Abstracts Engineering

Effects of Immiscible Compressor Oil in a Flooded PSHE Evaporator : A Visual Study Approach

by Joel Blomfelt

This paper deals with heat transfer in plate evaporators, with a focus on the effects caused by carried-over compressor oil ending up in the heat exchanger. A common standpoint in literature regarding the effect of immiscible oil contamination, in for example an ammonia PHE evaporator, is that the oil can accumulate as a layer on the plate surface causing an additional thermal conduction resistance. However, it is questionable whether this simplistic reasoning is sufficient to fully describe the effect of the oil in the evaporator. In any case, the often severe heat transfer degradation experienced with immiscible oil contamination should be worth analyzing thoroughly from different perspectives. Validated knowledge of what really happens to the boiling and the heat transfer in theunit under the influence of the lubricant can obviously be valuable. Let it be as a potential tool in designing or improving systems that avoid negative oileffects, or just to further validate current knowledge. The work presented in this report aims to emphasize that immiscible oil in a PHE plate gap not only affects the heat transfer in terms of an insulating layer, but can also show as changed appearance and characteristics of the boiling. A special flooded PSHE evaporator with a transparent end-wall was used inthe experimental part, tested in a thermosiphon-type of loop. Differences between an oil-free reference case and an oil-contaminated situation were revealed visually by using high-speed video capture. The boiling tests were limited to water as refrigerant as a first step. Similar studies with ammonia as refrigerant would be natural to aim for in a next step. Based on the visual study of the surface, some possible connections to the heat transfer were hypothesized. Observed phenomena were related to bubble nucleation, vapor/liquid movements, and flow patterns. Effects on heat transfer were mainly discussed in terms of changed surface characteristics, such as liquid repellency and wettability. The interpretations herein suggest that, to get the full picture of how immiscible oil influences the heat transfer, it may be worth to study or discuss the issue in terms of altered plate surface characteristics and interface interactions in the four-phase system (solid-oil liquid-vapor). Detta arbete behandlar värmeöverföring i plattförångare, med fokus på hurdenna påverkas när olja från systemets kompressor hamnar i värmeväxlaren. En vanlig utgångspunkt i litteraturen gällande effekten av oblandbar olja i förångaren, exempelvis i en plattvärmeväxlare för förångning av ammoniak, är att oljan ansamlas som ett skikt på platt-ytorna och att detta orsakar ett ytterligare värmeledningsmotstånd. Frågan är om detta förenklade resonemang är tillräckligt för att beskriva oljans påverkan i förångaren. I vilket fall som helst bör den ofta avsevärda försämringen av värmeöverföringen till följd av kontamination med oblandbar olja vara värd att analysera grundligt ur olika synvinklar. Validerad kunskap om hur kokningen och värmeöverföringen…