Refrigerant selection: matching fluid properties to application requirements

By DigiEntropy Engineering · 2026-01-11 · 11 min read

Choosing the right refrigerant is not just a regulatory box-ticking exercise — the fluid's thermodynamic properties directly determine the system's operating pressures, efficiency, compressor displacement, and long-term serviceability. This guide explains the key properties and how they map to application requirements.

About this post — Authored by an AI assistant using DigiEntropy's polynomial corpus, the universal compressor predictor, and the project's chart-generation tooling. Charts are produced by Python scripts that read the same database the live site queries; tables and formulas are pulled from the same engineering modules. Findings reflect the project's current dataset and methodology — send corrections or deeper questions to admin@digientropy.com.

Why refrigerant choice matters beyond regulation

The EU F-Gas Regulation (EU) 2024/573 has made GWP limits the most visible driver of refrigerant selection decisions in 2025–2026. But GWP is a single dimension of a multi-dimensional choice. The refrigerant determines:

The operating pressures at both high and low side — critical for component ratings and leak detection requirements.
The compressor displacement needed to deliver a target capacity — a high-density refrigerant needs a smaller, cheaper compressor; a low-density one may need a large, multi-cylinder machine.
The coefficient of performance at the design operating point — some refrigerants are thermodynamically better matched to specific temperature ranges.
The discharge temperature under real compression — a poorly matched refrigerant can overheat compressor oil and valves even at normal pressure ratios.
The safety classification (ASHRAE 34 group) — flammability (A2L, A3) and toxicity (B) impose installation, charge-size, and detection requirements.

All of these interact. A refrigerant that looks attractive on GWP alone may require a compressor 30 % larger, impose higher operating pressures that stress the system, and produce higher discharge temperatures that require additional cooling. The right choice balances all factors for the specific application.

3 - application bands - HBP, MBP and LBP

Refrigerants are broadly divided into three application bands based on the evaporation temperature the system needs to maintain.

30 % - larger compressor - the price of choosing on GWP alone

A refrigerant that looks attractive on GWP alone may require a compressor 30 % larger, impose higher operating pressures that stress the system, and produce higher discharge temperatures that require additional cooling.

31.1 °C - CO₂ critical temperature - above it, a gas cooler replaces the condenser

CO₂ has a critical temperature of 31.1 °C — in most European outdoor climates the ambient can exceed this, requiring the system to operate in transcritical mode with a gas cooler rather than a condenser.

73.8 bar - CO₂ critical pressure - R404A/R507 parts: ≈ 30 bar maximum

CO₂'s critical pressure is 73.8 bar — far above the working range of standard R404A/R507 components (≈ 30 bar high-side maximum). CO₂ systems use specialised high-pressure components.

50–55 °C - condensing temperature to reach - for ambient up to 40–45 °C

For a system operating in southern European climates (ambient up to 40–45 °C), condensing temperatures of 50–55 °C must be achievable.

148 - R454C GWP (AR6) - below the 150 GWP threshold

In the comparison table, R454C is an HFO blend with a GWP (AR6) of 148: below 150 GWP, A2L (mildly flammable).

The application temperature ranges

Refrigerants are broadly divided into three application bands based on the evaporation temperature the system needs to maintain:

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