From the Field to the Supermarket Post Harvest Cooling Part 3 of 4 By Dr. Robert Lamb Group Sales and Marketing Director Star Refrigeration Ltd., Derby, England This is the third of a series of articles by Dr. Robert Lamb and Neil Winney, covering the cold chain from the field to the supermarket store. Each article looks at a cooling process that food undergoes before it reaches the consumer. The next article will cover Modern Supermarket Refrigeration Technical Editor. Temperature Controlled Storage and Distribution Introduction In our first two articles we have looked at the journey of food as it leaves the field and undergoes post harvest cooling and inline cooling in food production facilities. The next stage in the cold chain will involve food passing though some form of temperature controlled storage. This article looks at different designs for storage facilities and the refrigeration systems used to maintain temperature. The Need for Temperature Controlled Storage and Distribution Food often travels hundreds and even C20 Cold Chain d July April - September June 2012 2012 thousands of miles from its point of origin to the consumer s table. It may undergo a number of value adding processes during this journey, which includes incorporation into other products including prepared meals. To prevent the food from spoiling, the cold chain must be maintained, often involving storage at chilled or sub-zero temperatures. The duration of storage depends on the type of product, but it could be for a few days or many months. Food may pass through a number of temperature controlled facilities on its journey to the consumer. Storage Temperature In simplistic terms, temperature controlled storage is at either chilled or frozen conditions. However, every product has its ideal storage temperature and in many cases, factors such as humidity, respiration and ripening need to be About the Author Robert Lamb is Group Sales and Marketing Director for Star Refrigeration Ltd. He joined Star in 1998 after completing a PhD in refrigeration at Leeds University, England. Star Refrigeration specialises in industrial cooling and heating solutions for the food, distribution, process HVAC, IT and retail markets. It is the parent company of Starfrost who design and manufacture inline freezing and cooling equipment for food processing applications. Starfrost have formed a joint venture with Star Coolers and Condensers Pvt. Ltd. based in Jalgaon, Maharashtra to design and manufacture inline solutions for the Indian and Far East markets.
Table 1: Recommended storage temperatures Product Storage Relative Ethylene Oxygen temperature ( C) humidity (%) production rate control % CO 2 control % Storage life Apples -1 to 4 90 to 95 Very High 2 to 3 1 to 2 1 to 6 Months Asparagus 2.5 95 to 100 Very Low N/A 5 to 12 2 to 3 Weeks Banana 13 to 15 90 to 95 Moderate 2 to 5 2 to 5 1 to 4 Weeks Beans (yard long) 4 to 7 90 to 95 Low N/A N/A 7 to 10 Days Broccoli 0 95 to 100 Very Low 1 to 2 5 to 10 10 to 14 Days Cauliflower 0 95 to 98 Very Low 2 to 5 2 to 5 3 to 4 Weeks Cucumber 10 to 12 85 to 90 Low 3 to 5 0 to 5 10 to 14 Days Garlic 0 65 to 70 Very Low 0.5 5 to 10 6 to 7 Months Grape -1 to 0 90 to 95 Very Low 5 to 10 10 to 15 1 to 6 Months Lemon 10 to 13 85 to 90 Very Low 5 to 10 0 to 10 1 to 6 Months Onion 0 65 to 70 Very Low 1 to 3 5 to 10 1 to 8 Months Peas 0 to 1 90 to 98 Very Low 2 to 3 2 to 3 1 to 2 Weeks Potatoes 4 to 12 95 to 98 Very Low Not Required Not Required 5 to 10 Months Sweet Potatoes 13 to 15 85 to 95 Very Low Not Required Not Required 4 to 7 Months Tomatoes 8 to 10 85 to 90 High 3 to 5 3 to 5 1 to 3 Weeks considered. Below are s o m e e x a m p l e s o f c o m m o n p r o d u c t types and their storage requirements: Fruit and vegetables These are typically stored at positive temperatures ranging from 0 C to 15 C, depending on the product. Control of oxygen, carbon dioxide, ethylene and humidity levels is often Figure 1: Banana ripening room necessary to prevent deterioration of product quality over time. It enables the product to be stored for many months and ensures availability throughout the year, not just at harvest time. Typical storage temperatures are detailed in Table 1. Where ripening time is required for a particular product, special chambers are often constructed which control the environment to meet the product specific requirements. Dairy After processing, fresh milk is typically transported to the point of sale with 24 hours but can be stored for a couple of days in some cases. To maintain product quality, storage areas are held between 1 and 5 C but no further controls are typically required. Cheeses are ideally stored between -1 C and +7 C, although higher temperatures are permitted depending on the type. Humidity control is also necessary to prevent drying out of product and also to remove moisture released from the cheese. Ice cream is stored at sub zero temperatures, typically between -25 C and -30 C. Higher storage temperatures are possible, but care needs to be taken based on fat content which affects the temperature at which the ice cream freezes. Meat and Poultry Storage temperature depends on holding time. For relatively short periods (less that a week) storage is typically between -2 C and +1 C. For long term storage, meat is typically packed and frozen to -18 to -20 C. Seafood Fresh sea water fish have a storage life of less than two weeks, with fresh water fish typically less than a week. Both are typically stored around 2 C in rooms with a relative humidity > 90%. Alternatively, the fish may be frozen soon after being caught to prolong its storage time. Quality loss is dependent on storage temperature and it can be seen from Table 2 that a small change can have a significant effect on longevity. Table 2: Effect of storage temperature on shelf life Product Storage Temperature Storage Life, C Months Whole blue fin tuna -12-18 to -20-29 4 8 12 Packaged mackerel fillets -9-18 -23 2 3 3 to 5 Warehouse Design The majority of temperature controlled warehouses are single storey. Methods and materials of construction along with building height, footprint and layout vary from one store to another. The building design is often determined by the location, cost of land, cost of construction and usage. Construction Material Key design requirements for any store are minimising the ingress of heat from outside during summer months and in extreme climates, retaining heat when the ambient falls below the store temperature. To achieve this, temperature controlled chambers are constructed from insulated panels. These are Cold Chain d July - September 2012 C21
From the Field to the Supermarket... Figure 2: Traditional store design with internal insulated chamber and roof void typically white in colour and consist of an insulated inner core with external, metal outer layers. Polystyrene has been widely used as the insulation material for panels for many years. Recent cases of fires at temperature controlled facilities (many food factories) have resulted in many insurance companies in Europe insisting on the use of insulation with a higher resistance for the spread of fire. The most widely used material is Polyisocyanurate (PIR). Construction Method Traditional warehouse design has an external structure with weatherproof cladding and roof. Inside this, temperature controlled chambers are constructed from insulated panel. The walls are typically free standing but the ceiling panels are hung from the main structure steelwork. There are roof voids above the insulated ceiling where services such as refrigeration pipe work, HVAC and fire suppression systems are suspended from the structure. The outer building roof protects both the insulated panels and services from the ambient conditions in terms of extreme heat, cold, rain and snow. In recent years, there has been a move towards a single layer method of construction where the insulated panel also provides the external weatherproofing. This has the benefits of eliminating a complete layer of construction (weatherproof cladding) and reducing capital cost. It does place greater emphasis on the building s long term integrity of construction as any damage to the vapour seal will result in moisture and heat tracking into the store. A notable difference in the store s construction and appearance is that the main building steelwork is inside the chamber and the insulation is fastened to the outside. Chamber Height The height of a temperature controlled chamber is dependent on the type of usage. Where there is a quick turn around of product which is only stored at floor level, the height is typically between 4m to 6m. This is sufficient for safe operation and keeps construction costs to a minimum. For longer term storage, product is palletised and put onto racking. A greater number of pallets can be stored per square metre by increasing the height and stores are typically 10m to 12m in height with 4 or more levels of racking. Further increases in storage space can be achieved by the use of mobile racking where the need for a corridor between each section of racking is removed. Sections of racking are stacked next to each until it is necessary to remove a pallet. The racking then moves apart to create a corridor for a forklift to travel down and collect a pallet. This method of storage is ideal for slow turn around product but can increase store capacity per square metre by 20% to 30%. As land prices have increased in Europe, warehouse operators have looked at other ways of maximising storage space in a limited footprint. This has resulted in the development of high bay warehouses. These are typically 25m to 30m high and constructed with the same external, weatherproof insulated panel discussed above. The racking in these stores serves a dual purpose of both storage space and the insulated panel support structure. This design of warehouse is for long term storage and is automated, with computer controlled cranes storing and picking pallets. There are few (if any) operators in the warehouse and many operate without lighting to reduce energy and heat load within the chamber. There is, usually, an adjacent low bay (10m to 15m) loading and despatch chamber, which is kept at a temperature Figure 3: Insulated panel section C22 Cold Chain d July - September 2012 Figure 4: External clad, high bay under construction Figure 5: Mobile racking continued on page C24
From the Field to the Supermarket... continued from page C22 Figure 6: High bay warehouse similar to the main storage area. This serves as an area for marshalling incoming and outgoing product. Most high bay facilities operate sub zero but some are used for chill products such as cheese and butter. Refrigeration System Design Warehouse size and customer preference are two key factors affecting the type of refrigeration system for a facility. Historically, R22 and ammonia have been used as the primary refrigerant for chill and frozen warehouses. Chill facilities have also used secondary refrigerants such as propylene glycol, although this reduces operating efficiency and increases capital cost. With the phase out of R22 in Europe, many customers have looked to find replacement refrigerants for existing installations and alternatives for new stores. For retrofit projects to direct expansion systems, R22 is often replaced with a drop in refrigerant such as the DuPont ISCEON 9 series. This can result in a change in cooling capacity and/or compressor absorbed power and requires careful consideration, as it may be necessary to replace components including compressor motors and relief valves. There is no direct replacement for R22 in pumped circulation systems and many have been replaced with direct ammonia or ammonia/glycol solutions. For new installations, the hydro-fluoro-carbon (HFC) refrigerants R404A and R502 are widely used in smaller (<10,000m 3 ), direct Figure 7: Low charge, low pressure receiver system for ammonia expansion systems. These typically have semi-hermetic reciprocating or screw compressors, air cooled condensers and coolers with either electric or hot gas defrost. The increased cost of HFC refrigerant and greater potential for leakage means that these refrigerants are less widely used for applications where there are large numbers of evaporators and long pipe runs. Air cooled packaged chillers are typically used with glycol circulated around the building to air coolers using either electric or warm glycol for defrosting. Larger freezer and chill warehouses typically use pumped circulation ammonia systems with open drive reciprocating or screw compressor, evaporative or air cooled condensers and evaporators with hot gas defrost. These systems require large refrigerant charges which is a health and safety concern to many customers due to the risk to staff and nearby businesses and residents in the event of a leak. To reduce system charge, glycol is widely used for chilled areas but this requires additional electrical power due to an additional level of heat transfer and the pumping energy required for glycol. Low charge, modular solutions (such as the low pressure receiver system shown in Figure 6) offer alternatives to pumped ammonia and glycol. The low pressure receiver systems deliver the efficiency of pumped circulation, but without the need for ammonia pumps and with as little as 20% of the refrigerant charge. The system design is simple and does not require the complicated valve stations or a high pressure receiver often associated with pumped circulation refrigeration plant. All expansion valves are located within the machinery room on the low pressure receiver package which is factory assembled prior to delivery to site. In many cases, the compressor(s) and drive motor(s) are also combined onto the package to reduce installation time, with only the evaporators and condenser to connect on site. Each system can have up to four evaporators, and a four way valve is used for reverse cycle defrosting. During defrost, the valve turns to reverse the functions of the evaporator(s) and condenser. This way, hot gas is forced through the evaporators to remove any frost or ice that may have formed during the refrigeration cycle. These systems have been installed for over 40 years and were originally designed for use with R502 and then R22. With the move towards natural refrigerant in Europe, ammonia solutions were developed and have been installed for more than 20 years. Packaged Ammonia Solutions Recent advances in technology have seen the development of packaged ammonia chillers similar in design to more main stream HVAC chillers. These incorporate a complete refrigera- C24 Cold Chain d July - September 2012
Figure 8: Factory built air cooled ammonia chiller tion system (compressor, condenser, evaporator, electrical panel, weatherproof housing) in a single factory built package which can be run tested prior to delivery to site. Site installation is simple, requiring only the connection of secondary flow and return pipe work and an electrical supply. This saves time and cost when compared to traditional solutions which are constructed on site and require purpose built plant rooms. The same approach has been taken to low temperature applications with the development of packaged ammonia systems which include the compressors, condensers, electrical panel and housing as a factory build unit. These ammonia condensing units require only piping to/from the cold store evaporators at site to complete the installation. Carbon Dioxide Carbon dioxide (CO 2 ) in cascade with ammonia was introduced around 10 years ago for large warehouse facilities. CO 2 has excellent thermodynamic properties and operates above atmospheric pressure at the -30 C to -35 C evaporating conditions typically required for low temperature storage. It is typically pumped to evaporators with a circulation ratio of 1.5 to 2 and the returning Figure 10: Carbon dioxide/ammonia cascade system Figure 9: Packaged ammonia solution for freezer applications Figure 11: Sub-critical CO 2 compressors vapour is compressed using sub-critical reciprocating or screw compressors. The discharge vapour from compression is condensed against ammonia refrigerant in a combined CO 2 condenser/ammonia evaporator. A low charge ammonia high side refrigeration plant then rejects heat to atmosphere. For chilled warehouse applications, liquid carbon dioxide is pumped to the evaporators as a volatile secondary refrigerant and condensed in the same manner. Figure 10 shows a simplified distribution warehouse circuit with both frozen and chill chambers. Energy data collected for installations in Europe indicate a 10% saving for this solution against similar warehouses using direct ammonia for the freezer and glycol to the chill chambers. Similar savings have also been reported in the US for cold storage facilities using CO 2 /ammonia cascade compared with pumped circulation ammonia even though theoretically, this should not be the case. Energy Efficiency The global increase in energy prices has resulted in greater emphasis on reducing refrigeration plant electrical consumption. Areas of particular focus are: Automation Sophisticated programmable logic controllers (PLCs) now available at relatively low cost, so new refrigeration plants now incorporate fully automated plant control that is designed to minimise energy consumption. Variable Evaporating and Condensing Pressure Power consumed by a refrigeration plant is directly related to the difference in evaporating and condenser pressure. Varying condensing pressure with changes in ambient conditions and increasing the evaporating pressure at reduced load conditions helps cut energy consumption and running costs. A 1 C reduction in condensing pressure can reduce annual energy consumption by 3%. Variable Speed Compressor Motors Part load efficiency for reciprocating and screw compressor increases when using variable speed control in place of cylinder off loading or Cold Chain d July - September 2012 C25
From the Field to the Supermarket... capacity slide control. Variable speed (frequency) control is being fitted both for new installations and as a retrofit solution to improve efficiency on systems which operate at part load for large portions of the year. Variable Speed Fan and Pump Motors The power consumed by fan and pump motors changes with the cube of the speed such that a motor operating at 90% speed will consume 27% less power. This is being applied to condenser and evaporator fan technology along with secondary liquid pumps to reduce energy consumption based on refrigeration load. Store Temperatures Traditional storage temperatures are being challenged in an attempt to reducing running costs. As with condensing pressure, a 1 C increase in air temperature within a cold store can improve efficiency by 3%. Recent work in the UK cold storage market has resulted in average temperatures increasing from -25 C to around -20 C, saving 10% to 15% on running costs. Lighting The replacement of traditional lighting with LED technology in warehouses is helping to reduce this continuous electrical consumer without affecting lighting levels. LEDs have a particular advantage in cold storage applications as they produce maximum lux levels immediately when switched on. This means that motion sensors can be applied within the store and lights can be switched off when aisles or a whole Figure 12: Loading dock before dehumidification Figure 13: Loading dock after dehumidification area of store is out of use. It was not possible to do this with traditional lighting solutions as they took a number of minutes to achieve their desired lux level when switched on. Dehumidification Moisture ingress into low temperature warehouses is a major contributor to cooling load (and power consumption) and is also a health and safety concern due to slippery floors and the risk of ice falling from coolers onto operatives below. Dehumidification of a loading dock area (or pods) leading into a cold storage facility has been shown to reduce moisture ingress by as much as 75%. Dehumidification systems extract air from inside the dock and pass it over a desiccant drier which reduces the moisture content to a dew point as low as -40 C. Air passing into the store is then dry, reducing the risk of ice formation on floors, walls and evaporators. Conclusion Temperature controlled storage plays a key role in the cold chain, ensuring food quality is maintained as it travels from the field to the consumer. The cost of land along with environmental legislation and energy costs have led to new developments in the design of warehouse facilities and associated refrigeration systems. The new solutions cut the carbon footprint of temperature controlled storage and distribution through reduced raw materials of construction, lower energy consumption and the use of naturally occurring refrigerants. Acknowledgements The author would like to thank ISD, ICS Energy and Star Refrigeration for their support and permission to use the photographs and images used in this article. v C26 Cold Chain d July - September 2012