Evaporator Coil Sizing for Refrigeration Systems

Direct expansion (DX) evaporator coils are critical components in refrigeration and air conditioning systems. Proper sizing ensures efficient operation, adequate capacity, and reliable performance.

Understanding Evaporator Operation

In a DX evaporator:

Low-pressure liquid refrigerant enters the coil

Heat from air causes refrigerant to boil (two-phase region)

Vapor continues to absorb heat (superheat region)

Superheated vapor exits to the compressor

Heat Transfer Zones

Two-Phase (Evaporating) Zone

Highest heat transfer coefficients

Typically 80-90% of coil area

Governed by boiling correlations

Superheated Zone

Lower heat transfer coefficients

Typically 10-20% of coil area

Ensures no liquid reaches compressor

Key Sizing Parameters

Evaporating Temperature

Determines refrigerant pressure

Affects compressor efficiency

Typical values:

- Air conditioning: 5-10°C
- Medium temp refrigeration: -5 to 0°C
- Low temp refrigeration: -35 to -25°C

Superheat

Protects compressor from liquid slugging

Typical values: 5-10 K

Higher superheat = lower efficiency

Approach Temperature

Difference between air outlet and evaporating temperature

Typical values: 3-8 K

Lower approach = larger coil

Two-Phase Heat Transfer

Shah Correlation

Widely used for evaporation in horizontal tubes:

h_tp = h_l × E

Where E is an enhancement factor based on:

Convection number (Co)

Boiling number (Bo)

Froude number (Fr)

Gungor-Winterton Correlation

Alternative correlation considering:

Nucleate boiling contribution

Convective boiling contribution

Flow regime effects

Refrigerant Selection Impact

RefrigerantGWPTypical h_tp (W/m²·K)Notes

R-410A20883000-5000Common in AC
R-134a14302500-4000Automotive, chillers
R-404A39222800-4500Commercial refrigeration
R-29033500-5500Natural refrigerant
R-326753200-5200Lower GWP alternative

Circuiting Design

Number of Circuits

More circuits = lower refrigerant velocity

Fewer circuits = better heat transfer but higher pressure drop

Balance for optimal performance

Circuit Length

Longer circuits = more pressure drop

Shorter circuits = better distribution

Typical: 3-6 m per circuit

Feed Method

Distributor: Best distribution, higher cost

Direct feed: Simpler, potential maldistribution

Pressure Drop Considerations

Refrigerant-Side

Two-phase pressure drop significant

Affects evaporating temperature along coil

Target: 20-50 kPa total

Air-Side

Affects fan selection

Consider frost accumulation

Target: 50-150 Pa

Superheat Control

Thermostatic Expansion Valve (TXV)

Maintains constant superheat

Self-regulating

Most common method

Electronic Expansion Valve (EEV)

Precise control

Variable superheat setpoint

Higher efficiency potential

Design Checklist

☐ Define cooling capacity requirement

☐ Select refrigerant and operating conditions

☐ Determine air flow rate and inlet conditions

☐ Calculate required surface area

☐ Select tube and fin geometry

☐ Design circuiting arrangement

☐ Verify pressure drops

☐ Check superheat adequacy

☐ Consider defrost requirements

☐ Validate with simulation software

Common Issues and Solutions

Insufficient Capacity

Increase coil size

Add rows or face area

Improve air distribution

Poor Superheat Control

Check expansion valve sizing

Verify sensor location

Consider EEV upgrade

Uneven Frost Formation

Improve air distribution

Check refrigerant distribution

Verify defrost coverage

Conclusion

Evaporator coil sizing requires careful consideration of thermal, hydraulic, and practical factors. Using validated calculation methods and professional software ensures reliable designs that meet performance requirements.