The PX G1300 can offer a solution to mitigate the failure of a CO₂ transcritical booster system during a heatwave. To understand how the PX G1300 can help, we must first examine why these systems are prone to breakdown under such conditions.
Most CO₂ systems are designed with 120 bar (1740 psi) pressure relief valves on the medium temperature (MT) compressor discharge. According to EN378 and ASHRAE 15, the high-pressure switches on the compressor discharge must be set between 108–110 bar (1566–1595 psi). This establishes a critical limitation for the system’s controls.
The control system, which regulates the gas cooler outlet pressure, must also account for the pressure drop across the gas cooler, oil separator, and piping to prevent the high-pressure switches from activating. This forces the control system to maintain the gas cooler outlet pressure within 95–105 bar (1377–1522 psi). Additionally, the high-pressure control system also regulates the receiver (flash tank) pressure, keeping it between 28–35 bar (400–500 psi).
Consider a heatwave where the ambient temperature reaches 43°C (110°F), and assume the gas cooler is optimally designed with a 2°C approach temperature. This means the gas cooler outlet temperature is 45°C (113°F).
Under these conditions, the control system tries to maintain:
At this operating point, 58% to 64% of the refrigerant flow passing through the compressors is recycled flash gas, which cannot be used for cooling. This excessive flash gas can lead to multiple failure scenarios like:
The PX G1300 provides a simple yet effective solution by reducing the amount of flash gas generated during high ambient conditions. It achieves this by subcooling the gas cooler outlet using only the available high pressure in the system.
Field and laboratory experiments show that the PX G1300 can subcool the gas cooler outlet by 3.3°C (6°F). Returning to the earlier example, this subcooling reduces the flash gas amount from 64%-58% to 53%-50% a 17% to 14% reduction.
This 17% to 14% reduction in flash gas can mean the difference between a functioning refrigeration system and one that fails under heatwave conditions.
In conclusion, the PX G1300 significantly enhances the reliability and resilience of transcritical CO₂ booster systems during extreme heat events, ensuring uninterrupted cooling performance.