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. More than 60 percent of all foods such as meat, milk and fruit would otherwise perish very quickly. 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 minimise the energy needed to deliver the vast amount of cooling required.

"Refrigeration systems that use natural refrigerants play an important role," says 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.“

Cooling distribution centres

The British supermarket chain Asda is one company that has benefited from this. Asda has been using a carbon dioxide and ammonia cascade system at one of its major distribution centres in Lutterworth since 2007. This is the latest of a number of such systems that Asda has had installed since 2002 at its distribution centres by the refrigeration systems manufacturer Star Refrigeration. The goal of this long term modernization program is to replace all of the HCFC R22 based systems in use to date. To cool the entire cold store, which has a volume of around 270,000 m3, Star Refrigeration installed a centralised refrigeration system consisting of two refrigeration circuits linked by a heat exchanger. This system has a charge of 1.6 tonnes of ammonia plus around 8 tonnes of carbon dioxide. It has a total refrigeration output of 3.2 megawatts and chills a frozen food cold store to -25°C as well as several blast chilling rooms at temperatures of between 1 and 13°C. For the cold store, carbon dioxide is used as the low temperature fluid in a vapour compression cycle, rejecting its heat, via the carbon dioxide condenser, to the ammonia circuit. The carbon dioxide is condensed at -5°C 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 carbon dioxide plant, which supplies liquid carbon dioxide at -31°C 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 carbon dioxide circuit incorporates two screw compressors, a surge drum and 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 computerised control system and the ammonia and carbon dioxide detectors, designed to detect any leaks early on.

The 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 in an environmentally friendly way. Trio Invest wanted five cold stores at a temperature of -24°C, and another five rooms were to be chilled to 0°C. The refrigeration systems manufacturer Johnson Controls installed a refrigeration system designed to meet these requirements using 2.9 tonnes of ammonia for the high-temperature level and 8.8 tonnes of carbon dioxide for the low-temperature level. The evaporating temperature of the coolant, carbon dioxide, is -32°C. The system, which has a refrigeration output of 2,500 kW and involves five carbon dioxide piston compressors and four ammonia screw compressors, entered service in 2007.

Storing fruit at controlled temperatures

The 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, to keep a total of about 100,000 m3 of storage space with ceiling heights of up to 20 meters at temperatures of between -3 and +13°C. GTI Koudetechniek installed a cascade system, with a capacity of 1.4 tonnes of ammonia and 11 tonnes of carbon dioxide. The system has been delivering refrigeration capacity of 1,600 kW since it was put into operation in 2007. It uses carbon dioxide as an evaporating coolant, with an evaporation temperature of -8°C. Since the temperature in each room should never vary by more than 1°C, 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 carbon dioxide 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 carbon dioxide gas, which eliminates the need for a conventional defrosting system.

A system for efficiently distributing cold air

The 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 the existing three warehouses by adding another one to provide an additional 65,000 m3 of storage space. Then, it was necessary to chill the new building to -28°C in the deep freeze store and -10°C in the adjoining automatic pick-and-pack plant. The engineers’ office SSP Kälteplaner designed a pumped ammonia system with Eco mode, which was built by the refrigeration systems manufacturer Johnson Controls (formerly York International). The system, which started operation in June 2007, has a refrigeration capacity of 540 kW and a capacity of 2.1 tonnes of ammonia. The refrigerant is used for direct evaporation and is recirculated repeatedly, at an evaporation temperature of -40°C. The main components used by the system are screw compressors, evaporative condensers, separators, economizers as well as ammonia pumps and air coolers. 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. With a storage capacity of 167,000 m3 this storage complex is the largest deep freeze cold store in Switzerland. The task was to ensure a constant temperature of -28°C throughout the interior, which has space for 25,800 pallets up to a height of 29 meters. To achieve this, the refrigeration systems manufacturer Johnson Controls (formerly York International) 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 1.9°C. 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 six percent. This resulted in a seven 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.

Switching to natural refrigerants is worthwhile

Natural 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”, says eurammon’s Thomas Spänich. “As the HCFC era in Europe is ending by 2015, the natural refrigerants ammonia and carbon dioxide are excellent alternatives. Cold store operators who do not yet use these refrigerants will find the switch is well worth it."

Annex

Ozone Depletion and Global Warming Potential of Refrigerants

Source: Eurammon

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