International Journal of Refrigeration 27 (2004) 215–224 www.elsevier.com/locate/ijrefrig
Oil presence in an evaporator: experimental validation of a refrigerant/oil mixture enthalpy calculation model M. Youbi-Idrissia, J. Bonjoura,*,1, M.-F. Terriera, C. Marvilletb,1, F. Meuniera,1 a
Laboratoire du Froid (E.A. 21), 292 rue Saint Martin, 75141 Paris cedex 03, France b GRETh, CEA Grenoble, 17 rue des martyrs, 38054 Grenoble, France
Received 9 July 2003; received in revised form 2 September 2003; accepted 3 November 2003
Abstract In this paper, the impact of the oil presence on the performances of a refrigerating machine is investigated both experimentally and numerically. To highlight the effect of oil, particularly on the evaporator behaviour, a theoretical model of enthalpy calculation for a refrigerant/oil mixture has been previously developed [Int J Refrigeration, 26(2003), 284]. In order to validate this model, tests were carried out on an industrial refrigerating machine working with R-407C. The lubricant is a polyol-ester oil whose solubility curves are given by the oil manufacturer. The oil circulating mass fraction is measured by a sampling technique and by an on-line density measurement method, whose advantages and drawbacks are presented. Both the model and the experiments show that the ratio of enthalpy change through the evaporator with to without the oil presence increases when the apparent superheat at the evaporator outlet increases. This is due to the presence of a non-evaporated amount of liquid refrigerant dissolved in the oil at this location, which is confirmed by visual observations. The numerical and experimental results are found to be in a good agreement as the maximum deviation is about 2.2%. # 2003 Elsevier Ltd and IIR. All rights reserved. Keywords: Modelling; Experiment; Mixture; Lubricant; Refrigerant; R407C; Performance
Pre´sence d’huile dans un e´vaporateur : validation expe´rimentale d’un mode`le de calcul d’enthalpie de me´lange de frigorige`ne/huile Mots cle´s : Mode´lisation ; Expe´rimentation ; Me´lange ; Lubrifiant ; Frigorige`ne ; R407C ; Performance
1. Introduction In the refrigeration and air conditioning mechanical vapour compression systems, oil is necessary for a correct * Corresponding author. Tel.: +33-1-58-80-85-51; fax: +33-1-40-27-20-47. 1 Members of IIR Commission B1. E-mail address:
[email protected] (J. Bonjour). 0140-7007/$35.00 # 2003 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2003.11.001
working of the compressor. It simultaneously ensures several roles among which the first is to protect the mobile elements (pistons, connecting rod/crank, valves, . . .) against wear. By lubricating the moving parts, the oil also serves as a tightness element and takes part in the compressor cooling. However, the use of oil in the compressor is often accompanied by undesirable effects on the machine, especially when the rejection quantity of oil in the circuit is high.
216
M. Youbi-Idrissi et al. / International Journal of Refrigeration 27 (2004) 215–224
Nomenclature a–f Cg Cp E h : m P: Q Rh R0h T X0 x
empirical constants oil mass fraction specific heat, J kg1 K1 experimental relative difference,% specific enthalpy, J kg1 mass flow rate, kg s1 pressure, Pa power, W enthalpy ratio experimental enthalpy ratio temperature, K vapour quality of refrigerant/oil mixture molar fraction in liquid phase
Greek letters density, kg m3 Subscripts in inlet L liquid m mixture oil lubricant out outlet r refrigerant ref reference sat saturation t total V vapour w water
The presence of oil in such systems is commonly considered as a major concern not only because it modifies the thermodynamic properties of the refrigerant (liquid– vapour equilibrium, enthalpy, viscosity, superficial tension, . . .), but also because it changes the flow configuration, and often decreases the heat transfer coefficients and increases the pressure drops. In the literature, the works carried out on the effect of oil circulation in the refrigerating circuit [1–5] show that the evaporator is the organ which is the most sensitive to the oil presence. Indeed, thermodynamic properties of lubricant are very different from those of refrigerants, notably the bubble temperature and dynamic viscosity. Consequently, the behaviour of a refrigerant/oil mixture is different from that of the pure refrigerant, particularly at the evaporator end where the effects of the temperature and viscosity occur: 1. The temperature increase (at a quasi-constant pressure) in the evaporator notably displaces the equilibrium point of the mixture (enriched in oil) compared with the pure refrigerant. Because of
oil presence, refrigerant/oil mixture becomes largely zeotropic, this equilibrium modification being generally given by the solubility curves. 2. The refrigerant/oil viscosity increases quickly with the refrigerant evaporation, the liquid phase (refrigerant + oil) contains less refrigerant, it is thus richer in oil which is accumulated at the evaporator end. As direct consequences of this phenomenon, a disastrous decrease in the heat transfer coefficient and an increase in the pressure drops at the evaporation end take place. Neglecting the oil under certain working conditions can lead to an overestimate of the real machine performances [1,3,4]. From this point of view, taking into account the oil presence in the calculation of enthalpy is an essential step to adjust the performance evaluation of a refrigerating machine. The aim of this paper is to validate a refrigerant/oil mixture enthalpy calculation model previously developed by the authors [1,3] from experiments carried out on an industrial refrigerating machine working with R-407C. This theoretical model will be briefly recalled prior to presenting the experimental results and their comparison with the numerical results.
2. Enthalpy of a refrigerant/oil mixture The refrigerating power is estimated in most cases by an enthalpy change calculation considering the refrigerant only. To analyse the oil effect on the enthalpy calculation, Youbi-Idrissi et al. [1,3] developed a simple thermodynamic model allowing to express the total enthalpy ht of a refrigerant/oil mixture by the following relation: ð1Þ ht ¼ 1 X0 Cg hL;r þ Cg hoil þ X0 hV where hL;r and hV are the saturated liquid and saturated vapour refrigerant specific enthalpy, respectively, for a given pressure, at the system temperature and corresponding liquid or vapour composition modified by the oil presence (solubility curves). The oil enthalpy at the system temperature ðhoil Þ is given by integration of the specific heat: ðT Cpoil dT ð2Þ hoil ¼ href þ Tref
X 0 is the vapour quality when the mixture contains oil, and is given by the following equation: : mV ð3Þ X0 ¼ : : : mL;r þ moil þ mV and Cg is the oil mass fraction circulating in the refrigerating machine:
M. Youbi-Idrissi et al. / International Journal of Refrigeration 27 (2004) 215–224
217
Fig. 1. Mollier diagram of R-407C and 2% of POE oil.
: moil Cg ¼ : : : mL;r þ moil þ mV
ð4Þ
The application of the model made it possible to plot Mollier-type diagrams for a mixture made of R-407C and a Polyol-ester oil (POE) at a given circulating mass fraction of oil. Fig. 1 shows an example of this diagram for R-407C mixed with 2% of the lubricant used in this study. Owing to this lnP-h-X0 diagram (Fig. 1), the relative contributions of latent and sensible heat for the refrigerant/oil mixtures can be pointed out. Indeed, when following the isotherms lines of refrigerant/oil mixture, we distinguish two different zones: in the first zone, up to a critical value of vapour quality, the refrigerant-oil mixture enthalpy-quality-pressure relationship is almost identical to that of the pure refrigerant, i.e. their isotherms are the same. On the contrary, when the quality is higher than this critical value, the isotherm lines separate. In the former zone, the enthalpy is controlled by the latent heat contribution [the first and third term in Eq. (1)], whereas it is controlled by the sensible heat in the latter zone [second term of Eq. (1)]. This model allows the calculation of the enthalpy change through an evaporator, with and without taking into account the effect of oil, and the prediction of the non-evaporated quantity of refrigerant at the evaporator outlet. A ratio of enthalpy change calculated with and without taking into account the oil was defined by the following relation: Rh ¼
Dhr;oil Dhr;only
ð5Þ
This theoretical study on an R-407C/POE oil pair showed that Rh is low (penalising effect of oil on the enthalpy change) when the apparent superheat is small and/or the circulating mass fraction of oil is high (Fig. 2). The term of ‘‘apparent superheat’’ should here be used to emphasize that at the evaporator outlet, the temperature of the mixture is higher than the pure refrigerant dew-point temperature while some refrigerant is still present under a liquid form dissolved in oil. It should be noted that, for given conditions of temperature and pressure, the enthalpy variation calculated for the refrigerant only is always lower than that for the refrigerant/oil mixture: as a matter of fact, the oil being regarded as a non-volatile component, the vapour quality with the oil presence is always less than one (X0
