Testing of a Falling-Film Evaporator for Adsorption Chillers

Abstract

In this work, the performance of an innovative evaporator based on water falling film was investigated. The studied evaporator has been equipped with a recirculation system to maximise the wetted surface. Tests have been carried out in a lab-scale adsorption unit connected to a test bench recently realised at Politecnico di Milano labs for evaluating heat transfer performances under realistic operating conditions. Several ad/desorption cooling cycles were performed, setting different liquid refrigerant initial contents (0.9–1.5 kg), different chilled water inlet temperatures (7–20 °C) and flow rates (200–1000 L/h) and different adsorbent bed temperatures (25–30 °C). Evaporation performance has been determined in delivered cooling capacity. Moreover, the experimental data were used to calculate the overall evaporator heat transfer conductance (UA). Experiments showed how the heat duty peaks are mainly due to the thermal level of the chilled water that enters the evaporator, not the water content inside it because this value only affects the duration of the process. Instead, the UA value does not depend on the evaporator inlet chilled water temperature and initial mass content inside the evaporator. UA is 540–570 W/K for temperatures of chilled water entering the evaporator, equal to 10–20 °C, and mass of refrigerant of 0.9–1.5 kg.

Adsorption Air Conditioning for Automotive Applications: A Critical Review

“…Adsorption technology relies on the desorption of water vapour from a solid material and this process is slower and less efficient than the compression and expansion of refrigerant gases used in standard air conditioners. As a result, adsorption systems may struggle to provide adequate cooling performance, especially in extremely hot climates or under heavy load conditions or they may rusult in bulk and heavy systems that could impact with integration process in small vehicles, even though the industry has made significant advancements to address this issue: compact and efficient evapourator designs [76], compact or innovative adsorber heat exchangers [77,78], the introduction of composite adsorbents [79,80], etc. Reported capacity density values for adsorption systems range from 70 to 110 W/kg (based on different studies [81][82][83]), while a typical VC system has an energy higher than 520 W/kg [82].…”

The Effect of Nozzle Configuration on Adsorption-Chiller Performance

“…A falling film evaporator is usually made of a stainless steel cylindrical tank, inside which chilled water flows through the pipes, while the refrigerant (usually water) is sprayed on the surface of the pipes [13]. Thus, the way in which the water is sprayed significantly affects the evaporation rate and overall evaporator performance.…”

Modelling of a falling-film evaporator for adsorption chillers

“…Adsorption chillers can be coupled with evaporators that use as heat transfer mechanism either pool boiling (Thimmaiah et al (2017)) or falling-film (Toppi et al (2022)). Flooded evaporators require that the tubes are completely immersed in the refrigerant pool; on the other hand, falling-film evaporators use a system in which the refrigerant is sprayed on top of the tubes, forming a layer of film on the tubes' surfaces.…”

“…According to literature review, the formation of dry patches is an important aspect to consider in the operation of falling-film evaporators. Even though a falling-film evaporator can provide more advantages compared to a flooded evaporator and, furthermore, it can effectively be inserted in an adsorption heat pump with water as refrigerant (Toppi et al (2022)), its performance can be negatively affected by a deficient amount of film flow rate and mass of film on top of the tubes. For the heat pump configuration used to validate the mathematical model in this article, this decline in performance was commented in Toppi et al (2022) through the evolution of the overall heat transfer conductance (𝑈𝑈𝑈𝑈) during the evaporation process.…”

“…The 𝑈𝑈𝑈𝑈 conductance includes the contributions of each individual thermal resistance involved in the process, and, moreover, contains the wetted surface (considered to be the exchange area). Toppi et al (2022) noted that the 𝑈𝑈𝑈𝑈 conductance decreased continuously as less refrigerant water fell on top of the tubes bundle (also decreasing the heat transfer flux between the chilled water and the refrigerant film).…”