In cascade systems designed for cold storage, ultra-low-temperature freezers, semiconductor wafer chuck cooling, pharmaceutical lyophilisation, and environmental test chambers operating at evaporation temperatures between -60 °C and -95 °C, the low-stage refrigerant selection has historically been dominated by R23 and R508B because both fluids are non-flammable, have adequate volumetric capacity, and can be handled in standard semi-hermetic compressors without ATEX-rated auxiliary equipment. The regulatory and economic constraints created by EU Regulation (EC) No 517/2014, the Kigali Amendment to the Montreal Protocol, and the high GWP of R23 at approximately 12,400 AR5 and R508B at approximately 13,396 AR5 have shifted attention toward ethane, designated R170 under ISO 817:2014 and classified as A3 under ASHRAE Standard 34-2019. Ethane has a normal boiling point of -88.6 °C at 101.325 kPa, a critical temperature of 32.2 °C, a critical pressure of 4.87 MPa, and a molar mass of 30.07 g/mol. R23 has a normal boiling point of -82.1 °C and a critical temperature of 25.9 °C, while R508B has a normal boiling point near -88.3 °C and a critical temperature near 12.4 °C. The lower critical temperature of R508B creates a specific limitation in high-side cascade heat rejection because the low-stage condenser must remain below the critical point under all load conditions, whereas R170 maintains a larger margin between typical low-stage condensing temperatures of -25 °C to -35 °C and its critical temperature. R170 is not a drop-in replacement for R23 or R508B because flammability, compressor motor compliance, oil miscibility, seal compatibility, and relief valve sizing all change when the working fluid is switched from an A1 fluorocarbon to an A3 hydrocarbon. The selection of R170 over R23 and R508B is therefore not solely a thermodynamic optimisation; it is an engineering risk-management decision that requires recertification of the low-stage compressor package, lubricant system, pressure relief system, and machinery room ventilation under the applicable safety codes.

Property	R170	R23	R508B
ASHRAE Standard 34 safety class	A3	A1	A1
Normal boiling point at 101.325 kPa	-88.6 °C	-82.1 °C	-88.3 °C
Critical temperature	32.2 °C	25.9 °C	12.4 °C
Critical pressure	4.87 MPa	4.84 MPa	4.04 MPa
Molar mass	30.07 g/mol	70.01 g/mol	95.4 g/mol
GWP (IPCC AR5)	5.5	12,400	13,396
Lower flammability limit in air	3.0 vol%	Non-flammable	Non-flammable

Does the Lower Molar Mass of Ethane Reduce Low-Stage Compressor Discharge Temperatures?

The discharge temperature of a reciprocating or screw compressor is governed by the pressure ratio across the stage and the effective adiabatic exponent of the fluid, with additional contributions from volumetric efficiency and internal superheat. At a representative low-stage envelope of -80 °C evaporating and -30 °C condensing, the saturated suction pressure of R170 is higher than that of R23 for the same evaporating temperature because the normal boiling point of R170 is lower; this reduces the pressure ratio across the compressor only if the condensing pressure is not adjusted. The actual discharge temperature depends on the compressor suction gas superheat, the pressure ratio, the built-in volume ratio in screw compressors, and the refrigerant-specific heat ratio. Compressor selection programs for low-temperature semi-hermetic and open-drive compressors generally show that R170 produces lower discharge temperature than R23 at the same evaporating and condensing temperatures when the compressor is resized for the lower mass flow and higher swept-volume requirement. Published service guidelines for R23 low-temperature systems commonly recommend a maximum discharge line temperature of 130 °C, with a preferred operating band below 120 °C, and a maximum compression ratio of 12 to 14 depending on suction superheat and condensing temperature. Halogenated refrigerants such as R23 and R508B can reach discharge temperatures above 120 °C at compression ratios exceeding 12, and the thermal decomposition of the refrigerant and the hydrolysis of POE oil accelerate when the discharge line temperature exceeds 130 °C. The lower discharge temperature potential of R170 is therefore relevant not because it is an inherent universal advantage, but because it may permit higher allowable compression ratios before the compressor safety limit is reached, provided the compressor is rated for hydrocarbon service. Because R170 has a molar mass of 30.07 g/mol, about 43 % of R23 and 32 % of R508B, the swept volume flow per kW of refrigeration capacity is higher. This changes compressor selection from a two-cylinder low-stage R23 compressor to a three-cylinder or larger displacement R170 machine in the same frame size, or requires increasing the motor power and speed. The lower mass flow of R23 at typical low-stage conditions is advantageous for minimising pressure drop in suction lines and evaporators, but the high discharge temperature of R23 imposes a ceiling on compression ratio. R508B was developed specifically to combine R23 and R116 to reduce discharge temperature relative to pure R23 while retaining similar vapour pressure. However, the GWP of R508B remains above 13,000, and its low critical temperature restricts condensing temperatures more severely than R170. The lower discharge temperature of R170 is therefore coupled with a larger volumetric displacement requirement and a higher suction-line sizing burden. The compressor selection must be performed using manufacturer software that includes R170 in the refrigerant database and that has been validated for flammable refrigerant applications; using an R23-rated displacement and simply changing the refrigerant name in the system controller is not sufficient. Published data comparing R170 and R508B in the same compressor frame is limited because compressor manufacturers have only recently released hydrocarbon-rated low-stage machines, so the selection must rely on certified performance data rather than field extrapolation. Typically, the most operationally disruptive change in a low-stage conversion from R23 or R508B to R170 is not the compressor discharge temperature but the behaviour of lubricating oil in the evaporator, suction accumulator, and oil separator. In systems operating below -70 °C, mineral oil and even low-viscosity POE can become highly viscous, and oil return from a gravity-flooded or pump-recirculated evaporator depends on the refrigerant’s ability to dilute the oil and reduce viscosity. R23 and R508B are miscible with POE oils across a limited temperature band, but at low-stage evaporator temperatures the solubility of refrigerant in oil decreases, and liquid refrigerant can separate from the oil-rich phase in the accumulator. The oleophilic nature of R170 can provide a different miscibility envelope with alkylbenzene or mineral oils, but the actual data for R170-oil miscibility at -85 °C is not as extensive as the published data for R23-POE systems. The low-temperature viscosity of the oil must be checked with a cold-cranking simulator or rotary viscometer according to ASTM D2983, and the pour point of the selected lubricant must be below -60 °C unless an oil recovery strategy is implemented. A documented failure mode on low-temperature cascade racks is oil logging in the cascade condenser when the oil separator is sized for an A1 refrigerant with lower oil carryover and the system is charged with ethane without resizing the coalescing element. The oil separator element must be replaced with a media type compatible with hydrocarbon refrigerant and the new oil, and the oil return needle valve must be reoriented to ensure that cold ambient conditions do not freeze residual water or high-viscosity oil. Without this mechanical audit, the replacement of R23 with R170 can lead to repeated low-oil trips, bearing wear, and compressor failure even though the thermodynamic performance of the refrigerant itself is acceptable. Polyol ester oils that meet compressor manufacturers’ approvals for R23 contain additives that may react differently with R170 because the absence of halogen eliminates the need for acid-scavenging chemistry. The oil selected for R170 is commonly a naphthenic mineral oil or alkylbenzene with a pour point below -60 °C and a kinematic viscosity at 40 °C of 32 mm²/s to 68 mm²/s depending on compressor type. The oil charge volume must be revalidated because R170 has a lower density and different oil solubility than R23, and the oil circulation rate must be measured by sampling the discharge gas and separating the oil using a calibrated refrigerant oil separator apparatus. The total acid number should remain below 0.15 mg KOH/g when tested per DIN 51558-1 or ASTM D974, and the moisture content should be kept below 50 ppm by Karl Fischer titration. R170 is less prone to hydrolytic acid formation than R23 because it contains no fluorine, but air ingress during service can oxidise mineral oil and create sludge; a nitrogen purge and pressure test with oxygen-free dry nitrogen is recommended before charging R170. The oil return system must also be reviewed for cold start conditions, because ethane’s low viscosity and lower liquid density affect the oil siphon pressure and the oil level switch behaviour in the compressor crankcase.

R23 Discharge Temperature Limits Accelerate POE Hydrolysis

The discharge temperature limit for low-stage R23 compressors is not set only by the thermal class of the motor insulation; it is set by the chemical stability of the lubricant and the refrigerant at the hot discharge valve and downstream pipe wall. At discharge temperatures above 130 °C, POE oils in the presence of trace moisture begin to hydrolyse, forming fatty acids that attack copper-plated steel surfaces. R23 is particularly unforgiving in this respect because it has a low molar mass, high discharge temperature potential, and hydrogen fluoride formation is possible when the molecule is exposed to hot surfaces in the presence of air and moisture. R508B was introduced to reduce discharge temperature below the R23 ceiling, but its GWP and its lower critical temperature create a separate set of constraints. R170 does not contain fluorine, so hydrofluoric acid formation is not a failure mode. However, R170 is a hydrocarbon that is flammable at concentrations between 3.0 vol% and 12.5 vol% in air, and it requires the compressor discharge temperature to be maintained below the auto-ignition temperature of 472 °C, which is far above actual discharge temperatures and is therefore not a direct thermal hazard. The deeper engineering interaction is that R170’s lower discharge temperature may allow a higher compression ratio before the 130 °C limit is reached, but the actual allowable compression ratio is ultimately determined by the compressor manufacturer’s hydrocarbon-certified operating map and the load-bearing capability of the motor at high pressure differentials. R23 and R508B are not flammable but their decomposition products under high temperature can include carbonyl fluoride and hydrogen fluoride, which are toxic and corrosive. This is a reason that discharge-line dew point and moisture control are critical for R23 systems. In contrast, R170 with properly dried mineral oil and nitrogen purging does not generate halogen acid decomposition products, but it introduces an explosion risk if the system leaks into a confined space. The removal of halogen acid risk cannot be used to justify R170 without simultaneously addressing the flammable atmosphere risk, because the safety classification changes from A1 to A3. A systematic safety assessment is required before charging R170, and the pressure relief valves, pressure switches, and compressor terminal box must all meet explosion protection requirements for the new fluid. The chemical stability difference is therefore not a simple one-way advantage; it is a shift from a toxic-corrosion failure mode to a flammable-ignition failure mode, and the system design must change accordingly.

Material Compatibility and Elastomer Swell in R170 Service

Field experience with hydrocarbon refrigerants in low-charge systems shows that NBR O-rings, chloroprene gaskets, and some EPDM materials can exhibit volume swell, hardness loss, and extrusion failure when exposed to hydrocarbon/oil mixtures at elevated pressure and low temperature. R23 and R508B systems typically use HNBR, FKM, and PTFE seal materials that are qualified for HFC exposure; those same materials may be acceptable for R170 but must be tested in the specific refrigeration oil and at the actual operating temperature. ISO 1817:2015 describes the immersion test method for determining volume change, hardness change, and tensile property change of vulcanised rubber after exposure to liquids, and the test is to be performed at the maximum expected seal temperature and pressure for the low-stage package. A volume swell of more than 15 % or a Shore A hardness reduction of more than 10 points is generally unacceptable for dynamic shaft seals in open-drive compressors, but published data for specific elastomer grades in R170 service is limited and must be obtained from the seal manufacturer. The shaft seal on an open-drive compressor must be a double mechanical seal with barrier fluid or a gas-lubricated seal rated for A3 refrigerants; the standard R23 single mechanical seal should not be reused without a documented compatibility test. Sight glasses, liquid-level indicators, and pressure gauge internals may contain elastomers and plastics that are not hydrocarbon-compatible. The replacement of all elastomer components is a mandatory part of the conversion because a leak in an R170 system creates an immediate flammability hazard, whereas the same leak in an R23 system creates an asphyxiation and future environmental compliance hazard. Equipment operators must also verify that the lubricants and thread sealants used during assembly are oxygen-safe and do not react with ethane; PTFE tape and anaerobic thread compounds are generally used, but chlorinated solvents must be avoided because they can contaminate the charge and cause corrosion.

When Ethane Replaces R508B in Existing Cascade Skids Without a Full Oil and Seal Audit

A field changeover from R508B to R170 should be treated as a new machinery room installation for flammable refrigerants, not as a refrigerant recovery and recharge event. The existing cascade skid may have the correct thermodynamic components for R508B but often lacks the safety controls, ventilation interlocks, and pressure relief paths required for ethane. The low-stage pressure relief devices must be recalculated for R170 because its critical pressure, molecular weight, and saturated pressure characteristics differ from R508B; the relief valve capacity must be certified for the new fluid by the valve manufacturer under the applicable pressure equipment directive or ASME code. The compressor contactor, overload relay, crankcase heater, and pressure transducers must be evaluated for use in a potentially flammable atmosphere if the machinery room is classified as a hazardous area under IEC 60079-10-1:2020. An R170 system normally requires an area classification of Zone 2 or Zone 1 for the low-stage skid unless the entire skid is located outdoors and ventilation is natural. Fixed gas detection using infrared or catalytic bead sensors should alarm at no more than 25 % of the lower flammability limit and initiate corrective action such as forced ventilation shutdown or liquid line solenoid de-energisation at 50 % of the lower flammability limit. The sensor calibration gas must be ethane, not methane or propane, because the detector response factors differ significantly. The safety chain must be hardwired, fail-safe, and testable via a periodic bump test; reliance on a programmable logic controller alone is not typical for flammable refrigerant safety functions. The refrigerant charge limit for an A3 refrigerant in an occupied space is determined by the lower flammability limit and the room volume, as set in EN 378-1:2016 Annex C. The lower flammability limit of ethane in air is 3.0 vol%, equivalent to approximately 0.038 kg/m³ at 25 °C and 101.325 kPa. A cascade system with a large low-stage charge may exceed the allowable charge if the machinery room is small or if the system is located below grade without adequate ventilation. The actual allowable charge must be calculated using the specific room volume, the floor area, and the ventilation rate; for non-ventilated spaces, the charge is often limited to a fraction of the lower flammability limit. This is a central advantage of R170 over R23 and R508B only in environmental metrics, not in safety metrics. The reduction in GWP from 13,396 to 5.5 is offset by the need to reduce charge, add leak detection, and install explosion-proof electrical components. The total lifecycle safety and environmental impact must be evaluated using an F-gas calculation and an area classification assessment before the conversion. Leak tightness testing after R170 charging must follow a vacuum decay protocol that is more stringent than that used for R23 or R508B because the flammability classification changes the acceptable leak rate. The system should be pressure tested with dry nitrogen to 1.1 times the maximum allowable pressure, and then evacuated to below 200 µm Hg absolute and held to confirm that the vacuum rise is less than 50 µm Hg over a defined time interval. These values are not universal and must be taken from the compressor manufacturer’s service manual and the relevant pressure equipment code. R170 has a smaller molecular size than R508B, so joints that are tight enough for R508B may still leak R170 at a rate that exceeds the leak detection threshold. Electronic leak detectors designed for hydrocarbons should be used, and they must be calibrated with an ethane reference gas. The leak rate at the bubble point of the system should be verified at joints, valve caps, and shaft seals. The system must be charged only after the oxygen content in the evacuated circuit has been confirmed to be below 0.5 vol% and after the system has been purged with dry nitrogen to remove non-condensables. Pressure transducers and temperature probes used in R23 systems are generally reusable, but pressure switches with internal elastomer diaphragms may not be suitable for ethane. The system controller must be programmed with the correct refrigerant property curves for R170, and the low-pressure and high-pressure cutout settings must be recalculated because R170 has different pressure-temperature saturation relationships. In a cascade condenser operating at -30 °C, a pressure switch set for R508B may not align with the R170 saturation temperature. The compressor oil pressure differential safety must also be verified because the density of suction gas changes with R170 and can alter oil pump net pressure. Pressure relief sizing for R170 must account for the fact that the low-stage vessel may be exposed to higher saturated pressure at the maximum allowable temperature than R508B at the same temperature. The relief path through the vent line must be routed to outdoors and terminated away from air intakes, per EN 378-3:2016. The low-stage receiver must be fitted with a dual relief valve changeover assembly to allow servicing without system shutdown, but the changeover valves must be locked and documented. The fill connection must be a unique hydrocarbon fitting that cannot be accidentally connected to an HFC cylinder; for example, a left-hand thread or a dedicated quick-connect. The use of sight glasses is restricted in some jurisdictions for flammable refrigerants, and liquid-level sensors may need to be non-contact ultrasonic or radar-based. The system logbook must record the refrigerant charge, lubricant grade, leak test results, detector calibration dates, and vent line inspection. The absence of these records during an inspection under the pressure equipment directive or the local fire code can render the system non-compliant even if the thermodynamic performance is verified. R170 is a technically viable low-stage refrigerant in cascade systems when the charge is limited, the compressor is approved for A3 service, and the safety architecture is designed for flammable gas. The decision to replace R23 or R508B cannot be based solely on GWP because the flammability hazard and the equipment cost must be integrated into the safety case.