Air Cooler / Fin-Fan Design

1. Air-Cooled Heat Exchanger Overview

Air-cooled heat exchangers (ACHEs), commonly called fin-fan coolers, reject process heat to ambient air rather than cooling water. They are the standard cooling solution in midstream gas processing, pipeline compressor stations, and remote production facilities where cooling water is unavailable, expensive, or impractical.

Components of an ACHE

A typical air cooler consists of finned tube bundles mounted in a structural frame above axial fans. The major components include:

• Tube bundle: Finned tubes arranged in rows (typically 3-6 rows deep). Tubes are usually 1-inch OD carbon steel or alloy with extruded or embedded aluminum fins.
• Headers: Pressure-containing boxes at each end of the tube bundle. Plug-type headers allow individual tube access; cover-plate headers allow easier cleaning.
• Fans: Axial flow fans, typically 4-16 ft diameter, driven by electric motors through V-belts or gear drives. Usually 2 fans per bay.
• Structure: Steel frame supporting the tube bundles at elevation (typically 10-15 ft above grade) with adequate clearance for air intake.
• Plenum: The enclosure between the fans and tube bundle that distributes air flow uniformly across the bundle face.
• Louvers: Adjustable blades for air flow control, winterization, and process temperature regulation.

Forced Draft vs. Induced Draft

Feature	Forced Draft	Induced Draft
Fan location	Below tube bundle	Above tube bundle
Air distribution	Less uniform	More uniform
Hot air recirculation	More likely	Less likely (higher exit velocity)
Fan motor environment	Ambient air (cool)	Hot discharge air
Maintenance access	Easy (ground level)	Difficult (elevated)
Weather protection	Bundle exposed	Bundle protected by fans
Power consumption	Lower (cooler air at fan)	Higher (hot air at fan, lower density)
Best for	Tprocess < 300°F	Tprocess > 300°F

Common Midstream Applications

Application	Typical Fluid	Process Temp (°F)	Typical U (Btu/hr·ft²·°F)
Compressor discharge cooler	Natural gas	200–350	5–12
Gas plant aftercooler	Sales gas	120–200	6–10
Amine cooler	Lean amine	150–200	40–70
Glycol cooler	Lean TEG	180–250	35–60
Produced water cooler	Produced water	140–200	50–80
Oil/condensate cooler	Light hydrocarbons	150–300	40–75
Process condenser	Mixed phase HC	100–250	20–50

U values are approximate design values including fouling. Actual values depend on fluid properties, flow rate, and fin geometry.

2. Thermal Design

The thermal design of an air cooler follows the fundamental heat exchanger equation Q = U × A × ΔT_m, but with important differences from shell-and-tube exchangers due to the crossflow arrangement and extended fin surface.

Fundamental Equation

Log Mean Temperature Difference (LMTD)

For an air cooler in crossflow arrangement, LMTD is calculated using the hot fluid and air temperatures:

Air Outlet Temperature

The air outlet temperature depends on the air flow rate and heat duty. A common initial estimate for sizing purposes:

LMTD Correction Factor (F)

Air coolers operate in crossflow, not true counterflow. The F correction factor accounts for this less-efficient arrangement. For ACHEs, F depends on the number of tube rows and pass configuration:

Tube Rows	F Factor (typical)	Notes
3 rows	0.85–0.90	Minimum for gas cooling
4 rows	0.88–0.92	Most common configuration
5 rows	0.90–0.94	Better for condensing service
6 rows	0.91–0.95	Maximum practical rows; high air-side DP

Overall Heat Transfer Coefficient (U)

The overall U for air coolers is much lower than for shell-and-tube exchangers because the air-side film coefficient is low (5-15 Btu/hr·ft²·°F). Extended fin surfaces compensate by increasing air-side area.

Typical Fouling Factors

Service	Fouling Factor (hr·ft²·°F/Btu)
Clean gas (natural gas, sales gas)	0.001
Lean amine, lean glycol	0.001–0.002
Light hydrocarbon liquid	0.001–0.002
Rich amine, rich glycol	0.002–0.003
Produced water	0.002–0.003
Heavy oil, crude oil	0.003–0.005
Asphalt, tar, heavy residuals	0.005–0.010

Approach Temperature

The approach temperature is the difference between the process fluid outlet temperature and the ambient air inlet temperature. It is the most critical economic parameter in air cooler design:

Approach (°F)	Design Impact	Typical Application
> 30	Small, economical unit	Compressor discharge gas cooling
20–30	Moderate sizing, standard design	Most gas plant services
15–20	Larger unit, higher fan power	Amine/glycol coolers
10–15	Very large, expensive	Critical condensing service
< 10	Impractical for air cooling alone	Consider trim cooler or water spray

3. Mechanical Design

Finned Tube Construction

Air cooler tubes use external fins to increase the air-side surface area, compensating for the low air-side heat transfer coefficient. The most common fin types in midstream service:

Fin Type	Max Temp (°F)	Bond Quality	Application
Extruded (integral)	500	Excellent	Most gas plant services; best thermal bond
Embedded (grooved)	750	Very good	High temperature; allows alloy tubes
L-foot (tension wound)	350	Good	Lower cost; adequate for moderate temps
Welded	850+	Excellent	High temperature refinery service

Standard Tube Geometry

Extended Surface Ratio

The extended surface ratio is the total external area (fin area plus exposed tube area between fins) divided by the bare tube external area. This ratio typically ranges from 15:1 to 25:1 depending on fin geometry:

Fin Density (fins/in)	Approximate Ratio	Air-Side ΔP
7	14–16:1	Low
9	17–20:1	Moderate
10	19–22:1	Moderate
11	21–24:1	Higher
14	26–30:1	High

Header Types

Headers are pressure-containing vessels at each end of the tube bundle:

• Plug-type header: Individual threaded plugs for each tube. Allows tube-by-tube access for cleaning or plugging. Required by API 661 for most services with fouling potential.
• Cover-plate header: Removable cover plate for bundle access. Easier inspection but requires gasket replacement.
• Manifold header: Used for high-pressure service. Individual tubes welded to manifold pipe.
• Box header: Rectangular box with removable cover. Good for multi-pass arrangements.

Bay Dimensions

Parameter	Typical Range	Most Common
Bay width	4–16 ft	8–12 ft
Tube length	12–40 ft	24–30 ft
Bundle height	10–15 ft above grade	12 ft
Fan diameter	4–16 ft	8–14 ft
Fans per bay	1–4	2

4. Fan System Design

Air Flow Rate

The required air mass flow rate is determined by the heat duty and the desired air temperature rise:

Fan Horsepower

Fan power depends on the volumetric air flow rate and the static pressure drop across the tube bundle:

Static Pressure Components

Component	Typical ΔP (in. WC)
Tube bundle (air-side)	0.2–0.8
Plenum losses	0.05–0.15
Louvers (if installed)	0.05–0.20
Guard screen	0.02–0.05
Total	0.3–1.2

Fan Types and Drives

• Axial fans: Standard for ACHEs. Fixed-pitch or adjustable-pitch blades. 4-16 ft diameter.
• Belt drive: V-belt from motor to fan shaft. Simple, allows speed variation by sheave change. Most common for fans under 30 HP.
• Gear drive: Right-angle gear reducer. Required for larger fans. More expensive but longer life.
• Variable frequency drive (VFD): Allows continuous speed control for energy optimization and temperature control. Increasingly common.
• Auto-variable pitch: Pneumatically adjustable blade pitch for automatic temperature control without VFD.

Face Velocity

The air face velocity is the volumetric air flow divided by the face area of the tube bundle. It affects heat transfer, pressure drop, and noise:

Face Velocity (ft/min)	Characteristics
300–400	Low noise, low ΔP, oversized unit
400–600	Typical design range, good balance
600–800	Higher ΔP, more fan power, compact unit
> 800	Excessive noise and power; not recommended

5. API 661 Requirements

API Standard 661 covers the minimum requirements for the design, materials selection, fabrication, inspection, testing, and preparation for shipment of air-cooled heat exchangers for use in the petroleum, petrochemical, and natural gas industries.

Key API 661 Requirements

Design Temperature and Pressure

• Headers designed per ASME Section VIII, Division 1
• Minimum design pressure: 150 psig or maximum operating pressure plus 10%, whichever is greater
• Design temperature: maximum operating temperature plus a suitable margin
• Minimum design metal temperature (MDMT) per ASME code

Materials

• Tubes: Carbon steel (SA-214, SA-179) for most services; alloy per process requirements
• Fins: Aluminum alloy (typical); steel fins for high-temperature service
• Headers: Carbon steel (SA-516 Gr. 70) or alloy as required
• Plugs: AISI 4140 or equivalent, minimum hardness BHN 235

Design Ambient Temperature

• Must be specified by the purchaser
• Typically the site maximum dry-bulb temperature (summer 1% exceedance value)
• For winterization: specify minimum ambient for fan and louver design

Noise Requirements

• Maximum sound pressure level typically 85 dBA at 1 meter from equipment
• Fan tip speed should not exceed 12,000 ft/min
• Consider low-noise fan blades for units near occupied areas

API 661 Data Sheet Information

The following information is required on the API 661 data sheet (typically filled out by the process engineer):

Data Sheet Item	Description
Process conditions	Fluid type, inlet/outlet temperatures, flow rate, heat duty
Design conditions	Design pressure, design temperature, MDMT
Ambient conditions	Design dry-bulb temperature, site elevation
Materials of construction	Tube, fin, header materials; corrosion allowance
Mechanical requirements	Header type, tube plugging criteria, fouling factor
Fan and drive type	Forced/induced draft, belt/gear drive, motor specifications
Controls	Auto-variable pitch, VFD, louvers, temperature control
Winterization	Louvers, steam coils, recirculation ducts

6. Worked Example

Size an air cooler for a compressor discharge gas cooling application at a midstream gas plant.

Step 1: Estimate Air Outlet Temperature

Step 2: Calculate LMTD

Step 3: Apply F Correction Factor

Step 4: Select Overall U

Step 5: Calculate Required Bare Tube Area

Step 6: Air Flow Rate and Fan HP

Step 7: Summary

Parameter	Value
Required bare tube area	~26,200 ft²
LMTD (corrected)	48.1°F
Overall U	7.94 Btu/hr·ft²·°F
Approach temperature	35°F (130°F - 95°F)
Bay configuration	2 bays, 10 ft wide × 30 ft long
Air flow rate	537,634 lb/hr
Total fan motor HP	30 HP (4 × 7.5 HP motors)

7. Operations & Troubleshooting

Performance Monitoring

Regularly monitor these parameters to detect fouling, fan degradation, or other performance issues:

• Process outlet temperature: Rising outlet temperature at constant ambient indicates fouling or reduced air flow.
• Air-side pressure drop: Increasing ΔP indicates external fouling (dust, insects, cottonwood seeds) on the fin surface.
• Fan motor current: Decreasing current may indicate belt slipping, blade damage, or bearing failure.
• Vibration: Increasing vibration indicates fan imbalance, bearing wear, or loose components.

Common Problems and Solutions

Problem	Likely Cause	Solution
High outlet temperature	Fouling, low air flow, high ambient	Clean fins, check fans, verify ambient is within design
Uneven cooling	Air recirculation, wind effects	Add wind walls, check louver position, verify fan rotation
High noise	Blade tip speed, mechanical issues	Reduce fan speed, check blade pitch, inspect bearings
Tube leaks	Corrosion, vibration fatigue	Plug failed tubes, evaluate materials upgrade
Fan vibration	Blade damage, ice buildup, imbalance	Inspect blades, de-ice, rebalance fan assembly
Frozen tubes (winter)	Inadequate winterization	Install louvers, add recirculation ducts, use steam coils

Winterization Strategies

In cold climates (ambient below 32°F), air coolers require winterization to prevent process fluid freezing or excessive cooling:

• Louvers: Adjustable blades on the air intake or discharge side. Can partially or fully close to reduce air flow.
• Recirculation ducts: Internal ducts that recirculate warm discharge air back to the fan intake. Effective but adds cost and complexity.
• Variable-pitch fans: Auto-variable pitch (AVP) fans adjust blade angle to reduce air flow as ambient temperature drops. Preferred method per API 661.
• Fan cycling: Turn off one or more fans during cold weather. Simple but gives step changes in cooling.
• Steam coils: Preheat inlet air using steam coils at the air intake. Effective but requires steam supply.
• VFD control: Variable frequency drives allow continuous speed adjustment. Best for precise temperature control and energy savings.

Maintenance Schedule

Task	Frequency
Visual inspection (leaks, vibration, noise)	Weekly
Belt tension check	Monthly
Fan blade inspection	Quarterly
Fin cleaning (external)	Semi-annually or as needed
Bearing lubrication	Per manufacturer schedule
Motor insulation test (megger)	Annually
Vibration analysis	Quarterly
Header plug inspection	During turnaround
Tube thickness measurement	During turnaround