DESCRIPTION OF ALTERNATIVE REFRIGERANTS WITH BACKONE EQUATIONS

The BACKONE equations area family of physically based equations of sta te, in which the Helmholtz energy (F) is written as a sum of contribut ions from characteristic intermolecular interactions. For dipolar flui ds F is given by the DIBACKONE equation as F = F-H + F-A + F-D, where F-H is the hard-body contribution, F-A the attractive dispersion force contribution, and F-D the dipolar contribution. For quadrupolar fluid s F is given by the QUABACKONE equation as F = F-H + F-A + F-Q, where F-Q is the quadrupolar contribution. F-D and F-Q have been determined on the basis of extensive molecular simulations [B. Saager, J. Fischer , Fluid Phase Equilibria, 72 (1992) 67-88]. Both the DIBACKONE and the QUABACKONE equation need only four substance specific parameters: a c haracteristic temperature T-0, a characteristic density rho(0), an ani sotropy parameter ct and either a reduced squared dipole moment mu(2) or a reduced squared quadrupole moment Q(2). In the present work the se parameters were determined for the alternative refrigerants R123, R 124, R125, R134a, R143a, R152a, R218, and R236ea by fitting them to sa turated liquid densities and vapour pressures at four temperatures. It turned out that all these substances can be quite well described by t he QUABACKONE equation with the exception of R152a which is better des cribed by DIBACKONE. Moreover, a description in the form of F = F-H F-A + F-Q + F-D with five parameters called D + QBACKONE is explored. Comparisons of the BACKONE results for the saturated liquid densities, the saturated vapour densities and the vapour pressures with experime ntal data or with data from reference Helmholtz function (RHF) equatio ns show satisfying agreement. For R123, R125, R134a, and R152a enthalp ies and entropies for coexisting Liquid and vapour states are also com pared with RHF equation results and show maximum relative deviations i n the enthalpy of less than 1.5% and in the entropy of less than 1.3%. Moreover, coefficients of performance for an idealized refrigeration and a heat pump cycle are compared with RHF equation results and show for R134a deviations less than 0.25% and for R152a deviations less tha n 1.00%. Finally, a thermodynamic table is given for R236ea on the bas is of BACKONE. (C) 1998 Elsevier Science B.V. All rights reserved.