Reefer journal: Container Refrigeration Temperature Recording Systems (Part 1)

By C. Maheshwar
Faculty, Training Ship Chanakya
Navi Mumbai

C.Maheshwar is a marine engineer, from Marine Engineering College, Kolkata, (DMET) 1980. He has sailed on board foreign - going ships of Shipping Corporation of India Ltd., from 1980 to 1997, the last 5 years of which were as chief engineer officer. Ashore, he has worked from 1997 to 1999 with the Taj Group of Hotels as chief engineer of Taj Connemara Hotel, Chennai, and as customer service manager, Reefer Container Group of Carrier Transicold for the region of South Asia from 1999 to 2001. Currently, he is working as engineering faculty at TS Chanakya, Navi Mumbai, a Merchant Navy Training Institute, belonging to the Govt. of India and affliated to University of Mumbai and IGNOU. He is a consultant for Anglo-Eastern Maritime Training Centre and conducts training programmes on reefer containers for seagoing engineers on a regular basis. He can be contacted at cmaheshwar@hotmail.com

This is the first of a series of articles on container refrigeration. These articles are intended to familiarize the reader with the basics of containerization, and will lead the reader through the complexities of the design and usage of refrigeration equipment when fitted on containers. This article in particular will deal with an introduction to containerization as a concept, container refrigeration, its evolution and commercial aspects. Subsequent articles will deal with various other aspects of interest to HVACR engineers.

Refrigerated containers will be henceforth referred to as reefer containers. It is to be understood that merely reading of these articles will not make the reader an expert or an authority on reefer containers and practical knowledge is essential. When working on any particular make or model of equipment, the appropriate Instruction Manual is to be consulted and instructions to be followed.

It is assumed that the reader is already familiar with the basic concepts of refrigeration.
Introduction to Containerization

Containerization dates back to the early 1950s. As the worldwide shipping trade grew by leaps and bounds, conventional break bulk cargo has been slowly replaced by containers. Commercial pressures and competition forced companies to reduce their cost of operations. This had a direct impact on the advanced countries where the cost of labour was more than the cost of technology. The cargo loading and unloading labourers get paid in dollars on an hourly basis with extra incentive in the form of overtime for working on weekends and holidays. There was an urgent need to bring down the labour component involved in cargo operations.


Additionally, cargo pilferage and losses were not accepted and tolerated any more. Shipping lines had to discharge the exact amount of cargo at the discharge port that was loaded on the ship at the loading port. The allowance due to loss/pilferage of cargo was drastically reduced. Further, the same conditions applied for the cargo during its onward journey from the discharge port to its final destination.

Unitization of cargo became an important concept for bringing down the labour costs. Pre-slinging of cargoes, palletization, containerization, barge-carrying ships, Ro-Ro ships are all examples of the unitization concept. Containerization was the need of the hour. Containerization became the most effective and efficient method of cargo handling. With containerization, per capita labour output improved dramatically.

A container is a box of internationally accepted standard dimensions. The standard sizes recommended by ISO (International Standards Organization) are 20ft (length) × 8 ft (width) × 8ft (height) and 40ft (length) × 8ft (width) × 8ft (height). The former was known as a TEU (Twenty Foot Equivalent Unit) and the latter was referred to as FEU (Forty Feet Equivalent Unit). One FEU is equal to two TEUs. Generally the capacity of a container ship is referred to in terms of TEUs. In addition to the above two sizes, the height may be also 8' 6." The length may also be 45 ft.

Material of Construction: The sides, roof and floor of a container may be made up of Steel, Stainless Steel, Aluminium, Fiberglass, Plywood, etc., depending upon the application. The structure is generally made of steel.

Features of a Container

1. It is of a permanent character, strong enough for repeated use.
2. It is designed to facilitate the transport of goods from one mode to another, i.e., from road to sea or rail to sea without intermediate reloading.
3. It is designed for easy stuffing and destuffing.
4. It is fitted with facilities to permit easy handling and transferring from one mode of transport to another.

Advantages of Container Transportation

1. Faster and reliable delivery
2. Greater protection of fragile and easily contaminated cargoes
3. Ensures original quality of goods
4. Reduces pilferage
5. Enables Physical seperation of dirty cargoes
6. Simplification of documentary procedures
7. Reduction in cost of cargo handling and ships' stay in ports.
8. Reduction in packing cost to the shipper.

A container has a strengthened deck (floor) and corners. The floor is lined with wood. The sides are corrugated to provide strength as the sides and roof of a container are highly susceptible to damage. The main strength of the container lies in the frame. The ICSC (International Convention for Safe Containers) specifies structural requirements for containers and contains regulations for inspection, approval and maintenance of containers. Most containers are built with fork lift pockets.




Types of Containers

1. Closed box or general purpose containers most commonly used for various types of cargoes.
2. Open sided containers which can be loaded from either side forward or aft and have hatch covers that drop down on either side.
3. Bulktainers (Dry bulk containers) have loading hatches in the roof and one or more discharge hatches
4. on the sides. Tanktainers (Tank Containers) are used for carrying bulk liquids.
5. Half height containers are normally used to carry high density cargo.
6. Reefer containers are used to carry refrigerated cargo and are equipped with refrigeration machinery.
7. Other special types are Pen Containers for livestock, Tiltable Containers for grain, Open Top Containers, Collapsible Containers, End Open Containers for carrying long cargo, Fantainers which are equipped with fans or blowers to blow air through the cargo, Hangtainers, which have hangers used for carrying garments and so on.

Refrigerated Containers

A refrigerated container (reefer container) is a container in which temperature can be maintained within certain limits that correspond to the storage conditions required for certain types of cargoes. They can be of two types – the Individual System in which the container is equipped with its own individual refrigerating machinery and the Collective System, in which a separate refrigerating source handles the distribution of cooling to a group of containers which is generally a permanently fitted refrigeration equipment on board the ship or a portable unit which is fitted on a temporary basis on the ship for that voyage or a charter connecting to a group of cargo carrying containers.

Question: By looking from outside, how can we make out if a container is a refrigerated container?

A. Yes, we can identify a reefer container from its external appearance, At the front end of the container, look for the presence of the refrigeration machinery. Also, the sides of a reefer container are smooth due to the external aluminium sheeting, where as, a normal GP (General Purpose) container will have corrugated sides made up of steel sheeting. The corrugations provide additional strength and stiffening to the container. A refrigerated container has external aluminium sheeting and internal stainless steel sheeting. There are intermediate strengthening vertical members spaced evenly apart. The space between the internal and external sheets is filled with insulating material, generally poly urethene foam (PUF) which is injected between the two layers at high pressure displacing air through vent holes at the top.

Question: Does the size of the container increase because of the additional refrigeration equipment?

A. No, the refrigeration equipment is fitted within the overall standard dimensions of the container, i.e., 40 × 8 × 8 ft or 20 × 8 × 8 ft. Some of the cargo carrying space is utilized to accommodate the refrigeration machinery. This is known as the picture frame type reefer container. Most of the marine reefer containers are of this type. This is because of the availability of limited space within the ship and the need to carry maximum number of containers requiring optimization of space. The other type of reefer containers are Over Hanging type in which the refrigeration equipment projects beyond the overall standard dimensions of the container. These are generally used ashore for over land transportation of the containers by trailer or rail.

Question: What about the power supply for the refrigeration machinery?

A. Generally power supply is taken from the ship's main power supply when the container is on the ship. While in port, the shore power supply is utilized to run the refrigeration machinery. When transporting across land in a truck, trailer or a train, power supply is generally taken through a genset from the main prime mover of the truck or trailer or main power supply of the freight carrying train. In some instances, an additional genset is permanently installed between the container trailer chassis and the driver's cabin. Some of the containers are installed with clip-on gensets which are dedicated to supply power only to the refrigeration machinery. These clip-on gensets can be removed when not required. Some of the older containers had gensets permanently fitted within the picture frame of the container. The genset with its own fuel tank occupied the lower half of one end of the container and the refrigeration machinery occupied the upper half.

Question: What about the extra power required for the reefer containers from the ship's main power supply?

A. Yes, the ship's main power supply should be able to cater to the extra load due to running of the reefer container machinery. Generally the ship's power supply will be designed after taking into consideration the maximum number of reefer containers it can carry at a time. However, if required, an additional generator is installed on board the ship exclusively to cater for the extra load due to running of the reefer container machinery.

Question: What about the extra deadweight, loss of volumetric space and the extra maintenance required for to the reefer container machinery?

A. Yes, the extra deadweight due to the reefer machinery results in a loss of freight for the ship. Similarly, a reefer container requires a lot of inspection and maintenance during the voyage which is a downtime equivalent of 1% per day. Similarly, there is a considerable loss of volume of cargo space which is required not only for the location and efficient functioning of the refrigeration machinery, but also for sufficient access space around the machinery for regular inspection and repairs if required. But all these factors are compensated more than adequately as the freight for reefer containers is much higher than a normal container. In fact, any shipping line will run after a reefer cargo and will be willing to carry out necessary modifications in the ship and providing additional machinery for power generation and other fixtures.

Question: What about the heat given out from the condenser of the refrigeration equipment?

A. Yes, a lot of heat is given out by the condensers, in fact, whatever heat has been removed from the inside of the container through the evaporator is given out by the condenser. When the reefer containers are stored on the deck in the open air, the heat from the condensers is discharged into the atmosphere. They use air-cooled condensers.

Question: Does it mean that they can store loaded reefer containers only on the deck?

A. No, not necessarily. Reefer containers can also be stored inside the cargo holds. But the heat given out by the condensers of all the running reefer containers inside the cargo hold should be led outside efficiently. Otherwise, there will be heat accumulation inside the cargo hold and the refrigeration machinery will not function efficiently. That means the ship's cargo hold ventilation system should be so designed to allow the required number of air changes so as to maintain the temperature inside the cargo hold within the permissible limits. Alternatively, they may use water-cooled condensers. In such a case, the cargo hold should be equipped with water circulating system with pipelines running along the sides of the cargo hold which can be connected to the individual reefer containers through a pair of flexible pipes – one for the inlet and the other for the outlet. Often, such containers are marked "IN HATCH STOWAGE ALLOWED". By looking at a reefer container, by noticing the water-cooled condenser inlet and outlet water pipe connection couplings, we should be able to determine its suitability for storage inside the cargo hatch. Generally, all reefer containers are equipped with air-cooled condensers, water-cooled condenser, if fitted, is an additional optional feature. It implies, that all reefer containers are by default designed to run as air cooled units. However, some of them are also designed optionally to run as water-cooled units.

Question: Can a reefer unit run as air-cooled and water-cooled unit at the same time?

A. No, to improve the heat transfer across the condenser coil, all reefer units are provided with a condenser fan. When fitted with an optional water-cooled condenser, and running as a water-cooled unit, the condenser fan cuts off when sufficient water pressure is available in the water pipelines of the water-cooled condenser – a pressure switch switches off the condenser fan and switches it on at low water pressures. In older units, there was a manual toggle switch which could be operated if water-cooled condenser was connected and was in operation.

Question: Does it have any bearing on the loading of the containers on the ship?

A. Yes, it should be noted that no two reefer containers should be loaded with the machinery facing each other. The hot air discharge from each of the condensers will enter the condenser of the other unit, affecting the performance of both the machineries adversely. Also, even when two reefer containers are loaded with their machinery not facing each other, enough space should be left near the condenser of each unit so as to facilitate free flow of air to and from the condenser, otherwise the performance of the machinery will be affected adversely.

Evolution of Container Refrigeration

Refrigerated containers have been employed on ships since more than 40 years. The earlier versions had only the basic components and a very narrow range of temperature applications. Setpoint could not be changed at will and even if it could be changed, the arrangement was very crude using a potentiometer arrangement. There was little scope for changing the defrost interval. There were no low voltage components resulting in lot of heat generation and power consumption. There were very few alarms and safeties, thus safety of the cargo, machinery and personnel was not ensured. There was no fool-proof and tamper-proof temperature recording mechanism, the only one which was available was a mechanically moving paper chart powered by a hand wound clock mechanism. There were frequent breakdowns caused by mechanical failures of components. Instances of cargo damage were high resulting in huge cargo insurance claims. Reefer container machinery has come a long way over the last 40 years and has evolved brilliantly. Each of the components has undergone a metamorphosis. Present systems are power efficient with foolproof and tamperproof recording, back ups for all important components, advanced warning systems in the form of alarms and cutouts, increased reliability of components resulting in enhanced safety of cargo, machinery and personnel. Now, with Remote Monitoring Modems and Radio Frequency Identification Systems, it is possible to monitor the performance of each reefer container from a shore office continuously and perform necessary adjustments
Commercial Aspects of Reefer Containers

Today, a 20 feet Dry (General Purpose) container box costs about US$ 2000. A reefer container consists of two distinct components – the refrigeration machinery and the box. The refrigeration machinery with standard features costs about US$ 10,000. A 20 ft. reefer box costs about US$ 5,000. The major reefer machinery manufacturers are Carrier, Thermo King, Daikin, Mitsubishi etc. The major reefer box manufacturers are Freuhauf, CIMC, Moon, GE, Transafe, Balmer Lawrie, etc. When a reefer container has to be bought, the orders are placed separately on the box manufacturer and the machinery manufacturer. The machinery manufacturer delivers the machinery at the location where the box is manufactured. The machinery is fitted on to the box , tested and the unit commissioned and delivered to the customer's representative by the machinery manufacturer's representative in the presence of the box manufacturer.
References

1. Cargo Work for Ship's Officers by Capt. Errol Fernandes.
2. Trends in World Seaborne Trade and Shipping-Present and Future by Dr. K V Hariharan.
3. Container Manual by Shipping Corporation of India
4. Marine Refrigeration Manual by Capt. AWC Alders.
5. www.carrier.transicold.com
6. www.thermoking.com
7. www.mhi.com
8. www.daikin.com

Reefer journal: Container Refrigeration Temperature Recording Systems (Part 2)

By C. Maheshwar
Faculty, Training Ship Chanakya
Navi Mumbai
Technical Advisor

C.Maheshwar is a marine engineer, from Marine Engineering College, Kolkata, (DMET) 1980. He has sailed on board foreign - going ships of Shipping Corporation of India Ltd., from 1980 to 1997, the last 5 years of which were as chief engineer officer. Ashore, he has worked from 1997 to 1999 with the Taj Group of Hotels as chief engineer of Taj Connemara Hotel, Chennai, and as customer service manager, Reefer Container Group of Carrier Transicold for the region of South Asia from 1999 to 2001.

Currently, he is working as engineering faculty at TS Chanakya, Navi Mumbai, a Merchant Navy Training Institute, belonging to the Govt. of India and affliated to University of Mumbai and IGNOU. He is a consultant for Anglo-Eastern Maritime Training Centre and conducts training programmes on reefer containers for seagoing engineers on a regular basis. He can be contacted at cmaheshwar@hotmail.com

In this issue, we will examine the various types of refrigerated cargo and the implications of the different types on the performance and operation of container refrigeration machinery.

Question : What are the various types of cargo that can be carried in a refrigerated container?

A. Refrigerated cargo carried in reefer containers can be classified as food items and non-food items. Examples of food items are fruits, vegetables, meat, fish, beverages, dairy products, ice cream, etc. Examples of non-food items are chemicals, explosives, leather, photo films, medicines, vaccines etc.

More importantly, refrigerated cargoes can be classified as chilled cargoes and frozen cargoes.

Chilled cargoes are those cargoes which are stored above –10°C. They are live cargoes with chemical reactions and processes going on within the product due to respiration, with continuous liberation of gases and heat. Chilled cargoes are also known as “perishable cargoes” in refrigeration parlance.


Frozen cargoes are those cargoes which are stored below –10°C. They are dead cargo, with no chemical processes or reactions taking place within the product and no liberation of gases or heat.

Chilled cargoes are temperature sensitive cargoes. The heat generated by the chemical reactions has to be led outside the cargo space faster than it is liberated to prevent any accumulation of heat and rise of temperature. Similarly, gases which are liberated as products of the chemical reactions have also to be drawn out of the cargo space, if found unsuitable for the cargo. Otherwise the cargo might deteriorate and get damaged. Chilled cargoes are generally fruits and vegetables, dairy products, chilled meat, beverages etc.



The temperature of chilled cargo needs to be maintained within a very narrow band of ± 0.5°C around the set point. Variation of temperature beyond this may enhance chance of cargo deterioration.

Myth: For all refrigerated cargoes, the lower the temperature, the better it is for the preservation of cargo.

Reality: For chilled cargoes, temperature must be maintained very close to the set point. If the temperature is allowed to fall lower than the set point, cargo damage may occur because of overcooling of cargo. Frozen cargoes do not get damaged even if temperature goes below the set point for prolonged periods of time. Instead of –20°C, if we maintain the temperature of ice cream at –25°C, it does not cause any damage or deterioration to the cargo. For frozen cargo, as there is no evolution of heat from the cargo, it is much easier to maintain the temperature.

Whenever there is a power breakdown or a planned and purposeful shutdown, it must be remembered that chilled cargoes are more vulnerable for damage and deterioration due to heat generation and accumulation inside the container resulting in temperature rise. Frozen cargoes do not get damaged because the temperature rise is very slow as there is not heat generation from the cargo. So, whenever possible, all chilled cargo reefer containers should be clustered together and connected to one power source or circuit breaker. Power should be restored as soon as possible for chilled cargo containers. Power to the frozen cargo containers can be restored later after all the chilled cargo containers have been activated.


Image 2

Question : Does the reefer machinery perform in the same way for both the chilled and frozen types of cargoes?

A. Obviously not. The refrigeration machinery behaves differently for chilled and frozen types of cargo.

Chilled Cargoes. When the set point temperature desired for the cargo is set to above –10°C, and the machinery connected and started, modulated cooling takes place, i.e., as the temperature inside the cargo chamber gets closer to the set point, the amount of refrigerant passing through the refrigeration circuit gets reduced, the additional refrigerant which is not under circulation gets stored in the receiver. When the temperature inside the cargo space reaches 0.2°C below the set point, the cooling is stopped. This is achieved by stopping the condensor fan and the compressor with a signal from the controller when a difference of 0.2°C is sensed between the set point and the actual temperature measured inside the container. This is a proactive step by the controller to prevent the temperature from falling far too much below the set point as chilled cargo will get damaged at temperatures much below the set point. The evaporator fans continue to run without the cooling circuit, circulating the same uncooled air and help in stabilizing the temperature closer to the set point due to heat generation from the cargo.

In case, the temperature of the cargo continues to fall further because of the momentum of the cooling cycle and the time taken for the temperature to stabilize along the entire length of the container, when the temperature reaches 0.5°C below the set point, the controller decides that the temperature must be prevented from falling any further, as a proactive step it activates the electric heaters. This is done by closing a heater relay inside the controller circuit, which energises a heater contactor and allows high voltage to flow through the heater coils. Since the evaporator fans are already running, hot air will be circulated through the cargo space inside the container.

As the temperature inside the container starts rising, when the temperature reaches 0.2°C below the set point, the heaters are stopped and only the evaporator fans are kept running. As the temperature continues to rise further, due to the heat generation from the cargo, once the temperature inside the container reaches 0.2°C above the set point, the cooling circuit gets activated through closing of the appropriate relays in the controller circuit, and the compressor and the condensor fan start running. The idea is to initiate proactive steps to keep the temperature of the cargo inside the container as close to the set point as possible, 0.2°C and 0.5°C being the limiting values.

Frozen cargoes. When the set point temperature desired for the cargo is set to below –10°C, and the machinery connected to power and started, nonmodulated cooling of the space takes place with the refrigerant flow kept fully (100%) in circulation through the refrigerant circuit. When the temperature reaches 0.2°C below the setpoint, as in the case of chilled cargoes, the cooling circuit gets switched off with the signal from the controller stopping the compressor and condenser fan. Only the evaporator fans continue to run. The evaporator fans continue to run until the temperature starts rising and reaches 0.2°C above the set point, when the cooling circuit gets activated again. In frozen mode, the heater coils do not get energized at all. The logic for this is that even if it takes longer time for the temperature to come closer to the set point or even if the cargo temperature falls below the set point for a prolonged period of time, cargo does not get damaged. So, the heater circuit need not be energized, thus achieving a saving in power.



Frozen cargoes run on non-modulated cooling so that the temperature can be brought down to the set point as soon as possible and saving power by cutting off the cooling circuit. It does not damage the cargo even if the temperature is lower than the set point for prolonged periods of time.

Evaporator Fans

Generally, there are two evaporator fans for a reefer container. Both of them are dual speed fans. Both of them run either at low speed or at high speed. (It is not possible to run one of the evaporator fans at low speed and the other at high speed). The running of the evaporator fans is controlled by two contactors with each contactor allowing power to be sent to both the fans either to low speed coils or to the high speed coils of both the evaporator fan motors.

Question : Is this cut-off temperature of –10°C between chilled cargoes and frozen cargoes sacrosanct? There may be some cargoes which are stored at more than –10°C and yet behave like frozen cargoes. How does the equipment treat and handle such cargoes – chilled or frozen?

A. Yes, there may be such cargoes which may be stored at more than –10°C, yet they may be frozen cargoes. The distinction point of –10°C is set as default by most of the manufacturers of reefer container machinery to control the operating logic of the various components. However, there is an option provided to change this setting from –10°C to –5°C. That means certain cargoes which are stored above –10°C, but below –5°C can be treated as frozen cargoes and the machinery will apply the logic of frozen cargoes.

Question : Why are two speeds required for the evaporator fans and when are the two different speeds used?

A. When carrying chilled cargo, both the evaporator fans run at high speed and when carrying frozen cargo, they run at slow speed.

Since chilled cargoes are temperature sensitive cargoes, as we need to maintain the temperatures within a very narrow range of ± 0.5°C, and as they are live cargo generating heat continuously, unless the heat generated is drawn away from the cargo space faster than it is generated, there will be heat accumulation and consequent cargo damage. To facilitate faster removal of the evolved heat, evaporator fans run at a higher speed.

Since frozen cargoes do not generate heat being dead cargoes, and cargo does not get damaged or deteriorated in spite of storage at temperatures much below the set point for prolonged periods of time, evaporator fans are run at lower speed, saving some power in the bargain.

Question : Where is the temperature measured inside the cargo space? Is there any difference in measuring points for the two different types of cargo?

A. The temperature sensors are located before the evaporator fans and are called “return air (temperature) sensors” and the ones after the evaporator coil are known as “supply air (temperature) sensors”. Usually, with a normal running container machinery, there exists a temperature difference of 2-3°C between the supply temperature and the return temperature, the supply being always cooler than the return as it is after the cooling effect of the evaporator. The circulation of the cold air from the evaporator coil is through the bottom of the container. The hot air from the cargo being lighter tends to rise to the top of the container. The suction to the evaporator fans is from the top. The evaporator fans force the air downwards through the evaporator coil, cooling the air and the cold air and passes eventually through the bottom of the container. To facilitate the flow of cold air across the entire length of 20 ft. or 40 ft. of the container, the evaporator fans are designed to provide sufficient air throw. Further facilitation of the same is provided by the T bars in the floor. Cold air passes through the spaces between the T bars and travels right till the end of the container. Cargo is placed on top of the T bar floor and even spacing of the cargo allows cold air to pass through the cargo and cool the cargo. It is important to keep the spaces between the T bars clear, otherwise there will be obstruction to the flow of cold air. Dunnage, paper, plastic and other packing material may block the passage between the T bars.


When carrying chilled cargo, the supply air temperature sensor acts as a controlling probe and when carrying frozen cargo, the return air temperature sensor acts as a controlling probe. This implies that in chilled mode, the temperature at the supply sensor acts as the controlling point to initiate cooling circuit, heating circuit or to run only the evaporator fans. Since at no point of time, any part of chilled cargo should be exposed to cold air at a temperature lower than the set point, the temperature at the supply side is the limiting factor. For frozen cargo, return temperature sensor is the controlling probe because the temperature of the cargo can be brought down below the set point without causing damage to the cargo. Hence temperature of the cargo will be brought to the set point and maintained with lesser running of the cooling circuit achieving significant saving in power.

Question : What about the packages of chilled and frozen cargoes? Can they both be both identical?

A. No. For chilled cargoes, each piece of the cargo (fruit or vegetable) has to be cooled by exposing it to the cooled air. Each piece of the cargo should be wrapped in material which can allow cold air to pass through. Generally, each piece of cargo is carried in plastic net or thin porous paper. Chilled cargo pieces should not be wrapped in completely enclosed plastic wrapper as plastic is an insulator. A totally sealed plastic wrapper does not allow the cargo piece to breathe. After a short passage of time, there will be a deficiency of oxygen and an excess of carbon-dioxide and accumulation of heat and gases like ethylene being liberated from the cargo because of ripening. If plastic is used as a wrapping material, it must have sufficient holes to allow the cold air to touch the cargo piece, and to allow breathing of each individual piece of cargo.

The cardboard or wooden boxes in which each individual piece of cargo is packed should not be air tight and a fully sealed container. They should have sufficient air holes all around the sides to allow cold air to pass through and cool each individual piece of cargo.

The same logic holds good for frozen cargo. The cold air should be allowed to touch the individual pieces of cargo. However, since frozen cargo does not generate heat nor gases as it is not a live cargo, the individual pieces of cargo may be packed in fully sealed air tight wooden or cardboard boxes.


Question : What are ethylene sensitive cargoes?

A. Ethylene is the sweet smelling gas which is evolved when fruits ripen. It is one of the products of chemical reactions taking place inside a fruit when fruits ripen. Other sources of ethylene are combustion of fuel in automobile engines, fluorescent light ballasts, and some fungi and bacteria. Though the quantity of ethylene generated is small (in ppm), it can affect the fruits to a considerable extent depending upon the type of fruit, and the amount and duration of exposure to the fruit. If not desired, ethylene produced while ripening of the fruit has to be drawn out from the atmosphere by using ethylene scrubbers which contain potassium permanganate which has the property of absorbing ethylene and converting it into harmless and ineffective products.

Depending upon their sensitivity to ethylene, fruits can be classified into climacteric or non-climacteric fruits.


Climacteric fruits are excessively sensitive to ethylene. They form an irreversible reaction which accelerates ripening uncontrollably. Once these fruits are exposed to even a small amount of ethylene, the ripening process cannot be retarded or controlled and they ripen at an accelerating pace. Such fruits must be consumed within a few days, otherwise they will become over-ripe and lose quality. Some fruits like tomatoes, bananas and honeydews are deliberately exposed to ethylene to ensure rapid and uniform cooling. On the other hand, for some fruits like kiwifruit and Bartlett pears, ethylene must be rigorously excluded, otherwise, they will become overripe and unusable.

Examples: apple, apricot, avocado, banana, blueberry, breadfruit, cherimoya, feijoa, fig, guava, jackfruit, kiwifruit, mango, muskmelon, nectarine, papaya, passion fruit, peach, pear, parsimmon, plantain, plum, sapote, soursop, tomato, watermelon etc.

Non-climacteric fruits are those fruits whose ripening process can be accelerated or retarded by exposing them to measured amounts of ethylene. By removing the ethylene at any stage of ripening, the process can be slowed down. The ripening process is not irreversible.

Examples: blackberry, cacao, cashew apple, cherry, cucumber, eggplant, grape, grapefruit, jujube, lemon, lime, loquat, lyches, olive, orange, pepper, pineapple, pomegranate, raspberry, satsuma mandarin, strawberry, summer squash, tamarillo, tangerine etc.

Examples: Some varieties of apples, apricots, avocados, asparagus, cantaloupes, snap beans, cherimoya, cranberry, cucumber, eggplant, flowering potted plants, foliage plants, grapefruit, kiwifruit, lemon, limes, olives, papaya, bell peppers, pineapple, tomatoes etc.

Download here (copy & paste address)
http://www.hamburgsud.com/group/media/sharedmedia/dokumente/brochures/Reefer_guide.pdf

This list is not exhaustive. There are many more products which are not covered in the above list. The shipper's guidelines must be followed for all refrigerated cargoes for the correct storage temperature.

Reefer journal: Container Refrigeration Temperature Recording Systems (Part 3)

Issue : April-June 2005








By C. Maheshwar
Faculty, Training Ship Chanakya
Navi Mumbai

C.Maheshwar is a marine engineer, from Marine Engineering College, Kolkata, (DMET) 1980. He has sailed on board foreign - going ships of Shipping Corporation of India Ltd., from 1980 to 1997, the last 5 years of which were as chief engineer officer. Ashore, he has worked from 1997 to 1999 with the Taj Group of Hotels as chief engineer of Taj Connemara Hotel, Chennai, and as customer service manager, Reefer Container Group of Carrier Transicold for the region of South Asia from 1999 to 2001. Currently, he is working as engineering faculty at TS Chanakya, Navi Mumbai, a Merchant Navy Training Institute, belonging to the Govt. of India and affliated to University of Mumbai and IGNOU. He is a consultant for Anglo-Eastern Maritime Training Centre and conducts training programmes on reefer containers for seagoing engineers on a regular basis. He can be contacted at
cmaheshwar@hotmail.com

In this issue, we examine the evolution of CA as a system, how it is different from Modified Atmosphere systems, various forms of Controlled Atmosphere systems available in the current reefer market and the various additional components involved.
Basic Concepts

When a fruit or vegetable is harvested from the tree, it is still living. Chemical reactions are going one within it, with liberation of gases and heat. In other words, it is still respiring. This respiration process contributes to aging or senescence of the fruit.








Respiration of fruit :
C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + Heat



If we are able to reduce the respiration rate, we can reduce the speed of chemical reactions going on within the fruit and can delay the aging of the fruit.


The idea of CA is to ensure conservation of perishables not only by refrigeration but also by changing the gas concentration of the air inside the refrigerated chamber. For the purpose of conservation the oxygen level is reduced to approx. 2-3% (normal air 21%) and the carbon dioxide level is increased to 5-15 % (normal air 0.03 %). The gas mixtures that provide the best conservation depend on the crop but may vary with variety, origin and harvest date. The most common fruits that are stored under CA on shore are apples and pears. It is to be remembered that CA is an additional feature provided to supplement refrigeration.


By changing the atmosphere in the correct way, the respiration of the fruit is reduced and preservation is better. The transit times can therefore be longer, so that fruit, normally transported by air, can be shipped by sea or truck. Another possibility is to harvest riper fruit in order to transport it in better quality than earlier.

The rate of respiration falls rapidly below 9% of Oxygen content in the atmosphere. At 2% Oxygen level, the rate of respiration is as low as 25%. Most of the products require Oxygen content close to 2% for better preservation in addition to refrigeration.

Atmosphere changes must however be made with care as oxygen levels that are too low, as well as carbon dioxide levels that are too high, can result in damage to the fruit. Ethylene, which is a gas that is produced by many fruits, but which also accelerates the ripening process, also has to be removed from the atmosphere (green bananas for example are treated with ethylene in the ripening stores to get them ready for sale).
What is Controlled Atmosphere? How is it Different from Modified Atmosphere?

* Controlled Atmosphere
– The control of the chemical composition of the atmosphere surrounding a commodity by the continual addition or removal of gases.
* Modified Atmosphere
– The one time surrounding of a commodity with a mixture of calibrated gases prior to shipment.

Applications of Controlled Atmosphere

For several years now, Controlled Atmosphere (CA) has become more and more important in the refrigerated transportation of perishables. While in the beginning CA was almost unknown, it has now become a quite common addition to normal refrigeration in several fruit trades. In the reefer vessel market CA is mostly used by banana companies like Chiquita and Dole for the transport of bananas and other fruits from Latin America to Europe. Apples from New Zealand to Europe and stone fruit (e. g. peaches, nectarines) from Chile to the US are also shipped in reefer containers under CA. Each product has its own requirement of Oxygen and Carbondi- oxide percentages ranging from 1 to 10% with varying degree of potential benefit, ranging from slight to very good.


Various Makes & Models of CA Systems

The first applications of CA on shore simply used the respiration of the fruit to lower the oxygen level and increase the carbon dioxide level. The fruit was placed in the gastight store, and the atmosphere changed by itself. With a normal CA store for apples it took several weeks to bring down the oxygen level to the desired 2%. To limit the carbon dioxide level, lime was simply placed inside the CA room. To increase the oxygen level if necessary, fresh air was supplied to the room.

Nowadays the pull-down of the atmosphere is usually achieved by flushing the room with nitrogen (normal air 78 % N2). The nitrogen is either separated from ambient air (Figure 1) or is delivered in tanks or trucks for an initial filling. Nitrogen separators can be of the membrane type or Pressure-Swing-Adsorption (PSA) systems. In the case of membrane separators, a membrane which has different permeability for oxygen and nitrogen is used to separate nitrogen from compressed air. In PSA systems, activated carbon is used to adsorb oxygen at high pressure and is reactivated at low pressure. Two adsorbers are combined with one adsorbing and the other reactivating in exchange.




For the limitation of carbon dioxide levels two different principles are used:

* flushing with nitrogen and
* carbon dioxide scrubbers.

The flushing system is normally used when a nitrogen separator is present. Due to the fact that the nitrogen produced by these separators still has oxygen content of approximately 2–3 %, the oxygen level cannot be reduced significantly lower than this level when carbon dioxide is flushed out of the room (Figure 1).

Carbon dioxide scrubbers are mainly PSA systems, but lime scrubbers are also used. When carbon dioxide scrubbers are used, attention must be paid to the ethylene level, which might increase in this case. Ethylene is not a problem for flushing systems, as both the ethylene and the carbon dioxide are flushed out of the room. The disadvantages of nitrogen flushing are the high-energy consumption of nitrogen separators and the fact that the capacity of the separator has to be designed for carbon dioxide limitation, where often more nitrogen is necessary than for atmosphere pull-down.

Some manufacturers offer ethylene scrubbers on a PSA base. Catalytic ethylene burners are also available.

The CA room has to be as gastight as possible in order to prevent ambient air (oxygen) from entering. Due to the injection of nitrogen for atmosphere pull-down or carbon dioxide flushing there has to be a pressure relief valve, otherwise the room would be blown up and destroyed.
TransFRESH

California based TransFRESH Corporation has been the biggest provider of CA services for some years. The Tectrol CA system came into the market at the end of 1990. Tectrol CA is used primarily with avocados and deciduous fruit. Other cargoes include asparagus, mangoes, melons and mixed vegetable shipments.

The Tectrol system is not able to build up its own atmosphere. An initial gas mixture is injected into the container after loading. The Tectrol system is designed to maintain this atmosphere by supplying fresh air and carbon dioxide scrubbing. This requires containers which are extremely gas tight with minimal leakage.


The main advantage of the Tectrol system is the low investment cost. Only an investment of about US $ 700 is necessary to fit a container with the basic installations. These include:

* Two port assemblies which are built into opposite ends of the container sidewalls. These plugs are temporarily removed to allow injection of the initial specified atmosphere.
* An extruded aluminium door track is built into the container door frame. In case of a transport, a large plastic curtain seals the rear container door opening. The curtain is snap-sealed into the track with a stiff plastic ribbon.
* Casing and wiring for the controller. The controller records atmosphere and other performance information during transit. It is only mounted to the container in case of a CA transport. Once the container has reached its destination the controller is removed and returned to TransFRESH for data retrieval.
* Facilities for fitting a lime based carbon dioxide scrubber

The disadvantage of the Tectrol system is that it is based on fruit respiration and therefore is not a standalone system. A TransFRESH station is needed at the loading port to supply the container with the necessary equipment and the initial atmosphere. On the other hand, this has turned out to be an advantage, as the containers as well as the cargo are inspected carefully before shipment commences. This results in a very good quality of transported goods and very few losses.
Freshtainer

Freshtainer, which is a part of the Austrian Welz group offers one of the more sophisticated CA systems that was available till recently: the INTAC IV. The system features a PSA nitrogen seperator, a carbon-di-oxide scrubber and an ethylene scrubber. Humidification and dehumidification are also available. Now, Freshtainer does not operate the INTAC IV containers itself anymore, but gives them on lease to large customers. Before introducing the INTAC IV system, the company already had an older INTAC III system in operation, which was based on carrying nitrogen and carbon dioxide in cylinders. Since the beginning of 1995 Freshtainer and Sabroe Reefer Cool have started to collaborate. The original Freshtainer's INTAC IV system has been redesigned to fit into a Sabroe TNE 508 refrigeration unit (formerly Klinge).

There are three options available:

* The basic option is designed for oxygen control and moderate carbon dioxide control by using a PSA nitrogen separator. The humidity level can also be modified. The unit is placed beside the reefer unit, so that no cargo space is lost.
* Option two offers the supply of carbon dioxide from two or four cylinders, which are placed just opposite the nitrogen separator.
* Alternatively option three adds a carbon dioxide and an ethylene scrubber instead of the CO2 cylinders. Depending on the installed option, the system weights 150 - 320 kg.

In order to guarantee the air tightness of the container, Freshtainer's own containers use a special one wing door instead of the normal two wing door. In case of the Freshtainer/Sabroe system, provisions concerning air tightness must be taken by the container manufacturer/ operator, for example, by using a plastic curtain.

With all options installed, the Freshtainer/Sabroe unit is able to build up and keep the inside atmosphere down to 1% of oxygen, between 0 % and 80 % of carbon dioxide at a humidity of 60 % to 98 %.

Carrier Transicold

In November 1994, Carrier Transicold unveiled its own CA system is based on Carrier's NT reefer machinery and fitted completely into it. The additional weight is 70 kg. The system consists of a membrane type nitrogen separator including the necessary oil-less air compressor. The membrane itself was developed in conjunction with Medal (DuPont). The controller for atmosphere control is of the same type as for temperature control, so that the two controllers can even be interchanged. The data logger records oxygen and carbon dioxide levels as well as the air humidity inside the container, in addition to the temperature setpoint, supply air and return air temperatures.
System Operation

Atmospheric air is drawn through an air intake filter to remove contaminants before entering an oil-less air compressor. Once it is compressed, air is feed into a condensing line where moisture is removed. It then passes through a filter assembly where the moisture is drained away. To enhance the efficiency of seperation, it is passed through an air heater before entering the membrane seperator. In the membrane seperator, nitrogen is separated from the other gases and delivered inside the container while the other separated gases are simply vented to the outside atmosphere.
Principle of Seperation

The membrane seperator contains thousands of small hollow-fibres, approximately twice the diameter of human hairs that make up a semi-permeable membrane bundle. Similar to the shell and tube heat exchanger, the membrane bundle is housed in a cylindrical shell with a feed inlet port at one end and two vent ports- one each at the opposite end and on the side of the housing. Compressed air used as a supply gas is fed at the inlet of the seperator and flows inside the hollow fibres towards the opposite end vent port. Each of the air components has its own permeation rate that is a function of its ability to dissolve and diffuse through a porous medium. This characteristic permeation rate allows fast gases like oxygen to separate out from slow gases like nitrogen.

The ability of a membrane to separate gases is determined by the rate of permeation of each feed gas component, which is a function of the individual components' solubility in the membrane material and the rate of diffusion through the membrane wall. Gases with a higher solubility in the membrane material and are small in molecular size, permeate faster than large, less soluble gases. The higher the solubility, the more efficient is the seperation process is. The driving force behind the seperation process is the difference between the gas components' partial pressures that make up the feed gas.


Both oxygen and carbon dioxide levels are controlled by flushing with nitrogen. An automatic purity control guarantees the proper function of the system.

It is also possible to connect a CO2 source to the container (cylinders inside and outside the container). The controller is already designed for CO2 supply, and a magnetic valve is installed to which the CO2 source can be connected.

Graaff

The German container manufacturer Graaff was known to have developed the "Pacas" container in conjunction with Permea Inc.

Graaff now offers a modular CA system. The simplest option provides only an uncontrolled nitrogen separator without data logging. By adding a controller, a data logger, a carbon dioxide scrubber and/or a humidification/ dehumidification unit, the CA system can be adapted exactly to customer's requirements.
Freshcon eshcon

Hamburg based Freshcon, a company belonging to the Sietas shipyard, also developed a CA unit in cooperation with Isolcell Italia (CA) and Noske-Kaeser (refrigeration). The system provides a membrane based nitrogen separator, a PSA carbon dioxide scrubber and a catalytic ethylene scrubber.
Isolcel

Isolcell is one of the biggest manufacturers of CA equipment and CA stores in Europe. Beside the Freshcon container, which uses Isolcell equipment for atmosphere control, the company offers its own CA container.

The CA system consists of a permeable membrane nitrogen separator, a carbon dioxide scrubber and an ethylene converter. The components are installed inside the container beside the reefer machinery casing, so no cargo space is lost. A controller and data logger are installed and are accessible from outside.

To increase the air tightness of the container, a plastic curtain is fitted magnetically into the door opening. This aims to reduce leakage to 600 l/h at 20 mm WG.
CONAIR plus

G+H Montage, who are known for building CONAIR stacks for cold air supply to porthole containers on vessels, offer a CA system for porthole containers. The CA equipment consists of a nitrogen separator producing 30 m3/h N2 at a purity of 97%, a carbon dioxide scrubber and a catalytic ethylene converter. This system is connected to the CONAIR stack, which has been specially sealed in order to improve gas tightness.

The main problem with this system is the air tightness of the containers, which are normal porthole containers without additional sealings. The capacity of the nitrogen separator is sufficient to lower the oxygen concentration to approx. 3 % even with not fully air tight containers, but high carbon dioxide levels will not be reached due to permanent nitrogen flushing.
Analysis

The most successful system at the moment, the TransFRESH Tectrol system, is the one with the lowest initial investment cost, but with the highest operating cost.

The different CA systems for container application show a wide range of different approaches. These relate to the technical equipment of the containers as well as the price. They range from the simple TransFRESH Tectrol system, which needs an initial atmosphere filling after loading, to Freshtainer's sophisticated INTAC IV system, which offers almost all opportunities for atmosphere control.

All in all, it can be found, that prices for well equipped CA containers like Freshtainer or Freshcon are still too high to be accepted on the market. The most successful TransFRESH Tectrol system is the simplest one with the lowest initial investment. Due to decreasing prices for stand-alone CA containers, it is expected that more of these units will get into operation in the future.
Safety

Since we are dealing with atmospheres which are deficient in oxygen, we need to be very cautious whenever we are working on Controlled Atmosphere systems. Awareness of the ill effects of exposure to Oxygen-deficient atmospheres is crucial.

The following are the effects of various degrees of reducing oxygen content in the atmosphere and the affects on human beings.

* 15 - 19% O2 – Co-ordination impaired
* 12 - 14% O2 – Perception and judgement impaired
* 10 - 12% O2 – Performance failure, poor judgement, onset of Cyanosis
* 8 - 10% O2 – Mental failure, unconsciousness
* 6 - 8% O2 – 100% fatal after 8 minutes exposure
* 4% O2 – Coma in 40secs, convulsions, death The following precautions are recommended when working on Controlled and Modified Atmospheres:
* Never assume the atmosphere is safe
* Vent before entering
– 20 minutes minimum
– Remain clear of open doors
– Remain clear of air vents
– Even if safe yesterday
* Work in pairs when entering Modified/Controlled Atmosphere Units

Most of the containers are now equipped with a safety door lock which allows the rear doors of the container to be opened only after the oxygen content in the container has reached 20.3% and locks the container at oxygen percentage below 19.8%. The locking and unlocking is achieved through a solenoid valve which pushes out a locking lever and the signal for the same is obtained from the controller based on the sensed value of the oxygen percentage inside the container.
Pre-Trip Kit

Prior to every loaded trip of the reefer container using CA System, the following items comprising the Pre-Trip Kit must be replaced.

* Rear Door Curtain
* Ribbon
* Air Filter Element
* Sample Air filter Element
* Safety Precautions Label (Decal)

Benefits of Controlled Atmosphere

* Reduces respiration
– Ethylene production retarded
– higher natural sugar content.
* Retards senescence (ripening / aging)
– associated softening and compositional changes
* Alleviates certain physiological storage disorders.
* Retards some pathogens and consequential decay.
* Provides insect control.
* Permits longer tree ripening
* Better product quality at final destination
* Less spoilage during transport
* Broader range of cargo can be shipped
* Permits extended post harvest shipping and storage times.

Controlled Atmosphere - A Panacea!

Controlled Atmosphere System is not a panacea. Temperature control is of paramount importance. Each product has its own requirement of oxygen and carbondi- oxide content to be maintained which needs to be known beforehand. All products are not equal. The product has to be of good quality at the time of loading inside the refrigerated chamber. Correct product quality is maintained by adoption of correct handling practices all through the product's post-harvest life - from the field to the consumer. Controlled Atmosphere Systems cannot improve the product quality. It can only maintain the product quality by prolonging the life of the product and retaining the freshness.

Individual pieces of fruit cargoes should be packed in cardboard boxes with holes all around so as to allow ingress of cold air inside the box and touch the individual pieces of cargo. Similarly each individual piece of fruit cargo should be properly protected to prevent any mechanical injury by wrapping with a suitable material. The ideal material of packing each individual fruit is thin porous paper which allows passage of cold air through the pores and touch the cargo. Alternatively, plastic nets may be used. Totally enclosed plastic bags should not be used, plastic being a non-conductor of heat does not allow passage of cold air to touch the individual pieces of cargo. As a result, heat accumulation and gas accumulation takes place inside the plastic bag causing deterioration and damage. If plastic bags are used, there must be sufficient number of holes to allow cold air to pass through.

Proper loading procedures should be followed along with the necessary precautions. Cargo pallets or crates should be evenly stacked with uniform space all around the boxes. Formation of hot pockets should be prevented. Proper lashing of stacked cargo should be done. Suitable separating dunnage should be used to obviate any possibility of shifting of cargo during transportation causing formation of hot pockets. Packing material should have sufficient stacking strength to withstand the weight of the upper tiers of cargo. This is particularly important if the cargo is to be transported across oceans in a refrigerated container, where it is subjected to severe weather conditions and mechanical movement due to rolling, pitching, pounding, panting, yawing and heaving.

If the product is of inferior quality at the time of harvesting or not packed in the right manner or not stowed in the right manner inside the refrigerated chamber or if the temperature is not maintained accurately within a narrow range, even Controlled Atmosphere System cannot prevent the cargo from deterioration or damage. GARBAGE IN – GARBAGE OUT!
References

1. www.carrier.transicold.com
2. www.thermoking.com
3. Dr. Ing. Yves Wild, Overview on Controlled Atmosphere Transportation in Containers, 19th Congress of International Institute of Refrigeration, The Hague, Netherlands, 23rd August 1995.
4. Brian M McGregor, Tropical Products Transport Handbook, United States Department of Agriculture, 1989.
5. Robert E Hardenburg, Alley E Watada and Chien Yi Wang, The Commercial Storage of Fruits, Vegetables and Florist and Nursery Stocks, United States Department of Agriculture, 1986.
6. Vick Kenneth W, For Economic, Abundant and Varied Fruits, Agricultural Research Journal, Jan 1993, Volume 43, Issue 1.
7. Devon Zagary and Adel A Kader, Controlled Atmosphere Handbook, Carrier Transicold, USA, 1999.

Reefer journal: Container Refrigeration Temperature Recording Systems (Concluding Part 4)

Issue : July-September 2005



Container Refrigeration
Temperature Recording Systems (Concluding Part 4)

By C. Maheshwar
Faculty, Training Ship Chanakya
Navi Mumbai

C. Maheshwar is a marine engineer, from Marine Engineering College, Kolkata, (DMET) 1980. He has sailed on board foreign - going ships of Shipping Corporation of India Ltd., from 1980 to 1997, the last 5 years of which were as chief engineer officer. Ashore, he has worked from 1997 to 1999 with the Taj Group of Hotels as chief engineer of Taj Connemara Hotel, Chennai, and as customer service manager, Reefer Container Group of Carrier Transicold for the region of South Asia from 1999 to 2001. Currently, he is working as engineering faculty at TS Chanakya, Navi Mumbai, a Merchant Navy Training Institute, belonging to the Govt. of India and affliated to University of Mumbai and IGNOU. He is a consultant for Anglo-Eastern Maritime Training Centre and conducts training programmes on reefer containers for seagoing engineers on a regular basis. He can be contacted at cmaheshwar@hotmail.com

In this concluding part we will examine the evolution of the various temperature recording systems used in reefer containers, right from the days of the manually maintained log sheets to the currently used remote monitoring systems and the possibility of using satellite transmission of data for monitoring and control from a shore establishment, which is already under trial.

Temperature Measurement implies on the spot instantaneous display of the temperature of the air inside the reefer container. The temperature measurement of air inside the reefer container is done at two locations - one at the return air i.e., at the entry to the evaporator coil and one at the supply air which is at the exit of the evaporator coil. Under normal working conditions, a temperature difference of 2-3°C is always permissible between the return and the supply. For a marine reefer container, the return is from top and supply is from bottom and for truck refrigeration, it is the other way around. When return is at the top, it aids the natural circulation of air, i.e., hot air goes upwards and cold air sinks to the bottom. The temperature difference is more pronounced in case of chilled cargoes like fruits and vegetables which liberate heat due to the chemical and physiological changes taking place within the cargo.

Temperature Recording is required for future reference. At the end of a loaded trip, if it is noticed that there is a deterioration or damage to the cargo, the record of the temperature maintained inside the container and performance of the reefer machinery is called for. This is done to ascertain the cause of the damage and extent of the damage to pinpoint the responsible party. The record of temperature maintenance is called for by the courts to find out the genuineness of cargo damage claims.

Evolution of Temperature Recording Systems
Manual Logsheet

In the earlier days, when there was no recording mechanism, the only available document was the reefer container temperature log sheet. Temperature was physically and individually checked and logged down in a pre-printed format. The temperatures were logged down at two fixed timings during the day - one at AM and one at PM. This was accepted as evidence in the court of law. However, this was not 100% reliable as the log sheet could be tampered with, the temperatures recorded could be wrong. An entire new log sheet could be fabricated to protect the ship's staff and the carrier. In spite of this, even in today's world, some shipping companies insist on mandatory maintenance of the daily temperature log.
Partlow Chart Recorder

The daily temperature log started to get supplemented by an additional feature - Partlow Recorder. This consisted of a temperature sensor (mercury filled) fitted to the return air side grill which would be permanently connected to a stylus. Variation in temperature sensed by the sensor would cause a corresponding and proportional movement of the stylus. A paper chart was fitted to a metal base on which the stylus would move and make a mark (an impression). The Partlow chart would be calibrated in 0C or 0F as desired by the shipper. The entire chart would rotate continuously about a center at a fixed speed, the movement of rotation provided by a hand wound clock mechanism. Once fully wound, the clock would complete one full circle and come back to the starting point after 31 days. The entire chart would be divided into 31 segments, each representing one day of 24 hours. Thus, with one chart, we could have a continuous record of 31 days.

Since there is no electronics involved and there is no requirement of external power source, this system would work as a stand alone system and the recording would go on irrespective of whether the container is loaded or not, the machinery is running or not, whether power supply is available or not. At the end of the 31 day period, the chart could be replaced with a new one. The required number of photocopies of the completed chart could be taken and distributed to the concerned parties and kept as a permanent record. Since the chart was exposed to the atmospheric elements, the markings being impermanent, would fade away and become incomprehensible after some time, like a thermal fax paper. So, it is important to take a photocopy of the completed Partlow chart at the earliest and preserve it.

A Partlow chart was accepted as evidence in the court of law for deciding the genuineness of cargo damage claims, ascertaining the extent of damage and awarding compensation to the aggrieved party. Also, at a glance at the Partlow chart, we could know the way the temperature was maintained inside the container for the last 31 days. We could identify temperature fluctuations due to defrosting cycles shown by periodic peaks, not running of the machinery due to power shutdowns or otherwise shown by a slow rise of temperature indicated by a gradual upward slope and so on.

However, even the Partlow chart was not foolproof. It could be tampered by simply lifting up the stylus for the required period and making the required temperature marking later. Ship's staff and carriers started protecting their interests by making their own charts to fudge the actual information. The actual temperature fluctuations due to machinery breakdown or power shutdown could be camouflaged and concealed. So, the Partlow chart was no longer accepted as foolproof and sacrosanct. Further, the Partlow mechanism itself needed regular calibration.

Subsequent variations of the Partlow mechanism were made by replacing the mechanically operated handwound clock mechanism by a replaceable battery. An electronic Partlow was developed which could give better precision as the temperature signal was converted into electronic signals and fed onto the chart. So, mechanical sluggishness of the Partlow was ruled out.

Electronic Data Recorders / Data Loggers

In view of the increased container trade, there was a tremendous pressure on the manufacturers to come out with a 100% tamperproof mechanism like the Black Box in the aircraft which could store the temperature signals as they were sensed without being tampered.

This gave rise to a Data Recorder also called Datacorder or Data Logger. Two additional sensors were fitted, one each at the supply side and the return side to record the supply and the return temperatures of the air after and before the evaporator coils respectively. These sensors were fitted next to the controller sensors. They would record the same temperatures as recorded by the controller sensors being located in the same vicinity and close proximity. The signals would be fed into an electronically sealed box called Datacorder fitted with a memory device. The capacity of the Datacorder would be sufficient to store data continuously from the container for about three years. In addition to the supply and return air temperatures, change of set point, alarms and all other events would be recorded in the Datacorder. Under normal conditions, the recording would be available with the main power supply, a low dc voltage signal being taken to power the recording unit. Back up power would be provided through a rechargeable, replaceable nickel cadmium battery pack with a fixed life. When main power supply is available, the battery would be charged through a charging circuit.


Unlike the circular temperature charts, the data loggers record the temperatures digitally which provides only a discrete number of measured values. The normal recording interval is one hour. However special events such as alarms, defrosting process etc. are all saved explicitly. Depending on the type of logger and the way it is programmed, the recorded temperature data may be snapshot values or may be average values over a defined period of time.

Today, a considerable proportion of refrigerated containers are equipped with data loggers of this type and not with the traditional chart recorders, which have become redundant. The trend today is away from chart recorders and toward data loggers. Some container owners, however insist on having a visual display of temperature in the form of Partlow chart, which when provided would be in addition to the electronic data logger, which has become a default feature.
Temperature Recorders Inside the Cargo

Nowadays, several additional data loggers are often placed within the load when transporting refrigerated cargo in containers. These are used to directly monitor the temperatures of the cargo inside the container during transport and allow this data to be made available to the recipient of the goods. These loggers can provide evidence of periods of insufficient refrigeration leading to damage. For certain highly sensitive products (for example, blood plasma) the use of such loggers is required as proof that the cold chain has not been broken at any time.


All the reefer containers carrying refrigerated cargo especially fruits and vegetables to United States are required to have special USDA (United States Department of Agriculture) sensors which are actually placed inside the cargo at various locations in the container. The unit is equipped with a receptacle to receive the temperature signals and connect to the data recorder to provide a continuous record. Only when the temperature fluctuations of the cargo during the period of passage is within limits, will the cargo be accepted and allowed to land inside United States. This is to prevent landing of fruit-fly infested cargo in US teritory.

The devices currently available on the market range from clockwork-driven analog recorders that write data on a strip of paper to digital data loggers that use infrared interfaces to transfer the data. The accuracy of the analog devices is approximately +1°C, and the digital devices are accurate to +0.1°C. Taking into account the value of the cargo and the potential costs of compensation for damaged goods, the cost of using a recorder is negligible and their use is highly recommended. The recorded data is not only useful in providing the consignor with data as evidence against the transporter, but is also useful in providing evidence for mistakes made by third parties like terminals.

When choosing where to place these loggers, care should be taken to ensure that they are placed in temperature-critical locations in the container so that they measure the actual cargo temperature. They can be located in a box on the top layer close to the door. However, it must be understood that placing recorders of this type on top of the cargo measures the temperature of the air rather than the temperature of the cargo giving rise to far-reaching discussions about whether the cold chain has been maintained. A clearly visible temperature peak in the middle of a printout shows an increase in temperature of approximately 10 °C within the space of 30 minutes, but can only reflect a change in air temperature since it would not have been physically possible for the cargo temperature to have increased this much during this period of time because of its mass and the heat capacity of the cargo. A sharp increase in air temperature, is perfectly possible in the event that the refrigeration system fails, since the air in the container very quickly starts to form layers where the warmest air is located just below the container roof.

Remote Monitoring Units

Since daily inspection of the large numbers of refrigerated containers carried on board a vessel takes a significant time, a number of shipping companies have started to use systems which enable remote monitoring of the containers. Data is exchanged between the ship's computer and the containers over the power cable of the containers. This includes information about current temperatures, any alarms that have occurred etc. The printed logs that are generated as a result of this exchange can effectively replace manually recorded temperature data. In addition, the crew is in a position to react to problems more quickly, since when relying on daily rounds it is possible that a container had an alarm for 24 hours before this is noticed.

With an increasing number of refrigerated containers being used in maritime transport, there is also a greater need for effective ways of monitoring these containers. On ships, many of which can nowadays transport over 1,000 refrigerated containers, using a remote monitoring system can cut the costs of inspecting the containers while also enabling the crew to react more rapidly to potential problems in the event of a refrigeration unit failing.

There are two basic types of Remote Monitoring System:

Four wire System. With the four-wire monitoring system, a separate monitoring cable with four wires is used to record the status messages "Compressor Running", "Defrost" and "Temperature in Range". Around 80-90% of all refrigerated containers have a socket to connect them to this type of monitoring system.

In the four-wire (4-pole) monitoring system, a 4-wire cable is used to transmit three signals as active 24V signals with a common return wire. The common return wire is generally connected to the chassis of the container. By checking for a connection between this return wire and the ground wire of the container, it is possible to determine whether a container is connected to the four-wire monitoring system.

One of the main disadvantages of four-wire technology is its contact problems. Although signal sockets on the containers and on the ships are equipped with protective flaps, the sockets still regularly suffer from corrosion due to the rough environment. The cables are also prone to damage as they are often subjected to rough treatment when twist locks and lashing rods are dropped. Sometimes the cables are also simply torn off, because no-one removed them before unloading the containers.


Since the signals are transmitted as voltage signals with the statuses 0 V and 24 V, it is impossible to determine whether or not there is a reliable electrical connection. It would have been better to use a procedure for monitoring whether a wire is intact (e.g. 4-20 mA signals).

Each of the three signals provided by four-wire technology has a different meaning. The most important signal is undoubtedly "Temperature in Range". If this is not issued, this triggers an alert. Very simple monitoring systems can therefore only evaluate this signal. "Defrost" and "Compressor Running", however, are status signals which are required to provide further information. The "Defrost" signal, for instance, can be used to suppress a temperature alarm (during defrosting, it is to be expected that the temperature will deviate from the nominal value). A cooling compressor which is constantly running (Compressor Running) in low-temperature mode can indicate a fault in the container.

During normal cooling operation, the cooling compressor runs in on/off mode. The compressor is switched off during defrosting and must immediately switch on again after defrosting and remain running for longer than usual, so as to dissipate the defrost heat which has accumulated. The "Temperature in Range" signal will not be issued during defrosting, since the air in the cooling unit is being heated, but must be issued again later.

Power Cable Transmission System (PCT). With the Power Cable Transmission (PCT) system, data is transmitted via the three-phase power cable of the containers. This enables an unlimited amount of data to be transmitted between the container and the receiver on board or on land. The data can be exchanged in both directions, so that it is also possible, for instance, to change the nominal value of the temperature of a container in this way. This offers a tremendous costsaving potential by the option of making remote pretrip inspections (PTI) of the containers on board or in the terminal, as well as to read out data logger information after a loaded passage.

Two different variants of the PCT system are currently available: Narrowband transmission, which operates at a fixed frequency to modulate data on to the power supply system, and Wideband transmission, in which data is transmitted over a frequency spectrum. Since both these systems are not compatible with each other, depending on what modems are fitted to the containers, both systems have to be installed on board to be able to communicate with all containers. Out of approximately 360,000 refrigerated containers worldwide in 1997, approximately 6.6% and 5.3% were equipped with narrowband modems and wideband modems, respectively, the choice depending much on the shipping line and the route.

Power cable transmission eliminates all the problems which arise from using an extra cable in the four-wire monitoring system. It also has a significantly larger range of functions, since any data can be transmitted as it merely depends on the transfer protocol used. The data is modulated onto the three-phase power supply system of the ship or the terminal as a high-frequency signal and received by one or more master modems. It is then transmitted from there via a bus system to the control computer.

Long Distance Systems

To cover longer distances, different types of technology are used by different providers. A system with a capacitive network only needs one master modem, which is connected to the three-phase power supply system over a kind of signal line and one or more capacitive bridge units (CBUs). To bridge transformers, transformer bypass units (TBUs) are available.

The other system uses a number of master modems which are connected to each other via a field bus The number of master modems required depends on the network configuration and the distances to be covered. The disadvantage of this is that no more than one master modem may be running at a time. This significantly reduces the effective average data transfer rate in the event of several master modems. In addition, it is fairly likely that some containers are positioned in the catchment area of a number of master modems, meaning that the same data is transferred several times unnecessarily and then needs to be filtered out.

Data transmission via narrowband is the older of the two methods. Data is modulated onto the power supply network at a fixed carrier frequency of approximately 55 kHz. It is transmitted at a rate of 1200 baud which is why this system is often also referred to as a "low data rate system".

Sealand were already carrying out their first trials with PCT at the end of the 1970s. At the beginning of the 1980s, ThermoKing collaborated with Sealand to develop the first marketable system, known under the name of ThermoNet. Sealand and Matson were the first to use this system on a large scale, principally in the relatively closed refrigerated container trade routes in the Pacific.

In the mid 1980s, the wideband system arrived, transmitting data in a frequency spectrum of approximately 140-400 kHz. Transmission over a number of frequencies was intended to ensure reliable transmission even with interference frequencies such as those generated by frequency converters. By distributing the signal over a frequency spectrum, the strength of a signal on any frequency is lower than with narrowband laying claim to a greater range.

Since the data transmission rate on wideband systems is theoretically 19,200 baud, it is also known as a "high data rate system", though in practice this speed benefit is barely discernible.

A monitoring system is useful only when there is a standard for data exchange, as genuine saving effects can only be achieved if all refrigerated containers can be monitored by PCT as far as possible. For this reason, an ISO sub-committee was set up between 1987 and 1990 to define a standard. This was finally published as ISO 10368. Since the various companies participating in the committee had different interests, no consensus was reached regarding hardware (i.e. the transmission frequency), and consequently there are still two systems available on the market. Only the frequency ranges for each system were defined, to ensure that they could both be operated simultaneously.

Apart from this, the standard primarily regulates the data transmission protocol (i.e. software) and defines the minimum range of functions for remote communication devices (RCDs).

Precise data protocols were not defined for all commands and room was left for subsequent extensions in the form of "private sessions", which can be used by individual manufacturers to transmit proprietary data. This extension facility was used excessively by certain manufacturers, to the extent that many functions available today are transmitted within these nonstandardized protocol sections. There is disagreement on which of the protocols should put in the public domain and therefore available to the competition for this type of transmission, and under what conditions.

The ISO Standard has only documented the two existing systems and prescribed some very basic queries. Even if all transmission protocols were put in the public domain, this would mean today that a separate software driver would have to be available for every modem type. Since the controllers of the refrigeration units and the data loggers which are used also have different ranges of functions and data formats, a large number of drivers is needed to support all potential configurations.

Another issue which was not dealt with by ISO is data protection. In accordance with ISO (and also in practice), all data on all containers equipped with modems is available on the power supply network. It is therefore possible theoretically that third parties with access to the power line network via a master modem can read out and even change information on the containers (e.g. the nominal values). This was never a problem while shipping companies were only using PCT on board their own ships and terminals. Once it began to be used on multi user terminals, however, the network operators (terminal operators) have had to ensure that only authorized persons have access to information on containers which pertains to them.

Power cable transmission usage has generally been restricted to shipping companies with a high proportion of refrigerated containers. There is no evidence of all refrigerated containers being generally equipped with modems. Two different systems will still be deployed, which means that the evaluation installations used on ships and terminals must be able to cope with both systems for the foreseeable future in order to effectively exploit the savings potential. Even if it may seem that recent investments are generally being made in wideband, there are still too many narrowband containers to expect them all to be converted to wideband. In the long term, however, the modems of the first generation at least must be upgraded, since the impedance values are too low and this interferes with dual band line transmission.

It can be expected that four-wire monitoring technology will be replaced by power line transmission in the near future.

On a ship or at a terminal, every refrigerated container slot must have the relevant sockets available to connect the signal cable. These sockets are often integrated directly into the refrigerated container power outlets. The signals from the individual containers are transmitted from there either via a field bus system or via the available power networks to the evaluation computer. The signal cables themselves must also be available.
Downloading and Interpretation of Recorded Data

It must be remembered that the data recorded in the Datacorder cannot be read directly at the container. It must be downloaded and converted into a readable and a comprehensible format. Datacorder is only a data storage device with a limited storage capacity. Data recorded in the Datacorder has to be regularly downloaded and preserved for posterity. Older data gets wiped out as newer data gets recorded and stored beyond the storage capacity.

Downloading is the act of transferring data from the container's Datacorder onto a portable storage device. This portable device could be either a laptop or a hand held unit. Carrier Transicold has called its hand held downloading device Datareader, which can download data only from Carrier units. Psion, a Hungarian company has developed a versatile hand held downloading device which is compatible for reefer units of all makes. Each container unit is provided with an Interrogation Port to which the hand held device can be connected using appropriate cables.

Using a hand held downloading device from Psion or a Datareader, a technician can download data from many containers on a ship or in a terminal. It would not be advisable to expect a technician to go up the different tiers of containers with a laptop, especially on a ship with continuous rolling and pitching in extreme cold climate and rain. The portable hand held device can be easily slid into a boiler suit pocket and can be conveniently carried around.

It must be remembered that the Datareader or Psion hand held downloading device is only a convenient gizmo and has a finite (about 2 MB) memory space. It can be used to download data from a limited number of containers. Older data gets wiped out as newer data gets stored beyond the storage capacity. Data has to be transferred onto a PC (desktop or a laptop) having the necessary software with which the downloaded data can be interpreted. The act of transferring downloaded data from the Handheld unit onto a PC containing interpreting Software is called Uploading. Carrier Transicold has developed a DOS based software called Dataview and recently, a Windows based software called Dataline to interpret data downloaded from containers with Carrier Transicold Reefer Machinery.

When transmitting Controlled Atmosphere data (oxygen content, carbon dioxide content, humidity etc.) by power line transmission, it is necessary for the controller of the refrigeration unit to forward this data to the slave modem and for the evaluation software to be able to process this data accordingly


Looking at the Future

Sooner or later, it will be possible to transmit data by radio frequency, e.g. using "wireless LAN" technology, as this promises higher transmission rates and lower interference.

Using satellites to monitor refrigerated containers generally fails, because when the containers are stacked they cover the antennas of the containers below them, thus making data transmission impossible. The same applies when containers are stowed below deck. In future, however, it will definitely be possible to send data transmitted by PCT via satellite from the ship to receiving stations on land, to enable the refrigerated containers to be accessed online.
References

1. www.carrier.transicold.com
2. Container Handbook, GDV, Berlin
3. Marine Refrigeration Manual by Capt. AWC Alders