CHILLERS & REFRIGERANT How to avoid an explosive situation T
Legislation, such as the Fluorinated Greenhouse Gases Regulations 2015 (F-Gas), prohibit the use of certain refrigerants which are ozone depleting or have a high GWP in heating, air conditioning and refrigeration equipment. This has led to an uptake in the use of alternative refrigerants, some of which are highly flammable. In the second feature this month on the subject of safety relays, Will Darby of Carlo Gavazzi considers the safety implications posed by these alternative refrigerants and the need for equipment to avoid the use of electronic components that create sparks
he UK government has set a target of Net Zero emissions by 2050 to help minimise the impact of climate change. To meet this commitment, the Committee on Climate Change has said that 19 million heat pumps will need to be installed to keep homes and buildings to keep them warm in winter. But, in addition to keeping people warm, as the climate inexorably warms there will also be an increased need for cooling and the use of air conditioning to help control temperature and humidity level in buildings in order to maintain comfortable spaces in which people can live and work. An uptake in the use of air conditioning combined with a boom in the roll-out of heat pumps has the potential to increase the use of refrigerants. However, legislation such as the Ozone-Depleting Substances Regulations 2015, and the Fluorinated Greenhouse Gases Regulations 2015 (F- Gas), prohibit the use of certain refrigerants, particularly those that are ozone-depleting or with a high global warming potential (GWP), such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). The F-Gas regulation has led to an increase in the use of alternative refrigerants, including naturally-occurring refrigerants such as propane (R290), in devices such as refrigerators, heat pumps and air conditioning equipment. In fact, there are reports that one large UK supermarket chain has now adopted propane as the preferred refrigerant for all refrigerated services in its stores. Hydrocarbons, such as propane, have no ozone depleting properties and low GWP. The challenge for refrigeration and heat pump equipment manufacturers, however, is that replacement refrigerants, such as propane, are often highly flammable. This means that the components used in these systems must be selected so that they do not pose a risk in the event of a release of the gas. Each type of refrigerant comes with its challenges. Propane, for example, is more dense than air, so if a leak does occur the propane will drop to the lowest point in a piece of equipment or a space, where it could potentially collect. Crucially, if the leaked gas comes into contact with a spark or other source of ignition from an electrical component there is a very high risk that the gas would explosively ignite. To help minimise the risk of an explosion, sections of the standard for home appliances EN60335 make reference to maximum surface temperatures of equipment components. EN60335-2-40
is the section of the standard for heat pumps, while EN60335-2-89 is the section for refrigeration cabinets. The standard includes an annex BB along with clauses that require specific tests to check for the surface temperatures of components that may be exposed to any leakage of the refrigerant. Propane ignites at 470℃. Clause 22.117 of EN60335-2-40, says that temperatures on surfaces that may be exposed to leaked flammable refrigerants shall not exceed 370℃, 100℃ lower than the ignition temperature. In addition to hot surfaces, there is a
requirement to avoid the placement of other ignition sources, such as components like electro-mechanical relays that create sparks when they switch, where they could be in contact with leaked refrigerant. An electromechanical relay (EMR) uses a physical moving part to connect contacts within the output element of the relay. While it is true that some electro-mechanical relays are available sealed inside a hermetically sealed box designed to stop the gas from entering the compartment where arcs are generated, a far simpler option is to use a Solid State Relay (SSR). By contrast, SSRs are switched electronically (rather than using physically moving parts) to provide arc-free switching when an external voltage is applied to the unit’s control terminals. SSRs are particularly useful for switching small compressors (<10Amp), such as those found in small refrigeration cabinets in supermarkets and vending machines where a high starting current is not required.
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Carlo Gavazzi make a wide variety of
relays, the type of load to be switched will affect the choice of SSR.
• Resistive loads, such as heating elements and light bulbs are best switched with a Zero-Crossing SSR, in which the output is activated at the first zero-crossing of the alternating line voltage sine wave.
• Inductive loads, such as solenoids, pumps and fan motors (anything built around a large coil of wire) work best with an instant-on SSR, the type of relay that is activated immediately after applying the control voltage.
• Heavy inductive loads, such as transformers, should be switched with peak-switching SSRs, where the relay is activated at the first peak of the line voltage.
In addition, Carlo Gavazzi’s NRG range of digital SSRs have transformed the solid state relay with the addition of a digital communication interface. This enables real-time measurement and diagnostic information, related to the status of the SSR and the load it controls, to be exchanged with the machine controller. What’s more, the NRG SSR occupies the same footprint as a traditional solid state relay, so its additional functionality does not require additional panel space or additional wiring.
BUILDING SERVICES & ENVIRONMENTAL ENGINEER JANUARY 2023 21
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