Supermarket refrigeration system with completely secondary loops

ASHRAE Journal, Sept, 2007 by Vasile Minea

Today, phaseout of ozone-depleting refrigerants and the improvement of waste heat recovery are the main issues of the supermarket refrigeration industry. The negative impact of refrigerant leakages on global warming and ozone depletion has stimulated the development of new supermarket refrigeration systems requiring less quantities of refrigerant.

The advanced system presented in this article involves secondary fluid loops on both refrigerating and condensing sides, and heat recovery with brine-to-air heat pumps and passive heat exchangers. This integrated concept has a considerable potential to reduce combined refrigeration and HVAC energy use in supermarkets located in northern climates compared to multiplex refrigeration systems with more conventional heat recovery approaches. It also may reduce up to 70% of the quantity of primary refrigerant required.

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Supermarkets are energy intensive commercial buildings with an average specific energy consumption of 1,000 kWh/[m.sup.2]/yr. Conventional multiplex refrigeration systems account for about 50% of this energy consumption and require large refrigerant charges: 1000 to 2500 kg (2,200 to 5,500 lb) of HCFC or HFC per store. Hundreds of meters of piping, and many valves and brazed joints provide 15% to 30% of refrigerant annual losses. Most of these systems recover by desuperheating between 30% and 40% of compressors' total heat rejection. In the Canadian cold climate, this amount of energy is not sufficient to completely eliminate the use of fossil fuels (natural gas, propane) for space and hot water heating.

Environmental issues have stimulated the development of new supermarket refrigeration systems that require less quantities of refrigerant. (1) Among these systems, decentralized compressors and completely secondary fluid loops can significantly reduce the length of pipes, primary refrigerant charges and leakage rates. (2,3)

Secondary loop refrigeration systems can take many forms. One uses secondary fluid loops on freezing and refrigeration (evaporators) zones, as well as on the heat rejection (condensers) sides. The Canadian government and several private and public partners (manufacturers, equipment suppliers and engineering firms, a large electrical utility and a national supermarket chain) have promoted and implemented a completely secondary fluids refrigeration system near Montreal. (4) The new 10 923 [m.sup.2] (117,578 [ft.sup.2]) supermarket with a total sales area of 6953 [m.sup.2] (74,842 [ft.sup.2]), representing about 64% of the total floor area, was designed for about 1,900 customers. It no longer circulates primary refrigerant throughout the store. The primary refrigerant (R-507), an azeotrope and non-flammable fluid, has no ozone depletion potential and is confined into a 131 [m.sup.2] (1,410 [ft.sup.2]) central mechanical/electrical room.

Low-Temperature Zone

The low-temperature (freezing) zone contains three distinct closed loops: a freezing loop for frozen food, a primary refrigerant loop and a heat rejection/heat recovery loop with heat pumps (Figure 1).

For secondary fluid, the low-temperature loop uses an inhibited potassium formate salt at an appropriate concentration for the working temperatures of this application. It is a non-toxic product having high heat capacity and low viscosity at very low temperatures. When not exposed to oxygen, the potassium formate has a good compatibility with aluminum, brass, bronze, carbon and stainless steel, cast iron and copper. However, it is incompatible with other materials, such as galvanized metals, magnesium and zinc. Also, with nonferrous alloys, the potassium formate can develop surface corrosion.

This secondary fluid eliminates the use of refrigerants on the freezing side of the refrigeration system. It is pumped through the display cases and cold storage rooms by three 5.6 kW (7.5 hp) parallel circulation pumps. The heat exchangers in display cases, designed to use such brine, operate with low-temperature differences between the thermal carrier fluid and the cooled air. The low-temperature primary refrigerant loop contains a rack with eight parallel semihermetic compressors, designed for a suction temperature of -28.9[degrees]C (-20[degrees]F) and total cooling capacity of 196 kW, and two evaporators close-coupled to the compressors' rack. Thermostatic expansion valves allow the primary refrigerant (R-507) to expand before entering refrigerant-to-brine evaporators.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

One compact liquid-cooled condenser transfers the condensing heat to the rejection loop. It is a compact arrangement, allowing the minimization of the refrigerant pressure drops and the compressors' suction superheating. A desuperheating coil installed on the compressor discharge line upstream of the condenser heats the potassium formate and supplies it via a four-pipe distribution system for defrosting the heat exchangers of freezing display cases. A 55 kW (15.7 tons) refrigerant liquid subcooler installed downstream from the condenser subcools a part of liquid from the medium-temperature rack. The heat recovered is then transferred to the medium-temperature primary refrigeration loop to superheat the compressors' suction gases, increasing the system efficiency. The low-temperature heat rejection/heat recovery loop rejects the condensing excess heat to the outdoor air by means of an air-cooled liquid cooler located on the roof. This closed-loop uses an ethylene glycol/water mixture (with a concentration of 50% by weight) as warm secondary fluid circulated by two parallel 11.2 kW pumps (15 hp). This fluid presents certain environmental risks, but they are minimal compared to the risks associated with common refrigerant leakages. Seven brine-to-air heat pumps connected on this loop recover heat for space heating and cooling purposes. Two 3.7 kW (5 hp) pumps and one 1.1 kW (1.5 hp) pump circulate a part of the warm secondary fluid through these heat pumps.