Spray evaporation of R1234ze(E)/POE-68 refrigerant-oil mixture on enhanced tube bundles

Abstract

Lubricating oil is used in compressors of vapor compression-based refrigeration systems. The oil that escapes from the compressors inevitably circulates in the heat exchangers, affecting their thermal performances. This paper presents a comprehensive study on the impact of lubricating oil on the heat transfer performance of spray evaporator tube bundles used in air conditioning and refrigeration systems. The research includes valuable data on the heat transfer coefficient of tube bundles when subjected to refrigerant and lubricant mixtures of R134a (HFC) and R1234ze(E) (HFO) with synthetic polyolester (POE) lubricant. The investigation delves into the influence of three crucial factors: heat flux, oil circulation ratio (OCR), and saturation temperature. The saturation temperature was examined across a range of −6.7 to 10 °C (20–50 °F), while the heat flux was varied from 6 kW/m2 to 17 kW/m2. OCR was controlled at 1.2 %. The effects of oil on spray evaporation of R1234ze(E)/POE mixture on enhanced tube bundle was studied by varying tube surface structure and bundle layout, that is, square and triangular configurations.The bundle heat transfer coefficient decreased by at least 10 % when lubricant was present in the mixture but, in some cases, varied by 50 % or more compared to the performances with refrigerant only (i.e., no lubricant). The data suggested that the oil penalized the heat transfer coefficient of the bottom tube rows of the bundle, which in many conditions, starved and experienced a partial-dry out state. Externally enhanced boiling-type tubes were more sensitive to the presence of oil than externally enhanced condenser-type tubes.