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- 5.2.2 Cold storage
- 5.2.3 Industrial cooling in buildings, power plant and IT centres
- 5.2.5 Industrial heat pumps and heat recovery
- 5.2.6 Leisure
- 5.2.7 Process Refrigeration
5.2 Applications5.2.1 Food processingRefrigeration is used for chilling and freezing food during processing, in order to prolong shelf life, but it can also be used to make handling or processing easier. For example hams are temporarily frozen to enable them to be sliced more thinly. Chilling also plays a part in the pasteurising process where the product is rapidly cooled after heat treatment to minimise spoilage. A wide variety of chilling and freezing techniques are used, including immersion in liquid, air blast freezing in batches or in a continuous process and contact freezing on tables or in blocks between metal plates. The choice of process depends on the form that the product takes, whether it is wrapped or unwrapped, robust or fragile, processed or raw. Some fruits and vegetables such as potatoes, apples and most soft fruit are notoriously difficult to freeze as the expansion of water destroys the cell walls, leading to mushiness when thawed. Other produce, such as peas, corn and beans, can be frozen in very small pieces using a fluidised bed of air to allow each individual piece to freeze without agglomerating. There is a negative public perception of frozen food, which is that thawed food will always be inferior quality to fresh. In fact if good quality food is frozen professionally immediately after harvest, catch or cooking it should offer increased shelflife and superior quality when thawed. Spoilage rates could be substantially reduced if a greater proportion of food were frozen before shipment. If the public perception of frozen food is improved then there could be a significant increase in this sector of the market. Freezing food requires a lot of heat transfer compared to storage, so refrigeration systems are large capacity and require a large power input although the freezing chamber may be physically quite small. There is a trade-off between the time required to complete freezing and the operating efficiency. Running the system at very low temperature is less efficient but results in a shorter freezing time Some food processes require careful control of humidity and temperature to ensure product quality and in these cases a cooling system is not enough. Examples include bakeries producing cakes and bread, fruit and vegetable storage and fruit ripening. The rate of fruit ripening is controlled by maintaining low ethylene levels in the atmosphere through the use of high rates of ventilation with fresh air. In periods of high ambient temperature the cooling load for the incoming air can be substantially higher than the product cooling load. Failure of the cooling system would cause the product to ripen too quickly resulting in a large loss of product value before it reached the point of sale. In some cases the air-conditioning system is connected to a central plant cooling system using R-717 or HFCs but in others a custom designed stand alone system in an air-handling unit serves the humidity control requirement. 5.2.2 Cold storageCold storage facilities usually operate at two temperature levels, frozen (well below 0oC) and chilled (above 0oC). Frozen produce must be stored below -18oC, and it is usual to maintain the store between -22oC and -26oC to provide a factor of safety in the event of major equipment failure. Some products require lower temperatures, for example ice-cream and similar produce is stored between -26oC and -29oC, and some niche market products such as some types of sushi must be kept significantly colder, even down to -60oC, in order to retain product quality. Chilled produce is typically held between 0oC and 4oC, although fruit, bakery products and vegetables are stored between 8oC and 12oC. Some stores offer long term storage contracts, in order to stock produce until it is “out of season” and therefore more valuable. Stock may be held for months in these warehouses. Other sites provide marshalling facilities in order to restock supermarkets on a daily basis; in these plants the product is not usually in the building for more than 24 hours. The cooling load on such a building is high because of the amount of traffic through the temperature controlled chambers, although product load is typically low because the residence time is not long enough for the air temperature to have any appreciable effect on the product. 5.2.3 Industrial cooling in buildings, power plant and IT centresSome production processes require tight control of the surrounding temperature, for example microchip production, paint spraying or injection moulding. These loads are relatively constant all year, and production output is affected if the chilling plant is inoperative, so both the reliability and the efficiency of the equipment are more important than for office air conditioning. This can sometimes lead to the specification of uniquely designed site-constructed systems to deliver the cooling in order to provide the high level of reliability required, or to achieve lower energy use. Where heat loads are too high to be handled by air- or water-based cooling systems, for example in some high density data centres and other IT cooling applications, other fluids including R-744 have been used in direct systems (Hutchins, 2005, Solemdal, 2014). Typical loads for these applications may be up to 2 kW per m2 in comparison to a typical office load of 40 W per m2. The industrial cooling load is typically almost entirely “sensible” cooling – reducing the air temperature without reducing the moisture content, in contrast with a typical commercial air conditioning load which is likely to involve more dehumidification, or “latent” cooling. Latent cooling requires lower temperatures to bring the air to its dewpoint. If an industrial cooling load is 100% sensible cooling, or if the cooling can be split into separate systems for sensible and latent cooling, then the operating temperature for the sensible cooling can be raised, making the system more efficient. In these cases it may also be possible to use indirect evaporative cooling to reject some or all of the process heat load. There is a significant benefit in pre-cooling the inlet air to gas-fired turbines in higher ambient climates. For example reducing the air inlet temperature from 40°C to 8°C will increase capacity by 28% (Kohlenberger, 1995). These systems usually use large chillers with R-22, R-134a or R-717, cooling glycol. 5.2.4 District cooling In the Middle East since the 1990’s district cooling applications have become common with the rapid rate of economic development in the Gulf area. Those applications serve office complexes, shopping malls, airports and call centres (Sarraf, 2012). Because of the high ambient temperatures throughout most of the year, the uninterrupted operation of those systems is “mission critical”. District cooling providers design multiple redundancies in their systems, since stoppage will necessitate vacating those premises incurring heavy fines on the operators. There is an estimated district cooling installed capacity of about 13.2 GW in the Middle East. The UAE have the largest share of this capacity at about 8.8 GW. Qatar is the second with the single largest district cooling plant with an installed capacity of 420 MW. Saudi Arabia has about 400 MW district cooling capacity and is the fastest growing market. All those systems use vapour compression technology and predominantly HCFC and HFC refrigerants. In Egypt, with an estimated 350 MW total installed capacity of district cooling, natural gas fired absorption chillers are mostly used. Some older district cooling applications in the region still use CFC-11 and CFC-12 but the majority of systems are now HCFC-22, HCFC-123 or HFC-134a. In the Gulf Cooperation Council countries (GCC), the total cooling demand is expected to triple between 2010 and 2030 (when HCFC are phased-out) reaching approximately 350 GW. This would be the equivalent to 60% additional power generation requirement in the region if the same mix of cooling technologies were used. The additional power generation required for this mix would consume the equivalent of 1.5 million barrels of oil per day so discussions are undergoing to increase the use of district cooling to enable electric power peak shaving and therefore reduce oil consumption for generating electricity. The potential for district cooling in the GCC counties between 2010 and 2030 is over 100 GW, existing capacity in GCC countries is 11.7 GW, and thus an increase of about nine folds of existing capacity is expected. This increase between 2012 and 2030 is divided as shown in Table 5-3 (Olama, 2012) Table 5-3: Estimated change of district cooling capacity in GCC countries, 2012 - 2030
District cooling systems are not restricted to the extreme tropical climate of the Middle East. Similar systems are installed in the United States serving business districts, hospitals and university campuses, and such systems are becoming more common in Scandinavia where the district heating network makes it easier to incorporate cooling into the existing infrastructure. Helsinki for example has a 120 MW district cooling system (Vartiainen, 2011), which is projected to grow to 280 MW by 2030. Several large systems have been installed in China, including steam-driven absorption. Where absorption chillers are used the district cooling system can be integrated with a combined heat and power plant, using the excess heat from generators to drive the cooling system. In some coastal sites, for example in Hawaii and Mauritius, deep seawater is being considered for district cooling offering a significant reduction in demand on an electrical infrastructure which might already be overloaded. 5.2.5 Industrial heat pumps and heat recoveryMany industrial processes including brewing, dairies, food factories and chemical processes require large amounts of heat in addition to a cooling load. Even if the primary use of heat, for example for cooking food, cannot be achieved by heat pumps or recovery there may be many uses for lower grade heat, such as pre-heating boiler feed water or heating wash water for the production area. When the application is collecting and redirecting waste heat from a refrigerating system it is called heat recovery. When it is performing a non-productive chilling process on a source of heat, whether it is at ambient temperature or is the waste heat stream from another process such as a cooker flue, it is a heat pump. Large heat pumps have also been used for heating public buildings, for example in Gardermoen Airport, Norway (8100 kW heating capacity) and Akershus hospital, Norway (8000 kW heating capacity). These systems are custom-designed, using R-717 as the refrigerant (Stene, 2008). Even larger systems are used for district heating systems, with many examples in Scandinavia. The smallest of these systems are about 5 000 kW. Most installations use HFC-134a in centrifugal compressors, with some (up to 15 000 kW) using R-717. The largest is in Stockholm, with a total capacity of 180 000 kW (180 MW) using HFC-134a in centrifugal compressors. This system takes heat from sea water to provide the thermal source; other similar installations have used waste water from the sewage system (Bailer, 2006). Steam-fired absorption systems (as described in section 5.3.6) can be used to raise condenser water temperature in power plants to provide heat to district heating networks and some industrial processes. Absorption can also be used to boost the temperature of a proportion of a medium temperature process stream by cooling the remainder of the stream. In this way a small part of a stream at 70°C could be raised to 120°C by cooling the rest of the stream and rejecting its heat to atmosphere at, say, 35°C. 5.2.6 LeisureThe principal use of refrigeration in the leisure market is for ice rinks, extended also to indoor ski-slopes, ice climbing walls and other ice features. Many older ice rink systems used direct CFC-12 or direct R-717. To change to an indirect system would require replacement of the floor slab, which is a considerable capital expenditure. Some CFC-12 systems have been converted to HCFC-22 despite the increased pressure. Similarly some R-717 systems in Central Europe have been converted to R-744. A few very large systems have been installed for bobsled and luge runs, typically associated with winter Olympics. These systems usually use pumped R-717. A recently installed cross-country ski track in Finland used R-744 for the track cooling, with circuits up to 1 km long. 5.2.7 Process RefrigerationCooling is used in a wide variety of process applications (in addition to food industry applications covered in section 5.2.1). The cooling can be applied by a direct refrigeration system with a coil in the process tank, or a jacket around the outside of a chemical reactor vessel or storage tank. Alternatively, a secondary fluid such as water, brine solution or glycol may be used. In these cases standard chillers as described in chapter 9 might be used, although there may still be other reasons for requiring the chiller to be specially designed for the project, for example location of the equipment within a hazardous area. In refineries refrigeration is used to remove light hydrocarbons from the process stream. Such systems can be extremely large and may use HCFC or HFC in centrifugal chillers. It is also possible to use the feedstock, particularly ethylene or propylene as the refrigerant, either in a closed-loop system or as part of the process flow. In very large systems, the use of HFC-134a enables centrifugal compressors to be used whereas ethylene typically requires screw compressors, which, at that size, are significantly more expensive. Some specialist processes including plastic forming, paper milling and precision machining require multiple small capacity systems and are typically constructed on site using HCFC or HFC. The use of multiple flexible hoses to connect to the moving parts of these machines presents a particular challenge due to high refrigerant leak rates. Where processes produce high grade waste heat, for example flue gases from glass production, power stations, steel mills, incinerators or cement factories, the heat can be used to drive absorption chillers, either directly or by raising steam which is fed to the chiller. Such systems require to be tailored to the application to ensure that the heat production and cooling demand are well matched. The cooling of deep mines presents another challenge because the operating conditions are arduous and the available space is severely constrained. Typical systems used centrifugal chillers underground. An alternative to the use of CFC-12 or HCFC-123 in centrifugal chillers was to produce cooling at the surface, either as cooled ventilation air or as chilled water or ice. However the depth to which surface cooling is effective is limited and for mines deeper than about 2,000m some form of underground cooling is required to counter the effects of air compression and the power required to transport the cooling effect from the surface to the workface. There is currently no acceptable alternative to HCFC-123 for underground applications to a depth of up to 4,500m (Calm, 2011). Some of the new fluid blends currently under development for other applications (see for example chapter 2, chapter 4 and chapter 9) may also be suitable for this application but it is unlikely that a fluid will be developed solely for this use.
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