Spray evaporation on enhanced tube bundles with LGWP refrigerant-miscible oil mixtures

Abstract

Greenhouse gas emissions stemming from refrigeration systems can be categorized into direct and indirect emissions. Indirect emissions arise as byproducts of electricity generation at power plants, while direct emissions occur when refrigerants are released into the atmosphere during refrigerant production, operation, and end-of-life disposal refrigerant in the equipment. To mitigate these emissions, researchers have proposed adopting refrigerants with low global warming potential (LGWP) and reducing the system charge yield, thus minimizing the direct contribution to emissions. The refrigerant R1234ze(E) (HFO), based on Hydrofluoroolefins, has emerged as a promising low global warming potential (LGWP) candidate for replacing the hydrofluorocarbon-based refrigerant R134a (HFC). Several experimental studies have been carried out to investigate the pool boiling heat transfer characteristics of R1234ze(E) and its comparison with other refrigerants. In commercial and industrial applications, chiller systems often employ flooded-type shell and tube heat exchangers, necessitating a substantial refrigerant charge to ensure efficient operation. In contrast, spray or falling film evaporators are seen as highly promising alternatives to flooded-type heat exchangers, as they demand a significantly smaller refrigerant volume. This attribute is particularly advantageous for mitigating direct emissions from chiller systems. In addition to the minimized refrigerant requirement, falling film evaporators also showcase the potential for elevated heat transfer coefficient (HTC) due to the substantial involvement of both nucleate boiling and convection. This differs from the primarily pool boiling mechanism characteristic of flooded type evaporators. Thus, using low-GWP refrigerants with spray evaporators in refrigeration systems mitigates the adverse effect of current cooling fluids on global warming.

Oil is often used in vapor compression systems for lubricating the moving parts in the compressor. If not removed completely, the oil will flow through the heat exchangers and other components of the system. In heat exchangers, this will affect the flow pattern and both the heat transfer and pressure drop depending on the concentration. In the existing literature, there is a lack of studies on spray evaporation of refrigerant/oil mixture on tube bundles. Even in studies focusing on single tube falling film evaporation, the presence of foam and the formation of dry patches have been reported. With tube bundles, the occurrence of foaming and the bundle effect complicates the heat transfer process, highlighting the importance of addressing these issues for the design of improved spray evaporators. Given the growing interest in low-GWP refrigerants and the significant impact of lubricating oil, this study specifically examines how lubricating oil affects the heat transfer performance of low-GWP refrigerants during spray evaporation on enhanced tube bundles.