Over the last 150 years or so, refrigeration’s great strides offered us ways to preserve and cool food, other substances and ourselves. Transport refrigeration has brought distant production centres and population together...
Refrigeration in general in its various applications from household refrigeration to transport refrigeration has become essential in today’s society. Use of transport refrigeration in its various modes e.g., road-based, sea-based and air-based for the transport of preserved food items to medicinal supplies and much more has been growing with a great demand. Transport refrigeration equipment, cold chain monitoring solutions and replacement components help protect our global food supply and other perishable cargo. Companies are offering complete range of transport refrigeration units designed to suite all types of vehicles, meet the needs for single or multi-temperature cooling and with qualities of unique performance, reliability, capacity, flexibility, ease of use and cost whilst also respecting the environment. Transport refrigeration is a vital part of every cold chain: perishable goods, mainly food but also medical supplies and other goods, have to be refrigerated on their way from harvest or production to the consumer. Refrigerated transport increases food safety and prevents economic losses due to spoilage. Trucks and trailers are in many parts of the world the main mode of transport for refrigerated goods. Fresh is not simply about how recently produce was harvested or products were created. It is also about the conditions in which they have been transported. Transport refrigeration solutions allow for precise control of temperature and humidity, preserving all types of perishable cargo no matter where it needs to go. With a robust portfolio of products and services designed to help monitor and manage the cold chain of the world’s most temperature-sensitive, perishable products, including food, pharmaceuticals, biologics, and industrial chemicals, transport refrigeration offers full-service supply chain visibility solutions that provide complete cargo transparency from origin to destination.
Over the last 150 years or so, refrigeration’s great strides offered us ways to preserve and cool food, other substances and ourselves. Transport refrigeration has brought distant production centers and population together. It tore down the barriers of climates and seasons. And while it helped to rev up industrial processes, it became an industry itself. Refrigerated transport, undoubtedly an essential link in the cold chain, aims at supplying the consumer with safe, high quality perishable goods. The goods concerned are perishable foodstuffs or non-food goods such as flowers, plants, pharmaceuticals or chemical products. There are three basic types of transport: sea transport (conventional ships, container ships), land transport (road, rail) and air transport. Intermodal transport combines more than one of these types of transport. Frozen goods are transported at a temperature of –18ºC or lower, chilled goods at a temperature above the freezing point. Driven primarily by the expansion of the chilled and quick-frozen foods market, with an annual sales level totalling 1200 billion USD worldwide in 2000 refrigerated transport is a major economic player. About 550 000 refrigerated marine containers and about 1 200 000 refrigerated road vehicles are currently in use and this clearly illustrates the importance of refrigerated transport worldwide. For each transport mode there is specialized refrigerated equipment to provide temperature control. Intermodal systems exist for all modes except for air transport. It must be noted that transport refrigeration systems not only cool, but also heat if necessary in order to achieve the correct transport temperature, an important consideration for chilled goods in cold climates. As a general rule, transport refrigeration equipment is designed to maintain temperature, not to cool down goods, so goods should be loaded already pre-cooled to carriage temperature. Exceptions are shipments of bananas and citrus fruit, for which appropriate cooling regimes are established. It is essential to establish rules governing hygiene, particularly with respect to cleaning of the body following transport. The majority of new transport refrigeration units use vapour-compression refrigeration with HFC refrigerants. Some equipment uses total-loss refrigerants (carbon dioxide or liquid nitrogen) for limited journey times. Road transport units may operate either from the vehicle engine or from an independent diesel engine.
Rail units may operate on electricity supplied from a generator wagon. Marine refrigeration is electrically driven from ships’ supplies. Intermodal containers are also electrically driven, but can have portable diesel generator sets attached to them. Refrigerated equipment in air transport is relatively rare, and commonly uses “dry ice” (solid carbon dioxide), though some battery-operated systems are available.
The technical requirements for transport refrigeration units are more severe than for many other applications of refrigeration. The equipment has to operate in a wide range of ambient temperatures and under extremely variable weather conditions (sun radiation, rain…); it also has to be able to carry any one of a wide range of cargoes with differing temperature requirements, and it must be robust and reliable in the often severe transport environment. For frozen goods, low temperature is needed, but generally a close range of temperature is not a critical requirement. Frozen foods at –18ºC may not suffer if they go down to a lower temperature, so a simple on/off control system may be used, which provides cooling whenever the temperature rises to the set point. However, especially for long journeys, frozen foods must not be exposed to large temperature variations, which can lead to moisture migration and loss of quality. Chilled foods, on the other hand, require close temperature control between two limits — too low a temperature will damage them; too high a temperature will reduce shelf life. For example, chilled meat may need to be maintained throughout a long journey within a temperature range of between 0ºC and –1.5ºC, which modern equipment can achieve. Close temperature control systems for chilled goods require continuous, modulated refrigeration combined with high rates of air circulation. This involves a greater power and energy requirement than for frozen foods with on/off control. Longer journey times need better control. Temperature control available in ship and intermodal container transport provides the best conditions; road transport can be comparable but less severe specifications are acceptable for shorter journeys or for frozen foods. Relatively poor temperature control can be acceptable for rapid air transport. Estimated diesel power requirements (as equivalent fuel use) for road units are from 11 kW for a typical van unit to 23 kW for a trailer unit, giving an overall figure of around 0.05 kWh per tonne km.6 This high power requirement is necessary to obtain a fast temperature pull down and fast recovery in distribution with many door openings. This corresponds to equipment with cooling capacity about 4 times the body heat loss cooled at –20°C and 10 times the body heat loss cooled at 0°C at 30°C ambient. Running hours may be between 1800 and 4000 hours per year. Intermodal container units may require electrical power of around 2 kW for frozen goods and 5 kW for chilled, but this is very dependent on unit design and operating conditions. It is notable that the close temperature control requirements and more rapid air circulation needed for chilled goods result in higher power requirements than for frozen goods, despite the lower temperature differences to be maintained.
Transport refrigeration technologies
A transport refrigerator vehicle is designed to carry perishable freight at specific temperatures. Like refrigerator cars, refrigerated trucks differ from simple insulated and ventilated vans (commonly used for transporting fruit), neither of which are fitted with cooling apparatus. Refrigerator vehicle can be ice-cooled, equipped with any one of a variety of mechanical refrigeration systems powered by small displacement diesel engines, or utilize carbon dioxide (either as dry ice or in liquid form) as a cooling agent. Different transport techniques are briefly explained as:
Water cooling: water cooling systems are expensive, so modern vessels rely more on ventilation to remove heat from cargo holds, and the use of water cooling systems is declining. Air cooling and water cooling are usually combined. Air cooling removes the heat generated by the reefers while water cooling helps to minimize the heat rejected by the reefers. The reefers are using some heat exchangers that behave as water cooled condensers.
Cryogenic cooling: Another refrigeration system sometimes used where the journey time is short is total loss refrigeration, in which frozen carbon dioxide ice (or sometimes liquid nitrogen) is used for cooling. The cryogenically frozen gas slowly evaporates, and thus cools the container and is vented from it. The container is cooled for as long as there is frozen gas available in the system. These have been used in railcars for many years, providing up to 17 days temperature regulation. Whilst refrigerated containers are not common for air transport, total loss dry ice systems are usually used. These containers have a chamber which is loaded with solid carbon dioxide and the temperature is regulated by a thermostatically controlled electric fan, and the air freight versions are intended to maintain temperature for up to around 100 hours. Full size intermodal containers equipped with these "cryogenic" systems can maintain their temperature for the 30 days needed for sea transport. Since they do not require an external power supply, cryogenically refrigerated containers can be stored anywhere on any vessel that can accommodate "dry" (un-refrigerated) ocean freight containers.
Redundant refrigeration: Valuable, temperature-sensitive, or hazardous cargo often require the utmost in system reliability. This type of reliability can only be achieved through the installation of a redundant refrigeration system. A redundant refrigeration system consists of integrated primary and back-up refrigeration units i.e., a container fitted with two refrigeration units and a single diesel generator. If the primary unit malfunctions, the secondary unit automatically starts. To provide reliable power to the refrigeration units, these containers are often fitted with one or more diesel generator sets.
Mechanical refrigeration: all mechanical transport refrigeration units normally includes a compressor, drive, and condenser combination; an evaporator or air-cooler; all necessary refrigerant lines and electrical wiring; and means whereby the unit can be suitably mounted and installed on a vehicle, used in transportation of perishable goods. The first successful mechanically refrigerated trucks were introduced by the ice cream industry in about 1925. There were around 4 million refrigerated road vehicles in use in 2010 worldwide.
Transport refrigeration units used in different transport modes and vehicles always meet the most rigorous criteria for sensitive goods transportation. High performances and fast temperature recovery provide superior protection of goods. Innovative microprocessor controls and cab command makes operations easy and safe. Different levels of protection rise reliability and lower life cost of unit. Therefore, requirement is a comprehensive one-stop solution for the transportation of temperature-controlled freight with the threefold benefit for the customer as: safety, efficiency and permanently low total cost. Further, constant checking of the system components, using a trailer telematics system, centralized monitoring and control of servicing, maximizes the availability of the customer's investment. The development of the transport refrigeration unit using the latest state-of-the-art technology, which sets the benchmark for the premium segment require:
• Precise temperature control throughout the entire interior with minimal fluctuation.
• Excellent economy combined with high-performance cooling through efficient motor management.
• High refrigeration performance for fast cooling down.
• The best heat output among the direct competitors for the shortest possible interruptions in goods cooling when defrosting.
• 50 % fewer defrosting cycles due to the ice-reducing evaporator design.
• Durable industrial motor with intelligent speed control and compressor with cylinder deactivation for low fuel consumption.
• Greater operational reliability and control thanks to the electronic control unit.
• Longer maintenance intervals due to the specially dimensioned wearing parts.
• Transport refrigeration systems designed specifically for make and model of vehicle.
• The size of all components is carefully calculated to give maximum cooling output without adversely affecting the vehicle’s fuel consumption for performance.
• To maintain the system in good working order and to minimize the loss of refrigerant, the refrigeration unit should be operated for a minimum of five minutes each week regardless of the season. This will assist in preventing the compressor seal from drying out; a condition which can cause loss of refrigerant and posible damage to the compressor.
• From the latest and best in refrigeration technology to the services required to keep units running efficiently.
• Next-generation transport refrigeration technologies require ever-higher standards in refrigerated container, truck and trailer systems while reducing fuel consumption, emissions and sound levels.
• Logistics security solutions in order to help to minimize product loss, ensure patient safety and protect product integrity by mitigating the risks associated with product distribution, such as theft, diversion, counterfeiting and chain of custody.
Challenges today, opportunities tomorrow
Transport refrigeration is one of the most demanding segments in commercial refrigeration. These refrigeration systems need to manage loads of varying sizes and types while dealing with exposure to harsh environmental conditions, ranging from sweltering desert heat to the corrosive salt of ocean swells.
Extreme ambient temperatures can cause interior temperatures to spike, posing a serious risk to food freshness and safety and significantly affecting fuel efficiency.
This application demands extremely high levels of engineering capable of simultaneously managing these competing concerns and helping to deliver high-quality foods at competitive prices. Unlike most commercial refrigeration applications for which many components are used interchangeably, the equipment used in transport systems is designed specifically to meet the challenges of navigating extreme conditions.
• Providing refrigerated products are handled carefully, using the right equipment, their size, freshness, firmness, texture, color, flavor, aroma and chemical balance are not compromised in transit.
• Different requirements for long-distance transport in trailers and distribution traffic in smaller vehicles leads to a range of cooling capacities. Similar to mobile ACs in passenger cars, the refrigerant leakage is high. These are in particular caused through constant vibrations during operation and at times when difficult road conditions lead to lose connections. Leakage rates can be as high as 20-30% per year.
• There are three key heat factors that impact on the temperature of products during transportation: outside temperatures entering the container during load and unload periods, the temperature of the product itself prior to loading, the ventilation is set according the cargo requirements. As a result, it’s important that goods are pre-cooled ahead of transportation to achieve optimal temperature levels before they are loaded into a container. This will reduce the impact of outside temperatures on the valuable products during the transportation period.
• It is important to note that there is currently no technology available to prevent or reverse the ageing of perishable goods.
• Palletized loads of refrigerated cargo must be capable of supporting a material’s stacking of a good height. The material used for packaging must also be able to withstand moisture and allow a vertical airflow through boxes in order to maintain the desired temperature.
• According to industry analysts 32% of refrigerated cargo loaded onto refrigerated vehicles is at the wrong temperature at the time of loading. The loss of temperature typically happens as a result of poor loading practices.
• Many times, cargo is left sitting too long on the loading dock, and the carrier runs the risk of the load being rejected by the receiver.
• Refrigeration units are not designed to alter the temperature of the cargo; they are designed to maintain the cargo at the loaded temperature.
• If the refrigeration unit fails, the sensitive cargo is at risk. Modern refrigeration units can have more than 200 alarm/fault codes, which could threaten the ability to maintain the temperature of the cargo being transported.
• Drivers must be required to set the mode of operation of the unit to either continuous run or start/stop mode. Drivers also need to perform manual defrost of the refrigeration unit when required. To perform these basic configurations, drivers must be familiar with the operation of the controller of the refrigeration unit. This poses a big challenge to carriers in terms of driver training. It also exposes them to risks of driver mistakes when operating the equipment.
• Saving a load when something goes wrong or fails requires real-time data, immediate notification and identification of the cause and prioritization of the issue.
• The temperature controls distribution and logistics industry are moving toward more stringent rules, reporting and record-keeping.
• Carriers of temperature-sensitive goods will need to develop and implement procedures to describe how they will comply with provisions for temperature control and how they will provide this information to both shippers and receivers on demand. Drivers will need to be trained on temperature management, and reporting requirements and temperature records for each shipment will need to be retained.
• Drivers may find it difficult to monitor, analyze and fix temperature problems in compartmentalized vehicles and multi-temperature and multi-zone loads. Instead, control from the dispatch office without involving the driver may be a more effective solution.
For the first time, the prestigious competition recognized the urgent need to develop sustainable transport refrigeration systems to replace the highly polluting diesel-powered Transport Refrigeration Units (TRUs) that dominate the industry today.
Companies are developing innovative cooling refrigeration technologies to reduce greenhouse emissions and local air pollution. With investment in cold logistics booming around the world, it is a timely recognition of the inevitable damage to the environment and human health if the exponential growth in diesel powered transport refrigeration goes unchecked, and of the arrival of a potentially multi-billion market in ‘clean cold’ technologies.
Transport refrigeration requires a high level of system performance in some of the most demanding environments imaginable with reliability, precision temperature control and efficiency. Ten years ago, less than 5% of the refrigerated container users employed scroll compressors. Now over half of the users employ scroll. Truck refrigeration and rail air conditioning have also seen a significant migration to scroll compressors.
• To understand the importance of temperature-controlled transportation in the time-sensitive perishables supply chain. This means moving perishables with the highest efficiency, retaining freshness and natural qualities from the point of produce to warehouses, to distributors and retail customers. Use of specialized temperature-controlled reefer/freezer trucks for refrigerated transport offers a seamless truck transportation chain for perishables and food items.
• Need to produce the transport refrigeration system that’s engineered entirely without compromise utilizing the ground-breaking, liquid nitrogen powered engine, the system is being developed to offer industry leading performance, zero-emission and quiet operations, all without having to compromise on cost.
• Switch to natural hydrocarbon refrigerant HC-290 or HC-1270
• Switch to natural refrigerant R-744
• Leakage reduction (design/maintenance); e.g. use a hermetic compressor in alternator/inverter systems
• Optimize components (e.g. compressor, heat exchanger)
• Use inverter technology coupled with an alternator to improve part-load efficiency
• Use HC refrigerant with favorable thermodynamic properties
• Reduce leakage, as fully charged systems are more effective
• Reduce cooling needs by improving the insulation of the vehicle, optimizing delivery routes and proper handling of goods
• Optimize dimension of refrigeration unit based on size, insulation and use of vehicle
• Use of thermal expansion valves with interchangeable components for ease of service in transport applications.
• Use of long-lasting protection from moisture and contaminants for a trouble-free, leak-free operation for transport refrigeration.
• Electronic control solutions allow for improved energy efficiency, tighter temperature control, remote monitoring and diagnostics for transport applications.
• By installing a remote monitoring modem in the refrigerated container, this will allow for notifications of shipping companies and terminals of deviations outside of optimal ranges so they can take appropriate actions.
The future of transport refrigeration
Transport refrigeration is not a one-size-fits-all market. Rather, it’s one in which environmental challenges and regulatory changes are rapidly driving the development of technologies specially adapted to provide maximum flexibility. To keep the transport industry moving forward efficiently and the food chain developing safely, it is critical that refrigeration component and equipment manufacturers establish close partnerships to develop solutions that help the industry meet its evolving performance objectives while anticipating future demands and challenges.
AUTHORS CREDIT & PHOTOGRAPH
Dr S S Verma
Department of Physics