The oil lubricating the compressor is always carried into the system to some extent, even if an oil separator is installed. To ensure lubrication of the compressor, the oil must be returned. Accumulation of oil in any part of the system could result in decreased functionality and/or efficiency. The solubility of oil in the refrigerant decreases with the temperature, whereas the viscosity and density increase. The evaporator is therefore critical, because it is the coldest point of the system. The flow in the evaporator is also generally upwards, which requires the oil to be lifted by the refrigerant flow. Because the evaporation temperature of the oil is much higher than that of the refrigerant, the oil must be carried out as droplets by the refrigerant vapor.
For normal concentrations (<3%) of oil inside the evaporator, the heat transfer coefficients will not vary significantly. A small concentration of oil may even affect the heat transfer positively. However, if oil accumulates, the thickening oil film will act as an insulating layer, and the evaporator efficiency will be noticeably reduced.
The most important parameters allowing the oil film to be carried are the refrigerant flow shear stress and the viscosity of the oil. It is difficult to give a precise minimum channel velocity to obtain a shear stress high enough to ensure oil return, because the geometries of the various models differ. The induced shear stress will be higher for a high-theta plate than for a low-theta plate at the same channel velocity. Consequently, a lower channel velocity is required for a high-theta plate than for a low-theta plate. The refrigerant vapor velocity in suction pipes to ensure oil return is often set to approximately 5-10 m/s, the higher value being for low temperature applications (<-20°C). The highly turbulent flow in BPHE evaporators allows channel velocities of less than a tenth of these values with assured oil return. SWEP recommend to have a channel velocity above 0.3m/s to ensure oil return. This warning should be a good indication of normal or high evaporation temperatures. However, for low evaporation temperatures (<-20°C), or for low-theta evaporators, the BPHE should be designed with a higher channel velocity.
Things to consider
Some measures can be taken to improve oil return in potentially problematic systems, i.e. those with very low channel velocities and/or low evaporation temperatures. Increasing the channel velocity by decreasing the number of plates, or selecting a different BPHE model, improves the shear stress. This may result in a larger temperature difference between the evaporating refrigerant and the secondary medium due to reduced heat transfer area. However, the increased pressure drop leads to a higher heat transfer coefficient, which may partly or completely compensate for the decreased heat transfer area.
The density of the oil is also important if the oil is insoluble in the refrigerant. If the oil is denser than the refrigerant, e.g. for NH3 and hydrocarbon refrigerants, it will sink to the bottom of the evaporator. If the oil is less dense than the refrigerant, it will float on top of the refrigerant as soon as the flow settles, e.g. in receivers or similar. If the oil is soluble in the refrigerant, the density is not critical, because oil and refrigerant then form a single phase. It is important to remember that the solubility of oil decreases with decreasing temperature. An evaporator operating at low temperatures may have problems with insoluble oil even if the oil and refrigerant are considered soluble.
For systems with an increased risk of oil retention, a good oil separator is recommended to minimize oil accumulation. A more radical solution is to turn the evaporator upside down and evaporate downwards. The oil will thus flow out of the evaporator due to the force of gravity. It is then important to remember that for dedicated evaporator models the refrigerant must still enter through the F3 port because the distribution system is located there. The force of gravity also increases the risk of liquid carry-over. Arranging the piping to collect potential liquid before entering the compressor is recommended.