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Cold storage is critical for food on its way from the producer to the consumer. To ensure consistent quality, it is essential to ensure an uninterrupted cooling chain.
Cooling these goods, which also require different temperatures, is a challenge for cold store operators: The method used needs to be safe, and the temperature of each store room has to be kept at a legally specified level, with as little fluctuation as possible. At the same time, economic pressures force them to minimize the energy needed to deliver the vast amount of cooling required.
“Refrigeration systems that use natural refrigerants play an important role,” said Thomas Spänich, who sits on the executive board of Eurammon, the European initiative for natural refrigerants. “Thanks to the thermodynamic properties of ammonia, carbon dioxide, and hydrocarbons, this type of system is very energy-efficient. Natural refrigerants are thus not only of interest to operators as an environmentally friendly alternative, they also offer economic benefits and cut operating costs.”
A SUPERMARKET APPROACHThe British supermarket chain Asda is one company that has benefited from this. Asda has been using a carbon dioxide (CO2) and ammonia cascade system at one of its major distribution centers in Lutterworth.
This is the latest of a number of such systems that Asda has had installed since 2002 at its distribution centers by the systems manufacturer Star Refrigeration. The goal of this long-term modernization program is to replace all of the HCFC-22-based systems.
To cool the entire cold store, Star installed a centralized refrigeration system consisting of two refrigeration circuits linked by a heat exchanger. This system has a charge of 1.6 tons of ammonia plus around 8 tons of CO2. It has a total refrigeration output of 3.2 MW, and chills a frozen food cold store to -13°F as well as several blast chilling rooms at temperatures of between 34° and 55°.
For the cold store, CO2 is used as the low-temperature fluid in a vapor compression cycle, rejecting its heat - via the CO2 condenser - to the ammonia circuit. The CO2 is condensed at -23° and is then circulated as a high-temperature volatile secondary refrigerant, providing cooling for a number of large chill rooms and for the building’s air conditioning system.
The cooling capacity of the low-stage CO2 plant, which supplies liquid carbon dioxide at -24° to six air coolers in the cold store, is 820 kW. The volatile secondary refrigerant plant serves 20 air coolers in three chill rooms and has a capacity of 2,400 kW. The low stage CO2 circuit incorporates two screw compressors, a surge drum, pump set, two plate-and-shell condensers, and a high-pressure receiver.
The refrigeration system also comprises two separate high-stage ammonia systems, each incorporating two screw compressors, a surge drum/evaporator unit, and an evaporative condenser. The refrigeration system’s special features are the computerized control systems and the ammonia and CO2 detectors, designed to detect any leaks early on.
FOOD DISTRIBUTION PROJECTThe food distributor Trio Invest uses a similar system at its new food distribution centre in Domodedovo, Russia. In the warehouse complex, which has a total capacity of 22,000 tons, the company aims to chill meat products, fish, and poultry. Trio Invest wanted five cold stores at a temperature of -11°, and another five rooms were to be chilled to 32°.
Johnson Controls installed a refrigeration system designed to meet these requirements using 2.9 tons of ammonia for the high-temperature level and 8.8 tons of CO2 for the low-temperature level. The evaporating temperature of the coolant, CO2, is -26°. The system has a refrigeration output of 2,500 kW and involves five CO2 piston compressors and four ammonia screw compressors.
STORING FRUITThe cold store operator H. M. de Jong commissioned refrigeration system manufacturer GTI Koudetechniek to build a powerful and economical refrigeration system for its cold store in the small Dutch town of Ridderkerk. The family business particularly needed a solution for storing hard and soft fruit between -27° and 56°. GTI Koudetechniek installed a cascade system, with a capacity of 1.4 tons of ammonia and 11 tons of CO2.
The system has been delivering refrigeration capacity of 1,600 kW since it was put into operation. It uses CO2 as an evaporating coolant, with an evaporation temperature of 18°. Since the temperature in each room should never vary by more than 34°, a combination of electronic expansion valves and a carefully designed air distribution system ensures that the chilled air is distributed with more than 100 air coolers.
When this system was being developed, the airflows were initially simulated in a computer model. The model was then used to achieve a low air circulation rate. The main components of the ammonia circuit include two screw compressors, three air-cooled condensers, and an evaporative condenser. In addition to this, GTI Koudetechniek used two CO2 thaw condensers and two cascade evaporators or condensers for the system. In addition to the high level of energy efficiency of the natural refrigerant used, parts such as the economizer and the frequency converter contribute to the system’s low power consumption.
The operator can also save money by defrosting iced-up parts using hot CO2 gas, which eliminates the need for a conventional defrosting system.
DISTRIBUTING COLD AIRThe refrigerated and deep-frozen goods logistics provider Frigosuisse uses a pumped ammonia system at one of its deep-freeze warehouses in the small Swiss town of Möhlin. The company expanded storage space by adding a fourth warehouse. Then, it was necessary to chill the new building to -18° in the deep freeze store and -14° in the adjoining automatic pick-and-pack plant. SSP Kälteplaner designed a pumped ammonia system with Eco mode, which was built by Johnson Controls.
The system has a refrigeration capacity of 540 kW and a capacity of 2.1 tons of ammonia. The refrigerant is used for direct evaporation and is recirculated repeatedly, at an evaporation temperature of -40°. The main components used by the system are a screw compressor, evaporation condenser, a separator, an economizer, as well as an ammonia pump and aerial photographs. The coolers distribute the cold air vertically in the room, forming a “cold reservoir.”
In the Swiss village of Neuendorf, Migros Verteilbetrieb AG, a company whose core business is deep-freeze logistics for foods, was likewise looking for a solution to the problem of how to distribute the temperature in its deep-freeze store.
The storage complex is the largest deep freeze cold store in Switzerland. The task was to ensure a constant temperature of -18° throughout the interior, which has space for 25,800 pallets. To achieve this, Johnson Controls installed a two-stage ammonia refrigeration system with a refrigeration capacity of 1,074 kW. A descending air system distributes the cold air and ensures that the temperature can only fluctuate by up to 35°. The benefits of this system are a drop of about 85 percent in the amount of power needed for the fan, which reduced the amount of cooling required by about 6 percent.
This resulted in a 7 percent reduction in the total power used by the system, and investment costs have been cut by 25 percent. Since the mid-1990s, Johnson Controls has installed other, similar, systems in 15 deep-freeze warehouses. The warehouse in Neuendorf is by far the largest to date to have been fitted with a descending air system.
GOING NATURALNatural refrigerants have traditionally been used in Europe to process and store foodstuffs. For example, ammonia is used 90 percent of the time in countries such as the UK, Denmark, and Austria. “With ammonia and carbon dioxide, we can achieve energy savings of up to 30 percent,” said Spänich. “As the HCFC era in Europe ends, the natural refrigerants ammonia and CO2 are excellent alternatives. Cold store operators who do not yet use these refrigerants will find the switch is well worth it.”
For more information, visit www.eurammon.com.
Publication Date: 02/04/2008