CO2 transcritical booster (TCB) systems — which are designed to use CO2 (refrigerant name R-744) as the sole refrigerant for both medium- (MT) and low-temperature (LT) refrigeration loads — are among the leading sustainable architectures being trialed and evaluated by supermarket owners/operators.
In a CO2 TCB system, MT compressors discharge into a gas cooler, which is typically located on the roof. LT compressors discharge into the MT compressors, thereby allowing the MT compressors to boost the refrigerant to the gas cooler.
The gas cooler is impacted by ambient temperature and humidity levels of the installation climate. In subcritical mode, it acts much like a traditional condenser. As temperatures rise above 75 °F and push the system beyond CO2’s critical point (87.8 °F), the system transitions to transcritical (aka supercritical) mode and relies on the gas cooler to cool and/or condense R-744 before it can be used in the system. After exiting the gas cooler, it passes through a high-pressure valve (HPV), which drops the refrigerant pressure to approximately 550 psig.
Next, a flash tank receives a mixture of vapor and liquid refrigerant at 40 °F equivalent saturation, with vapor rising to the top and liquid settling at the bottom. Liquid is circulated through insulated lines that feed the MT and LT cases — all of which are equipped with an electronic expansion valve (EEV). LT loads could be as low as -20 °F.
MT compressors manage three sources of system mass flow:
- The MT evaporator suction gas feeds the MT compressors.
- LT compressors discharge into the MT suction group.
- Excessive flash tank gas (vapor) feeds into the MT suction via a bypass line equipped with a bypass gas valve (BGV).
Understanding CO2 TCB system load requirements is critical for determining the sizing of key system components and achieving the required system capacity.
System loads — Determine maximum refrigeration requirements for both MT and LT loads and circuits.
MT and LT compressors — Select MT compressors that can manage the system load from three sources, calculating the total heat of rejection from the LT compressors.
Evaporators — Undersizing will decrease suction pressures, resulting in a higher temperature differential (TD) and the removal of case moisture. Oversizing will increase pressures and cause excess case humidity.
Flash tank —As the liquid receiver that serves both MT and LT loads and feeds vapor through the bypass line, flash tank sizing is critical to proper TCB system operation.
EEVs — EEVs for CO2 systems are sized on the estimated pressure from the flash tank to the evaporator, plus:
- Refrigeration load
- Liquid temperature at the inlet of an EEV
- Pressure drop across the EEV and distributor
Gas cooler — Must handle the total heat of rejection from MT compressors and capacity requirements at the design conditions of the installation location (hours in subcritical and transcritical modes).
HPVs/BGVs — Valve sizing is based on the total system capacity at saturated temperature for MT and LT loads, flash tank temperature, and the maximum gas cooler outlet temperature.
