Understanding Subcritical and Transcritical Refrigeration Cycles

In refrigeration and heat pump systems, the terms subcritical and transcritical describe how a refrigerant behaves in relation to its critical point—the temperature and pressure beyond which it cannot exist as a distinct liquid or gas. These concepts are especially important when working with carbon dioxide (CO₂), known as R-744, as a refrigerant.

Subcritical Refrigeration Cycle

A subcritical refrigeration cycle operates entirely below the refrigerant’s critical temperature and critical pressure. This means the refrigerant undergoes a traditional phase change—it evaporates into a gas at low pressure and condenses back into a liquid at high pressure. Most conventional refrigeration systems (using refrigerants such as R-134a or R-410A) operate subcritically, where the condenser allows heat to be rejected efficiently as the refrigerant transitions from gas to liquid.

For CO₂, the critical temperature is 31.1°C (87.8°F), which is relatively low. This means CO₂ systems used for refrigeration in warm environments often exceed this threshold, forcing them into transcritical operation instead.

Transcritical Refrigeration Cycle

A transcritical cycle occurs when part of the refrigeration process takes place above the critical temperature. In this state, the refrigerant does not condense in the usual way; instead of a condenser, the system uses a gas cooler to remove heat. The refrigerant remains in a dense fluid state rather than transitioning sharply between liquid and gas.

Transcritical CO₂ systems are used in supermarkets and industrial cooling, where CO₂’s high pressure can still deliver efficient heat transfer despite operating above its critical point. These systems require specialized components, such as high-pressure compressors and expansion valves, to handle the extreme operating conditions.

Understanding the difference between subcritical and transcritical cycles is crucial when designing refrigeration or heating systems that use CO₂, ensuring optimal efficiency and performance in varying temperature conditions.

For more information on how subcritical CO₂ heat pumps can transform cold-climate electrification, read the article Reinventing Indoor Heating.