# Performance Study and Efficiency Improvement of Ice Slurry Production by Scraped-Surface Method
## 1 Introduction  
Over the last three decades, interest in using phase-change ice slurry coolants has grown significantly [1; 2]. Ice slurry refers to a mixture of small ice crystals (typically 0.1 to 1 mm in diameter) and carrier fluid. Ice slurry cooling is a promising technology, because of the high energy storage density associated with the latent heat of phase change and the fast cooling rate due to the large surface area available for heat transfer created by its numerous crystals. Due to these features, ice slurry can be used in many applications, such as comfort cooling of buildings, mine cooling, food cooling and medical cooling [3; 4; 5]. The major requirement for the widespread use of ice slurry cooling is a reliable, energy efficient and cost-effective production technology. Various methods have been proposed for ice slurry production, including the scraped-surface method [6; 7], supercooling method [8], direct-injection or direct heat exchange method [9; 10], fluidized-bed method [11], and vacuum method [12; 13]. Among these methods, the scraped-surface method is the most technologically developed and widely accepted method. The major advantage of existing scraped-surface ice slurry generators over other ice slurry generation technologies is that the mechanical agitation results in exceptionally high heat transfer rates that translate into rapid cooling rates providing an excellent end product. Furthermore, scraped-surface ice slurry generators offer a modular design, which allows for easy expansion with growth in demand. The limitation of scraped-surface ice slurry generators is the minimum concentration of the freezing point depressant that can be used on the ice slurry side to generate the ice. At very low depressant concentrations the continuous accumulation of ice layerson the ice slurry generator walls cannot be prevented which would eventually block the rotation of the scraper-blades and cause the freeze-up of the ice slurry generator. Increasing the additive concentration prevents freeze-up, however, it would reduce heat transfer rates and affect the coefficient of performance (COP) of the system. One additional drawback of existing scraped-surface ice slurry generators is that the rotating scrapers, brushes and orbital rods wear over time and have to be replaced at a given time interval. Existing studies on the scraped-surface method for making ice slurry mainly focus on the heat and mass transfer characteristics of the ice crystal formation process [14; 15; 16; 17]. [PERSON] et al. [14] studied the heat transfer of the scraped-surface heat exchanger via laboratory experiments, and found that the heat transfer coefficient with phase change was about three to five times greater than that without phase change. [PERSON] et al. [15] investigated the effects of two different scrapers on the heat transfer coefficient and found that the use of adaptable scrapers greatly improved the heat transfer coefficient. However, existing research also shows that the overall energy consumption of this method is higher than that of the current heating, ventilation, air conditioning and refrigeration (HVAC and R) system due to the high energy demands of ice makers, and this is also an important reason why this method has not been widely used [18]. Researchers generally believe that the main reason for the high energy consumption of scraped-surface method is that the continuous rotation of the scraping system during the ice-making process requires a large amount of energy [19]. In this context, [PERSON] et al. [20] studied the scraping force of ice growing on cooling surface by varying the degree of supercooling, ice formation times, concentrations of the solution, surface temperatures, surface roughness and number of scrapings. The results showed that these factors affected the scraping force to some extent, but the power consumption of the scraping system has not yet been reported. Up to now, few studies have evaluated the scraping power consumption.