Experimental and theoretical analysis of refrigerant absorption in lubricant oil

HVAC & R Research, Jan, 2008 by Jader R. Barbosa, Murilo A. Ortolan

INTRODUCTION

In small-capacity refrigeration systems, cooling capacity is controlled by a succession of ON/OFF compressor cycles whose frequency depends on thermal load and cold ambient temperature requirements. During each one of such cycles, immediately after the compressor turns off, a process of vapor refrigerant absorption in the oil inside the hermetic compressor shell is initiated. The oil is present as a stagnant layer at the bottom of the compressor shell (carter) and the height of the oil layer depends on the amount of oil present in the system. Refrigerant absorption inside the compressor is one of the processes that determine the equalizing pressure, i.e., the pressure attained by the whole system while the compressor is off. When the equalizing pressure is high at compressor start-up, the refrigerant vapor density entering the cylinder through the suction valve is also high. Consequently, because of the larger mass of vapor in the cylinder, the discharge pressure is reached before the piston reaches the top dead center and is maintained for longer during the extended discharge process. This results in higher pressure forces on the face of the piston and in a higher resisting torque that must be overcome by the electrical motor. Thus, despite unwanted complications associated with excessive lubricant viscosity reductions and foaming phenomena inside the shell, high refrigerant absorption rates are often desirable since lower equalizing pressure means reduced torque and power required for compressor start-up.

This paper reports a series of tests aimed at systematically investigating vapor-refrigerant absorption in lubricant oil. A rig constructed from a section of transparent glass tube was used to enable visual recording of the absorption process. Three synthetic polyolester (POE) oils of different viscosity grades (approximately 5, 7, and 10 cSt at 40[degrees]C, respectively) were combined with refrigerants R-134a and R-600a. In absorption of R-134a in lubricant oil, the liquid refrigerant is heavier than the oil, and the liquid flow field is a result of natural mass convection. Convection in the liquid promotes higher absorption rates and leads to a more pronounced decay of the system pressure as a function of time compared with the absorption of R-600a, in which only pure mass diffusion takes place. In the present experiments, the effects of initial height of the oil layer (aspect ratio) and refrigerant-oil combination on the flow field and on the refrigerant absorption rate were investigated. In addition to pressure measurements, temperature recordings of the liquid at the liquid-vapor interface were carried out in some experimental runs. The temperature increase as a function of time during refrigerant absorption (latent heat release) was observed.

Fukuta et al. (1995) investigated both experimentally and analytically the absorption of R-22 in three mineral oils of different viscosity grades (ISO 32, ISO 46, and ISO 85, where the figures correspond to the oil kinematic viscosity in cSt at 40[degrees]C) at constant temperature and pressure in stagnant oil layers of different height to base-diameter aspect ratios. The absorption rates decreased with increasing oil viscosity. The data were predicted using an apparent diffusion coefficient model, which was correlated in terms of the liquid layer aspect ratio and of an equivalent Grashof number.

Rates of absorption of pure refrigerants and refrigerant blends in a polyolester oil (POE 68) were measured by Goswami et al. (1998) (also Leung et al. [1998]) over wide ranges of pressure and temperature. They correlated the instantaneous volume-averaged liquid mass fraction with a complementary exponential decay function. Recently, Gessner and Barbosa (2006) revisited the analysis of Yokozeki (2002) for predicting absorption of single component refrigerants through a one-dimensional apparent mass diffusion model. The model was validated with the data of Goswami et al. (1998) for absorption of R-125, R-32 and R-134a in POE 68 at pressures ranging from 239 to 584 kPa (33.66 to 84.70 psi) and at a constant temperature of 24[degrees]C (75.2[degrees]F).

As far as studies on flow field visualization during absorption are concerned, there is a lack of published studies in the open literature. Perhaps the most complete study of such kind is that by Okhotsimskii and Hozawa (1998), who employed Schieleren techniques to visualize absorption and desorption of [CO.sub.2] in 23 systems. In addition to the Rayleigh stability condition, they also investigated the effect of Marangoni (surface tension driven) stability on the mass convection flow pattern as well as on interfacial turbulence. A recent paper on visualization of absorption (and desorption) of R-600a in several types of oil was put forward by Fukuta et al. (2005). They performed experiments in which the refrigerant absorption rates were measured for a number of conditions. A diffusion model was also proposed. In addition to the fact that liquid R-600a is lighter than the oil and absorption takes place by diffusion alone, the most significant difference between the present study and that of Fukuta et al. is that in their study the pressure is kept constant during refrigerant absorption. Here, in contrast, the total mass of refrigerant is constant and, hence, system pressure is allowed to decrease with time as a result of vapor absorption.