Air Cooled Heat Exchanger

Professional Engineering Calculator for preliminary sizing, estimating Finned Area, Plot Size, Air Flowrate, and Total Fan Power.

US CUSTOMARY METRIC / SI

1. Project Data

Project Name

Tag Number

Engineer

Tube Material

Design Code

Design Press. (psi g)

Design Temp. (°F)

2. Process & Air Parameters

Service / Fluid Type

Heat Duty Input Mode:

Direct Entry From Flowrate & Cp

Heat Duty ($Q$, MMBtu/h)

Overall Heat Transfer Coeff ($U$, Btu/h.ft².°F)

Process Inlet Temp (°F)

Process Outlet Temp (°F)

Air Inlet Temp (°F)

Approach checks: Hot approach: — Cold approach: — Air rise: —

Air Face Velocity (ft/min) —

Site Elevation (ft)

3. Geometry & Efficiency

Standard Tube Presets:

Number of Tube Passes

Number of Tube Rows

Tube Length (ft)

Tube OD (in)

Wall Thickness (in)

Fin Height (in)

Tube Pitch (in)

Fins Per Inch (FPI)

Fan Efficiency (%)

Motor Efficiency (%)

Number of Fans

Fouling Resistances

Rf process-side (h·ft²·°F/Btu)

Rf air-side (h·ft²·°F/Btu)

—

Tube-Side Velocity Check

Process Fluid Density (lb/ft³)

Nozzle Velocity Limit (ft/s)

Enter fluid density to check velocity.

Nozzle Sizing

Nozzle Allowable Velocity (ft/s)

Enter fluid density and mass flow to size nozzles.

Bay Sizing

Standard Bay Width (ft)

Run calculation to see bay sizing.

Annual Energy Cost Estimate

Operating Hours / Year

Electricity Cost ($/kWh)

—

Calculation Summary

Thermal Performance

Air Outlet Temp -

LMTD (Uncorrected) -

Correction Factor (F) -

LMTD (Corrected) -

Air Temperature Rise -

Fin Efficiency

η_fin (Schmidt) -

η_overall (surface) -

Geometry Details

Finned Area -

Face Area -

Total Tubes -

Tubes per Row -

Air Cooler Width -

No. of Bays -

Oversurface % -

Plot Footprint -

Power Density -

Tube-Side Velocity

Tube ID -

Total Flow Area -

Aerodynamics & Power

Air Flowrate -

Fan Diameter -

Total Pressure Drop -

Fan Brake Power (Total) -

Motor Power (Total) -

Motor Power (Per Fan) -

Note: Calculation relies on an iterative energy balance to match LMTD-based duty with air-side mass flow duty, dynamically incorporating the structural layout and pitch provided.

Engineering Reference & Technical Basis

1. Heat Transfer Relationships

The fundamental design equation relates required heat transfer area to Duty, U-value, and Corrected Log Mean Temperature Difference (MTD). The iterative solver establishes equilibrium between heat duty and air-side thermal absorption.

A_{bare} = \frac{Q}{U \cdot MTD} \qquad MTD = LMTD \times F$$ $$Q = m_{air} \cdot C_{p, air} \cdot (T_{air, out} - T_{air, in})

Where LMTD for counter-current flow is:

LMTD = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)}

U-value convention: The overall heat transfer coefficient U entered here must be referenced to the total finned (extended) surface area — i.e., the same area as $A_{bare}$ computed by this calculator. Typical values for air coolers on a finned-area basis range from 4–12 Btu/(h·ft²·°F) [23–68 W/(m²·K)]. Do not enter U on a bare-tube basis (which would be 15–21× higher for standard API 661 geometry with 10 FPI, 0.625" fins on 1" OD tubes — the finned area is approximately 21× the bare tube area for this configuration).

2. Fin Geometry & Extended Surface

The surface area per unit length of tube $A_f$ is derived from the user-provided tube OD ($OD_{in}$), fin height ($H_f$), and Fins Per Inch (FPI). Using $t_f = 0.016"$ fin thickness (standard aluminium):

D_f = OD_{in} + 2 H_f$$ $$A_{fin/ft} = (FPI \cdot 12) \cdot \frac{2 \cdot \pi (D_f^2 - OD_{in}^2)}{4 \times 144}$$ $$A_{bare/ft} = \frac{\pi \cdot OD_{in}}{12} \cdot (1 - FPI \cdot t_f)$$ $$A_f = A_{fin/ft} + A_{bare/ft}

The Face Area Factor relates the total extended finned surface to the frontal flow area, directly incorporating the tube pitch ($P_t$) and rows ($N_{rows}$):

AreaF = \left(\frac{12}{P_{t, in}}\right) \times N_{rows} \times A_f

3. Correction Factor (F)

Because air coolers represent crossflow conditions, a correction factor $F$ based on Bowman, Mueller, and Nagle charts is numerically evaluated across the iteration loop using polynomials corresponding to the temperature efficiency ($P$) and heat capacity rate ratio ($R$).

R = \frac{T_{hot, in} - T_{hot, out}}{T_{air, out} - T_{air, in}} \qquad P = \frac{T_{air, out} - T_{air, in}}{T_{hot, in} - T_{air, in}}

4. Aerodynamics & Elevation Correction

Air density is corrected for site elevation using the barometric formula. Required fan brake power is determined from the volumetric airflow rate and total pressure drop (static + velocity).

\rho_{alt} = \rho_{sea} \times e^{-\frac{29 \cdot Z}{1545 \cdot (T_{air}+459.67)}}$$ $$BHP = \frac{Q_{air} \cdot \Delta P_{total}}{6356 \cdot \eta_{fan}}

Static pressure drop across the tube bank:

\Delta P_{static} = \frac{6 \times 10^{-8} \cdot G_a^{1.825} \cdot N_{rows}}{D_R}

ΔP correlation: The static pressure drop equation uses an empirical correlation (C = 6×10⁻⁸, exponent 1.825) based on Hudson/Brown preliminary design charts for standard 10 FPI finned tube bundles (Serth, Process Heat Transfer, Ch. 12). Accuracy is ±30% — suitable for preliminary fan power sizing only. $D_R = \rho_{air}/0.075$ is the air density ratio relative to standard air at sea level.

5. Assumptions & Limitations

Air Cp = 0.241 Btu/lb·°F — dry air at ambient conditions. Humid air on high wet-bulb sites may deviate by 1–3%.
Fin efficiency — computed using Schmidt (1945) approximation with assumed air-side film coefficient h = 10 Btu/h·ft²·°F and aluminium fin conductivity k = 110 Btu/h·ft·°F. Actual h depends on face velocity and fin geometry.
Fan coverage — fan diameter is computed assuming 40% of face area per fan (API 661 §4.2.2 minimum). Actual coverage is design-specific.
Static ΔP — empirical Hudson/Brown correlation, ±30% accuracy, calibrated for standard 10 FPI aluminium finned bundles. Accuracy decreases for non-standard FPI.
LMTD method — assumes pure counter-current flow with F-factor correction. Valid for standard multi-row bundles. Does not account for maldistribution.
All results are for preliminary sizing only and must be verified by a qualified engineer before use in detailed design.

References

API Standard 661, 7th Edition: Petroleum, Petrochemical, and Natural Gas Industries — Air-cooled Heat Exchangers for General Refinery Service. American Petroleum Institute.
GPSA Engineering Data Book, 13th Edition: Section 9 — Heat Exchangers. Gas Processors Suppliers Association.
Mukherjee, R. (1997): "Effectively Design Air-Cooled Heat Exchangers", Chemical Engineering Progress, Vol. 93, No. 2. — Primary reference for ACHE preliminary sizing methodology, U-value ranges, and FPI selection.
Brown, R. (1978): "Design of Air-Cooled Exchangers — A Procedure for Preliminary Estimates", Chemical Engineering, March. — Source of empirical air-side pressure drop correlation constants.
Serth, R.W. & Lestina, T.: Process Heat Transfer: Principles, Applications and Rules of Thumb, 2nd ed., Academic Press. — Fin efficiency, extended surface area, and pressure drop methods.
Schmidt, T.E. (1945): "La production calorifique des surfaces munies d'ailettes", Bulletin de l'Institut International du Froid, Annexe G-5. — Schmidt approximation for annular fin efficiency (η_fin = tanh(mL)/(mL)).
Bowman, R.A., Mueller, A.C., & Nagle, W.M. (1940): "Mean Temperature Difference in Design", Trans. ASME, Vol. 62. — Basis for F-factor correction charts; polynomial coefficients from Mason (1954).
AMCA Standard 210: Laboratory Methods of Testing Fans for Aerodynamic Performance Rating. — Basis for fan BHP formula: BHP = Q(CFM) × ΔP(in H₂O) / (6356 × η_fan).