Low-GWP Refrigerants and Their Impact on Heat Exchanger Design

The HVAC and refrigeration industry is undergoing a major transition away from high-GWP (Global Warming Potential) refrigerants like R-410A (GWP = 2,088) and R-134a (GWP = 1,430) toward alternatives with significantly lower environmental impact. This transition has profound implications for heat exchanger design, materials selection, and system safety.

The Regulatory Landscape

The Kigali Amendment to the Montreal Protocol (2016) mandates an 80-85% reduction in HFC consumption by 2047. In the United States, the AIM Act requires a phasedown of HFC production and consumption. The European F-Gas Regulation has been even more aggressive, with bans on high-GWP refrigerants in many applications already in effect.

Starting January 1, 2025, new residential and commercial air conditioning systems in the US must use refrigerants with GWP below 750, effectively ending the use of R-410A in new equipment.

Key Low-GWP Alternatives

R-32 (GWP = 675)

R-32 (difluoromethane) is a single-component refrigerant that is actually one of the two components of R-410A (which is a 50/50 blend of R-32 and R-125). Key characteristics:

Higher volumetric capacity: ~10% higher than R-410A, allowing more compact heat exchangers

Higher discharge temperature: Requires careful compressor and system design

Mildly flammable (A2L): Requires safety measures but is manageable in most applications

Better heat transfer: Higher thermal conductivity improves two-phase heat transfer coefficients

R-454B (GWP = 466)

R-454B is a zeotropic blend of R-32 (68.9%) and R-1234yf (31.1%). It was designed as a near drop-in replacement for R-410A:

Temperature glide: ~1.5°C, which must be considered in heat exchanger design

Similar operating pressures to R-410A

A2L classification: Similar safety requirements to R-32

Slightly lower capacity than R-410A, requiring ~5% more heat exchanger surface

R-290 (Propane, GWP = 3)

R-290 is a natural refrigerant with near-zero GWP:

Excellent thermodynamic properties: High COP potential

High flammability (A3): Limits charge size and requires special safety measures

Lower operating pressures: May allow thinner tube walls

Charge limits: IEC 60335-2-40 limits charge based on room size, typically restricting use to smaller systems

Impact on Heat Exchanger Design

Tube Sizing and Material

R-32's higher operating pressures (about 5-10% higher than R-410A) may require thicker tube walls or smaller tube diameters. The trend toward 5 mm and 7 mm tubes (instead of traditional 3/8" = 9.52 mm) is partly driven by the need to handle higher pressures while reducing refrigerant charge.

For R-290 systems, the primary concern is minimizing refrigerant charge. This favors:

Smaller tube diameters (5-7 mm)

Microchannel heat exchangers

Optimized internal volume design

Heat Transfer Coefficients

Different refrigerants have different two-phase heat transfer characteristics. The Chen correlation and its derivatives show that:

R-32 typically has 10-20% higher evaporation heat transfer coefficients than R-410A due to its higher thermal conductivity and latent heat

R-454B has slightly lower coefficients due to the R-1234yf component, but the temperature glide can be used advantageously in counterflow arrangements

R-290 has excellent heat transfer properties, often 15-25% better than R-410A

Pressure Drop Considerations

The pressure drop characteristics of low-GWP refrigerants differ from R-410A:

R-32: Higher vapor density reduces pressure drop in the vapor phase, but higher mass flow rates (per unit capacity) can increase liquid-phase pressure drop

R-454B: Similar pressure drop to R-410A in most configurations

R-290: Lower density means higher volumetric flow rates, potentially requiring larger tube diameters or more circuits

Circuiting Design

Refrigerant circuiting becomes more critical with low-GWP refrigerants:

R-32's higher discharge temperature requires careful superheat control and may benefit from subcooling circuits

R-454B's temperature glide means the circuiting should be designed to take advantage of counterflow arrangements where possible

R-290's charge limits demand minimized internal volume, favoring fewer, shorter circuits with smaller tubes

Safety Considerations for Heat Exchangers

All A2L and A3 refrigerants require additional safety measures:

Leak detection: Heat exchangers must be designed and manufactured to minimize leak risk. This includes proper brazing techniques, pressure testing at higher safety factors, and consideration of vibration-induced fatigue.

Charge minimization: Reducing the internal volume of heat exchangers directly reduces the refrigerant charge, which is especially critical for A3 refrigerants like R-290.

Material compatibility: Some low-GWP refrigerants (particularly HFO-based blends) may have different compatibility requirements with elastomers, lubricants, and metals. Verify material compatibility before specifying components.

Design Software Adaptation

Heat exchanger design software must be updated to handle the specific properties of new refrigerants. Key requirements include:

Accurate thermodynamic property databases for new refrigerants and blends

Proper handling of temperature glide in zeotropic mixtures

Updated heat transfer correlations validated for the specific refrigerant

Charge calculation capabilities to verify compliance with safety standards

ExCoil includes property databases for R-32, R-410A, R-134a, R-22, and other common refrigerants, with correlations validated for each fluid's specific behavior in finned tube heat exchangers.

Conclusion

The transition to low-GWP refrigerants is reshaping heat exchanger design practices. Engineers must understand the unique properties of each alternative refrigerant and adapt their designs accordingly. While the transition presents challenges, it also offers opportunities for improved efficiency and more compact system designs. Staying current with evolving regulations and refrigerant technologies is essential for successful HVAC system design.