ODP is a relative measure of how much damage a substance can cause to the stratospheric ozone layer, compared with trichlorofluoromethane (CFC-11, R-11) which is assigned an ODP of 1.0. A higher ODP means greater ozone destruction. Commonly used refrigerants with high ODP include CFCs (R-11, R-12) with ODP around 1.0 and HCFCs (R-22) with ODP of about 0.055. GWP is a relative measure of how much heat a gas traps in the atmosphere over a given time horizon (usually 100 years) compared with carbon dioxide (CO2) which has a GWP of 1. High-GWP refrigerants include HFCs such as R-134a (GWP ~1430), R-404A (GWP ~3900) and R-410A (GWP ~2088). Ozone-friendly but high-GWP HFCs were introduced to replace CFCs but are themselves potent greenhouse gases, so the industry is now moving to low-GWP natural refrigerants.

• R-134a (HFC) has been the standard marine refrigerant but is being phased down under the Kigali Amendment to the Montreal Protocol because of its GWP of about 1430.
• R-404A and R-410A (HFC blends) used in some systems.
• Natural refrigerants with near-zero ODP and very low GWP:
• R-717 Ammonia, GWP ~0, used in large industrial/fishery plants.
• R-744 Carbon dioxide (CO2) transcritical systems, GWP = 1.
• R-290 Propane and R-600a Isobutane hydrocarbon refrigerants, very low GWP but flammable, posing safety constraints in machinery spaces.
• R-718 Water, R-728 Nitrogen in special applications.
• Low-GWP HFO/HFO blends such as R-1234yf, R-513A, R-448A are also being introduced as transitional low-GWP substitutes.

In practice onboard, the choice depends on machinery space fire safety, gas detection arrangements and applicable MARPOL Annex VI restrictions which prohibit use of virgin ozone-depleting gases after defined dates.

Normal operation:

• Carry out routine leak checks using an electronic leak detector or a foaming agent at all joints, valves, flanges and service points; log results.
• Avoid unnecessary opening of the system; maintain charge books and keep records of any topping up so that unexplained losses indicate a leak.
• Keep the plant fully liquid-charged and correctly superheated/subcooled to avoid fluttering of expansion valves which can cause avoidable recharge.
• Ensure relief valves, gauge connections and purge valves are properly seated and capped.
• Schedule periodic sniffer checks and maintain the compressor area well ventilated so leakage is noticed promptly.

During maintenance:

• Recover all gas into a dedicated refrigerator reclaim/recovery unit before opening any circuit; never vent gas to atmosphere.
• Use proper service and recovery cylinders correctly labelled and weigh the gas recovered to quantify any loss.
• Keep tools, fittings and spare gaskets ready so the system is open for the minimum time.
• Before breaking any joint, pump the section down and isolate it with closed valves; fit caps on open lines to avoid moisture ingress and loss.
• After overhaul, evacuate with a vacuum pump and pressure test with dry nitrogen before recharging so leaks are found before refrigerant is introduced.
• Encourage the use of calorific/infra-red analysers to monitor for leaks and to arrest loss from safety reliefs.
• Comply with MARPOL Annex VI (Regulation 12) and the ODS/record requirements, maintaining the Refrigerant Log / ozone-depleting substances record showing virgin/recycled gas and any discharge.