Study finds R717/CO₂ cascade refrigeration system offers 35% higher COP than R404A systems to Indian seafood processors

March 01, 2024

A new study on deep freezing in the Indian fish processing industry has found that an ammonia (NH₃/R717)/CO₂ (R744) cascade refrigeration system has an annual COP 35% higher than an R404A refrigeration system.

The study modeled the impact of switching a blast freezer with 60kW (17.1TR) of cooling capacity operating at −42°C (−43.6°F) from an R404A refrigeration system to an ammonia/CO₂ cascade system. The model calculated the cooling capacity, input power and refrigerant mass flow rate using compressor polynomial equations. The compressor data was sourced from a Bitzer ammonia screw compressor and a Bitzer subcritical CO₂ compressor.

The performance data was modeled across the cities of Veraval, Mumbai and Cochin on India’s west coast and the eastern coastline cities of Chennai, Vizag and Kolkata. These cities are in tropical climates with nearly constant ambient temperatures of between 26 and 35°C (78.8 and 95°F) and have a relative humidity of 55–85% throughout the year.

“For the coastal cities of India’s western and eastern shores, the findings are promising,” the study said. “The NH₃/CO₂ cascade systems show a consistent annual performance, with COP ranging from 1.33 to 1.42 in the west and 1.28 to 1.45 in the east. The R404A systems show lower efficiency, with COP values between 0.81 to 0.95 for the west and 0.72 to 1.03 for the east, respectively.”

The study, “Retaining Natural Refrigerants in Seafood Processing Industries in India,” was written by a team of 10 researchers, including Arun B. Sa, Sumit Kumar and Kristina Norne. The paper was supported by the Indo-Norwegian project Future Refrigeration India: INDEE+. INDEE+ is funded by the Norwegian Ministry of Foreign Affairs and supports the refrigeration and air-conditioning sector in India to transition to more environmentally friendly technologies.

According to a LinkedIn post from Cochin Food Tech, an ammonia/CO₂ cascade system has been successfully implemented at NAS Fisheries. The cooling system implemented uses an ammonia/CO₂ cascade arrangement, where ammonia condenses CO₂ at −5°C (23°F) within a gravity-flooded cascade condenser. The CO₂ is transferred via a motorized valve to a low-pressure vessel. From there, it is pumped at −40°C (−40°F) to the IQF flooded evaporators at a ratio of 2.5:1.

The setup includes a Bitzer compressor providing 350kW (99.5TR) of refrigeration capacity using R744 at −40°C SST for the low side and a Bitzer screw compressor for the low-charge ammonia on the high side. The system also features an evaporator coil sourced from Thermofin.

Introducing a third fluid

In addition to a traditional ammonia/CO₂ cascade system, the study also found similar COP improvements when introducing a third fluid, HyCool, into the mix compared to the R404A system. The HyCool fluid is moved through the system using a low-capacity pump and requires an additional extra heat exchanger, which the study found led to an efficiency loss of 10–14% compared to the ammonia/CO₂ cascade system.

Despite potential issues in ammonia/CO₂ systems like dry expansion and the risk of leaks in the ammonia/CO₂ cascade condenser that could form ammonia carbamate, the benefits, including the avoidance of significant system overhaul costs and the minimal need for additional control systems to manage the ammonia and CO₂ compressors, often outweigh the drawbacks, the study found.

HyCool is a non-toxic, low-temperature heat-transfer fluid used in the indirect cooling system’s secondary circuit. It is made up of 30–50% potassium formate, deionized water and a corrosion inhibitor. It has a freezing point of between −20 and −50°C (−4 and −58°F) depending on the formulation.

Transcritical CO₂ also an option

In addition to the ammonia/CO₂ cascade system, the study also modeled the performance of a transcritical CO₂ system with heat recovery compared to a similar system using R404A. Unlike the cascade system, the transcritical CO₂ system was modeled for a 90kW (25.6TR) cold storage unit operating at −18°C (−0.4°F). The modeled system includes an air-cooled condenser, a liquid receiver, Dorin semi-hermetic reciprocating compressors and oil separators, and it incorporates a heat recovery system for processing heat.

The study found that the CO₂ system had a COP averaging between 3.28–3.69 for cities on the western coastline and 3.15–3.86 for cities on the eastern coastline.

The study notes that ammonia has traditionally been used by the Indian seafood industry for storage purposes. However, as residential areas have built up around these seafood facilities, companies have switched to synthetic refrigerants out of fear of ammonia leaks. The Indian seafood industry exported 17.4 million metric tons of seafood in fiscal year 2022–23 worth $8.09 billion (€7.5 billion), according to the country’s Ministry of Commerce & Industry.

“This study will aid in the retention of natural refrigerants such as NH₃ and promotion of CO₂ for industrial applications in the high ambient temperatures that exist in many seafood cities of India,” the study said.