Comparing temperature fluctuation attenuation capabilities and feasible parameter regions of indirect- and direct-contact heat exchangers for an ultra-high precision constant-temperature chilled water system: An experimental and Modelica-based study

Constant temperature and humidity rooms have widespread applications in high-end manufacturing and scientific research. Conventional wisdom primarily focuses on controlling the air temperature at the terminals, i.e., the supplied outlets. For achieving ultra-high precision (e.g., ±0.1 °C), it becomes essential to pre-control the temperature of the chilled water of the air-conditioning system. Nevertheless, there is still limited understanding of this matter. This paper investigated two common temperature fluctuation attenuators for the chilled water system: indirect- and direct-contact heat exchangers, e.g., a plate heat exchanger (PHE) and a water mixing pump (WMP), respectively, as follows. First, Modelica models of the two systems were established and validated by an experimental set-up capable of switching between the PHE and WMP loops. Second, an orthogonal analysis was conducted to evaluate the attenuation effects of four key factors. Results indicated that both attenuators can provide the required constant temperature precision with nuanced differences. The MWP system demonstrated faster settling time, primarily affected by valve adjustment accuracy. In contrast, PHE exhibited superior fluctuation control, with valve adjustment accuracy and input chilled water temperature fluctuation amplitude as critical factors. Finally, feasible parameter regions for the two types of attenuators under ultra-high precision targets were delineated and analyzed.