M.R. Jones,     MIRA Ltd, UK

ABSTRACT

The new refrigerant R-1234yf is on the verge of widespread acceptance as the preferred alternative fluid for use in automotive air conditioning systems. In view of this, much work has recently been conducted in the fundamental thermophysical analysis of the refrigerant, as well as in its practical application in automotive air conditioning systems.Much of the property analysis work published to date has been directed at establishing accurate coefficients for use in well-known relationships such as the Martin Hou and Peng-Robinson equations of state. This enables the computation of a range of useful properties based on, for example, temperature and pressure conditions, and aids refrigerant cycle analysis and simulation. However, these equations of state and their corollaries can be complex, and generally require the use of iterative numerical techniques in their solution. Various proprietary tools are available to enable these computations, but they are not generally applied in the domain of the practicing automotive climate control engineer.This paper draws together much of the published thermophysical data relating to R-1234yf and reduces it to a set of discrete equations, presented with coefficients, which enable direct calculation of all relevant properties. This facilitates relatively easy analysis of A/C cycles within a spreadsheet environment, as demonstrated in the paper by application to a system incorporating an internal heat exchanger.

1 INTRODUCTION

R-1234yf is confirmed as the preferred replacement automotive refrigerant in all markets where R134a is now unacceptable on the grounds of environmental impact. All vehicle manufacturers selling new models into the European market from 2011 onwards have to comply with the Global Warming Potential (GWP) upper limit of 150 laid down in the legislation (1); R-1234yf achieves this comfortably, with a published GWP value of 4 (2).

However, although the GWP of R-1234yf is clearly acceptable, other thermodynamic properties of the fluid are inferior as compared with R134a. Most notably, enthalpy of vaporisation at typical evaporating pressures is around 18% lower for R-1234yf as compared with R134a, although this is offset to some extent by the higher gas densities of R-1234yf (3). In view of these differences, the automotive community has been, and continues to be, active in A/C system performance development using the new fluid.

This paper will briefly discuss some of the measures which are being adopted in order to claw back the potential performance degradation relating to the new fluid, but the main objective of the paper is to present a set of thermodynamic property relationships which enable the practicing engineer to analyse the R-1234yf A/C loop. Whilst evaporator air-side observations and calculations are useful in assessing the ultimate performance of the new refrigerant, detailed analysis of the refrigerant loop is also essential in the development of any new system. Vehicle development engineers conducting whole-vehicle refrigerant charge determination tests, typically in the Climatic Wind Tunnel (CWT), need to be able to calculate condenser subcool and evaporator superheat accurately. Similarly, laboratory-based A/C studies, which generally seek to develop a more fundamental understanding of the refrigerant cycle, also need accurate saturation data, as well as comprehensive refrigerant enthalpy information. Some of the data required has been published recently, with discrete laboratory-based measurements of refrigerant properties and extended tables of data derived from complex equations of state (4, 5, 6, 7). However, the data presented is not sufficiently detailed for use in analysis of A/C cycles, and the mathematical relationships and coefficients given generally require the use of relatively complex iterative numerical solutions.

This paper gathers together property data which has been published for R-1234yf, and condenses the information into a set of highly useable curve fits which can be implemented by the practicing automotive engineer in everyday simulation and analysis environments. The logarithmic and 2D and 3D polynomial relationships presented enable direct calculation of refrigerant saturation properties and enthalpy over the normal operating range of the fluid in an automotive A/C system, and generally recover properties to an accuracy of within 0.2% as compared with the published data.

2 DEVELOPMENT OF THERMOPHYSICAL RELATIONSHIPS

The relationships developed here are generally of relatively simple polynomial form, and they are presented along with their respective coefficients. As some of the relationships extend to sixth order terms care needs to be taken with respect to the accuracy of coefficients used; these are quoted to 12 significant figures where appropriate, which was found to give acceptable accuracy and stability.