Thermodynamic Phase-Equilibrium and Non-Linear Temperature Glide Modeling of Next-Generation Zeotropic A2L Refrigerants (R-454B & R-32)

HVACLogic Research Group; Miad S.

doi:10.7910/DVN/SR1NZO

HL-TR-2026-A2L05DOI: 10.7910/DVN/SR1NZOPublished: 2026-09-04

Thermodynamic Phase-Equilibrium and Non-Linear Temperature Glide Modeling of Next-Generation Zeotropic A2L Refrigerants (R-454B & R-32)

An applied thermodynamics study of vapor-liquid phase equilibrium, bubble and dew saturation boundaries, and systemic charging diagnostics under the EPA AIM Act.

By HVACLogic Research Group • Miad S.

📄Download Official PDF Whitepaper

Abstract

Under global climate regulations, including the Kigali Amendment to the Montreal Protocol and the U.S. EPA American Innovation and Manufacturing (AIM) Act, the HVAC/R industry is undergoing a mandatory phase-down of legacy hydrofluorocarbons (HFCs), specifically R-410A. The dominant replacement refrigerants entering residential and commercial heat pump systems are ASHRAE Class A2L lower-flammability fluids: R-454B and R-32. This paper presents an exact thermodynamic phase-equilibrium modeling framework calibrated against NIST REFPROP 10.0 extended Helmholtz-energy formulations. We derive the discrete mathematical boundaries separating bubble-point liquidus curves from dew-point vaporus curves, formulate the exact differential governing equations for superheat and subcooling diagnostics, and quantify the empirical consequences of legacy trade heuristics.

Key Technical Findings & Code Impacts

Evaluating liquid-line subcooling on R-454B systems using the saturated vapor (dew-point) curve introduces a systemic mathematical error of +2.2°F (+1.22°C), resulting in an 8.5% refrigerant undercharge.
Operating undercharged zeotropic systems elevates compressor discharge temperatures by up to +14.2°F (+7.9°C), risking ester lubricant thermal breakdown and degrading seasonal COP by 5.4%.
Single-component A2L refrigerants (R-32) exhibit zero temperature glide and identical bubble/dew saturation states, whereas zeotropic binary blends (R-454B: 68.9% R-32 / 31.1% R-1234yf) display non-linear temperature glide between 1.5°F and 2.5°F.
Differential fractionation during slow vapor-phase leaks shifts remaining liquid mass fractions toward R-1234yf, causing minor glide expansion (<0.4°F) while maintaining Class A2L lower flammability margins.

Governing Industry Standards & Codes

🏛️EPA AIM Act (40 CFR Part 84)

🏛️ASHRAE Standard 34-2022 (Designation and Safety Classification of Refrigerants)

🏛️ASHRAE Standard 15-2022 (Safety Standard for Refrigeration Systems)

🏛️AHRI Standard 210/240-2023

🏛️NIST Standard Reference Database 23 (REFPROP 10.0)

Mathematical Formulations & Governing Equations

Helmholtz Free Energy Residual Formulation

governing_model_eq_1.math
PEER-REFERENCED
\alpha(\delta, \tau, \mathbf{x}) = \alpha^0(\delta, \tau, \mathbf{x}) + \alpha^r(\delta, \tau, \mathbf{x}) = \sum_{i=1}^m x_i \alpha_i^0(\delta_i, \tau_i) + \sum_{i=1}^m x_i \ln x_i + \alpha^r(\delta, \tau, \mathbf{x})

💡 Fundamental multi-fluid thermodynamic state equation defining residual free energy from reduced density, inverse reduced temperature, and constituent mole fractions.

Zeotropic Phase-Change Temperature Glide

governing_model_eq_2.math
PEER-REFERENCED
\Delta T_{\text{glide}}(P) = T_{\text{dew}}(P, \mathbf{y}) - T_{\text{bubble}}(P, \mathbf{x})

💡 Quantifies the temperature variance between saturated vapor and saturated liquid boundaries at constant absolute saturation pressure.

Governing Diagnostic Equations for Zeotropic Systems

governing_model_eq_3.math
PEER-REFERENCED
\text{SC}_{\text{zeo}} = T_{\text{bubble}}(P_{\text{liquid}}) - T_{\text{line,liquid}}, \quad \text{SH}_{\text{zeo}} = T_{\text{line,vapor}} - T_{\text{dew}}(P_{\text{suction}})

💡 Strict differential thermodynamic relations requiring bubble-point saturation for condenser subcooling and dew-point saturation for evaporator superheat.

Companion Calculation Engines & Simulation Models

A2L Refrigerant PT Chart (R-454B, R-32, R-410A)

Interactive saturation pressure-temperature chart with dual bubble and dew curves.

Launch Live Simulator →

Refrigerant Superheat & Subcooling Calculator

Validate evaporator superheat and condenser subcooling with exact zeotropic glide compensation.

Launch Live Simulator →

Refrigerant Charge & Add-On Line-Set Calculator

Compute factory weigh-in charges and long line-set liquid additions.

Launch Live Simulator →

Academic Citations & BibTeX

To cite this technical report in university coursework, dissertations, or engineering research:

APA Format:

HVACLogic Research Group, & S., M. (2026). Thermodynamic Phase-Equilibrium and Non-Linear Temperature Glide Modeling of Next-Generation Zeotropic A2L Refrigerants (R-454B and R-32) (Technical Report No. HL-TR-2026-A2L05). HVACLogic Open-Access Building Science. https://doi.org/10.7910/DVN/SR1NZO

BibTeX Entry:

@techreport{hvaclogic_2026_a2l_glide,
  author = {{HVACLogic Research Group} and S., Miad},
  title = {Thermodynamic Phase-Equilibrium and Non-Linear Temperature Glide Modeling of Next-Generation Zeotropic A2L Refrigerants (R-454B and R-32)},
  institution = {HVACLogic Open-Access Building Science Monograph Series},
  year = {2026},
  number = {HL-TR-2026-A2L05},
  doi = {10.7910/DVN/SR1NZO},
  url = {https://hvaclogic.com/research/thermodynamic-modeling-a2l-refrigerant-glide-r454b}
}