Danfoss Multi-Ejector HP with Parallel Compression Reduces CO₂ System Energy Use by 21% Compared to Traditional Boosters

Danish component manufacturer Danfoss reported in a white paper that pairing its multi-ejector HP (high-pressure) with parallel compression in transcritical CO₂ (R744) refrigeration reduced energy use by 21% compared to traditional booster systems in two U.S. field tests.

The company tested its multi-ejector in two new supermarkets across different climate zones: a mild environment in the Midwest and a warmer site in the South. The technology capitalizes on the Venturi effect to generate free compression.

“Test results show that the Danfoss ejector provided significant energy saving even when ambient roof temperatures exceeded 95°F [35°C],” said Chris Brown, Application Engineering Manager at Danfoss. “In cold ambient conditions, the HP ejector serves the same functions as a conventional high-pressure valve, controlling the system at the COP optimum.”

Danffos offers a range of ejectors for CO₂ refrigeration systems, including models for low-pressure booster systems without parallel compression, high-pressure systems with parallel compression and liquid ejectors for both configurations. The company first piloted the technology in 2015. To secure UL certification for the device, it started collaborating with U.S.-based OEMs in 2016.

How it works: Instead of using a high-pressure valve to reduce the pressure of the CO₂ gas leaving the gas cooler, Danfoss’s multi-ejector harnesses the expansion energy to produce free compression.

• High-pressure CO₂ gas from the gas cooler is funnelled into the ejector, where a narrowed throat accelerates the flow.
• The gas exits the nozzle at supersonic speed, creating a low-pressure zone and drawing in saturated vapor from the medium-temperature (MT) evaporator suction line.
• The two streams mix in the chamber and are discharged to the parallel compressor, with the kinetic energy of the flow raising the pressure without the need for mechanical compression.

U.S. tests: In the two pilot cases, Danfoss installed a six-cartridge multi-ejector HP, CTM 6, with parallel bypass piping that integrates a redundant high-pressure valve to facilitate comparative testing. A Danfoss controller pack manages the ejector’s on/off staging.

• Refrigeration design included: low-temperature compressors with a suction inlet of −15°F (−26°C) at 220psi (15bar); MT compressors at 18°F (−7°C) at 395psi (27bar); an intermediate compressor at 36°F (2°C) at 520psi (36bar); a receiver at 36°F and 520psi; a flash gas bypass valve; a suction accumulator; and a discharge/gas cooler operating with refrigerant temperatures from 50 to 95°F (10 to 35°C) at 700 to 1300psi (48 to 90bar).

The conclusions: At 95°F ambient conditions, parallel compression reduced energy usage by 8% compared to a traditional CO₂ booster system, and the multi-ejector cut energy use by an additional 13.4% ‒ for total savings of 21.4%.

• Performance gains from the device begin to appear in temperatures as low as 50°F, Danfoss said, adding that the sharpest decrease in energy usage occurs between 65 and 70°F (18 and 21°C).
• With stabilized CO₂ rack performance, the multi-ejector cuts compressor cycling by as much as 47%, helping to extend equipment life.

Quotable: “Combining parallel compression and an ejector in an integrated system is the special ingredient that gets more useful work from carbon dioxide’s refrigeration cycle,” said Brown. “The solution also potentially reduces compressor cycling and cumulative current inrush, enhancing the reliability of the supermarket’s refrigeration system.”