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Brine and Glycol Solution
Brine is a salt solution or an aqueous glycol solution used as a heat-transfer medium. Its freezing point is lower than that of water, and depends on the concentration of salt or glycol in solution.
Brine is also used as a liquid coolant to absorb or to store heat energy in refrigeration and thermal storage systems. Ethylene glycol and propylene glycol are brines. They are colorless, nearly odorless liquids. They are often mixed with water at various concentrations and used as freezing point depressants to lower the freezing point of water. Inhibitors must be added to ethylene and propylene glycols to prevent metal corrosion.
The freezing point of an aqueous ethylene glycol solution with a concentration of 25 percent by mass drops to 10°F ( 12.2°C), and its rate of heat transfer is about 5 percent less than that of water. The freezing point of a propylene glycol solution with a concentration of 25 percent by mass drops to 15°F ( 9.4°C).
The physical properties of aqueous ethylene glycol solution are more appropriate for thermal storage systems than those of aqueous propylene glycol solution. In certain applications, toxicity considerations may be dictated by federal EPA requirements and local codes and regulations.
Ice Storage Tank
In an ice storage system, ice making or charging is the process in which compressors are used to produce ice. Ice burning (ice melting or discharging) means that ice in the storage system is melted in order to produce chilled water to offset the required refrigeration load.
In an ice-on-coil, internal-melt ice storage system, ice is produced, or charged, in multiple
storage tanks where closely spaced multicircuited polyethylene or plastic tubes are surrounded by water, as shown in Fig. 31.3. Brine, an aqueous ethylene glycol solution with 25 to 30 percent ethylene glycol and 70 to 75 percent water, circulates inside the tubes at about 24°F ( 4.4°C). The
water surrounding the tubes freezes into ice up to a thickness of about 0.5 in. (12.7 mm). Tubes containing glycol solution entering and leaving the tank are arranged side by side alternately to provide more uniform heat transfer.
Brine typically leaves the storage tank at 30°F ( 1.1°C). Plastic tubes occupy about one-tenth of the tank volume, and another one-tenth is left empty to accommodate the expansion of ice during ice making. Multiple ice storage tanks are always connected in parallel. During ice burning or ice melting, brine returns from the cooling coils in the air-handling units at a temperature of 46°F (7.8°C) or higher. It melts the ice on the outer surface of the tubes and is thus cooled to 34 to 36°F (1.1 to 2.2°C). The brine is then pumped to the air-handling units to cool the air again.
In the storage tank, ice is stored and the high-pressure brine inside the tubes is separated from the water, usually at atmospheric pressure, surrounding the tubes in the storage tank.
Case Study: Operating Modes of Ice-on-Coil Ice Storage System
In a typical ice storage system using ice-on-coil, internal-melt storage tanks in a 550,000 ft2 (51,115 m2) office building near Dallas, Texas, as described by Tackett (1989), there are two centrifugal chillers. Ethylene glycol is used as the coolant. Each chiller has a refrigeration capacity of 568 tons (1997 kW) when it produces 34°F (1.1°C) brine at a power consumption of 0.77 kW /ton (COP 4.56). If the brine leaves the chiller at 24°F ( 4.4°C), the refrigeration capacity then drops to 425 tons (1494 kW) with a power consumption of 0.83 kW / ton (COP 4.24).
A demand-limited partial-storage strategy is used; i.e., one chiller is operated during on-peak hours, as shown in Fig. 31.1c. Meanwhile, ice is also burned during on-peak hours to reduce the demand charge. Ice is charged during off-peak hours to reduce energy costs. The system uses 90 iceon-coil, internal-melt storage tanks.