Low Temperature Freezing

The low-temperature freezing (for frozen food) and the medium-temperature cold (for chilled food) secondary loops use theoretically noncorrosive and nontoxic brines with high heat capacities and low viscosities at low temperatures. They eliminate the use of refrigerants on both cold and freezing sides of the system and are separately pumped to the display cases and cold storage rooms. The display case heat exchangers were designed to use secondary fluids and, consequently, the temperature differences between the brines and the air were minimized.

As secondary fluid on the low-temperature freezing loop, an inhibited potassium formate solution with a concentration of 100%--a nontoxic product compatible with the majority of most common metals and alloys--was chosen. Some analytical studies indicate thermal and pressure drop advantages for HFE-7100 over other potassium formate solutions for low-temperature applications (below -20[degrees]C or -4[degrees]F) (IEA 2003). However, it was recently demonstrated that corrosion had been a problem associated with the use of potassium salts, particularly when galvanized materials were used in display cases. At the same time, the HFE-7100 is an unnatural global warming substance and is expensive. For future applications, a valuable deep-freeze coolant would be carbon dioxide (C[O.sub.2]). As a two-phase secondary coolant, C[O.sub.2] has no ozone degradation potential and negligible GWP. It is universally available, uses very little energy for pumping, and has low costs. In the presented system, the low-temperature secondary fluid (pure potassium formate) is circulated through the freezing secondary loops by two 37.5 hp parallel pumps. Although propylene glycol (35%) has a relatively high level of viscosity at low temperatures, it was chosen for the medium-temperature cold secondary loop. It is also circulated through the loop by two 37.5 hp pumps installed in parallel.

Two warm secondary loops reject the condensing excess heat to the outdoor air by means of remote air-cooled liquid coolers located on the roof (Figure 1). Both loops use ethylene glycol (with a concentration of 50%) as warm secondary fluid. This fluid presents certain environmental risks, but they are minimal compared to the risks associated with common refrigerant leakages.