The growing focus on environmental sustainability has driven the refrigeration, air conditioning and heat pump (RACHP) industry towards adopting refrigerants with lower global warming potential (GWP). The new applications of refrigerants, while beneficial for the environment, often come with new risk considerations compared with traditional choices.
While traditional refrigerants are considered non-flammable (although most of them can burn under certain circumstances), some modern options are classed as flammable. This necessitates stricter safety protocols throughout the entire life-cycle of the RACHP system, from selection and installation to maintenance and disposal.
The presentation by Takizawa1 provides a useful overview of the properties that impact refrigerant flammability, which include the limits for upper and lower concentrations in air (upper flammability limit (UFL) and lower flammability limit (LFL)); the burning velocity (higher burning velocity signifies a faster burning rate, translating to a faster spread of fire); the minimum ignition energy (MIE) (higher means more difficult to ignite); the quenching distance (the closest a flame can get to a cool surface, such as a metal cabinet, before it goes out); and the flame extinction diameter (which helps explain how heat loss and flame size influence flame stability).
According to the 2020 paper2 that followed on from an ASHRAE and AHRI-funded research project involving extensive laboratory testing of A2L refrigerants, it was discovered that when the refrigerant concentration was increased slowly, open flames from candles, matches and cigarette lighters extinguished rather than initiated significant explosions (deflagrations). (The study excluded lubricating oil and high humidity effects on ignition, and did not account for open flames from gas hobs or room heaters.)
However, a still mixture of refrigerant and air (known as a ‘quiescent premixture’) above the LFL can ignite when exposed to very hot (740°C) elements (compared with a cooker element at 480°C) and open flames (matches and butane). This highlights the need for appropriate ventilation for spaces with A2L refrigerants to ensure that the LFL is never reached.
The researchers discovered that other sources likely to be found in occupied premises did not ignite the A2L refrigerant even when in a quiescent premixture. These included a smouldering cigarette, a butane lighter, friction sparks, a mains plug and socket, a light switch, a bread toaster, a hair dryer, a hot plate, and a space heater. The difficulty in igniting an A2L in air is partly attributable to its relatively long quenching distance of approximately 8-25mm that compares with propane at approximately 1.5mm.
Additionally, the minimum ignition energy for a typical A2L is 10J, compared with approximately 0.0003J for methane3 and propane. Under some conditions, the tested A2L refrigerants were observed to act as flame suppressants. (There are interesting videos linked from the paper2 that illustrate the test results.)
Several standards influence the application of refrigerants that have their origins in standards organisations, such as the International Standards Organisation (ISO), the International Electrotechnical Commission (IEC), the European Committee for Standardisation (CEN), and the American National Standards Institute (ANSI).
Although some standards are considered global, they are frequently adopted by national, regional, and local standards authorities, sometimes with local deviations. Also, since the development timeline for standards is not common, they do not necessarily agree on specific guidance at any one time.
Standards are informally referred to as ‘vertical’ when applying to a specific industry or group of products and ‘horizontal’ when they are referenced by a wide range of industries. Horizontal standards will, for example, likely account for the requirements of a wide range of system types during the design, installation, commissioning, servicing and end-of-life processes.
BS ISO 817:20144 establishes a system for assigning the safety classification commonly used for refrigerants based on toxicity and flammability data, and provides a means of determining refrigerant concentration limits. The classifications – shown in Table 1 – are synchronised with those of ANSI/ASHRAE Standard 34 Designation and Safety Classification of Refrigerants.
Ensuring a safe system may be by ‘intrinsic safety’ and ‘extrinsic safety’ methods. Intrinsic safety limits the quantity of refrigerant so that any leaks into the space cannot create an unsafe condition.
Extrinsic safety employs alternative measures – such as the physical arrangement of the system, additional safety equipment, and operational procedures – to ensure that a dangerous situation cannot arise. Some equipment, such as refrigerant gas detectors and alarms, may be included as part of the product and some sourced separately and installed on site.
The horizontal standard BS EN 378 Refrigerating systems and heat pumps – Safety and environmental requirements (which is currently under review) is intended to minimise possible hazards to persons, property and the environment across the whole array of refrigerating systems and refrigerants.
It effectively acts as a foundation for risk management, establishes safety benchmarks, and promotes best practices for working with refrigerants in systems that cover many product groups. This standard engages with the work of the various building engineering professionals when working on UK and European projects. (ISO 51495 may be more appropriate for work outside that geographic area.)
The vertical standard BS IEC EN 60335-2 Household and similar electrical appliances – Safety focuses on the specific safety requirements for the appliances (or products) themselves. (Despite its title, this standard relates to commercial applications.)
The recent 2023 revision to BS EN IEC 60335-2-40,6 which specifically covers electrical heat pumps, air conditioning and dehumidifiers, included many revisions relating to the safe application of A2L refrigerants. It provides manufacturers with a clear and concise set of guidelines to follow, ensuring that their products are safe, reliable,
and efficient.
In the UK, all refrigerants are subject to Dangerous Substances and Explosive Atmosphere Regulations7 (DSEAR). Identified risks must be eliminated or minimised as far as reasonably practicable. Conducting and documenting relevant risk assessments is essential, along with ensuring the proper provision of safety equipment such as leak detection, ventilation, shut-off valves and alarms.
As highlighted by the Federation of Environmental Trade Associations8 (FETA) in the Pressure Equipment (Safety) Regulation (PE(S)R), A2L refrigerants are classified as ‘dangerous’ owing to their flammability. Split air-conditioning systems using A1 refrigerants are more likely to be in PE(S)R Category 1 (or possibly exempt and therefore only required to be constructed in accordance with ‘sound engineering practice’ (SEP)).
For these systems, the contractor can self-certify its compliance with the regulations. In contrast, systems with A2L and A3 refrigerants are more likely to be in Category 2 or above, and so will require some form of assessment by an Approved Body before a UK Conformity Assessed (UKCA) mark can be applied to the installed system.
This body must verify the design and technical information and witness a portion of the strength pressure tests. The Cool Concerns briefing note9 advises that the contractor acts as the ‘manufacturer’ of the complete system and is usually responsible for the final conformity assessment (see IoR Guidance Note 3610 for more detail).
One of many ways that manufacturers can achieve a UKCA (or CE) mark is by demonstrating conformity to the requirements of a harmonised safety standards, such as relevant parts of BS EN IEC 60335. BS EN 378-1 notes that product family standards dealing with the safety of refrigerating systems take precedence over horizontal standards covering the same subject, including limits on refrigerant quantities for a particular application. BS EN 378 applies to a far wider, generic set of applications that are outside the scope of individual product standards.
One of the key issues of employing A2L refrigerants in room units, such as would be used in split air conditioning and variable refrigerant flow (VRF) systems, is the allowable charge of refrigerant in a particular space. Fortunately, recent editions of BS EN 378-1 and BS EN IEC 60335-2 are generally consistent on refrigerant quantity limits if the same assumptions are applied to both standards. However, in specific applications, there may be different areas of nuance in the horizontal and vertical standards, and the standards should be consulted for full details.
Both current versions of BS EN 378‑1 (equation C.2) and BS EN 60335-2-40 (equation GG.9) use the same (empirical) intrinsic safety equation to establish the minimum room floor area Amin (m2) that can be used to install an appliance with refrigerant charge mc (kg) where the room is unventilated, Amin = (mc/ 2.5 × LFL1.25 × h0)2 where:
h0 is assumed release height of leaking refrigerant, greater of (hinst+hrel) or 0.6m
hrel is distance (m) from bottom of appliance to point of release
hinst is the reference installed height of the unit (0m for floor-mounted, 1.8m for wall-mounted, and 2.2m for ceiling-mounted)
For example, applying a floor-mounted room unit, such as that shown in Figure 1, charged with 2.4kg R32 (an A2L refrigerant) that has an LFL of 0.307kg.m-3,the minimum allowable room area for the room unit where there is no ventilation would be (2.4/(2.5 x 0.3071.25 x 0.6))2 = 49.02m2 in unventilated areas.
Figure 1: An example of a floor-mounted room unit, charged with 2.4kg R32, capable of delivering up to 5kW sensible cooling and 6kW heating (Source: Mitsubishi Electric)
However, when a fan that is incorporated into the unit is either continuously operated, or through an appropriate refrigerant detection system, is able to deliver a sufficient recirculation airflow rate (of at least 30 x mc/LFLm³.h–¹, according BS EN 60335-2-40), the allowable minimum room area can be smaller, as it is assumed that the recirculation will prevent potentially leaking refrigerant reaching the LFL, while alarms will also alert users to the leak.
BS EN 378 and BS EN 60335-2-40 suggest that leak detectors should be located where leaking refrigerant may stagnate or concentrate, but they (currently) differ in specific detail. However, the intent is the same in the two standards and, although using different calculation methods, they appear consistent (and the upcoming revisions to BS EN 378 may provide increased similarity in method).
From BS EN 60335-2-40 equation GG.11, the simplified empirical relationship is Amin = mc/(0.75 × LFL × hra) where hra is the estimated reaching height of the airflow (m). So, repeating the previous example for a floor-mounted unit with 2.4kg R32, and an estimated reaching height of 0.6m, the minimum room area = 2.4/(0.75 x 0.307 x 0.6) = 17.4m2 for the unit with a circulation fan and an inbuilt leak detector.
This provides opportunity for applying the unit in a smaller room by applying extrinsic safety measures where, in the event of a leak, the indoor unit must be capable of increasing the fan speed to maximum and triggering an alarm. (The installation could also comply with BS EN 378-311 if the system leakage alarm has an independent power source, such as a battery-backed supply.)
In the calculations undertaken above, a key variable is the mounting height of the unit – this should be considered carefully to ensure that the minimum areas are properly representative of an installation. A site variation to the mounting height can significantly impact the installed system, as it determines the extent to which refrigerant, if it leaks out of the system, will disperse through the whole space rather than pooling in a concentrated layer at floor level.
Meeting the requirements of the comprehensive product safety standard may well be considered as appropriate for compliance regardless of the horizontal standard. Indeed, the introductory text to BS EN IEC 60335‑2‑40:2023 explains there is no need to refer to horizontal standards for products within its scope, since they have been taken into consideration when developing the general and particular requirements of this vertical standard.
As with any engineering solution, manufacturers, installers and operators have a responsibility to ensure that installations meet the safety levels established by industry standards. It is crucial to understand how to evaluate and mitigate risks associated with the use of refrigerants, and the systems that incorporate them.
To help address concerns such as refrigerant leakage and detection, while providing flexible heating and cooling solutions, manufacturers have introduced hybrid VRF systems. These systems place all refrigerant-containing components outside of commonly-occupied spaces and use water for heat distribution, thereby minimising both leakage risks and the amount of refrigerant required.
In all cases, the designer should have a clear understanding of why decisions are made and apply the standards that are most appropriate to the application.
© Tim Dwyer 2024.
References:
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