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Compared to systems based on legacy hydrofluorocarbon (HFC) refrigerants, CO₂ transcritical booster (TCB) systems have unique characteristics, high-pressure management strategies and design considerations.

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1. Booster design — CO₂ TCB systems run on R-744 for both medium- (MT) and low-temperature (LT) refrigeration loads. Instead of LT compressors discharging refrigerant into the condenser, LT compressors discharge R-744 into the MT compressors, which boost the refrigerant to a gas cooler.

2. Gas cooler (aka condenser) — The gas cooler (on the roof) must be sized to handle the system’s total heat of rejection from MT compressors per installation design conditions. Variable-speed fan motor control is typically deployed, and adiabatic cooling pads can be used to improve system efficiencies in warm ambient climates.

3. High operating pressures — CO₂ TCB system operating pressures can exceed 1,400 psi when MT compressors discharge into the gas cooler on a hot summer day (e.g., 100 °F). MT discharge lines must be constructed with stainless steel or special ferrous alloy copper to handle these pressures.

4. Low critical point of 87.8 °F — Below the critical point, R-744 is at saturation and the system operates in subcritical mode. When the ambient temperature rises above ~75 °F, refrigerant exits the gas cooler as a supercritical fluid (at or above 87.8 °F) and the system enters transcritical mode.

5. High-pressure valve — As R-744 exits the gas cooler, its pressures must be reduced before being reintroduced to the refrigeration circuits. A high-pressure valve (HPV) reduces the refrigerant to around 550 psi and transfers it to a receiver (aka flash tank) that separates vapor from liquid.

6. Flash tank (aka receiver) — The flash tank receives a mixture of vapor and liquid refrigerant at around 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 (as low as -20 °F) equipped with electronic expansion valves (EEVs). Vapor is fed through a bypass gas valve (BGV) and bypass line.

7. Reliance on electronics — Because of R-744’s potential reactivity to various conditions, electronic controls are required to: manage system pressures; control variable fan speeds; modulate HPVs, BGVs and EEVs; maintain consistent flash tank pressure; and provide smooth compressor staging.

8. High triple point (-69.8 °F and 60.4 psig) — When charging with liquid while system pressures are below the triple point of 60.4 psig, R-744 turns into dry ice, stops the refrigerant flow, and causes a variety of potential system problems. Technicians should charge with vapor until the system reaches 100 psig.

9. Rising standstill pressures — Managing rising system pressures during power outages or planned shutdowns is an important CO₂ TCB design consideration. Technicians must understand the maximum pressure ratings of cases, valves, evaporators and all system components.

10. System safeties and pressure-relief valves — High-pressure safety and/or mitigation strategies provide system- and compressor-specific protection. Pressure-relief or isolation valves should be installed in various system sections to prevent a full loss of refrigerant charge. Refrigerant should be vented outdoors per applicable codes.