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become state of the art. Other application areas have followed or are under development, where especially the commercial refrigeration sector is relevant.
For both protected as well as unprotected cooling systems, amongst others, it needs to be evaluated if a possible leakage of refrigerant could lead to explosions in areas outside the storage compartment were system components possibly producing arcs or sparks are mounted. Clause 22.109 of the standard explains exactly how these evaluations are to be performed. In short the test comprises that a certain gas flow (simulation of a leakage) at positions of pipe work joints of the cooling system is created. During this leakage simulation, the gas concentration near hazardous electrical components is to be measured. The measured concentration shall not exceed 75% of the lower flammability limit at all and shall not exceed 50% of the lower flammability limit for a period longer than 5 minutes.
Prescribed gas leakages were created on different positions (near pipe work joints) and the gas concentration has been measured at the hazardous electrical components indicated in Figure 2.
For ultra-low temperature freezers down to -80°C, it is particularly relevant that ethane is available as a low boiling point HC. By mixing it with other HCs, properties can be obtained which allow replacing HFC based low temperature refrigerants. HCs were already known to be added to the HFC refrigerants to assist oil return. In general HC refrigerants can be used with mineral oils, which eases their use compared to HFC refrigerants that require other lubricants, such as polyolesters (POE), which are more difficult to handle in the field due to their hygroscopic nature. For low temperature freezers down to -40°C, it is possible to apply propane or, when the cooling capacity is not sufficient, propylene.
RELEVANT SAFETY STANDARDS AND APPLICATION
In several European electro-mechanical safety standards, already special considerations for products containing flammable refrigerants are included or are being discussed within the relevant standardisation bodies. Here an example is discussed which relates to the conversion of an existing laboratory freezer (DF8514GL, make Skadi Europe in The Netherlands,
www.skadi-europe.com) to hydrocarbon refrigerants. The relevant European standard applied is EN 60335-2-24 [2].
According to clause 22.106 of this standard, the mass of flammable refrigerant shall not exceed 150 gram in each separate refrigeration circuit. For the appliance under evaluation containing two separate cascade refrigeration systems this limit of 150 gram was not a problem.
The standard makes a difference between so-called protected and unprotected cooling systems. For the specific freezer evaluated no parts of the cooling system were placed inside the storage compartment and therefore this freezer is considered to be a protected cooling system.
In the original configuration, too high gas concentrations were measured near several hazardous electrical components positioned at the bottom of the electrical component box and also near the thermostat. The electrical component box contained openings for cable transits and component fixtures. Through these holes the gas apparently enters the box. It was concluded that applying a sealed box with sealed cable transits would be the major part of the solution to convert this cabinet to a hydrocarbon refrigerant. Components that could not be placed inside this sealed box had to be replaced by electrical components complying with IEC 60079-15:2005, section 4. In the new updated Skadi Green line product, these modifications have been applied.
Next to the components producing arcs or sparks it also needs to be evaluated if the system contains any surfaces, which may be exposed to flammable refrigerants that become too hot. According to clause 22.110 temperatures should remain below the ignition temperature of the relevant refrigerant reduced with 100°C. Regarding this clause especially (but not only) heaters should be verified (e.g. evaporator defrost heaters and water condensation heaters).
Obviously these are only examples of the evaluations needed to be performed. For a conversion to flammable refrigerants of any appliance all relevant clauses of the EN 60335-2-24 should be carefully followed.
BENEFITS IN ENERGY USE AND CO2 EMISSION In many applications hydrocarbons outperform HFC refrigerants due to favourable thermodynamic properties. Though a full analysis may require a very detailed investigation, it is possible to calculate the efficiency of a theoretical refrigeration loop using different refrigerant. In praxis the efficiency will be influenced by matters as compressor optimisation towards each refrigerant, but it is a well know method to screen refrigerants on first principles. The results are shown over an evaporating temperature range in Figure 3 for a typical single stage freezer. In praxis the evaporation and condensation temperature may change due to differences in heat transfer between refrigerants. This works out generally favourable for the hydrocarbons, so the efficiency difference as shown in the diagram is somewhat conservative.
In principle the use of a laboratory freezer during its lifecycle has two effects on global warming: a direct effect due to the refrigerant emission at end of life (if not recovered) or during
servicing and an indirect effect due to the CO2 emission involved with the electricity generation to drive the freezer. These factors are taken together in the so-called TEWI factor. If an example case is taken for the same single stage low temperature freezer that is converted from R-404A to propane the following calculation can be made (a 15% energy consumption reduction is assumed as derived from Figure 3):
CONCLUSIONS
As current refrigerants employed in laboratory freezers or coolers do have a contribution to the greenhouse effect when emitted, their use is under continuous increasing pressure. It is possible that limitations for using these refrigerants in certain countries or regions will be put in place, which may also have a negative impact on future servicing. Hydrocarbons form an excellent solution for this application, provided these are applied with careful consideration to the safety of the products. Current electro-mechanical safety standards do already contain provisions for using such, flammable, refrigerants. The current paper has shown a few examples of safety testing and redesign needed to accommodate these refrigerants. As hydrocarbons do generally lead to energy savings compared to the current, HFC based, refrigerants for the laboratory freezers, these refrigerants must be seen as good alternatives to reduce the environmental impact of these products.
REFERENCES
1. UNEP 2006 Report of the Refrigeration, Air Conditioning and Heat Pumps Technical Options Committee, 2006 Assessment
2. EN60335-2-24 Household and similar electrical appliances – Safety – Part 2-24: Particular requirements for refrigerating appliances, ice-cream appliances and ice-makers.
3. Regulation (EC) No 842/2006 of the European Parliament and of the Council of 17 May 2006 on certain fluorinated greenhouse gases.
Bottom of cabinet with refrigeration system and some electrical components
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Figure 2. Possible ignition source locations which were updated in order to obtain a safe hydrocarbon refrigeration system.
Figure 3. Efficiency for different refrigerants, the conditions applied are: condensation temperature 45°C, liquid subcooling 2 K, superheat in the evaporator 5 K, internal heat exchanger with an efficiency of 70%.
Contact Tamsyn Cox on +44 (0)1727 855574 or email:
tamsyn@intlabmate.com
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