compressors
4. Power plants
5. Chemical industries

Correct design calculation is important.
If size is wrong → fluid will not cool properly and power consumption increases.

Step 1 — Heat Duty (How Much Heat to Remove)

First calculate how much heat must be removed.

Formula

Q = m × Cp × (Tin − Tout)
Where:

• Q = heat load (kW)
• m = flow rate (kg/s)
• Cp = specific heat
• Tin = inlet temperature
• Tout = outlet temperature

Example

Flow rate = 5 kg/s
Cp = 2.1
Tin = 120°C
Tout = 80°C

Q = 5 × 2.1 × (120 − 80)
Q = 420 kW
👉 The cooler must remove 420 kW heat

In real applications this heat duty decides the size of the air cooler unit required for the process.

Step 2 — Temperature Difference (LMTD)

Cooling depends on temperature difference between hot fluid and air.

ΔT1 = Hot in − Air out
ΔT2 = Hot out − Air in
LMTD = (ΔT1 − ΔT2) / ln(ΔT1/ΔT2)

Example

Hot in = 120°C
Hot out = 80°C
Air in = 35°C
Air out = 55°C
ΔT1 = 120 − 55 = 65°C

ΔT2 = 80 − 35 = 45°C

LMTD ≈ 54°C

Step 3 — Overall Heat Transfer Value (U)

Typical values:

Fluid	U Value (W/m²°C)
Gas	20–40
Oil	30–70
Water	50–100

Assume:
U = 45 W/m²°C

Step 4 — Required Surface Area

Area formula:

A = Q / (U × LMTD)

A = 420000 / (45 × 54)
A ≈ 172 m²

Required finned tube area = 172 m²

The surface area directly influences how many tubes are arranged inside the cooling bundle.

Step 5 — Air Flow Required

Q = m_air × Cp_air × (Air out − Air in)

Cp air ≈ 1.005
420 = m × 1.005 × (55−35)
m ≈ 20.9 kg/s
Air density ≈ 1.2 kg/m³
Air flow ≈ 17.4 m³/s
This decides fan size

Step 6 — Number of Tubes

A = π × D × L × N

D = tube diameter
L = tube length
N = number of tubes
Example:

D = 25 mm
L = 6 m
N ≈ 365 tubes

Important Points

1. Choose correct fin type
2. Consider summer ambient temperature
3. Avoid air recirculation
4. Select proper fan power
5. Use correct tube material

Conclusion

Air cooled heat exchanger design includes:

1. Heat duty calculation
2. LMTD calculation
3. Surface area calculation
4. Air flow calculation
5. Tube count selection

Proper calculation gives better cooling, low power use and long equipment life.

Air Cooled Heat Exchanger — FAQ

1. What is an air cooled heat exchanger?

An air cooled heat exchanger is equipment that cools hot fluid using atmospheric air. Fans blow air over finned tubes and remove heat without using cooling water.

2. Why use an air cooled heat exchanger instead of water cooled?

Because it saves water and reduces corrosion problems. It is best for dry areas and remote plants where cooling water is not available.

3. What is the basic formula for heat duty calculation?

Heat duty is calculated using:

Q = m × Cp × (Tin − Tout)
This tells how much heat must be removed from the fluid.

4. What is LMTD in air cooled heat exchanger?

LMTD (Log Mean Temperature Difference) is the average temperature difference between hot fluid and air.
It decides the size of the heat exchanger.

5. What affects air cooled heat exchanger performance?

Main factors:

1. Ambient air temperature
2. Air flow rate
3. Fin surface area
4. Fouling on tubes
5. Fan speed

6. What is the typical overall heat transfer coefficient (U value)?

Typical values:

1. Gas cooling: 20–40 W/m²°C
2. Oil cooling: 30–70 W/m²°C
3. Water cooling: 50–100 W/m²°C

7. How do you select fan size?

Fan size depends on required air flow rate (m³/s).
Higher heat load requires larger airflow and higher fan power.