An air-cooled exchanger is used to cool fluids with ambient air. 1 inch OD tube is the most popular diameter and the most common fins are 1/2 inch or 5/8 inch high. Tube configuration used in this guideline : 1 inch OD tube, 5/8 inch fin height, 10 fins per inch and 2.5 inch triangular pitch.

**APSF – ** External area in ft²/ft² of bundle face area.

Rows | APSF | Face Velocity, ft/min |
---|---|---|

3 | 80.4 | 700 |

4 | 107.2 | 660 |

5 | 134.0 | 625 |

6 | 160.8 | 600 |

**APF – ** Total external area/ ft of fintube in ft²/ft ~ 5.58.

**Face Velocity – ** Typical air face velocities (V_{Face} ) used in design are tabulated above, these value result in optimum cost of exchanger.

**Obtain Process Parameters**

Obtain process duty (Q), hot process side inlet (T1) and outlet (T2) temperature. Select an overall heat transfer coefficient (U) from literature based on type of fluids. Select an air inlet temperature (t1) that is not exceeded during a certain percentage of time over the year (e.g. 95% of the time).

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**Calculate Air Density**

Density at air inlet temperature and site elevation

`ρ`

_{o}= 14.696 x 29 /(10.7316 x (t1 + 459.67))`ρ`

_{Air}/ ρ_{o}= exp(-29 x z/ (1545 x (t1 + 459.67)))

where,

t1: Air Inlet Temperature, °F

z : Elevation above sea level, feet

**Calculate MTD**

Assume an Air outlet temperature (t2) and calculate LMTD.

`LMTD = ((T1-t2) -(T2-t1))/ ln((T1-t2)/(T2-t1))`

`R = (T1 - T2)/(t2 - t1)`

`S = (t2 - t1)/(T1 - t1)`

LMTD Correction factor (F) is estimated based on following graphs

**Cross Flow Single Pass**

**Cross Flow Two Pass**

**Calculate Air Flowrate**

Finned area is estimated using following equation :

`A`_{Finned} = Q / (U * F * LMTD)

Bundle Face area is estimated using following equation :

`A`_{Face} = A_{Finned} / APSF

Air Flow is estimated using following relation :

`V`_{Air} = A_{Face} * V_{Face}

Air Mass flowrate is estimated :

`M`_{Air} = V_{Air} * ρ_{Air}

Air temperature rise is calculated :

`ΔT = Q / (M`_{Air} * Cp_{Air} )

Revised air outlet temperature is calculated :

`t2 = t1 + ΔT`

This temperature is again used in above steps to re-estimate air outlet temperature. Above steps are iterated till there is no change in air outlet temperature.

**Air Cooler Dimension**

Air cooler width is calculated :

`W = A`_{Face} / L_{Tube}

Number of Tubes are calculated :

`N`_{Tube} = A_{Finned} / (APF * L_{Tube})

Number of Tubes per Row are calculated :

`Nr `_{Tube} = N_{Tube} / No of Rows

where,

L_{Tube} : Length of Tube

**Air Side Pressure Drop**

Air Side pressure drop is calculated as following :

`ΔP `_{Total} = ΔP_{Static} + ΔP_{Velocity}

**Static Pressure Drop**

Pressure drop across tube bundle.

`ΔP`

_{Static}= F_{P}* No of Rows / D_{R}`F`

_{P}= 6*10^{-8}* ( G_{Face})^{1.825}

where,

D_{R} : ρ_{Air} / ρ_{Air at seal level and 70°F}

G_{Face} : Air face mass velocity in lb/h.ft² face area

ΔP_{Static} : Static pressure drop in inch of H2O

**Velocity Pressure Drop**

Typically 2 fans are used in air cooler. Fan area per fan (FAPF) is calculated as following :

`FAPF = 0.4 * A`_{Face} / No. of Fans

Fan Diameter is calculated :

`D = (4 * FAPF / π )`

^{0.5}`ΔP`

_{Velocity}= (ACFM / (4005 * (π* D^{2}/4)) )^{2}* D_{R}

where,

ACFM : Air flowrate in Actual Cubic Feet per Minute

ΔP_{Velocity} : Velocity pressure drop in inch of H2O

**Power Calculation**

Break power for fan is calculated as following :

`BHP = ΔP `_{Total} * ACFM / 6356 / η_{Fan}

Motor power is calculated as following :

`Power = BHP / η`_{Motor}

Spreadsheet for Shortcut Air Cooled Heat Exchanger Design

### References

- Process Heat Transfer: Principles and Applications,2007, Robert W Serth
- Handbook of Chemical Engineering Calculations, Nicholas P Chopey
- Rules of Thumb for Chemical Engineers, Carl R Branan
- GPSA, Engineering Databook, 12
^{th}Edition FPS