The calculation of the maximum permissible refrigerant charge for R-E170 (dimethyl ether, ASHRAE safety class A3) in an evaporator cabinet is not a fixed mass but an occupied-space limit derived from the release-height and floor-area method in EN 378-1:2016, Annex C. The governing expression is m_max = 2.5 × LFL^5/4 × h₀ × A^1/2, where LFL is the lower flammability limit expressed in kilograms per cubic metre, h₀ is the installation height factor in metres, and A is the occupied-space floor area in square metres. For R-E170, the volumetric lower flammability limit is approximately 3.4 vol% at 20 °C and 101.325 kPa. Converting this volumetric concentration to a mass concentration through the molar mass 46.07 g/mol and molar volume 24.0 L/mol yields approximately 0.065 kg/m³, although a rounded value of 0.064 kg/m³ is also encountered in refrigeration safety literature for two-significant-figure charge calculations. This value is below the 0.100 kg/m³ threshold used in ASHRAE 34-2022 to define Class 3 flammability, and it produces a more permissive charge limit than many hydrocarbon refrigerants but still a severe restriction compared with A2L fluids.

What Minimum Floor Area Must an Integral Evaporator Cabinet Meet for a 300 g R-E170 Charge?

When the total circuit charge is fixed at 0.300 kg and the cabinet is floor-mounted, the height factor is 0.6 m. The term LFL^5/4 for R-E170 at 0.064 kg/m³ is 0.0322. The minimum floor area is obtained by rearranging the occupied-space expression as A_min = (m_charge / (2.5 × LFL^5/4 × h₀))². Substitution gives (0.300 / (2.5 × 0.0322 × 0.6))² = 38.6 m². For a wall-mounted cabinet with h₀ = 1.8 m, the same charge requires 4.3 m². This outcome illustrates that a floor-standing integral display cabinet charged to 300 g is not acceptable in a small stockroom or service area below 38.6 m², whereas an elevated wall-mounted cabinet with the same refrigerant mass can be installed in a room only slightly above 4.3 m². The calculation is a room-hazard criterion and does not replace the internal volume, fill ratio, or component charge verification required for the refrigerating circuit.

R-E170 has a vapor density approximately 1.59 times that of air, so a release from a low-level evaporator does not become uniformly mixed through the full room height when natural ventilation is limited. The 0.6 m floor-level height factor in EN 378-1 conservatively accounts for this heavier-than-air behaviour. The total charge in an evaporator cabinet is the sum of the liquid and vapour masses in the evaporator coil, liquid line, condenser, receiver, accumulator, and compressor shell, multiplied by the saturated liquid density of R-E170 at the maximum design ambient temperature. A saturated liquid density of approximately 0.66 kg/L at 20 °C is used in preliminary charge estimates, but the value must be confirmed from refrigerant thermodynamic property data for the maximum operating pressure of the cabinet. The fill ratio must not exceed the component manufacturer’s maximum allowable liquid fill at the maximum operating temperature. Published point-by-point charge distribution data for R-E170-specific evaporator cabinets is limited, so pre-production charge verification usually requires direct displacement measurement or an isochoric calculation on the assembled circuit. The safety calculation does not credit refrigerant dissolved in lubricant, because pressure decay after a leak can release dissolved refrigerant into the occupied space.

Installed Height, Release Direction, and the 1.25 Exponent Change Allowable Charge Nonlinearly

The exponent 5/4 on LFL is an empirical nonlinearity that makes charge limits sensitive to small changes in the lower flammability limit. If the LFL is halved, the allowable charge is reduced by a factor of (0.5)^1.25 = 0.420, not by 0.5. For R-E170 compared with R-290, the LFL ratio is 0.064 / 0.038 = 1.684, so the allowable charge ratio at identical installation conditions is approximately 1.684^1.25 = 1.92. This is why R-E170 can permit roughly twice the charge of R-290 in the same room and at the same release height. The height factor h₀ assumes discrete values: 0.6 m for floor-mounted units, 1.0 m for window or sill-mounted units, 1.8 m for wall-mounted units, and 2.2 m for ceiling-mounted units. Because the charge expression is linear in h₀ but square-root in A, increasing the installation height from 0.6 m to 1.8 m increases the allowable charge by a factor of 3 at constant area, while increasing floor area from 10 m² to 40 m² only doubles the allowable charge. For R-E170 in a 20 m² room, the floor-mounted allowable charge is 0.216 kg, and the wall-mounted allowable charge is 0.648 kg. The results are summarised in the following table for three A3 refrigerants.

IEC 60335-2-89:2019, Annex BB imposes additional construction, marking, and testing requirements for commercial cabinets using flammable refrigerants. The 150 g charge threshold is frequently referenced for sealed refrigeration circuits in occupied spaces, but charges above 150 g are not automatically prohibited. Instead, the manufacturer must demonstrate through the room-area calculation that the marked installation location has a floor area sufficient for the charge, and the cabinet must comply with enhanced leak-simulation and ignition-source requirements. Electrical components inside the cabinet must be assessed for ignition risk during a refrigerant leak; typical protection measures include spark-free evaporator fans, sealed connection boxes, and solid-state controls that do not generate arcs. For an integral cabinet with the compressor compartment separated from the evaporator, the charge is defined as the complete circuit charge, including the condenser and compressor shell, because a leak at any pressure boundary can release the full charge into the occupied space. A floor-standing cabinet with a top-mounted evaporator does not automatically qualify for a higher height factor if the primary pressure boundary remains near the floor; the release height used in the calculation must correspond to the location of the most probable leak source.

When the Cabinet Is Installed in an Unventilated Room Below 10 m², the Calculated Limit Falls Below 150 g

For a floor-mounted R-E170 cabinet with h₀ = 0.6 m, the allowable charge in a 10 m² room is 0.153 kg, only 3 g above the 150 g threshold. The minimum room area for a 150 g floor-mounted charge is 9.65 m². If the room area falls to 4 m², the allowable charge drops to 0.0966 kg, which is below 150 g. This creates a practical process conflict in small service rooms, under-stair storage spaces, and walk-in cooler ante-rooms: a nominal 150 g charge is not acceptable in a floor-mounted cabinet unless the free floor area is at least 9.65 m². Elevating the release height changes the result significantly; a wall-mounted cabinet at 1.8 m in the same 4 m² room permits 0.290 kg. If the room is below grade with no natural ventilation, the charge calculation still applies, but local fire and building regulations may impose additional gas detection and ventilation interlocks. The room-area calculation must be repeated after final installation because wall penetrations, floor obstructions, and final cabinet elevation can alter the effective release height and occupied-space area from design-stage assumptions.

For remote evaporator cabinets connected to a compressor pack in a machinery room, the EN 378-1 occupied-space calculation may be applied only to the portion of the charge that can be released into the sales area. The compressor, receiver, and condenser in the machinery room are assessed separately under EN 378-3 for machinery room ventilation, refrigerant detection, and charge limits. The interconnecting liquid and suction lines routed through the occupied space are included in the occupied-space charge when they are outside a ventilated duct or riser. Published point-by-point charge distribution data for long supermarket piping runs with R-E170 is limited; commissioning records should include the calculated liquid-line charge per metre and the measured evaporator coil charge from the manufacturer’s drawings or displacement tests. Field measurements on production-scale retail cabinets indicate that evaporator coil volumes can vary by ±10–15% between manufacturers for the same nominal duty, directly affecting total charge. Therefore a nominal charge value cannot be substituted for a measured or verified value when the margin to the allowable limit is less than 10%. Where two independent circuits serve the same cabinet, the limit applies per circuit only if a single leak cannot release charge from both circuits; otherwise the combined charge must be used.

Refrigerant Detector Set Points and Ventilation Interlocks for R-E170 Display Cabinets

R-E170 releases from an evaporator cabinet can accumulate near the floor because the vapor is 1.59 times heavier than air. Fixed gas detection is typically set to alarm at 25% of the lower flammability limit. For R-E170 at 0.064 kg/m³, 25% of the LFL corresponds to 0.016 kg/m³ and 8,500 ppm by volume. Detector heads serving floor-mounted cabinets should be installed approximately 0.15 m to 0.30 m above the finished floor and near the cabinet base, while detectors serving wall-mounted cabinets should also account for the thermal plume produced by the evaporator fans. The alarm function must interlock with the room ventilation system and, where specified by the cabinet standard, interrupt the liquid-line solenoid valve to stop continued release. Ventilation airflow is not determined by room air change rate alone; the rate must be sufficient to dilute the maximum credible leak mass flow to below the alarm set point before the occupied-space concentration reaches the LFL. No single fixed air change rate is stated in EN 378-1 for all R-E170 installations; the design must be based on the specific charge mass, room volume, and release height. Operating the cabinet with the ventilation interlock disabled places the installation outside the marked safety envelope. Material compatibility is also a separate constraint: dimethyl ether can swell certain natural rubber and silicone compounds, so valve seats, diaphragms, and cabinet gaskets should be validated for R-E170 exposure at the maximum operating temperature and pressure.

Evaporator cabinet leak simulations under IEC 60335-2-89, Annex BB require the refrigerant charge to be released inside the cabinet and the resulting concentration measured at potential ignition sources. Published validation reports for R-E170-specific cabinets with glass doors, night blinds, and low-energy fan motors are limited, so compliance for new configurations is usually established by testing the cabinet with a surrogate refrigerant of matched density, followed by calculation of the charge limit using the room-area formula. The leak simulation does not replace the EN 378-1 charge calculation; it confirms that no internal ignition source is exposed to a flammable concentration during a contained leak. For larger display cabinets above 150 g, the room-area calculation, the internal leak simulation, and the ventilation interlock verification are parallel requirements, and all must be satisfied before the cabinet is placed in service.