# Performance Study and Efficiency Improvement of Ice Slurry Production by Scraped-Surface Method
## 3 Results and Discussion  
### Influence of Concentration of Sodium Chloride Solution  
The temperature change characteristics in the ice slurry storage tank were measured by varying the sodium chloride concentration in the range of 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, and 6 wt% under the condition of the fixed rotary scraping speed (n = 29 rpm) and flow rate of the solution (q = 1.0 m\({}^{3}\)/h). When the temperature of the aqueous solution dropped to 5 \({}^{\circ}\)C, the temperature change began to be recorded. Taking 3 wt% as an example, the relationship between temperature and time is shown in Figure 4. The temperature of the solution kept falling over time. When the temperature dropped to the freezing point temperature (\(-\)1.8 \({}^{\circ}\)C), it was found that the ice crystals became continuously formed and the temperature was basically unchanged. The formation temperature and generation time of ice crystals at different concentrations were compared, as shown in Figure 5. It can be seen that as the concentration of the solution increased, the time required for the appearance of ice crystals increased and the freezing point \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Mass Concentrations (\%)** & 1 & 2 & 3 & 4 & 5 & 6 \\
**Specific Heat (J/kg K)** & 4112 & 4053 & 4005 & 3962 & 3918 & 3874 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The specific heat of aqueous sodium chloride solution. Figure 4: Variations in temperature during ice slurry production at different initial NaCl concentrations. temperature decreased. The sodium chloride concentrations of 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, and 6 wt% corresponded to the ice slurry formation temperatures of \(-0.6\), \(-1.2\), \(-1.8\), \(-2.4\), \(-3.0\), and \(-3.6\)\({}^{\circ}\)C, and the theoretical freezing point temperatures of \(-0.6\), v1.2, v1.8, v2.4, \(-3.0\), and \(-3.6\)\({}^{\circ}\)C, respectively. Therefore, the ice slurry formation temperatures were consistent with the theoretical freezing point temperatures. This indicates that the supercooling degree of this ice-making method is almost nonexistent. Figure 6 displays the relationship between the IPF of the ice slurry and ice-making time. At the same ice-making time, higher initial concentration of aqueous sodium chloride solution resulted in lower IPF of ice slurry, suggesting that the formation of ice slurry became slower. This is mainly because the increase in sodium chloride concentration lowered the freezing point of the solution and increased the system cooling load. The total power consumption of the ice-making system and the power consumption of the scraping system were monitored. Taking 3 wt% sodium chloride solution as an example, the experimental results are shown in Figure 7. When the IPF of the ice slurry reached 13.9%, the total power consumption was 1.9 kWh. The power consumption of the scraping system was 0.1 kWh, which accounted for only 5.26% of the total power consumption. Experimental results at other sodium chloride concentrations also indicated that the scraping power consumption accounted for about 5% of the total power consumption of the system. Thus, the results suggest that the scraping system does consume a certain amount of energy, but it does not account for a large proportion. Compared with the supercooling method, the scraping method has the advantages of simple equipment, small floor space, and easy temperature control, so this method is a more promising method for ice slurry production. Figure 5: Ice slurry formation characteristics at different initial NaCl concentrations. Figure 6: Relationship between ice packing fraction (IPF) and ice-making time at different initial NaCl concentrations. The COPs invariably declined as the concentration of sodium chloride increased, as shown in Figure 8. The COP reduced because the increase in solution concentration caused a corresponding decrease in the freezing point of the solution and the required evaporation temperature of the refrigerant.