CO2 vs ammonia refrigeration is the first argument in every low-temperature plant project, and it is usually settled by three numbers: the lowest evaporation temperature, the highest ambient condensing temperature, and the space the machine actually gets. Neither fluid wins on its own. CO2 (R744) is non-toxic and non-flammable, which makes it the comfortable choice near occupied or hazardous areas, but below zero it runs transcritical and its pipework sees pressures an ammonia system never does. Ammonia (R717) does low temperatures efficiently at low pressure and costs less per tonne, yet it is toxic and flammable, so sites keep it out of occupied rooms and behind gas detection. The NH3/CO2 cascade is the compromise that lets each fluid do the job it is good at.
The comparison below uses the published AIST specification as the concrete example, because it is a machine that carries both fluids and publishes both charge figures.
Why one machine carries two refrigerants
In a cascade, the two loops share only one heat exchanger. The CO2 loop evaporates at the target temperature — the AIST module unit runs a −38 °C nominal low-temperature condition — and rejects its heat into the NH3 loop. The ammonia loop then condenses to ambient. Each refrigerant stays inside the pressure and temperature window where it behaves best.
Two consequences follow. The CO2 charge can be large in mass — 2,700–5,200 kg in the AIST series — because the loop is pressurized throughout, and because CO2 is non-flammable that mass is a filling and handling issue, not a fire issue. The NH3 charge stays small — 110–220 kg in the same units — which keeps the classified-volume calculation for the machinery space manageable. Cascade units exist for that reason first: a risk split with defined boundaries, not a single efficiency number being chased.
What the split does to your installation
The short version, with the AIST charge figures as the concrete row. The rest is behaviour any plant using these fluids will meet.
| Ammonia (R717) | CO2 (R744) | |
|---|---|---|
| Role in a cascade | Heat-reject (high-temperature) stage | Evaporation (low-temperature) stage |
| Pressure behaviour | Low operating pressure at plant conditions | Transcritical below zero; gas cooler can exceed 100 bar on hot days |
| Safety profile | Toxic and flammable — classified space, detection, drainage | Non-flammable, non-toxic — oxygen displacement in sealed rooms |
| Charge in AIST unit | 110–220 kg | 2,700–5,200 kg |
Six checks before the spec is signed
- Operating window, not a nameplate point. Work out how many hours a year the CO2 stage runs transcritical. A plant that evaporates at −38 °C against a 40 °C summer ambient — the AIST nominal condition — spends its warm months in high-pressure mode, and the gas cooler must be sized for that, not for the shoulder season.
- Pressure rating of the CO2 side. Verify the design pressure of piping, valves and the gas cooler against the worst ambient you will see, not the rated operating pressure. It is the most common gap found during review of transcritical projects.
- Cascade heat exchanger balance. Heat rejected by the CO2 loop must equal heat absorbed by the NH3 loop, at temperatures that actually match. A mismatch shows up as drifting stage pressures — the control system then fights a hydraulic problem it was not meant to solve.
- Ammonia zone rules. Confirm the machinery space classification, leak-detection placement and drainage for the NH3 loop before the unit position is fixed; moving a plant after construction is far more expensive than adding a detection circuit.
- CO2 accumulation indoors. A multi-tonne CO2 charge in a sealed room is an oxygen-displacement risk even though the fluid is non-flammable. Indoor CO2 rooms need ventilation, and monitoring where people are present.
- Start and control strategy. The AIST series starts on frequency conversion with no inrush current and runs on an integrated control system. Decide who controls what: which loop follows process demand, and which follows the cascade exchanger.
Where cascade plants already run
The AIST CO2 module unit is documented in low-temperature food processing, low-temperature process cooling in the pharmaceutical and chemical industry, refrigeration in hazardous operating areas, and artificial-environment applications. MOON-TECH has undertaken phases of the national hydrochlorofluorocarbon-substitution demonstration programme and holds the United Nations Ozone Layer Protection Certificate — natural-refrigerant systems have been its core field for decades.
Questions plant managers ask
Is CO2 refrigeration safer than ammonia?
For the people around the machine, usually. CO2 is non-flammable and non-toxic at the concentrations a small leak would produce, so it can sit closer to occupied areas. Its risk is volume: a large charge in a sealed room displaces oxygen, so indoor CO2 rooms still need ventilation and monitoring. Ammonia is toxic and flammable, so it belongs in a classified machinery space with detection and drainage. The cascade split is designed to keep each fluid where its specific risk is manageable.
Can we add a CO2 stage to an existing ammonia plant?
That is exactly what a cascade is — an added CO2 loop with a cascade heat exchanger. Before specifying one, check three things: physical space and pipe routing for the new loop, whether the existing NH3 loop has capacity margin at the new operating point, and how the two control loops will hand demand back and forth.
Why do CO2 units carry thousands of kilograms of refrigerant?
Because the loop is pressurized throughout. A transcritical CO2 system holds refrigerant at high pressure in both liquid and gas states, so a given heat load needs more mass than in a low-pressure ammonia circuit. In MOON-TECH’s AIST units the CO2 charge is 2,700–5,200 kg; since CO2 is non-flammable, that mass is a filling, handling and control issue rather than a fire risk.
Specifying a cascade unit?
Send your evaporation and condensing temperatures plus the machine-room constraints. The engineering team responds with a matched configuration and the full AIST data.