10.5 CO₂ (R744) systems
CO₂'s critical point is about 31 °C and 73.8 bar. Above this point there is no separate liquid and vapour. Heat is rejected in a "gas cooler" instead of a condenser.
This is called a transcritical cycle. Because the hot gas cools over a wide temperature glide, CO₂ heats water very well from a cold inlet to 65–90 °C. That suits sanitary hot water.
Typical pressures (approximate): gas cooler side 80–130 bar, evaporator side 25–45 bar. Standstill pressure on a warm Indian day can exceed 60 bar.
| Property | R744 (CO₂) | R410A (for comparison, typical) |
|---|---|---|
| Safety class | A1 | A1 |
| GWP | 1 | ~2000 |
| High-side pressure | 80–130 bar | 25–35 bar |
| Critical temperature | ~31 °C | ~71 °C |
| Best use | Hot water, large water temperature lift | Space heating and cooling |
Safety: high pressure needs rated pipes, fittings and gauges. CO₂ is heavier than air and displaces oxygen in pits and plant rooms. Dry ice can form when liquid CO₂ is vented quickly, blocking lines.
Remember
- Use gauges and hoses rated for CO₂ pressures.
- Vent CO₂ slowly to avoid dry ice formation.
- Fit a CO₂ detector in enclosed plant rooms.
- Efficiency depends strongly on cold water inlet temperature.
CO₂ is non-flammable and has GWP 1, but it runs at very high pressure and often above its critical point.
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