Dipartimento di Fisica Tecnica – Università degli Studi di Padova
Over the last several years, much research and development effort has been focused on potential refrigerants possessing low Global Warming Potentials (GWPs). The catalyst for much of this effort can be attributed to European regulations regarding the use of R134a in automotive applications.
In particular, the European Union’s F-gas regulations specify that beginning on January 1, 2011 new models and on January 1, 2017 new vehicles fitted with air conditioning cannot be manufactured with fluorinated greenhouse gases having global warming potentials (GWP) greater than 150. Possible candidate refrigerants that possess GWP < 150, and that are being considered, include R-152a, R-744 (CO2), and R1234yf. R1234yf has a normal boiling temperature approximately 3.7 ºC lower than that of R134a. It has a GWP of 4 and is being widely considered as a possible replacement for R134a in automotive applications. Recently it is also under scrutiny as a component in blend with R32 for R410A replacement in stationary air – conditioners.
To date, researchers and manufacturers have focused their R1234yf research and development efforts primarily on characterizing its flammability, toxicity, environmental impact, materials compatibility, oil compatibility, air conditioning system performance, thermophysical property data, and in the developing of simple equations of state (EoS). In this paper an update about the most recent analyses on the compatibility of R1234yf with commercial oils is presented.
Review of lubricant specifications-properties affected by the refrigerant change to R1234yf
The rather low GWP of R1234yf is achieved thanks to the very short atmospheric life of the molecule as a consequence of a C=C double-bond in the molecular structure. The mentioned double carbon bond (sometimes called “olefinic” bond) can be quite easily “broken” in atmospheric environment. R134a molecule contains the same number of fluorine and hydrogen atoms as R1234yf but does not include the olefinic structure and has an incredible long term stability in atmosphere, if compared with R1234yf. So, according to this rather “rough” consideration, one would expect that R1234yf molecule should in general be more slanted to instability also during operation inside a vapour compression unit where the olefinic bond can be attacked by several substances in circulation.
This assumption is often present in the bloggers discussion in several web sites, but cannot be considered correct.
The actual question is to have a clear scenario of the rather complex possible interactions of the refrigerant with the lubricant and all the other materials present inside a refrigeration circuit. Figure 1, adapted from Randles (2005), attempts to outline the mentioned scenario. Much of the published work has been carried out in terms of R1234yf/lubricant stability by refrigerant producers (Spatz, 2009, among others), organizations (SAE through CRP-1234yf and JAMA, see for example Ikegami et al., 2008), research institutions (Grimm 2010, Bobbo et al. 2011), lubricants producers (Dixon, 2010) or MAC systems manufacturers. On the contrary the present author could not find significant work related to the other possible interactions as shown in Figure 1 (i.e. R1234yf with additives, system extractables, debris).
Figure 1. Schematic of refrigerant interactions in a refrigeration circuit (adapted from Randles, 2005)
The main part of the tests was run according to ASHRAE Standard 97: tubes were charged with lubricant and refrigerant and then heated at 175°C for 14 days with added aluminium, copper and steel strips.
The reported results are almost unanimous in indicating that the off-the-shelf lubricants (PAG or POE) stability with R1234yf is usually lower than with R134a or with R410A (Fujitaka et al., 2010) since the formation of higher amount of breakdown acid products observed. The results however vary markedly between PAG types and between PAG and POE, more than previously seen with R134a. Grimm (2010) seems to indicate R1234yf purity grade as one of the possible causes.
Dixon (2010) and Fujitaka et al. (2010) proposed two different possible mechanisms of chemical instability in R1234yf linked to the presence of –OH groups or initiator radicals, respectively. Both mechanisms lead to the olefinic bond splitting. According to the proposed mechanisms, the presence of ester groups accelerate the mentioned split, so POE (poly-ol-ester, precisely) might be considered to some extent more critical than PAG.
Another issue that strongly affects the reliability of the refrigeration system is the miscibility/solubility of the refrigerant in the oil. Recent studies (Spatz 2009, Bobbo et al. 2011) indicate a rather different behaviour of R1234yf in comparison with R134a in PAG lubricants originally developed for R134a.
In general R1234yf shows lower solubility than R134a in the same lubricant, under the same thermodynamic conditions and displays larger immiscible regions sometimes with the formation of two liquid phases in equilibrium with the refrigerant vapour. This phenomenon is recognized to affect the oil circulation through the refrigeration circuit, the oil return to the compressor and, consequently, the oil retention in condenser and evaporator, thus reducing the heat transfer efficiency. A detailed experimental analysis about a minichannel condenser operating with R1234yf and a commercial PAG developed for R134a MAC will be presented by the present author at the IIR conference in Prague, next August (Zilio et al. 2011).
In summary, the work appeared in the open literature in the last two years indicate that the off-the-shelf lubricants (POE or PAG) developed for R134a or R410A are not suitable for the use with R1234yf.
Accordingly, lubricant manufactures have been working both on the molecular structure of the base components of the oil blend and on the lubricant additives (anti-corrosion, anti-oxidation, anti-wear, anti-seize, with particular care to the anti-oxidation additives, since moisture presence is more critical in R1234yf systems). New “improved” PAG oils for mobile air conditioning with R1234yf have been proposed by several producers. Durability tests on compressor run by the major MAC systems producers are rather promising. According to the literature (very often presentations made by the producers) the new lubricants behave like R134a in the original oil blends in terms of thermal stability and solubility. Furthermore, the new lubricants are usually suitable also for R134a.
A systematic test campaign for the measurement of R1234yf solubility in a PAG oil expressly developed for R1234yf is undergoing at the Istituto per le Tecnologie della Costruzione – Consiglio Nazionale delle Ricerche in Padova (Italy) and the preliminary results seems to confirm the good solubility of R1234yf.
The analyses appeared so far do not explore in details the behaviour of the lubricants in terms of compatibility with resins and insulating materials for electric windings (motors in hermetic/semi-hermetic compressors).
PAG and POE lubricants developed for R134a or R410A can be considered critical in terms of stability and solubility/miscibility in some applications with R1234yf and more systematic studies with different R1234yf/oil are advisable, as well as compatibility with elastomers and resins (including electric motor winding insulation in hermetic or semihermetic compressors). However, the works recently appeared in the open literature seem to indicate that the refrigeration industry shall not have to face an “epochal” transition in lubricant type, as it happened at the dawn of HFCs, following the ban of CFC.
List of Acronyms
MAC: Mobile air conditioning POE PolyOlEster
Bobbo S., Groppo F., Scattolini M., Fedele L., 2011, R1234yf as a substitute of R134a in automotive air conditioning. Solubility measurements in commercial PAG, to be presented at IIR Int. Conference Refrigeration ICR2011, August 2011, Prague. Dixon L., 2010, Results of Shrieve evaluations of 1234yf refrigerant on mobile A/C lubricant performance and system chemistry, SAE 2010 Alternate Refrigerant & System Efficiency Symposium, July, Scottsdale, Arizona.
Fujitaka A., Shimizu T., Sato S., Kawabe Y., 2010, Application of low global warming potential refrigerants for room air conditioner, Int. Symposium on next-generation air conditioning and refrigeration technology, 17-19 February 2010, Tokyo, Japan. Grimm U., 2010, Complex interactions of low GWP refrigerants, A/C oils, and materials in MAC circuits, SAE 2010 Alternate Refrigerant & System Efficiency Symposium, July, Scottsdale, Arizona.
Ikegami T., Iguchi M., Aoki K., Iijima K., 2008, JAMA-JAPIA new refrigerants evaluation results, SAE 2008 Alternate Refrigerant & System Efficiency Symposium, 10-12 June, Phoenix, Arizona. Randles S. J., 2005, Refrigeration Lubricants, in Rudnick L.R. editor. Synthetics, Mineral Oils, and Bio-based lubricants: Chemistry and Technology, CRC Press, ch. 30, pp. 493-516 Spatz M., 2009, HFO-1234yf Technology update, VDA 2009 Winter meeting, Saafalden, Austria.
Wieschollek F., 2009, Compressor testing results & findings with the usage of HFO-1234yf, VDA 2009 Winter meeting, Saafalden, Austria.
Zilio C., Brignoli R., Brown J.S., 2011, Experimental analysis of a minichannel air cooled condenser operating with R1234yf, to be presented at IIR Int. Conference Refrigeration ICR2011, August 2011, Prague.