In a comparison of an adiabatic CO2 (R744) gas cooler and a dry gas cooler using both simulation software and real field experiences, Italian component manufacturer LU-VE Group showed the efficiency advantage of the adiabatic system.
The comparison was explained by Giovanni Mariani, LU-VE USA Technical Manager, LU-VE Group, during his presentation at the ATMOsphere (ATMO) America Summit 2023 on natural refrigerants. The conference took place June 12–13 in Washington, D.C., and was organized by ATMOsphere, publisher of R744.com.
In adiabatic gas cooling, the air enters via the coil through a cellulose matrix. The top of the matrix is injected with water. The air is cooled down by the “adiabatic effect,” which means the dry bulb temperature decreases, and the relative humidity increases (without any change in enthalpy). This is very efficient in hot and dry climates.
Mariani performed a simulation for a refrigerated warehouse in two climatic zones: the warmer Dallas, Texas, and the colder Philadelphia, Pennsylvania. The simulation was performed using in-house LU-VE software. A standard LU-VE XDHV (“V-shape”) gas cooler operating exclusively in dry mode was compared with an adiabatic gas cooler, which was the same model as the dry unit but upgraded with adiabatic panels.
“Dry and adiabatic gas coolers show similar behavior for pressure inside the gas cooler until water is activated,” said Mariani. When the water is activated at 26°C (78.8°F) ambient temperature in an adiabatic gas cooler, the gas cooler pressure is kept below 80bar (1,160.3psi), even at 42°C (107.6°F) ambient, which increases COP by 80% compared to a dry gas cooler operation, he added.
Mariani highlighted that the air temperature of the gas-cooler coil is reduced by up to 3–5K (3–5°C/5.4–9°F) above the wet bulb temperature. The CO2 outlet temperature is reduced by over 10K (10°C/18°F) during the hottest hours.
“The compressor power consumption is reduced by 70% using an adiabatic gas cooler over a dry gas cooler solution,” Mariani noted, adding that until the switch point at 26°C (78.8°F), the dry and adiabatic gas coolers had equal energy consumption.
“If the extra cost of water is more, it can easily be compensated by a significant reduction of the energy consumption of the compressors,” he said.
Overall, in Dallas, the operational costs saw a reduction of up to 38%, while Philadelphia enjoyed a 25% cost cut when the adiabatic unit was used.
Assuming that the adiabatic gas cooler requires double the maintenance cost than the dry cooler, “we found 9% savings for Dallas and 3% savings for Philadelphia,” said Mariani.
“If we want to speak about the payback time, let’s consider adiabatic gas cooler as 10–15% more costly than the dry solution; then the payback period would be six months or at maximum one year considering other circumstances,” he explained.
The refrigerated warehouse in the simulation had a capacity of 200kW (57TR) and was designed to ensure optimal performance even under demanding conditions. The evaporation temperature operated at -10°C (14°F), facilitating rapid cooling to maintain the quality of stored items.
Central to efficiency is the intermediate pressure receiver, which is stabilized at 38bar (551.1psi), streamlining the flow of refrigerants throughout the system. The system employs a 2K (2°C/3.6°F) ΔT approach for the gas cooler, enhancing the heat exchange. In subcritical mode, the system minimizes condensation pressure to meet the desired evaporator capacity. Additionally, the compressor’s isentropic efficiency is determined by pressure ratio, ensuring optimal energy use.
Real-world applications
In a real-world comparison, in 2018 LU-VE gas coolers were installed in a leading Dutch retailer’s supermarkets in Gorinchem, the Netherlands.
During the summer, when the ambient temperature rose above 37°C (98.6°F) for extended periods, the real-world benefits of adiabatic panels became strikingly evident. In one supermarket, the adiabatic gas cooler pressure stabilized around 80bar (1160.3psi), which corresponds to an outlet CO2 temperature of a relatively cooler 32°C (89.6°F). Factoring in a suction temperature in the medium line of -8°C (17.6°F), the system’s COP stood at around 2.
Another supermarket installation in the same region with a dry gas cooler performed significantly differently on the same day. The gas cooler pressure for this system was at 95bar (1,378psi), leading to a warmer outlet CO2 temperature of 39°C (102.2°F) and a reduced COP of approximately 1.5.
Thus, by simply integrating the adiabatic panels, the refrigeration system’s efficiency was boosted by about 30%.
In Dallas, the operational costs saw a reduction of up to 38%, while Philadelphia enjoyed a 25% cost cut when the CO2 adiabatic dry cooler unit was used.
Giovanni Mariani, LU-VE USA Technical Manager, LU-VE Group
