Answer EKG Question 43

Q43. Sketch and describe the refrigeration system of a container carrying bananas only. How does the controlled atmosphere of the container extend the green life and shelf life of bananas? How is the airflow system designed.

Answer: Recommended transport conditions:

• Desired transit temperature: 56° to 58°F (13° to 14°C)

• Desired relative humidity: 90 to 95 percent

• Highest freezing point: 30.6°F (-0.8°C)

Bananas are shipped green and usually ripened at destination, although some-in-transit ripening is also done. They are very temperature sensitive; lower than desired temperatures will cause chilling injury, and higher than desired temperatures may cause rapid and improper ripening or ‘cooking’. Proper air circulation is required to maintain uniform temperatures throughout the load, since fluctuating temperatures are detrimental. Provide a fresh air vent at minimum to prevent ethylene gas buildup inside the container or trailer or use an ethylene scrubber. Ethylene is produced by bananas and will cause premature ripening. Also, do not ship bananas with other cargo that is not temperature compatible or that produces high amounts of ethylene.

Recommended loading methods:
• Fiberboard cartons — Nearly all bananas are packed in heavy duty, plastic film-lined fiberboard cartons at the country of origin. The gross weight of the cartons is 40 pounds (18 kg). The cartons are usually palletized and secured by glue, straps, or netting. They then are transported under carefully controlled temperature and humidity conditions in refrigerated marine containers or, less commonly, on break-bulk ships.

Since bananas are easily bruised, do not throw or drop the cartons during handling. Place the cartons on their bottoms, and do not invert or stack them on their sides. Use airflow or center loading with spacer blocks for pallets in trailers; block stow individual cartons and use 9-11 loading of pallets in marine containers, being sure to use spacer blocks in units with flat walls. If not palletized, block stow the cartons crosswise or lengthwise and stack tightly together to get a dense load. In extremely cold weather, transfer the cartons onto floor racks or pallets in vehicles without deep T-rail floors so as to prevent freezing or chilling injury.

Transport reefer units are equipped with microprocessors that are interfaced with temperature sensors controlling the supply air and return air temperatures that are located inside the reefer unit (not inside the insulated cargo box). Supply and return air temperatures and other parameters are generally controlled and measured from inside the refrigeration units and not inside the insulated cargo box. These sensors can control the conditioned air as it is delivered to the refrigerated cargo space in the insulated box and returns back to the reefer unit to be re-cooled (or heated) and then re-circulated back to the cargo space. These sensors do not control the micro-environments that develop inside the cargo compartments of insulated boxes.

A Controlled Atmosphere reefer technology is used for sensitive fruits and vegetables, because of the following reasons:
• After harvesting, fruits and vegetables consume oxygen and release carbon dioxide& ethylene gas
• Release of moisture and heat generation provide specific challenges
• Process of respiration results in deterioration of the product
• Fruits and vegetables are still “alive” after harvesting
• In a standard reefer container such products start to degrade and deteriorate immediately
• Fruits and vegetables require an adjusted environment to preserve their condition
• Atmosphere control slows down the ripening process and increases the shelf life of perishable products
The technologies used especially for live cargo represent the following specific advantages to prolong shelf life
• Slow down of ripening process and reduce risk of decay
• Decrease of dehydration (no weight loss)
• Extension of market range from harvest to consumption
• Maintenance of freshness, colour and condition to maintain product quality
• Prevention / minimization of wastage and financial losses
• Ocean transport to become a viable alternative to airfreight
 

For banana or similar live cargo transportation containers with special fittings are being used for example  Liventus and Maxtend fitted containers. They are special supplement for reefer containers. They create a tailor-made atmosphere via pre-trip gas injections. An additional controller is monitoring & recording the atmospheric composition inside the reefer. Such “Controlled Atmosphere” containers are ideal for low respiring cargo.
• Pre-injection gas mixture (N2 and CO2) the system can modify and preserve desired atmosphere.
• Equipped with oxygen (O2) and carbon dioxide (CO2) sensors.
• Equipped with automatic ventilation to let in fresh air, when O2 levels drop below set point.
• Ethylene scrubbers are available for sensitive cargoes.
Air Exchange Management is an automated ventilation system designed to regulate the oxygen and carbon dioxide level inside the container via the fresh air vents. This technology takes advantage of natural respiration processes of the fruit or vegetables and therefore does not require gas injections.

A refrigeration cooling system is basically comprised of a refrigeration compressor, an evaporator, a condenser, an expansion valve, piping, and refrigerant. The refrigeration system operates on the principle of the vapour-compression cycle. This refers to the physical principle that all substances must gain energy (heat) in order to change from solid to liquid and liquid to gas phases, and release energy (heat) when changing phase in the opposite direction. The temperature at which phase changes occur are characteristic of the substance and vary according to pressure – increasing the pressure increases the temperature at which the phase change occurs. Refrigerants are chosen based on their high heat capacity and ability to change phase between liquid and gas at the desired temperature.
Mechanical refrigeration units are rated according to their ability to remove or produce heat. The cooling capacity of a unit is expressed in the number of Btu’s per hour a unit can remove.
Today’s refrigeration units are equipped with microprocessors programmed to control the operation of the unit so that both refrigeration and fuel efficiency are maximized. Air temperatures are monitored at the discharge and return locations and adjusted to demand for refrigeration at the thermostat set-point.
This reduces temperature spread around the thermostat set-point, which reduces dehydration and maintains product quality. The microprocessors also can be programmed to provide diagnostic tests and automatically run through a ‘Pre-Trip’ mode. Some of the microprocessors are radio-equipped and may be contacted via satellite to monitor performance of the refrigeration unit, pinpoint geographic location of the trailer, monitor product temperatures, and perform other functions.

Refrigerated Containers
A refrigerated container, also known as an integral reefer container, is an intermodal shipping container that is used for the transportation of temperature sensitive cargoes. The container’s reefer unit is located inside the cargo compartment Refrigerated containers can be transported over land and sea by trains, trucks, and vessels.
The three main components of a refrigerated container are the refrigeration system, the microprocessor controller, and the air circulation system (chilled or heated air system). A reefer container is a metal box with polyurethane insulation and an attached electric-powered cooling (refrigeration) unit. Reefer containers are equipped with a microprocessor/data logger function that controls the operation of the reefer machinery, documents the pre-trips (system maintenance and diagnostic checks), and records temperatures and various events during transit.
Reefer containers are equipped with a bottom-air delivery system (also known as reverse air flow). The conditioned air is delivered from the reefer unit located at the front of the reefer container, along the floor, through the ‘T’ floor channels, and up vertically through the cargo so that the air is driven around and through the load to maintain product temperatures. This type of airflow system pressurizes the ‘T’ floor with conditioned air when the cargo is properly stowed in the container. The pressurized air is forced a short distance up and through the cargo vertically from the ‘T’ floor to the top of the load and back to the reefer unit. A bulkhead (plenum or false wall) at the front of the container directs the discharge air out at floor level and allows the return air to flow back to the reefer only at ceiling level.
Reefer containers have an adjustable fresh air exchange opening that is set to avoid potentially injurious depletion of oxygen (O2) or build-up of carbon dioxide (CO2) and ethylene (C2H4) gases. The fresh air exchange is usually set manually, but automated systems are also available that are controlled by the refrigeration system microprocessor. Automated fresh air exchange systems are able to keep the fresh air exchange closed as much as possible to improve temperature pull-down and reduce fuel usage, maintain a beneficial atmosphere of O2 and CO2, or avoid infiltration of freezing or chilling outside air that may damage the products being carried.
The reefer unit is self-contained, but it does require a power source. The operation of integral reefer units relies on electrical power sources and connected  to an electrical supply through the container’s electrical plug. The electrical power supply must be either 380 volts/50 hertz or 440 volts/60 hertz and the power cables must have standard ISO plugs.

Transport temperature management
Temperature management plays the most significant role in extending the market life of perishable foods. Bringing the product to its desired carrying temperature as quickly as possible and maintaining uniform temperatures is paramount for domestic and global distribution. The practice of maintaining optimum temperatures throughout the perishables distribution system without any breaks is often referred to as ‘maintaining the cool chain.’ Refrigeration units for containers, trailers, and railcars are equipped with computerized controls and recording capabilities. The computer or microprocessor is sometimes referred to as a Data Management System (DMS) or a data logger. Computerized refrigeration units offer significant benefits to transportation companies, drivers, shippers, and receivers. To wit, the microprocessor controls, tracks, and records operations of the refrigeration system, including, in part, the temperature set point, return air sensing, discharge air sensing, operating modes, safety alarms, and probe sensing. Temperatures can be recorded in either degrees Celsius (°C) or Fahrenheit (°F). The microprocessor also performs pre-trips and diagnostics of the refrigeration unit and records operational events and alarm codes. The refrigeration unit’s computer system offers various levels of guarded access, thereby protecting the refrigeration unit from tampering and unwelcome changes. Trip data can be retrieved (but not erased) from the microprocessor memory.
The DMS also permits the operator to set up pre-determined temperature management conditions including upper and lower critical control limits for perishable foods. Product storage guidelines can be utilized to help set up carrying temperature parameters for perishable and temperature-sensitive products transported using containers, trailers, and rail cars equipped with these refrigeration units.

Air flow Management
In a refrigerated transport unit, a temperature differential exists between the air entering the refrigeration unit, (i.e., the return air), and the air exiting the unit, (i.e., the supply air). This temperature differential continues into the cargo space, creating micro-environments within the cargo and the air surrounding the cargo.
Air circulation is one of the most important factors in protecting refrigerated loads of perishable foods. Air circulation is important to ensure uniform temperatures throughout the load. Refrigeration capabilities are meaningless if the refrigerated air is not properly circulated to maintain product temperature. Air circulation carries product heat and the heat which penetrates the walls, floors, and ceiling of the trailer to the refrigeration unit where it can be removed. Heated air may be circulated to keep fresh produce from incurring chilling or freezing injury.
There are two major methods of circulating air in refrigerated vehicles. Top air delivery is the conventional method for refrigerated truck and rail cars. The second method is bottom-air delivery, which has been employed extensively in seagoing van containers for several decades, but only to a limited extent in highway trailers. Ribbed (fluted) sidewalls or spacers at least 1 inch (3 cm) thick to allow top airflow down the sides of the load. This reduces the amount of heat conducted across the walls to or from the product. Up to 20% of the top airflow should bleed off down the sidewalls. Vertical channels or fluted rear doors and sidewalls facilitate air flow and prevent the blocking of air circulation between the load and the rear doors and sidewalls