How to Size an Air Cooled Heat Exchanger for Cooling

Identify the Cooling Duty and Process Constraints

Start with the heat load. The cooling duty is the amount of thermal energy the exchanger must remove from the process stream.

Q = m * Cp * ΔT

Where Q is the heat load in watts, m is the mass flow rate in kg/s, Cp is the specific heat capacity in J/kg K, and ΔT is the temperature difference between the inlet and outlet of the process fluid.

Before sizing, confirm the operating conditions. Record the process fluid inlet and outlet temperatures, the flow rate, and the fluid properties at those temperatures. If the fluid changes phase, such as condensation, use the latent heat of vaporization instead of the sensible heat equation.

Also define the ambient air conditions. The maximum ambient dry-bulb temperature is the primary constraint. Humidity affects the wet-bulb temperature, which is the lower limit for air-cooled rejection. If the design must operate at high ambient temperatures, the required area increases.

Select a Flow Configuration

The flow arrangement changes the approach temperature and the required surface area.

Parallel flow is simpler to install but has a lower log mean temperature difference (LMTD) than counter flow. Counter flow maintains a more uniform temperature difference along the length of the tube bundle.

For air-cooled shells, the air passes across the tubes, and the process fluid flows inside the tubes. This is a cross-flow arrangement. The air velocity across the tube bank is the main variable you control.

Consider the tube orientation. Horizontal tubes are easier to drain and clean. Vertical tubes allow air to flow upward through the bundle, which can improve heat transfer in some cases. Choose based on the site layout, maintenance access, and the fluid type.

Estimate the Required Heat Transfer Area

The heat transfer area is the total surface area available for heat exchange between the process fluid and the air.

A = Q / (U * LMTD)

Where A is the heat transfer area in square meters, U is the overall heat transfer coefficient in W/m² K, and LMTD is the log mean temperature difference in K.

The overall heat transfer coefficient U combines the internal fluid resistance, the tube wall resistance, and the external air resistance. For air-cooled exchangers, the external air resistance is usually the largest contributor.

Typical U values for air-cooled exchangers range from 30 to 120 W/m² K depending on the air velocity and the process fluid. Higher air velocity increases U but also increases the pressure drop and fan power.

Use a conservative U value for the initial estimate. If you are unsure of the exact fluid properties, assume a lower U. This gives a larger area, which is safer than a smaller area that fails under real conditions.

Determine the Tube Bundle Dimensions

Convert the required area into physical dimensions.

Count the number of tubes. Each tube has an outer diameter and a length. The total area is the sum of the outer surface area of all tubes.

A = π * D_o * L * N

Where D_o is the tube outer diameter in meters, L is the tube length in meters, and N is the number of tubes.

Choose a tube diameter. Smaller tubes increase the surface area per unit volume but raise the pressure drop. Larger tubes reduce the pressure drop but reduce the surface area per unit volume. A common tube diameter for general service is 19 mm to 25 mm.

Set the tube length. Longer tubes reduce the number of tubes needed and lower the overall footprint. However, longer tubes are harder to clean and can have higher pressure drops. A tube length of 1.5 m to 2.5 m is common for air-cooled shells.

Select the Air Velocity and Fan

Air velocity is the speed of the air moving through the tube bundle. It directly affects the heat transfer coefficient U.

A typical air velocity for air-cooled exchangers is 1.5 to 4.5 m/s. Lower velocities reduce fan power and noise but require more area. Higher velocities increase heat transfer but increase fan power and can cause vibration or clogging if the fins are small.

Calculate the required air flow rate. The air must carry away the heat load. The air flow rate is determined by the heat load and the air temperature rise.

Q = m_air * Cp_air * ΔT_air

Where m_air is the mass flow rate of air in kg/s, Cp_air is the specific heat capacity of air in J/kg K, and ΔT_air is the temperature rise of the air.

The mass flow rate of air is related to the air velocity and the frontal area of the tube bundle.

m_air = ρ_air * v_air * A_frontal

Where ρ_air is the air density in kg/m³, v_air is the air velocity in m/s, and A_frontal is the frontal area of the tube bundle in square meters.

Select a fan that delivers the required air flow at the system resistance. The system resistance is the pressure drop caused by the tube bundle, the inlet and outlet headers, and any filters or screens. The fan curve must intersect the system resistance curve at the design point.

Apply the Safety Factors and Fouling

The calculated area is for clean, ideal conditions. Real systems are not ideal.

Apply a fouling factor to the U value. Fouling reduces the heat transfer coefficient. The fouling factor is a temperature difference correction. If the clean U is 80 W/m² K and the fouling factor is 0.8, the dirty U is 64 W/m² K.

Apply an area safety factor. A common practice is to increase the calculated area by 10% to 30% to account for fouling, manufacturing tolerances, and the maximum ambient temperature.

Also check the pressure drop. The pressure drop across the tube bundle must be compatible with the fan and the process fluid. If the pressure drop is too high, the fan power increases. If the pressure drop is too low, the heat transfer may be insufficient.

Verify the Final Design

Run a final check. Confirm the heat load matches the process requirements. Confirm the U value is appropriate for the selected air velocity and fluid properties. Confirm the LMTD is calculated correctly.

Check the fan operation at the maximum ambient temperature. The air density drops at higher temperatures, so the mass flow rate decreases. The heat transfer coefficient also changes. The fan must still deliver enough air to meet the duty.

Check the noise level. The air velocity and fan speed determine the noise. Keep the air velocity below the level that causes excessive noise at the operating point.

Finally, review the maintenance access. Can the tubes be cleaned? Can the fins be straightened? Can the fan be replaced? A well-sized exchanger that cannot be maintained will fail in the field.

Common Mistakes in Air-Cooled Sizing

A common mistake is using a single U value for all conditions. The U value changes with air velocity and fluid properties. If the process fluid is viscous, the internal resistance is higher, and the U value is lower.

Another mistake is ignoring the wet-bulb temperature. If the design must operate at high humidity, the air cannot cool below the wet-bulb temperature. This reduces the LMTD and increases the required area.

A third mistake is underestimating the fan power. The fan power is proportional to the air flow rate and the pressure drop. If the pressure drop is higher than expected, the fan power increases. Select a fan with a margin for the expected pressure drop.

A fourth mistake is not checking the tube length. If the tubes are too long, the pressure drop is too high. If the tubes are too short, the area is too small. Balance the tube length with the number of tubes.

Final Verification Checklist

1. Confirm the heat load calculation.
2. Confirm the flow configuration and LMTD.
3. Confirm the U value and fouling factor.
4. Confirm the required heat transfer area.
5. Confirm the tube bundle dimensions.
6. Confirm the air velocity and air flow rate.
7. Confirm the fan curve and system resistance.
8. Confirm the pressure drop and fan power.
9. Confirm the maintenance access.