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Safety testing of domestic refrigerators using flammable refrigerants
International Journal of Refrigeration 27 (2004) 621–628 www.elsevier.com/locate/ijrefrig
Safety testing of domestic refrigerators using flammable refrigerants Andrew Gigiel* FRPERC, University of Bristol, Churchill Building, Langford, Bristol BS40 5DU, UK Received 11 November 2002; received in revised form 1 March 2004; accepted 1 March 2004
Abstract A domestic refrigerator with flammable refrigerant was tested according to the methods specified in the safety Standard, IEC/EN 60335-2-24. The tests were all carried out as specified in the Standard. Some of the test specifications were straightforward but some tests ambiguous and gave different results depending on the method used. The method of testing the protection of the refrigeration circuit does not simulate the type of damage that could be caused by defrosting with a knife. The simulation of a leak in a protected cooling circuit is not specifically defined. The concentration of refrigerant in the compartment with the protected circuit depended on the method used to prevent foam from entering the capillary tube (either 140 or 17,500 ppm). The position and direction of a simulated leak in the compressor compartment is not specified in the Standard but had a significant effect on the concentration distribution (643 – 240,000 ppm). The sudden release of an accumulation of refrigerant caused peaks in the concentration that could not be measured by the response time of the measuring instrument specified (28,500– 8000 ppm in 1.5 s). q 2004 Elsevier Ltd and IIR. All rights reserved. Keywords: Domestic refrigerator; Test; Safety; Flammability; Refrigerant
Re´frige´rateurs domestiques fonctionnant avec des frigorige`nes inflammables: e´tude sur la se´curite´ Mots-cle´s: Re´frige´rateur domestique; Essai; Se´curite´; Inflammabilite´; frigorie`ne
1. Introduction The safety of domestic refrigerators that use flammable, natural refrigerants was a concern when they were first introduced 10 years ago. The European Standard concerned with the safety of domestic refrigerators was EN 60335-2-24: “Safety of household and similar appliances—Part 2-24: Particular requirements for refrigerators, food freezers and icemakers”. This was amended in 1994 to include specifica* Tel.: þ44-117-928-9239; fax: þ44-117-928-9314. E-mail address: [email protected] (A. Gigiel).
tions for the safety of refrigerators using flammable refrigerants. During the next few years, several amendments have been made to this Standard and today the current international version is EN/IEC 60335-2-24:2001 [1] (The Standard). The Standard is based on the principle that a leak of refrigerant will not result in a concentration of flammable gas of more than 75% of its lower explosive limit (LEL) or, if it does, there will be no source of ignition on the refrigerator itself. Harun Iz et al. applied the tests specified in the 1994 Standard on a refrigerator and a freezer [2]. They found that on these units concentrations of R600a (isobutane) greater
0140-7007/$ - see front matter q 2004 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2004.03.001
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than the LEL did not occur in areas outside the units resulting from an external leak. They did not test the external concentration of R600a origination from a concentration inside the food compartments. Concentrations of R290 (propane) from simulated leaks were measured by Clodic and Cai [3], who found that they could ignite explosive mixtures of R290 and air flowing out of a food compartment in which there was a leak. This paper describes testing domestic refrigerators using isobutane according to the methods described in the current Standard and discusses the practicality of the tests and their reproducibility.
2. The safety precautions specified in the Standard for refrigerators with flammable refrigerants The Standard deals with four types of incident; the ability of the refrigeration circuit to contain an excessive pressure without leaking, the possibility of a leak from the refrigeration circuit, the possibility of the leaked refrigerant forming an explosive mixture and the possibility of an explosive mixture being ignited. In the latter case, the Standard is only concerned with ignition sources on the refrigerator. In the event of warm temperatures around the refrigerator, that may be caused by exceptional circumstances such as blocking the airflow to the condenser, the Standard requires that the refrigeration circuit should withstand the high pressures that will be caused inside the circuit by the elevation of the saturated vapour pressure, without leaking. The Standard defines how compliance with this is tested. The Standard considers that all joints in the refrigeration system are potential sources of leaks and that there is a reasonable probability of a leak occurring from one. It also considers that if a refrigeration circuit is likely to be physically damaged there is also a reasonable probability of a leak occurring. Such an occurrence could be caused by scraping the circuit to remove ice. The Standard does not consider that other leaks are reasonably probable. Where there is a probability of a leak the Standard considers the probability of an explosive mixture forming. An explosive mixture is one where the concentration of the flammable refrigerant in the air is sufficient to be ignited, i.e. greater than the LEL. The Standard considers that the possibility of an explosive mixture forming inside the food storage compartment when there is a leak depends on whether the refrigeration circuit can be considered as protected. Protected circuits are, in concept, where no part of the cooling system is inside a food storage compartment. Therefore, if a leak occurs, an explosive mixture will not form inside the compartment. Circuits that have a part of the cooling system inside the food storage compartment and can be considered to be as leak-proof as a protected circuit are also defined in the Standard. Because an explosive mixture
will not occur special precautions need not be taken to prevent a spark from occurring in the compartment. The Standard requires that this concept be tested by simulation of a leak and measuring the concentration in the compartment and by testing that a leak will not occur if the protection is deliberately damaged. Generally, an unprotected circuit is one that is inside the food storage compartment. However, any refrigeration circuit that is not protected, or which fails the test to demonstrate that it is protected, is unprotected. If electrical components, such as the thermostat, are inside an unprotected food storage compartment, they must be constructed so as not to ignite the mixture. Leaks from an unprotected circuit into a food storage compartment will increase the concentration of the flammable refrigerant in the compartment. When the door is opened, the gas will flow out. The Standard requires that this flow of gas will not cause an explosive concentration to form around the vicinity of the external electrical components of the refrigerator. If it does, then these components must be constructed so as not to ignite the mixture. The Standard defines how the refrigerator shall be tested for this. The Standard requires that leaks from an external joint in the refrigeration circuit will not cause an explosive mixture to form around the external electrical components. If an explosive mixture is formed, then these components must be constructed so as not to ignite the mixture. The Standard defines how the refrigerator shall be tested for this. The Standard requires that the temperatures of surfaces that may be exposed to leakage of the flammable refrigerant shall not exceed the ignition temperature of the refrigerant. Finally, the Standard requires additional information be marked on the cabinet and included in the instructions than that required for refrigerators with non-flammable refrigerants.
3. Method Each test was carried out following the specifications in the Standard. 3.1. The refrigerator The refrigerator used for the tests was a 218 l, 2-door fridge-freezer operating with R600a. A single 8.8 cm3 compressor cooled both compartments with a single evaporator split into two parts, the first part parallel with the shelves in the freezer compartment, and the second part behind the rear wall of the chilled compartment, separated from it by the thermoformed liner and an aluminium plate. The refrigeration circuit contained 55 g of R600a. Two models were specially prepared for the tests. One was without the compressor or electrical components and
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the other had capillary tubes, with one end fixed next to the joint between the evaporator and the suction pipe prior to foaming, the other being outside the refrigerator.
3.2. Test locations Tests that required a controlled environment were carried out in an insulated test room 6 £ 3.5 £ 2 m3. The air was circulated round the room and through an air handling plant. The air was introduced into the room through a perforated ceiling and left via eight grills at low level in the sidewalls. The air was controlled to 20 ^ 1 8C and the humidity to 65 ^ 10% RH. In tests requiring no drafts, the fans were switched off. During these tests the air temperature increased to 23.5 8C.
3.3. Pressure test The refrigerator without the compressor was used. A deadweight pressure tester was connected to a pressure vessel with an oil – water interface. The water circuit was connected in turn to the high and low pressure parts of the refrigeration circuit via valves and the air bleed out of the system. The valves on the refrigeration circuit were opened and the pressure increased gradually to the specified test pressure. The pressures were maintained for 1 min and the circuit then examined for signs of leakage.
Fig. 1. Scratch tool used for scratch tests as specified in the Standard.
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3.4. Scratch test for protected circuits Two scratching tools as specified in the Standard (Fig. 1) were made and each was used to scratch the rear vertical face of the liner in the chilled compartment of the compressor-less refrigerator. Each tool was mounted so that it was free to move vertically in a tube. The tool was prevented from rotating by a squared plate fixed to the tube in which a squared part of the tool shaft was free to move vertically. The tube was fixed on a platform 100 £ 100 mm2 mounted on rollers so that the platform could be moved freely over a flat surface. The refrigerator was laid on its back and the tool was drawn across the rear surface at a rate of approximately 1 mm/s by hand. The force at right angles to the surface was always 35 ^ 3 N, maintained by a weight placed on the end of the tool. The force parallel to the surface was always less than 250 N, a human not easily exceeding this limit inside a refrigerator. The low-pressure circuit was then tested as for the pressure test above, the test pressure being reduced by 50%.
3.5. Leak test for protected circuits Refrigerant vapour was injected through a capillary tube at each joint. The capillary tube was positioned before foaming the appliance and had a diameter of 0.68 ^ 0.05 mm and a length of 2.5 m. The tested was carried out twice, once with the compressor switched off and once on. During the test the doors of the appliance were closed and for the test with the compressor operating, gas injection was started at the same time as the appliance was switched on. The refrigerant injected was taken from the vapour side of a gas bottle that contained 80% of the charge of the refrigerator. The gas bottle was weighed and connected to the capillary tube via a quick-coupling connector. It was placed in a water bath which was maintained at 32 ^ 1 8C. The gas bottle was weighed continuously, together with the water bath on scales accurate to ^ 0.5 g. The gas bottle was disconnected at periods during injection and weighed on scales accurate to ^0.5 g. Each disconnection lasted for less than 1 min. The gas bottle was removed completely 1 h after the start of injection. Initially, no special protection was used at the open ends of the capillary tubes (diameter 0.68 ^ 0.05 mm, length 2.5 m) to prevent foam from entering during foaming. After foam was found to enter the tubes during foaming, porous flexible foam was wrapped over the ends of the capillary tubes prior to foaming. Refrigerant vapour was injected through the capillary tubes at each point. Each test was carried out twice, once with the compressor off and once on.
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3.6. Leak test for unprotected circuits Four tests were carried out in the test room, two with the compressor operating and two with it off. At the start of each test 44 g of R600a, 80% of the refrigerant charge, was injected into the frozen food compartment in 9 min. The injection point was 5 mm away from the centre of the rear wall at one-third of the height from the top. The air movement around the refrigerator when the door was opened was investigated with powder during repeat tests. 3.7. Leak test from external joints Two external leak tests were carried out, one with the refrigerator operating and one with the refrigerator switched off. In a third test, with the compressor off, the concentration of R600a was measured at several locations in the compressor compartment. At the start of each test 27.5 g of R600a was injected into the compressor area from the end of the service pipe, in line with it, at a constant rate of 0.51 g/min. 3.8. Measurement of concentration The concentration of R600a was measured with an Inficon Ecotec II detector. The sampling time was approximately 250 ms.
4. Results 4.1. Pressure test The test pressure of 35.1 bar gauge in the high-pressure side was maintained for 1 min without change. The pressure of 10.2 bar gauge was maintained in the low-pressure side. The circuits showed no signs of leakage.
Fig. 2. The concentration of R600a in the chilled compartment after the start of injection at a critical leak point to test a compartment with a protected refrigeration circuit.
the polyurethane insulation around the area of the simulated leaks removed exposing the end of the capillary tube and the test repeated. No refrigerant flowed through the capillary tube. The end of the capillary tube was cut off in stages of approximately 3 mm and the flow checked. No flow occurred until approximately 15 mm had been removed. The first 15 mm of tube had been blocked with foam. The leak test was repeated with the tube placed back into position with a small piece of open celled flexible foam taped over the open end of the tube. The outer steel skin of the refrigerator cabinet was fixed back into position with tape and the cavity refoamed. The entire leak charge was injected in 30 min. The concentration in the chilled compartment increased as injection started, reaching a maximum in 2340 s (Fig. 2). After the test had finished, the inside and outside of the cabinet were tested for escaping R600a. R600a was entering the chilled compartment via one of the holes in the liner, used for fixing the thermostat housing (Fig. 3). The gas was
4.2. Scratch test for protected circuits When one of the scratching tools was drawn across the surface of the liner covering the evaporator the amount of damage was negligible. When the second tool was used, the depth of the scratch was significantly greater but the tool still did not penetrate the liner. The subsequent pressure tests confirmed that no damage had been done to the refrigerant circuit. 4.3. Leak test for protected circuits In the first tests, no refrigerant was injected into the joint area and the concentration of R600a in the chilled compartment did not increase above the background level of 140 ppm. The back of the refrigerator was cut open and
Fig. 3. View of the top right corner of chilled compartment showing fixing holes for the thermostat housing. R600a entered the chilled compartment through the lower rear hole when simulating a leak from a protected circuit.
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Fig. 4. Concentration of R600a by the compressor after the door is opened (time ¼ 0) in the unprotected leak test.
also escaping from the insulation where the suction pipe entered the compressor compartment. 4.4. Leak test for unprotected circuits With the compressor off, the concentration of R600a around the compressor increased immediately to a maximum of 8000 ppm when the door was opened. It then decreased rapidly to less than 400 ppm, 60 s after the door was opened. With the compressor operating, the concentration of R600a increased rapidly from 450 ppm to a maximum of 28,500 ppm, 1.3 s after the door started to open (Fig. 4). During the first 20 s the concentration oscillated between 7000 and 22,700 ppm. When the compressor was off and the door closed, the air velocity around the refrigerator was approximately 0.1 m/s without any steady direction. With the compressor operating, prior to the door opening there was a draft under the refrigerator from front to back of 0.8 m/s caused by heat rising at the rear of the refrigerator. When the door was opened, the cold gas flowed rapidly from the freezer compartment across the floor away from the refrigerator. The current of air under the refrigerator from the front to back also increased. 4.5. Leak test from external joints With the compressor off, the capillary tube discharging from the end of the charging tube and a flow rate of 0.51 g/min (slightly greater than 80% of the charge in 1 h), the concentration of R600a was a maximum of 3200 ppm and a mean of 1230 ppm. With the compressor running the maximum concentration was 1105 ppm with a mean of 643 ppm. The concentration around the compressor compartment varied from 240,000 ppm, immediately in the path of the jet from the capillary tube, to the background level. The velocity of the discharge from the capillary tube was calculated to be approximately 5 m/s. The direction in which the tube pointed had a big effect on the concentration of R600a in the immediate vicinity of the discharge (Fig. 5).
Fig. 5. View of the end of the capillary tube in line with the end of the service tube of the compressor, simulating a leak from an external joint.
The test was repeated with the jet dissipated by covering the end of the tube with porous foam. The R600a then spread out from the end of the capillary tube, upwards or downwards depending on whether the compressor was running or not. The concentration reduced to less than the LEL within 2 mm of the foam.
5. Discussion 5.1. Pressure test The test as specified in the Standard was straightforward to carry out. The purpose of the test is to demonstrate the mechanical integrity of the refrigeration system when it is subjected to excessive temperatures. The Standard does not require pressure tests if non-flammable refrigerants are used. With flammable refrigerant it requires that the test pressure on the low side correspond to the saturated pressure at 22 8C. The European Standard for the safety and environmental requirements of refrigeration systems [4] requires that the minimum design pressure for the low-pressure side corresponds to the saturated pressure of at least 32 8C. If the compressor on a domestic refrigerator is not operating, the low-pressure side will be at the same saturated temperature as the high-pressure side, considerably warmer than 22 8C (Table 1). 5.2. Scratch test for protected circuits The scratching of the protecting surfaces as specified in the Standard was difficult to carry out. The damage done to the surfaces was dependent on the sharpness of the scratching tool, which was not defined in the Standard.
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Table 1 Table of tests, their purpose, method and results Test
Purpose
Method
Result
Pressure test Scratch test
Prove physical integrity of system Prove protection of circuit
35.1 bar in high-pressure side, 10.1 bar in low-pressure side Surface over evaporator scratched with specified tool
Pressure test after scratching
Prove protection of circuit
Scratch test not specified in Standard Leak test for protected circuit
Prove protection of circuit
17.5 bar high-pressure side, 5 bar low-pressure side Scratched with knife
Pressure held .1 min, no leakage Scratch did not penetrated deep enough to damage evaporator Pressure held for .1 min, no leak Evaporator pierced with knife
Leak test for protected circuit
Leak test for unprotected circuit, compressor off
Leak test for unprotected circuit, compressor running
Leak test from external joint
Prove concentration of refrigerant does not exceed 75% of LEL in protected compartment in the event of a leak Prove concentration of refrigerant does not exceed 75% of LEL in protected compartment in the event of a leak Prove concentration of refrigerant does not exceed 75% of lower explosive limit by possible ignition sources on cold appliance Prove concentration of refrigerant does not exceed 75% of lower explosive limit by possible ignition sources on cold appliance Prove concentration of refrigerant ,75% of LEL by possible ignition sources on cold appliance
The specified scratching tool has a wide, long footprint. The area in contact with the surface increases as the depth of the scratch increases. When the tool has penetrated to a depth of 1 mm, the area of contact is 12 mm2. The tool did not mimic the most likely cause of damage. Trials were carried out using a knife to scrape the evaporator area. When the knife slipped it penetrated the protected circuit. The test pressure was reduced after scratching as specified in the Standard. The probability of a scratched refrigerator being subjected to excessive temperatures is the same as for an ‘unscratched’ refrigerator. A new refrigerator (unscratched) would be more likely to be looked after well than an old, damaged one. 5.3. Leak test for protected circuits The Standard does not specify how to prevent foam from entering the capillary tube during the foaming operation; it only states that care should be taken to
80% of charge injected by joint via capillary tube, no protection against foam
No leak
80% of charge injected by joint via capillary tube, protection with porous foam with a surface area of 200 mm2 80% of charge injected into unprotected compartment over 9 min. The door opened after 30 min
75% of LEL exceeded in compartment
80% of charge injected into unprotected compartment over 9 min. The door opened after 30 min
Maximum concentration 28,500 ppm varying between 7000 and 28,500 ppm
Release 80% of charge over 1 h adjacent to external joints
Maximum concentration 3200 ppm while compressor off. The concentration much greater in the path of the leak
Maximum concentration 8000 ppm (,75% of LEL)
prevent it. When porous foam was used to prevent foam from entering the capillary tube, the flow rate of refrigerant was 1.43 g/min. The method used to prevent foam from entering the capillary tube is important for the reproducibility of the test. The force exerted by the refrigerant vapour is proportional to the surface area over which it acts. When porous foam was used, the area over which the refrigerant acted was approximately 200 mm2. If the foam had been prevented from entering the capillary tube, but not interfering with its adhesion to any other surface, then the area available for the refrigerant pressure to act on would have been the crosssectional area of the capillary tube, 0.4 mm2. It is possible that in this case the refrigerant would not have escaped so quickly, or at all. In the tests carried out by Harun Iz et al. the method of preventing the foam from entering the capillary tube was not given. It is necessary to define a method of leak simulation that is reproducible rather than leaving the Standard open to an unlimited number of
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methods of preventing foam from entering the capillary tube, some of which will show leakage and some not. Once the leak had started, an escape path was propagated between the evaporator and the liner of the chilled compartment. There should therefore be no openings in the liner through which refrigerant could pass from the insulation space into the food chamber. The leaking refrigerant also passed along the suction pipe to the compressor compartment. 5.4. Leak test for unprotected circuits If there had been no dilution by air infiltrating past the door seals, the concentration of R600a would have been 140,000 ppm immediately before the door was opened. In a new refrigerator the performance of the door seals is, in part, specified in the performance standard, ISO8187:1991. In an old refrigerator, the infiltration would be greater, reducing the refrigerant concentration. When the door was opened, the heavy air – vapour mixture flowed out across the floor. When the compressor was not operating there was a very little flow of refrigerant under or round the sides of the refrigerator and the concentration of gas at the rear of the refrigerator, by the electrical components, was less than 75% of the LEL. When the compressor was operating the vapour in the freezer compartment was colder and heavier than when the compressor was off. When the door opened the flow of vapour from the compartment was faster. Because of the flow under the refrigerator from front to back, driven by the hot air rising from the operating compressor, the concentration of refrigerant by the electrics was greater than when it was not operating, and greater than 75% the LEL (13,500 ppm for R600a). The peak in the concentration lasted for less than 2 s. The Standard requires that the instrument for measuring the concentration should have a fast response, typically 2 – 3 s. If an instrument with a response time of 3 s had been used in these tests, the peak value of concentration could have been missed. In these tests, this would not have been important as the concentration remained above 75% of the LEL for 18 s but on different refrigerators this may not be the case. The sudden release of an accumulation of refrigerant can cause a short peak in concentration. To avoid missing this would need an instrument with a faster response than specified in the Standard. In order to conform to the Standard the electrical equipment in the compressor area of the refrigerator tested must be non-sparking (to conform to at least section 3, clauses 16 and 17, and section 4 of IEC 60079-15). In normal households, there are sources of ignition other than the refrigerator. The sudden release of vapor when a door is opened flows away from the refrigerator and is therefore more likely to be ignited by an ignition source remote from the refrigerator. Clodic and Sai [3] used an
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external ignition source and found that ignition was possible. The Standard does not address this point. 5.5. Leak test from external joints The concentration did not increase to more than 75% of the LEL during the tests nor more than 50% for a period of 5 min. These criteria mean that the electrics would not have to be spark proof to conform to the Standard with respect to leaks from external joints. The concentration in the jet from the capillary tube was greater than the LEL. The concentration distribution depended on the position and direction of the outlet of the capillary tube. If the concentration had been measured in a different position or if the simulated leak had been directed in a different direction then the result would have been different. The Standard is not specific enough on the direction and exit velocity of the simulated leak for the test to be reproducible.
6. Conclusions Some of the tests specified in the Standard were not reproducible within a reasonable interpretation of the Standard. This could result in different test laboratories reaching different conclusions about a refrigerator’s compliance with the Standard. The simulation of a leak from a joint in a protected cooling circuit is not specifically defined in the Standard. The concentration of refrigerant in the compartment with the protected circuit depended on the method used to prevent foam from entering the capillary tube. The position and direction of a simulated leak in the compressor compartment is not specified in the Standard but had a significant effect on the concentration distribution. When there is a sudden release of an accumulation of refrigerant, the concentration at the measuring point can peak more rapidly than the response time of the measuring instrument specified in the Standard. The method, sampling rate and accuracy of the refrigerant gas concentration instrument should be specified to measure rapid changes in concentration. This paper has presented experience of testing refrigerators according to the Standard concerned with their safety. It has not addressed the question of whether the refrigerator is safe. A cautious approach for making safe the refrigerator tested for this paper would be to: † use non-sparking electrics for low-level compressors, regardless of the results of the leak tests specified in the Standard. † use a continuous liner with no holes in the chilled food compartment with the protected cooling circuit or have no sources of ignition in the compartment.
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† give guidance to users on the need for ventilation and the position of ignition sources other than the refrigerator.
References [1] EN 60335-2-24:2001. ‘Safety of household and similar appliances—part 2-24: particular requirements for refrigerating appliances, ice cream appliances and ice makers’.
[2] Harun I, Tekin Y, Yalcin T. Experimental results of the safety tests on domestic refrigerators for refrigerant R600a. Proc. IIFIIR Commissions B1, B2, E1 and E2, Aarhus, Denmark; 1996– 3. p. 321–28. [3] Clodic D, Sai W. Tests and simulations of diffusion of various hydrocarbons in rooms from air conditioners and refrigerators. Proc. IIF-IIR Commissions B1, B2, E1 and E2, Aarhus, Denmark; 1996–3. p. 299–308. [4] EN 378-2:2000. ‘Refrigerating systems and heat pumps— safety and environmental requirements’.
Safety testing of domestic refrigerators using flammable refrigerants Andrew Gigiel* FRPERC, University of Bristol, Churchill Building, Langford, Bristol BS40 5DU, UK Received 11 November 2002; received in revised form 1 March 2004; accepted 1 March 2004
Abstract A domestic refrigerator with flammable refrigerant was tested according to the methods specified in the safety Standard, IEC/EN 60335-2-24. The tests were all carried out as specified in the Standard. Some of the test specifications were straightforward but some tests ambiguous and gave different results depending on the method used. The method of testing the protection of the refrigeration circuit does not simulate the type of damage that could be caused by defrosting with a knife. The simulation of a leak in a protected cooling circuit is not specifically defined. The concentration of refrigerant in the compartment with the protected circuit depended on the method used to prevent foam from entering the capillary tube (either 140 or 17,500 ppm). The position and direction of a simulated leak in the compressor compartment is not specified in the Standard but had a significant effect on the concentration distribution (643 – 240,000 ppm). The sudden release of an accumulation of refrigerant caused peaks in the concentration that could not be measured by the response time of the measuring instrument specified (28,500– 8000 ppm in 1.5 s). q 2004 Elsevier Ltd and IIR. All rights reserved. Keywords: Domestic refrigerator; Test; Safety; Flammability; Refrigerant
Re´frige´rateurs domestiques fonctionnant avec des frigorige`nes inflammables: e´tude sur la se´curite´ Mots-cle´s: Re´frige´rateur domestique; Essai; Se´curite´; Inflammabilite´; frigorie`ne
1. Introduction The safety of domestic refrigerators that use flammable, natural refrigerants was a concern when they were first introduced 10 years ago. The European Standard concerned with the safety of domestic refrigerators was EN 60335-2-24: “Safety of household and similar appliances—Part 2-24: Particular requirements for refrigerators, food freezers and icemakers”. This was amended in 1994 to include specifica* Tel.: þ44-117-928-9239; fax: þ44-117-928-9314. E-mail address: [email protected] (A. Gigiel).
tions for the safety of refrigerators using flammable refrigerants. During the next few years, several amendments have been made to this Standard and today the current international version is EN/IEC 60335-2-24:2001 [1] (The Standard). The Standard is based on the principle that a leak of refrigerant will not result in a concentration of flammable gas of more than 75% of its lower explosive limit (LEL) or, if it does, there will be no source of ignition on the refrigerator itself. Harun Iz et al. applied the tests specified in the 1994 Standard on a refrigerator and a freezer [2]. They found that on these units concentrations of R600a (isobutane) greater
0140-7007/$ - see front matter q 2004 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2004.03.001
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A. Gigiel / International Journal of Refrigeration 27 (2004) 621–628
than the LEL did not occur in areas outside the units resulting from an external leak. They did not test the external concentration of R600a origination from a concentration inside the food compartments. Concentrations of R290 (propane) from simulated leaks were measured by Clodic and Cai [3], who found that they could ignite explosive mixtures of R290 and air flowing out of a food compartment in which there was a leak. This paper describes testing domestic refrigerators using isobutane according to the methods described in the current Standard and discusses the practicality of the tests and their reproducibility.
2. The safety precautions specified in the Standard for refrigerators with flammable refrigerants The Standard deals with four types of incident; the ability of the refrigeration circuit to contain an excessive pressure without leaking, the possibility of a leak from the refrigeration circuit, the possibility of the leaked refrigerant forming an explosive mixture and the possibility of an explosive mixture being ignited. In the latter case, the Standard is only concerned with ignition sources on the refrigerator. In the event of warm temperatures around the refrigerator, that may be caused by exceptional circumstances such as blocking the airflow to the condenser, the Standard requires that the refrigeration circuit should withstand the high pressures that will be caused inside the circuit by the elevation of the saturated vapour pressure, without leaking. The Standard defines how compliance with this is tested. The Standard considers that all joints in the refrigeration system are potential sources of leaks and that there is a reasonable probability of a leak occurring from one. It also considers that if a refrigeration circuit is likely to be physically damaged there is also a reasonable probability of a leak occurring. Such an occurrence could be caused by scraping the circuit to remove ice. The Standard does not consider that other leaks are reasonably probable. Where there is a probability of a leak the Standard considers the probability of an explosive mixture forming. An explosive mixture is one where the concentration of the flammable refrigerant in the air is sufficient to be ignited, i.e. greater than the LEL. The Standard considers that the possibility of an explosive mixture forming inside the food storage compartment when there is a leak depends on whether the refrigeration circuit can be considered as protected. Protected circuits are, in concept, where no part of the cooling system is inside a food storage compartment. Therefore, if a leak occurs, an explosive mixture will not form inside the compartment. Circuits that have a part of the cooling system inside the food storage compartment and can be considered to be as leak-proof as a protected circuit are also defined in the Standard. Because an explosive mixture
will not occur special precautions need not be taken to prevent a spark from occurring in the compartment. The Standard requires that this concept be tested by simulation of a leak and measuring the concentration in the compartment and by testing that a leak will not occur if the protection is deliberately damaged. Generally, an unprotected circuit is one that is inside the food storage compartment. However, any refrigeration circuit that is not protected, or which fails the test to demonstrate that it is protected, is unprotected. If electrical components, such as the thermostat, are inside an unprotected food storage compartment, they must be constructed so as not to ignite the mixture. Leaks from an unprotected circuit into a food storage compartment will increase the concentration of the flammable refrigerant in the compartment. When the door is opened, the gas will flow out. The Standard requires that this flow of gas will not cause an explosive concentration to form around the vicinity of the external electrical components of the refrigerator. If it does, then these components must be constructed so as not to ignite the mixture. The Standard defines how the refrigerator shall be tested for this. The Standard requires that leaks from an external joint in the refrigeration circuit will not cause an explosive mixture to form around the external electrical components. If an explosive mixture is formed, then these components must be constructed so as not to ignite the mixture. The Standard defines how the refrigerator shall be tested for this. The Standard requires that the temperatures of surfaces that may be exposed to leakage of the flammable refrigerant shall not exceed the ignition temperature of the refrigerant. Finally, the Standard requires additional information be marked on the cabinet and included in the instructions than that required for refrigerators with non-flammable refrigerants.
3. Method Each test was carried out following the specifications in the Standard. 3.1. The refrigerator The refrigerator used for the tests was a 218 l, 2-door fridge-freezer operating with R600a. A single 8.8 cm3 compressor cooled both compartments with a single evaporator split into two parts, the first part parallel with the shelves in the freezer compartment, and the second part behind the rear wall of the chilled compartment, separated from it by the thermoformed liner and an aluminium plate. The refrigeration circuit contained 55 g of R600a. Two models were specially prepared for the tests. One was without the compressor or electrical components and
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the other had capillary tubes, with one end fixed next to the joint between the evaporator and the suction pipe prior to foaming, the other being outside the refrigerator.
3.2. Test locations Tests that required a controlled environment were carried out in an insulated test room 6 £ 3.5 £ 2 m3. The air was circulated round the room and through an air handling plant. The air was introduced into the room through a perforated ceiling and left via eight grills at low level in the sidewalls. The air was controlled to 20 ^ 1 8C and the humidity to 65 ^ 10% RH. In tests requiring no drafts, the fans were switched off. During these tests the air temperature increased to 23.5 8C.
3.3. Pressure test The refrigerator without the compressor was used. A deadweight pressure tester was connected to a pressure vessel with an oil – water interface. The water circuit was connected in turn to the high and low pressure parts of the refrigeration circuit via valves and the air bleed out of the system. The valves on the refrigeration circuit were opened and the pressure increased gradually to the specified test pressure. The pressures were maintained for 1 min and the circuit then examined for signs of leakage.
Fig. 1. Scratch tool used for scratch tests as specified in the Standard.
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3.4. Scratch test for protected circuits Two scratching tools as specified in the Standard (Fig. 1) were made and each was used to scratch the rear vertical face of the liner in the chilled compartment of the compressor-less refrigerator. Each tool was mounted so that it was free to move vertically in a tube. The tool was prevented from rotating by a squared plate fixed to the tube in which a squared part of the tool shaft was free to move vertically. The tube was fixed on a platform 100 £ 100 mm2 mounted on rollers so that the platform could be moved freely over a flat surface. The refrigerator was laid on its back and the tool was drawn across the rear surface at a rate of approximately 1 mm/s by hand. The force at right angles to the surface was always 35 ^ 3 N, maintained by a weight placed on the end of the tool. The force parallel to the surface was always less than 250 N, a human not easily exceeding this limit inside a refrigerator. The low-pressure circuit was then tested as for the pressure test above, the test pressure being reduced by 50%.
3.5. Leak test for protected circuits Refrigerant vapour was injected through a capillary tube at each joint. The capillary tube was positioned before foaming the appliance and had a diameter of 0.68 ^ 0.05 mm and a length of 2.5 m. The tested was carried out twice, once with the compressor switched off and once on. During the test the doors of the appliance were closed and for the test with the compressor operating, gas injection was started at the same time as the appliance was switched on. The refrigerant injected was taken from the vapour side of a gas bottle that contained 80% of the charge of the refrigerator. The gas bottle was weighed and connected to the capillary tube via a quick-coupling connector. It was placed in a water bath which was maintained at 32 ^ 1 8C. The gas bottle was weighed continuously, together with the water bath on scales accurate to ^ 0.5 g. The gas bottle was disconnected at periods during injection and weighed on scales accurate to ^0.5 g. Each disconnection lasted for less than 1 min. The gas bottle was removed completely 1 h after the start of injection. Initially, no special protection was used at the open ends of the capillary tubes (diameter 0.68 ^ 0.05 mm, length 2.5 m) to prevent foam from entering during foaming. After foam was found to enter the tubes during foaming, porous flexible foam was wrapped over the ends of the capillary tubes prior to foaming. Refrigerant vapour was injected through the capillary tubes at each point. Each test was carried out twice, once with the compressor off and once on.
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3.6. Leak test for unprotected circuits Four tests were carried out in the test room, two with the compressor operating and two with it off. At the start of each test 44 g of R600a, 80% of the refrigerant charge, was injected into the frozen food compartment in 9 min. The injection point was 5 mm away from the centre of the rear wall at one-third of the height from the top. The air movement around the refrigerator when the door was opened was investigated with powder during repeat tests. 3.7. Leak test from external joints Two external leak tests were carried out, one with the refrigerator operating and one with the refrigerator switched off. In a third test, with the compressor off, the concentration of R600a was measured at several locations in the compressor compartment. At the start of each test 27.5 g of R600a was injected into the compressor area from the end of the service pipe, in line with it, at a constant rate of 0.51 g/min. 3.8. Measurement of concentration The concentration of R600a was measured with an Inficon Ecotec II detector. The sampling time was approximately 250 ms.
4. Results 4.1. Pressure test The test pressure of 35.1 bar gauge in the high-pressure side was maintained for 1 min without change. The pressure of 10.2 bar gauge was maintained in the low-pressure side. The circuits showed no signs of leakage.
Fig. 2. The concentration of R600a in the chilled compartment after the start of injection at a critical leak point to test a compartment with a protected refrigeration circuit.
the polyurethane insulation around the area of the simulated leaks removed exposing the end of the capillary tube and the test repeated. No refrigerant flowed through the capillary tube. The end of the capillary tube was cut off in stages of approximately 3 mm and the flow checked. No flow occurred until approximately 15 mm had been removed. The first 15 mm of tube had been blocked with foam. The leak test was repeated with the tube placed back into position with a small piece of open celled flexible foam taped over the open end of the tube. The outer steel skin of the refrigerator cabinet was fixed back into position with tape and the cavity refoamed. The entire leak charge was injected in 30 min. The concentration in the chilled compartment increased as injection started, reaching a maximum in 2340 s (Fig. 2). After the test had finished, the inside and outside of the cabinet were tested for escaping R600a. R600a was entering the chilled compartment via one of the holes in the liner, used for fixing the thermostat housing (Fig. 3). The gas was
4.2. Scratch test for protected circuits When one of the scratching tools was drawn across the surface of the liner covering the evaporator the amount of damage was negligible. When the second tool was used, the depth of the scratch was significantly greater but the tool still did not penetrate the liner. The subsequent pressure tests confirmed that no damage had been done to the refrigerant circuit. 4.3. Leak test for protected circuits In the first tests, no refrigerant was injected into the joint area and the concentration of R600a in the chilled compartment did not increase above the background level of 140 ppm. The back of the refrigerator was cut open and
Fig. 3. View of the top right corner of chilled compartment showing fixing holes for the thermostat housing. R600a entered the chilled compartment through the lower rear hole when simulating a leak from a protected circuit.
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Fig. 4. Concentration of R600a by the compressor after the door is opened (time ¼ 0) in the unprotected leak test.
also escaping from the insulation where the suction pipe entered the compressor compartment. 4.4. Leak test for unprotected circuits With the compressor off, the concentration of R600a around the compressor increased immediately to a maximum of 8000 ppm when the door was opened. It then decreased rapidly to less than 400 ppm, 60 s after the door was opened. With the compressor operating, the concentration of R600a increased rapidly from 450 ppm to a maximum of 28,500 ppm, 1.3 s after the door started to open (Fig. 4). During the first 20 s the concentration oscillated between 7000 and 22,700 ppm. When the compressor was off and the door closed, the air velocity around the refrigerator was approximately 0.1 m/s without any steady direction. With the compressor operating, prior to the door opening there was a draft under the refrigerator from front to back of 0.8 m/s caused by heat rising at the rear of the refrigerator. When the door was opened, the cold gas flowed rapidly from the freezer compartment across the floor away from the refrigerator. The current of air under the refrigerator from the front to back also increased. 4.5. Leak test from external joints With the compressor off, the capillary tube discharging from the end of the charging tube and a flow rate of 0.51 g/min (slightly greater than 80% of the charge in 1 h), the concentration of R600a was a maximum of 3200 ppm and a mean of 1230 ppm. With the compressor running the maximum concentration was 1105 ppm with a mean of 643 ppm. The concentration around the compressor compartment varied from 240,000 ppm, immediately in the path of the jet from the capillary tube, to the background level. The velocity of the discharge from the capillary tube was calculated to be approximately 5 m/s. The direction in which the tube pointed had a big effect on the concentration of R600a in the immediate vicinity of the discharge (Fig. 5).
Fig. 5. View of the end of the capillary tube in line with the end of the service tube of the compressor, simulating a leak from an external joint.
The test was repeated with the jet dissipated by covering the end of the tube with porous foam. The R600a then spread out from the end of the capillary tube, upwards or downwards depending on whether the compressor was running or not. The concentration reduced to less than the LEL within 2 mm of the foam.
5. Discussion 5.1. Pressure test The test as specified in the Standard was straightforward to carry out. The purpose of the test is to demonstrate the mechanical integrity of the refrigeration system when it is subjected to excessive temperatures. The Standard does not require pressure tests if non-flammable refrigerants are used. With flammable refrigerant it requires that the test pressure on the low side correspond to the saturated pressure at 22 8C. The European Standard for the safety and environmental requirements of refrigeration systems [4] requires that the minimum design pressure for the low-pressure side corresponds to the saturated pressure of at least 32 8C. If the compressor on a domestic refrigerator is not operating, the low-pressure side will be at the same saturated temperature as the high-pressure side, considerably warmer than 22 8C (Table 1). 5.2. Scratch test for protected circuits The scratching of the protecting surfaces as specified in the Standard was difficult to carry out. The damage done to the surfaces was dependent on the sharpness of the scratching tool, which was not defined in the Standard.
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Table 1 Table of tests, their purpose, method and results Test
Purpose
Method
Result
Pressure test Scratch test
Prove physical integrity of system Prove protection of circuit
35.1 bar in high-pressure side, 10.1 bar in low-pressure side Surface over evaporator scratched with specified tool
Pressure test after scratching
Prove protection of circuit
Scratch test not specified in Standard Leak test for protected circuit
Prove protection of circuit
17.5 bar high-pressure side, 5 bar low-pressure side Scratched with knife
Pressure held .1 min, no leakage Scratch did not penetrated deep enough to damage evaporator Pressure held for .1 min, no leak Evaporator pierced with knife
Leak test for protected circuit
Leak test for unprotected circuit, compressor off
Leak test for unprotected circuit, compressor running
Leak test from external joint
Prove concentration of refrigerant does not exceed 75% of LEL in protected compartment in the event of a leak Prove concentration of refrigerant does not exceed 75% of LEL in protected compartment in the event of a leak Prove concentration of refrigerant does not exceed 75% of lower explosive limit by possible ignition sources on cold appliance Prove concentration of refrigerant does not exceed 75% of lower explosive limit by possible ignition sources on cold appliance Prove concentration of refrigerant ,75% of LEL by possible ignition sources on cold appliance
The specified scratching tool has a wide, long footprint. The area in contact with the surface increases as the depth of the scratch increases. When the tool has penetrated to a depth of 1 mm, the area of contact is 12 mm2. The tool did not mimic the most likely cause of damage. Trials were carried out using a knife to scrape the evaporator area. When the knife slipped it penetrated the protected circuit. The test pressure was reduced after scratching as specified in the Standard. The probability of a scratched refrigerator being subjected to excessive temperatures is the same as for an ‘unscratched’ refrigerator. A new refrigerator (unscratched) would be more likely to be looked after well than an old, damaged one. 5.3. Leak test for protected circuits The Standard does not specify how to prevent foam from entering the capillary tube during the foaming operation; it only states that care should be taken to
80% of charge injected by joint via capillary tube, no protection against foam
No leak
80% of charge injected by joint via capillary tube, protection with porous foam with a surface area of 200 mm2 80% of charge injected into unprotected compartment over 9 min. The door opened after 30 min
75% of LEL exceeded in compartment
80% of charge injected into unprotected compartment over 9 min. The door opened after 30 min
Maximum concentration 28,500 ppm varying between 7000 and 28,500 ppm
Release 80% of charge over 1 h adjacent to external joints
Maximum concentration 3200 ppm while compressor off. The concentration much greater in the path of the leak
Maximum concentration 8000 ppm (,75% of LEL)
prevent it. When porous foam was used to prevent foam from entering the capillary tube, the flow rate of refrigerant was 1.43 g/min. The method used to prevent foam from entering the capillary tube is important for the reproducibility of the test. The force exerted by the refrigerant vapour is proportional to the surface area over which it acts. When porous foam was used, the area over which the refrigerant acted was approximately 200 mm2. If the foam had been prevented from entering the capillary tube, but not interfering with its adhesion to any other surface, then the area available for the refrigerant pressure to act on would have been the crosssectional area of the capillary tube, 0.4 mm2. It is possible that in this case the refrigerant would not have escaped so quickly, or at all. In the tests carried out by Harun Iz et al. the method of preventing the foam from entering the capillary tube was not given. It is necessary to define a method of leak simulation that is reproducible rather than leaving the Standard open to an unlimited number of
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methods of preventing foam from entering the capillary tube, some of which will show leakage and some not. Once the leak had started, an escape path was propagated between the evaporator and the liner of the chilled compartment. There should therefore be no openings in the liner through which refrigerant could pass from the insulation space into the food chamber. The leaking refrigerant also passed along the suction pipe to the compressor compartment. 5.4. Leak test for unprotected circuits If there had been no dilution by air infiltrating past the door seals, the concentration of R600a would have been 140,000 ppm immediately before the door was opened. In a new refrigerator the performance of the door seals is, in part, specified in the performance standard, ISO8187:1991. In an old refrigerator, the infiltration would be greater, reducing the refrigerant concentration. When the door was opened, the heavy air – vapour mixture flowed out across the floor. When the compressor was not operating there was a very little flow of refrigerant under or round the sides of the refrigerator and the concentration of gas at the rear of the refrigerator, by the electrical components, was less than 75% of the LEL. When the compressor was operating the vapour in the freezer compartment was colder and heavier than when the compressor was off. When the door opened the flow of vapour from the compartment was faster. Because of the flow under the refrigerator from front to back, driven by the hot air rising from the operating compressor, the concentration of refrigerant by the electrics was greater than when it was not operating, and greater than 75% the LEL (13,500 ppm for R600a). The peak in the concentration lasted for less than 2 s. The Standard requires that the instrument for measuring the concentration should have a fast response, typically 2 – 3 s. If an instrument with a response time of 3 s had been used in these tests, the peak value of concentration could have been missed. In these tests, this would not have been important as the concentration remained above 75% of the LEL for 18 s but on different refrigerators this may not be the case. The sudden release of an accumulation of refrigerant can cause a short peak in concentration. To avoid missing this would need an instrument with a faster response than specified in the Standard. In order to conform to the Standard the electrical equipment in the compressor area of the refrigerator tested must be non-sparking (to conform to at least section 3, clauses 16 and 17, and section 4 of IEC 60079-15). In normal households, there are sources of ignition other than the refrigerator. The sudden release of vapor when a door is opened flows away from the refrigerator and is therefore more likely to be ignited by an ignition source remote from the refrigerator. Clodic and Sai [3] used an
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external ignition source and found that ignition was possible. The Standard does not address this point. 5.5. Leak test from external joints The concentration did not increase to more than 75% of the LEL during the tests nor more than 50% for a period of 5 min. These criteria mean that the electrics would not have to be spark proof to conform to the Standard with respect to leaks from external joints. The concentration in the jet from the capillary tube was greater than the LEL. The concentration distribution depended on the position and direction of the outlet of the capillary tube. If the concentration had been measured in a different position or if the simulated leak had been directed in a different direction then the result would have been different. The Standard is not specific enough on the direction and exit velocity of the simulated leak for the test to be reproducible.
6. Conclusions Some of the tests specified in the Standard were not reproducible within a reasonable interpretation of the Standard. This could result in different test laboratories reaching different conclusions about a refrigerator’s compliance with the Standard. The simulation of a leak from a joint in a protected cooling circuit is not specifically defined in the Standard. The concentration of refrigerant in the compartment with the protected circuit depended on the method used to prevent foam from entering the capillary tube. The position and direction of a simulated leak in the compressor compartment is not specified in the Standard but had a significant effect on the concentration distribution. When there is a sudden release of an accumulation of refrigerant, the concentration at the measuring point can peak more rapidly than the response time of the measuring instrument specified in the Standard. The method, sampling rate and accuracy of the refrigerant gas concentration instrument should be specified to measure rapid changes in concentration. This paper has presented experience of testing refrigerators according to the Standard concerned with their safety. It has not addressed the question of whether the refrigerator is safe. A cautious approach for making safe the refrigerator tested for this paper would be to: † use non-sparking electrics for low-level compressors, regardless of the results of the leak tests specified in the Standard. † use a continuous liner with no holes in the chilled food compartment with the protected cooling circuit or have no sources of ignition in the compartment.
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† give guidance to users on the need for ventilation and the position of ignition sources other than the refrigerator.
References [1] EN 60335-2-24:2001. ‘Safety of household and similar appliances—part 2-24: particular requirements for refrigerating appliances, ice cream appliances and ice makers’.
[2] Harun I, Tekin Y, Yalcin T. Experimental results of the safety tests on domestic refrigerators for refrigerant R600a. Proc. IIFIIR Commissions B1, B2, E1 and E2, Aarhus, Denmark; 1996– 3. p. 321–28. [3] Clodic D, Sai W. Tests and simulations of diffusion of various hydrocarbons in rooms from air conditioners and refrigerators. Proc. IIF-IIR Commissions B1, B2, E1 and E2, Aarhus, Denmark; 1996–3. p. 299–308. [4] EN 378-2:2000. ‘Refrigerating systems and heat pumps— safety and environmental requirements’.