In the design of refrigeration systems in low-temperature constant temperature baths (such as 40 ° C to room temperature range), the selection of refrigerants should comprehensively consider temperature requirements, environmental friendliness, safety, and system efficiency. The following are key factors and recommended options:

1. Core considerations
Temperature range: Ensure that the refrigerant still has appropriate evaporation pressure (avoid vacuum) and condensation pressure (avoid excessive pressure) at the target low temperature.
Environmental friendliness: Prioritize the use of refrigerants with low GWP and ODP.
Safety: Focus on toxicity (A1/A2L/A3 classification) and flammability (B1/B2L/B3 classification).
Thermodynamic performance: high refrigeration efficiency (COP value), appropriate pressure ratio, and refrigeration capacity per unit volume.
Compatibility: Compatibility with compressors, lubricants, and system materials.

2. Recommended common refrigerants
#(1) Traditional refrigerants (gradually phased out but still in use)
R404A
Temperature range: above 46 ° C, widely used for low temperatures.
Disadvantage: High GWP (≈ 3922), currently being replaced.
R22 (gradually phased out)
Temperature range: above 40 ° C, but ODP ≠ 0, restricted.
#(2) Environmentally friendly alternative refrigerants
R448A/R449A
Replace R404A, GWP ≈ 1400, A1 safety level, suitable for temperatures above 45 ° C.
R507A
Low temperature performance similar to R404A, GWP ≈ 3985, environmental friendliness needs to be balanced.
R454C（A2L）
GWP≈148， Low flammability, suitable for temperatures below 30 ° C, requiring leak proof design.
#(3) Natural refrigerant
CO₂ （R744）
Supercritical cycle can be used for temperatures above 40 ° C, environmentally friendly (GWP=1), but for high-pressure systems (requiring pressure resistant design).
Ammonia (R717)
Efficient and environmentally friendly, but toxic (B2L), suitable for large-scale industrial systems.
Propane (R290)
High energy efficiency, GWP≈3， But highly flammable (A3), strict explosion-proof measures are required.

3. Special low-temperature requirement plan
Overlapping system:
Extremely low temperatures (such as 80 ° C) can be stacked in two stages, with R404A/R448A used for high temperatures and R23 (high GWP, caution should be exercised) or R170 (ethane, A3 class) used for low temperatures.

4. Selection suggestions
40 ° C to 10 ° C: R4481/R449A or R454C (balance environmental protection and performance) are preferred.
Lower temperature: Consider cascade systems or CO ₂ transcritical cycles.
Environmental mandatory scenario: Choose R744 (CO ₂) or R290 (requiring safety design).