CO2 Transcritical Gas Cooler Optimum Pressure

ScopeAbove CO2's critical point -- 87.8 degF and about 1,071 psia -- there is no condensation, so the high side is a single-phase gas being cooled and pressure and temperature become INDEPENDENT, unlike a condenser where fixing one fixes the other. Raising discharge pressure at a fixed gas cooler outlet temperature moves the cycle into a region where CO2's isotherms bend sharply, which increases the refrigerating effect per pound substantially; it also increases compressor work. The two effects cross, and the crossing point is the optimum this reports, from the widely used correlation P = 2.6 x T_out + 7.54 in bar and degC. The optimum depends almost entirely on the gas cooler OUTLET temperature, which is ambient plus the gas cooler's approach -- so the control strategy is to measure that outlet temperature and float the high-side pressure to match it, continuously. A fixed high-side setting is leaving efficiency on the table at every ambient except one. On a cool day the outlet falls below the critical temperature and the system reverts to ordinary subcritical condensing, where this arithmetic does not apply at all, and a controller that kept applying it would command a pressure hundreds of psi above what the cycle needs. That is why the regime is reported first and the pressure is withheld below the critical temperature rather than printed with a caveat. The pressures involved are why transcritical CO2 equipment is built to ratings no other supermarket refrigerant needs and why its service procedures differ. A correlation-based screen: it does not compute COP, capacity, or discharge temperature, model the flash gas bypass or ejectors, and it is not a substitute for the manufacturer's control algorithm.

Formula or tablegas cooler outlet = ambient + approach; the cycle is transcritical when that outlet exceeds CO2's 87.8 degF critical temperature; above it the optimum high-side pressure is 2.6 x the outlet in degC + 7.54, in bar, converted at 1 bar = 100,000 Pa and 1 psi = 6,894.757293168361 Pa.

Source and editionCO2's critical point at 87.8 degF and about 1,071 psia, with the widely used transcritical optimum-pressure correlation. The correlation applies only above the critical temperature and is WITHHELD below it rather than printed with a caveat. It does not compute COP, capacity or discharge temperature, and does not model flash-gas bypass or ejectors.

Free accessOne published correlation and two exact pressure conversions.

What governsEstimate. AHJ and licensed professional govern.

API field namesambient_f, gas_cooler_approach_f, evaporating_psig, gc_outlet_f, gc_outlet_c, p_opt_bar, p_opt_psia, p_opt_psig, pressure_ratio

• A correlation, not a cycle model COP, capacity and discharge temperature are not computedthe equipment manufacturer's control algorithm
• Withheld below the critical temperature the relation has no meaning where the high side condensesCO2 property data
• The approach is entered gas cooler approach depends on the coil and the ambient conditionsthe gas cooler manufacturer
• Which temperature the correlation takes secondary sources disagree whether Kauf (1999) wrote it on the gas cooler OUTLET or on AMBIENT with a 2.9 K approach (2.6 x 2.9 = 7.54); read on ambient, the optimum is about 7.5 bar (110 psi) lower than shown, which sits closer to the Liao and Sawalha correlations; the original paper governsACS Omega 2020 (PMC7409259); Purdue IRAC 2018; Brunel PhD thesis Fig. 3.13