Air-Cooled Condenser Design for HVAC Applications

Air-cooled condensers reject heat from refrigeration cycles to ambient air. This guide covers the design principles for efficient and reliable condenser coils.

Condenser Heat Transfer Zones

1. Desuperheating Zone (5-15% of area)

Vapor cooling from discharge to saturation

Single-phase heat transfer

Highest temperature difference

2. Condensing Zone (70-85% of area)

Two-phase condensation

Constant temperature (at saturation)

Highest heat transfer coefficients

3. Subcooling Zone (5-15% of area)

Liquid cooling below saturation

Single-phase heat transfer

Prevents flash gas in liquid line

Design Methodology

Step 1: Define Operating Conditions

Ambient temperature (design day)

Condensing temperature target

Refrigerant and capacity

Step 2: Calculate Heat Rejection

Q_cond = Q_evap + W_comp

Or using COP:
Q_cond = Q_evap × (1 + 1/COP)

Step 3: Determine Zone Loads

Desuperheating: Based on discharge superheat

Condensing: Latent heat of condensation

Subcooling: Based on desired subcooling

Step 4: Size Each Zone

Using appropriate correlations for each heat transfer mode.

Condensation Heat Transfer

Nusselt Correlation (Film Condensation)

For horizontal tubes:

h_cond = 0.725 × [ρ_l × (ρ_l - ρ_v) × g × h_fg × k_l³ / (μ_l × D × ΔT)]^0.25

Shah Correlation (In-Tube Condensation)

More accurate for refrigerants:

h_tp = h_l × [(1-x)^0.8 + (3.8 × x^0.76 × (1-x)^0.04) / p_r^0.38]

Air-Side Design

Face Velocity

Typical range: 2-3.5 m/s

Higher velocity = smaller coil but more fan power

Consider noise requirements

Fin Spacing

Standard: 10-14 FPI

Wider spacing for dusty environments

Consider cleaning accessibility

Number of Rows

Typical: 1-4 rows

More rows = higher capacity but diminishing returns

Balance with pressure drop

Subcooling Importance

Benefits of Subcooling

Prevents flash gas in liquid line

Increases system capacity

Improves expansion device performance

Typical Subcooling Values

Standard systems: 5-10 K

Long liquid lines: 10-15 K

High ambient variation: 8-12 K

Ambient Temperature Considerations

Design Conditions

Use 1% or 2% design temperature

Consider diurnal variation

Account for heat island effects

Part-Load Performance

Condensing pressure floats with ambient

Consider head pressure control

Evaluate annual energy consumption

Fan Selection

Axial Fans

Most common for air-cooled condensers

Lower pressure capability

Higher efficiency at low static

Centrifugal Fans

Higher pressure capability

Quieter operation

Used for indoor units

EC Motors

Variable speed capability

Higher efficiency

Better part-load performance

Practical Design Tips

Provide adequate clearance

- Inlet: 1.5× coil height minimum
- Outlet: 2× fan diameter minimum

Consider recirculation

- Avoid short-circuiting
- Use discharge hoods if needed

Plan for maintenance

- Access for coil cleaning
- Filter options for dusty environments

Account for altitude

- Air density decreases with altitude
- Adjust fan selection accordingly

Performance Verification

Key Metrics

Condensing temperature vs. ambient

Approach temperature (typically 10-15 K)

Subcooling achieved

Fan power consumption

Testing Standards

ASHRAE Standard 20

ARI Standard 460

EN 327/328

Conclusion

Effective condenser design balances thermal performance, fan power, space constraints, and cost. Modern calculation tools enable optimization across these parameters for efficient, reliable systems.