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Saturday, December 12, 2009

REEFER CONTAINER TRANSPORT 1 (THE CARRIAGE OF REEFER CONTAINERS)

The purpose of this bulletin is to provide Members with a set of guidelines on the carriage of temperature controlled cargo in reefer containers to ensure maximum protection during transit. It was originally written in response to a claims analysis finding that the numbers of reefer container claims were increasing, and evidence that the casual transportation of reefer containers by non-specialised operators was a growing practice.

The complex nature of the subject precludes advice for every eventuality, and consequently these guidelines are outlined in broad and general terms. It should be understood that exceptions can and will arise, and in such circumstances specialist advice should be sought. The common goal however is the prevention of claims, and to ensure that the cargo transit is carried out in a safe and efficient manner with minimum loss of product quality.

To achieve this it is vital that all concerned appreciate the importance of maintaining the specified cargo temperature throughout the journey. It is also essential to understand that the primary purpose of transport equipment is to maintain temperature. Such equipment is not designed to cool warm cargo other than extremely slowly.

Members operating vessels on which reefer containers are carried on an occasional basis are therefore asked to heed the following recommendations and to note the subsequent advice concerning the operation, stowage and transportation of refrigeration units in general.

Guidelines

a. Vessels should always carry basic reefer spares, suitable tools and repair manuals specifically relating to the type(s) of refrigeration units carried, for use by the reefer technician in case it is necessary to effect emergency repairs during the voyage. There are many different types of reefer unit in general use, each having individual repair and maintenance characteristics. The provision of a working platform is essential if containers are to be stowed more than one tier high.

b. If insulated containers powered by clip-on refrigeration units or reefer containers powered by clip-on generator sets are to be carried, similar precautions regarding spares and repair manuals should be taken.

c. The ship's crew should make certain that the spares provided are of the correct type and quantity before loading commences.

d. Prior to shipment it is essential that written confirmation is obtained from the shippers addressing all cargo conditions of carriage including temperature, ventilation and humidity requirements. International Cold Chain Technology (ICCT) recommendations regarding the specification of carriage conditions are available from the Loss Prevention department on request.

e. Members should give precise instructions to their vessels, listing details of all parties to be contacted in the event of a malfunctioning reefer container.

f. Members are urged to provide their vessels with a reefer operations manual specifying the carriage particulars of various commodities shipped in refrigerated containers, possible problems and a summary of trouble shooting procedures. If there is any doubt, the advice of cargo care experts should be obtained.

g. Before loading commences, crews should reconfirm that the vessel's reefer plugs are compatible with all reefer containers planned for shipment.

h. A number of reefer extension leads should be carried as a precaution against the failure of individual plugs.

i. Members should ensure that where chart recorders are fitted, the recording charts are removed before the container is released to the consignee and are retained for a period of at least twelve months. If electronic logging is incorporated, procedures for downloading records should be established and observed.

j. If stuffing is to be carried out at the loadport terminal itself, a surveyor should be appointed to monitor the arrival temperature of the cargo and to note details of any departure from specification.

k. Many reefer container losses have arisen from confusion between Fahrenheit and Celsius temperature scales, and also between plus and minus temperatures. Great care should be exercised to ensure that carriage temperatures are correctly set as soon as containers are placed on board. Any discrepancy between container settings as received and bill of lading instructions should be reported immediately.

l. At sea, reefer units should be inspected at intervals of not more than six hours, recording the times of such inspections and noting details of any problems in an appropriate logbook. Alternatively, automatic logging systems may transmit signals via power cables to a central point. Such systems should be checked for error messages on a regular basis.

m. Basic instruction in customary reefer container practices such as on-board monitoring and recording procedures, the checking of seals and the shifting of containers at intermediate ports would clearly benefit inexperienced crews. Members are asked to consider implementing such training schemes where applicable.

n. If circumstances permit, Members are requested to keep their crews informed of the values of the commodities shipped so that the significance of a potential loss can be fully appreciated.

Reefer Containers

A reefer container is designed to maintain cargo at the pulp temperature prevailing at the time of stuffing. Although the container machinery over a period of time can bring cargo delivered at too high a temperature down to (or closer to) the designated temperature, this is not the primary function of a reefer container.

If a container is loaded with a cargo where the pulp temperature exceeds the carriage temperature stipulated by the shippers, the "warm" cargo will cause the temperature of the delivery air to rise very rapidly when passing up and through the cargo. Eventually, the return air may reach a temperature level whereby the refrigeration machinery cannot cool it down sufficiently prior to re-circulating it as delivery air. In this event, the tracking pattern on the chart or logger will show a temperature higher than that of the temperature control setting. The delivery/return air differential will in most cases narrow as the continuous circulation of air, being cooler than the cargo, brings the cargo temperature down towards the desired level. Any rise in return air temperature will be arrested as the refrigeration unit begins to run in standard operational mode.

In cases where the stuffing temperature is higher than the stipulated carriage temperature, the refrigeration unit will cool down the surface layer of the cargo relatively quickly (within days). However, the centre of the stow will not reach the desired temperature for a considerable period of time. The temperature of a cargo stuffed into a refrigerated container should not, in general, deviate by more than 3ºC (5ºF) from the specified carriage temperature. Chilled cargo (excluding bananas) should not deviate by more than 0.4ºC (1ºF). This does not mean that even these deviations should be encouraged; the objective is to receive and deliver the cargo at the carriage temperature.

Defrosting

During the operation of a refrigeration unit, a layer of ice will form on the evaporator coils depending on the temperature set, the temperature of the cargo, the amount of fresh air ventilation and the cargo humidity. The unit periodically enters a phase where heat is produced by a series of electrical bars, allowing defrosting to take place. At such times, all fans are turned off automatically in order to prevent heat from entering the cargo compartment.

However, the return air temperature sensor is so closely located to the refrigeration machinery that the temperature record will inevitably register some of this rise. The record will therefore display periodic temperature increases in keeping with the defrost periods. It must be stressed that these increases, which are conspicuous on paper chart recorders, have no immediate effect on the actual temperature of the cargo and are not an indication of an unstable refrigeration unit. Electronic loggers usually indicate the timing and duration of defrost periods in addition to temperatures.

If, as described in the previous section, a cargo is loaded into a container in a "warm" condition exceeding the specified temperature, the refrigeration unit will automatically work to bring the cargo temperature down towards the correct level. This unintentional strain on the unit may result in a heavier accretion of ice on the evaporator coils, leading to an increase in the defrost patterns recorded.

Recorder charts do not identify refrigeration unit defects, but do give useful indications of correct operation. Data logger records may give detailed information about system faults in addition to set point, delivery and return air temperatures. Container temperature recording systems do not usually record actual cargo temperature, only air temperatures, but cargo temperature may be recorded by shippers’ loggers within the stow.

Malfunction of a reefer unit

Should a refrigeration unit cease to operate, the chart or logger will register a gradual but steady rise in temperature to the point where eventually the ambient temperature is recorded. Again, the sensor will record an air temperature and the record will not accurately reflect the true position regarding the cargo itself. The cargo will be reasonably well protected from the influences of the external air temperature by the surrounding insulation.

There are many other situations where the record may not be a precise representation of the temperature or condition of the cargo within. These examples are given merely to warn that conclusions should not be drawn automatically from the temperature tracking pattern alone.

Equipment

The standard terms of measurement for refrigerated containers are the "20 foot equivalent unit" (TEU) and the "40 foot equivalent unit" (FEU). These terms do not distinguish between containers of differing heights. Two principal types of reefer container are in use today:

i. Insulated containers requiring an external refrigeration source. These units are often referred to as being of "port hole" or "Con-air" design. It is expected that they will be gradually phased out.

ii. The second, and by far the largest group, consists of insulated containers each fitted with an integral reefer machinery unit, often known as "integrals" or "reefers".

Prior to delivery to a shipper, an integral unit container must be subjected to a Pre-Trip Inspection (PTI) arranged by the carrier or his local agent, which involves the refrigeration machinery being run and tested by a specialist engineer, usually within the port area. During a PTI the machinery is checked, faulty parts are repaired or replaced, and thermostatic temperature recorders (if fitted) are wound up and calibrated (normally at 0ºC). In such cases a copy of the test programme results is left inside the temperature chart recording box for the benefit of the vessel's crew, declaring the outcome of the tests and any repairs carried out.

Modern electronic controllers usually incorporate a self-checking PTI procedure which may be carried out quite simply. Attention by a fully trained refrigeration engineer may be required only if the automatic system indicates a fault. These systems retain a record of the previous PTI.

The PTI should be repeated if stuffing is to take place after more than a 31 day delay.

In some cases, generating sets capable of providing independent power may be used to facilitate the completion of "cold chain" operations from the premises of the shipper until the moment the container is loaded aboard the ship. Generators consist of three types:

i. Permanent fixed units
ii. Top clip-on units
iii. Under-slung clip-on units

Generator units must undergo similar tests before the accompanying containers are released from the depot, and serial numbers must be recorded. The inspection also verifies that a generator unit has adequate fuel supplies for the return journey. It is important that the container temperature settings are checked prior to a container being shipped out, and that the correct temperature scale (ºC/ºF) is selected, particularly in the case of digital displays.

The external temperature recording equipment of a refrigerated container may consist of a Partlow chart fitted to a circular recording disc. Discs must be checked to ensure they are fully wound. A check should also be made to ascertain that the Partlow chart and the thermostat setting correspond to the same temperature scale as specified by shippers. All relevant shipping details must be entered on the Partlow chart by the carrier's representative or agent, and commencement of tracking must correspond with the date and time of the hook-up to the external power source.

If electronic logging is used rather than a chart recorder, an appropriate "start of journey" code may need to be keyed in.

Checks should also be carried out to ensure that ventilators and humidity controls are set to the levels requested. If data logger probes are being used in order to comply with the USDA Cold Treatment Programme, they should be inspected both before and after fitting by the carrier's representative or agent on completion of the PTI to ensure they are suitably calibrated and are correctly monitoring the appropriate importation carriage temperature.

If the cargo is to be carried under controlled atmosphere (CA) conditions, gas controllers must be correctly set and fresh air vents must be closed. Instructions should be issued regarding the steps to be taken in the event of gas control failure, which may include opening fresh air vents when switching off the CA system.

In tropical or sub-tropical regions, it is preferable that containers are loaded in a temperature controlled environment (eg chilled warehouse). However, if loading in ambient conditions, containers should not be pre-cooled before stuffing except in exceptional circumstances as this may lead to the development of excessive condensation on the inner surfaces of the container.

Refrigeration machinery should always be switched off when the container doors are open to minimise the accumulation of moisture on the evaporator coil, the only exception being loading or devanning using a cold store tunnel.

Container shipment/On-carriage
When a reefer container is awaiting shipment or on-carriage from the place of receipt or terminal, it must always be hooked up to a static power jack point or independent generating unit so that the reefer machinery can continue to operate.

The reefer unit should be monitored by the carrier's representative or terminal operator at least 4 times every 24 hours and monitoring reports should be completed and handed to the agent just prior to the container being dispatched. Temperature settings and temperature records must always be cross-checked. Each time a reefer unit is monitored, an external check of the complete unit should be made.

During such an examination it is essential that all affixed seals, including veterinary seals, are thoroughly inspected by pulling and twisting. Seal numbers must also be checked against the monitoring records. In the event of any irregularities, owners and/or agents must be informed immediately both to initiate remedial action and to mitigate a potential cargo loss.

Temperature recording

A Partlow recorder registers temperature on a pressure sensitive circular chart over a 31 day period. If the voyage transit is expected to exceed 31 days, care must be taken to ensure charts are replaced by a ship's engineer before expiry. It is imperative the device is rewound with the fixed key attachment whenever a new chart is affixed. Replacement charts must always be used when a transfer from vessel to vessel takes place. The first chart should be placed underneath the new chart in order to build up a complete temperature record for the entire voyage up until arrival at the final destination.

These charts should always carry the following endorsements:

Name of Vessel
Voyage Number
Container Number
Temperature Setting
Load Port
Discharge Port
Date of Stuffing/Change
Ventilators: Closed/Open (degrees)
Humidity Controls (HMC) (percentage)

Data logger recorders may monitor both air and cargo pulp temperatures within a reefer unit, and the data is stored in an electronic memory. The memory also logs PTI results, alarms and transit details (eg shifting of the container in port), together with serial data communication to both the controller and the power unit. The data can be transmitted directly to an IBM compatible PC, from where information can be either printed out or transferred to disk. The information is generally more comprehensive and accurate than indicated by a Partlow chart alone.

Where electronic recording systems are incorporated, the "start of trip" information should include details of origin and destination. Date and time should also be checked for accuracy. The use of any portable recorders within the cargo space should be noted on all cargo documents. These portable recorders may be disposable or returnable units using bimetallic strip sensors, or may be electronic memory recorders. The latter type may have cargo probes attached by leads, or may incorporate an internal sensor.

When installed at a Container Freight Station (CFS) to accompany prepared shipments, it is vital the portable recorder charts are filled in correctly by the carrier's representative or agent. The location of a recorder should be noted on all documents together with the time and date of its activation.

Seals and security

In CFS operations, the carrier's seal should be attached immediately stuffing has been completed, recording the serial number on all shipping documents. In shipper-stuffed units, it is not normally possible for the container to be sealed by the carrier or his representative until the container has been returned to the container yard for shipping out. On receipt, a seal should be affixed without delay and the details again noted on all documents.

It is particularly important where veterinary seals are attached to containers that all details are noted and the seals checked for signs of interference on arrival at the container yard. Imports to the EU, USA and Japan will only be permitted if veterinary seals are intact on arrival, thus confirming the cargo has not been tampered with in any way during the transit. At intermediate ports, the vessel or her agents must reconfirm the security of all such seals and this fact must be noted on the accompanying documentation.

For cargoes classified and labelled as "Quick Frozen", there are special EU importation requirements which demand correct temperature maintenance from the point of production, which may be prior to receipt by the carrier. In such cases, the carrier needs to have evidence of previous temperature maintenance.

Commodities
In general, refrigerated commodities may be divided into two distinct categories;

i. Chilled
ii. Frozen

Many chilled cargoes (e.g. fruit) are regarded as a "live" cargo since they continue to respire post harvest and as such are susceptible to desiccation (wilting and shrivelling). This is not the case with commodities such as chilled meat or cheese. The minimum fruit carriage temperature is usually no lower than -1.1ºC (30ºF). Frozen cargo is regarded as "inert" and is normally carried at or below -18ºC (0ºF).

However, both categories are highly perishable and require care in handling to ensure arrival in optimum condition. In chilled commodity transportation, the ventilators are normally left in an open position, with a limited number of exceptions (eg meat, chocolate, film, chemicals, dairy products, and controlled atmosphere shipments). Some cargoes may require controlled humidity (eg flower bulbs). It should be remembered in such cases that many refrigeration units are only capable of reducing humidity within the cargo space and the settings should be applied accordingly. Those units which can increase humidity may incorporate water tanks with special cleaning and hygiene requirements to avoid contaminating the cargo. Controlled atmosphere carriage involves the use of specialised containers capable of substituting the oxygen levels with nitrogen and carbon dioxide in order to extend the post harvest shelf life of the product. This method is suited to many soft and stone fruits, but requires specialist knowledge to determine the most appropriate gas concentration levels.

Stowage
Correct stowage is extremely important to the carriage of containerised reefer cargo. However, this is seldom under the control of the carrier, who often receives a sealed container "said to contain" a specific cargo. With frozen cargo, the objective is to provide a circulation of cold air around the cargo to reduce the possibility of temperature variations at the boundaries (eg walls, floor and roof). With chilled live cargoes (eg fruit and vegetables), the air flow must be allowed to permeate up and through the cargo stow, removing product heat, carbon dioxide, ethylene (if present), moisture and other residual gases in the process.

Cargo must never be loaded above the red line marked inside the container. This space must always be left to allow an uninterrupted flow to the front air intake. The ideal stowage pattern should permit free movement of delivery air whilst restraining any movement of the cargo. Adequate space must be left above and below the stow to allow free air circulation (see Figures "A" & "B").






Frozen products require a very simple stowage arrangement provided they are loaded at the specified carriage temperature. This can be achieved by a solid block stow, with no space between the stow and the container walls. When carrying frozen cargo, the fresh air ventilation hatches must always be closed.

It is important to ensure that the cargo stow covers the entire floor area, projecting beyond the rear floor restrictions of the "T" bars in order to prevent the air short circuiting, and to facilitate an effective flow of return air. In larger containers, if the cargo volume is less than the space in the container, the stow should be of uniform height. As stowage plays an important part in maintaining the quality and security of the cargo during transit, it is essential that specialist advice is sought should there be any doubts when a cargo is booked.

Inspection of cargo

Many bills of lading will allow a carrier to open a sealed container in order to mitigate a potential loss. The method of inspection will depend on the type of cargo and, in most cases, will require the assistance of a specialist.

The pulp temperatures of chilled fruit and vegetable cargoes and core temperatures of frozen cargo must always be measured, where possible, before a reefer unit is stuffed. Fruit and vegetables should also be checked for pre-cooling damage, mould, wilt, dehydration, shrivel, discolouration, soft spots, skin break and slip, bruising, chill damage and odour. Frozen cargoes should be checked for dehydration, desiccation, fluid migration, odours, black spot, colour and flavour changes, and should also be examined for signs of any upward temperature deviation and subsequent re-freezing. Cartons, trays and other packaging should be scrutinised in respect of their suitability to protect the cargo during a long sea transit.

In the event of a reefer machinery unit appearing to function erratically, the vessel should advise its owners and the agents at the next port prior to arrival so that arrangements can be made to rectify the problem ashore if nothing can be done during the sea transit. In such circumstances, a surveyor should be appointed to attend the vessel on arrival in order to inspect and report on the condition of the cargo and the container.

If a container sustains physical damage, the agent and/or the surveyor must ensure that action is taken to rectify the problem without delay so that a potential cargo loss can be minimised.

REEFER CONTAINER TRANSPORT 2 (BASIC OPERATIONAL GUIDELINES FOR REEFER VESSELS)

An analysis of major claims recently carried out by the Club has highlighted a disproportionate number of cargo claims generated by reefer vessels over ten years of age during the past five years. The findings revealed that approximately 50% of reefer vessel claims occurred due to "Reefer Plant Failure" (40%) or "Electrical Failure" (10%) and almost 40% of the claims were directly attributable to the errors of engineer officers or deck officers. Since the failure of plant can often jeopardise an entire cargo, and considering the valuable nature of the commodities generally conveyed, a resulting loss can be immense.

The volume and magnitude of reefer vessel claims has persuaded the Club to introduce positive measures in an effort to minimise reefer losses. Now that it has been established that most claims have arisen due to the breakdown of machinery, the Club will be introducing a policy whereby all reefer vessels of more than ten years old will be inspected on an annual basis. The inspections will be carried out by specialist reefer surveyors and will be directed towards the reefer plant, ancillary equipment, cargo chambers and those responsible for its operation and use.

This Bulletin has been written for the benefit of those Members who are operating reefer vessels, addressing common problems and offering basic practical advice. Such Members are also requested to place copies on board their vessels for the guidance of their crews.

Operation and Maintenance

Perhaps the most crucial factor in the prevention of reefer claims is the appointment of suitably experienced senior officers. It is considered essential that crew changes do not result in the sudden loss of expertise, particularly if personnel are switched between reefer and non-reefer vessels on a regular basis. At least one senior officer in both the deck and engine departments should have several years’ reefer experience.

The majority of reefer plants are automated to a certain degree. In general, sudden faults are infrequent and are usually preceded by an abnormal deviation or trend. Routine attention to operational matters will frequently highlight such trends. For example:-

• A running log should be kept.

• Logs and computer print-out sheets should be monitored and figures compared at regular intervals to pinpoint any adverse trends.

• Adequate local pressure gauges and thermometers should be installed and marked with approved limits.

• The function of all indicator and warning lights should be understood and recorded, and not simply ignored if activated.

• In manually operated plants, the manufacturers control limits should be adhered to.

• All faults or breakdowns should be recorded, however minor.

• All operations staff should be familiar with the activation of standby plant.

Basic day to day maintenance as well as adherence to manufacturers instructions is essential if claims are to be reduced. User maintenance of automatic and non-automatic equipment must include attention to filters, oil levels, leak checks, the running of standby plant and the monitoring of control equipment. Attention should also be given to any other items noted in this Bulletin. It is also important with fully automated plant that operators are capable of handling the plant in manual mode. Standing instructions concerning the operation of the plant should be posted in the main plant room. A well-maintained and operated generating plant is vital to the safe carriage of reefer cargoes. Attention should also be given to the monitoring of electrical equipment, including the regular recording of insulation resistance (megger) readings.
The general cleanliness of the plant is equally important, and is an indication of the care and interest taken by the ship's staff.

Common Faults

A basic understanding of how the plant functions and the close scrutiny of the plant in operation is important. As already noted, the majority of plant failures relate to abnormal activity for some time before a fault actually occurs.
Some common faults in brine and direct expansion systems are as follows;

• Choked sea water cooling filters.
• Dirty heat exchangers (sea water side).
• Choked filters or pipes in freon/CO2 leakage detection systems.
• Contamination in refrigerant filters after overhaul or repair work.
• Drains from battery coils choked. (This can lead to serious cargo contamination in hot sea water defrost systems).
• Inadequate attention to defrosting, particularly in non-automatic systems. (The compartment battery should be independently inspected before and after defrosting).
• Choked hold bilge suctions.
• Dirty air filters on generators and motors.
• Leaking glands in brine pumps.
• Inadequate sounding of hold bilges.
• Misinterpretation of lubricating oil levels in compressors. (A low level or no show in the sight glass is often a sign that lubricating oil has been pumped over into heat exchangers).
• Slack or missing drive belts in motor/compressor systems.
• Inaccurate, missing or faulty direct reading thermometers or pressure gauges.
• Inaccurate setting of alarm and safety cut-out systems.
• Inadequate replacement of driers in gas systems.
• Failure to retain faulty or replaced components for manufacturers assessment.
• Over-reaction to compartment temper-ature changes. (This is particularly applicable in the case of direct expansion systems).
• Lack of a comprehensive fault check list or chart on board. (Members may seek the Club's advice in this respect).

Spares

In general, Classification Societies no longer specify a list of spares to be retained on board. Societies merely recommend that adequate spares together with the tools necessary for maintenance or repair should be carried. Spares themselves must therefore be selected by the shipowner according to the design of the plant and the intended service. The provision of spares is consequently the responsibility of the owner and the Club considers it essential that Members keep an updated inventory. Except in the case of a planned overhaul, spare parts will only be wanted in an emergency, and then in a hurry. It is therefore most important that spares are clearly labelled and identified, and stored in a safe, clean area.
It is also essential that vessels are furnished with an adequate stock of common consumable supplies. On several occasions the Club has found reefer vessels to be without sufficient reserves of spare refrigerant gas, compressor oil, thermometers, gaskets/joints and spare phials for Draegers.

General Advice

In addition to the foregoing comments, the following sound practices are also important;

• All manufacturers’ manuals and instructions should be provided on board. If necessary, these should be professionally translated into the appropriate language spoken on board.

• Members should place instructions and advice on board regarding the stowage, carriage and constraints of various reefer cargoes. Similar guidance should be given regarding the operation of the plant. These instructions should be updated regularly and recorded as read by joining officers.

• All defects or breakdowns should be recorded and reported to Members.

• Preload checks should be carried out and recorded.

The measures outlined above will, if adhered to, minimise the risk of a reefer claim. The Club's Loss Prevention Officer will be glad to assist any Member in need of further guidance.

Wednesday, November 25, 2009

REEFER CONTAINER AIR-DELIVERY SYSTEMS

All reefer containers are bottom-air delivery units, which means that air is consistently supplied from the bottom of the container through the T-bar floor.
The advantage of technology advanced reefer airflow system is that it delivers uniform and consistent air temperature across the entire floor and throughout the entire cargo area.
The precise air temperature, in combination with high evaporator airflow protects fresh products against moisture loss and shrinkage so that sensitive perishable products will stay fresher, longer and in better quality when they arrive at the final destination.

POWER SOURCES AND PORTABLE GENERATORS

ALL refrigerated containers operate on external power sources. Power supply used must be either 380 volts/50 Hz or 440 volts/60 Hz.

Some reefers are equipped with dual voltage supply inclusive of power supply of 190 volts/50 Hz or 220 volts/60 Hz.

Containers may be plugged into a vessel's main power supply or equipped with portable generators for land use. Two types of portable generators are available:

Clip-on generator
For use with refrigerated cargo moving on trains or trucks; attached to the front (nose) of the container.

REEFER CONTAINER MODIFIED/CONTROLLED ATMOSPHERE

When using the modified atmosphere technology, a container is purged of most gases first before a new mixture of gases - at optimum levels and amounts for the commodity being shipped - is injected into the container after it has been sealed. This modified atmosphere technology is a supplement to temperature management that provides more precise control than fresh air exchange. It is effective in slowing respiration and retarding the production of ethylene in horticultural commodities. Lettuce, for example, can be transported across the Pacific using modified atmosphere with excellent results. The modified air content of a container, however, can change during a trip due to the respiratory activity of the commodities and pressure changes in the container, thereby reducing its effectiveness at retarding the deterioration of fresh produce.

Controlled atmosphere is the most technologically advanced process that is used to precisely control the atmospheric composition within the container throughout a shipment's entire journey. Controlled atmosphere can increase the post-harvest life of some perishables by two to three times longer than other methods.

Controlled atmosphere technology uses computer systems to monitor and control the atmosphere in the container and make adjustments during the trip. These systems also record changes in the atmospheric composition during a container's journey and provide a printout for quality-control purposes.

Excellent out-turn of asparagus after 18 days of voyage under 10% O2 and 10% CO2 controlled atmosphere condition.

The composition of the controlled atmosphere transport is commodity-specific. Atmospheric conditions are custom-tailored to provide an optimum environment for each commodity. The availability of controlled atmosphere is important in opening new markets for shippers, allowing the ocean carriage of commodities previously transported by airfreight. Controlled atmosphere can also be used to meet the needs of a particular market by manipulating the ripening rate of fruits and vegetables. Some of the products benefiting from controlled atmosphere include sweet cherries, nectarines, peaches, broccoli, asparagus, avocados, mangoes, cut flowers and chilled meat.

Successful shipping of these commodities is supported by many years of university and industry research. MA and CA differ only in the degree of control. However, CA is more precise in controlling the level of gases. In summary, the following are the key potential benefits of MA/CA shipment which of course is dependent upon the commodity, variety of cultivars, duration of storage, correct post-harvest handling and proper temperature management (most critical factor).

Retard ripening and slow down respiration, ethylene production rates.

Direct or indirect effect on post-harvest pathogens, reduce decay incident and severity.

Alleviate certain fruit physiological disorders such as chilling injury.

Useful tool for insect control in some commodities to meet quarantine requirements of importing countries.

Appropriate level of gases (O 2 & CO 2 ) is essential to attain the potential benefits.

REEFER CONTAINER COLD TREATMENT

Cold treatment is a special post-harvest handling process for perishables to meet local quarantine requirements (e.g. US Department of Agriculture). It is a means of insect control by exterminating Mediterranean and certain tropical fruit flies and larvae.

This is achieved by maintaining a sufficiently low temperature, uninterrupted for a pre-determined duration of time. Cold treatment protocol includes the number of probes, treatment temperature and duration which vary according to different kinds of fruits and depend on the agreement between the country of export and import.


The use of cold treatment process is to eliminate the need for fumigation and the use of certain insecticides, which may be illegal in some countries due to environmental reasons. In general, cold treatment is primarily applicable for various types of citrus fruits such as oranges, lemons and is also common for apples, pears, lychees, star fruits, avocados and kiwi fruits.

Rigid adherence of appropriate cold treatment procedures - product pre-cooling, correct loading and packaging, probe calibration and continuous temperature monitoring - is very critical in preventing process failure. To ensure successful cold treatment shipment, dedicated and experienced APL staff will assist and supervise the installation, loading and monitoring during the entire voyage.

REEFER CONTAINER DEHUMIDIFICATION MODE

The purpose of dehumidification is to reduce or lower the relative humidity in the container during transportation.

Low relative humidity ensures dry packaging, prevents rotting and reduces incident of fungal development and is essential for cargo such as cigarette, electronic components, photographic films, onions, ginger and garlic etc.

Almost all BLUE DOLPHINS reefer equipment are equipped with dehumidification systems, where relative humidity can be set between 65% and 95% and can be controlled according to cargo requirements.

Bulb mode

Bulb mode is an extension of dehumidification mode and upon activation, the user has the option to select the evaporator fan speed (low, high or alternate) and defrost termination sensor setting.

In addition, the relative humidity setting is available from 60% to 95% (instead of the normal 65% to 95%).

Bulb mode is utilized mainly for transportation of flower bulbs, and BLUE DOLPHINS provides containers that are certified by ATO (Agrotechnological Research Institute).

Tuesday, November 24, 2009

REEFER CONTAINER FRESH AIR VENTILATION

Fresh-air exchange helps prevent unwanted ripening and the accumulation of odors, and ensures longer shelf life for many perishables. It is particularly useful for commodities that produce high levels of ethylene, like tomatoes and apples.

The amount of fresh air needed depends on the tolerance of the commodity to low levels of oxygen and high levels of carbon dioxide and ethylene, as well as the rate at which the commodity respires and produces ethylene.  


Fresh-air exchange systems can be set at different levels and are measured in cubic feet per minute (cfm) and in cubic meters per hour (cmh).

Fresh-air exchange involves flushing the container with air from the outside through vent holes. The fresh air ventilation technique is essential to protect some agricultural products by removal of unwanted heat, gases, and carbon dioxide produced by the cargo. However, excessive ventilation may result in freezing of the evaporator coils and will need additional defrost to remove the ice build-up.

POWER PLUG COIL BURN - Reefer Container Return To Depot

marine container
Do not cut power plug safety pin.

power plug coil burn during plug-in at customer premises. This burn happen due to female plug at premises ready kaput.

marine refrigerated container


marine refrigerated container

marine refrigerated container

marine refrigerated container

marine refrigerated container

marine refrigerated container

marine refrigerated container

marine refrigerated container

marine refrigerated container

Wednesday, July 29, 2009

Gas And Pressure

By S. Raha
Manager – Engineering
YORK Refrigeration India Ltd., Pune
Subhankar Raha, a graduate mechanical engineer has worked for York Refrigeration India (Formerly Sabroe Refrigeration / ABB Alfa Stal Refrigeration / Alfa Laval India Ltd) for six years. Earlier he worked for NDDB Anand for five years and Development Consultants Kolkata for seven and a half years. He is a life member of Institution of Engineers, India and can be contacted at sraha@yorkrefindia.com

The industrial refrigeration field, like any other field, is becoming more and more competitive. We therefore need to evaluate and carefully check alternative systems before deciding on a particular refrigeration system for a specific application.

For food processing, fisheries, meat packing or any other similar industry, where process temperature requirement is around (–)40°C, generally a two-stage ammonia refrigeration system has been used.

But we need to rethink the economic viability of such two-stage ammonia systems. One good alternative to consider is a cascade refrigeration system of two refrigerants with carbon dioxide on the low temperature side and ammonia on the high temperature side. This article makes a detailed techno-economic analysis of a CO2 / NH3 cascade system to check for its advantages and disadvantages in comparison with a conventional two-stage ammonia system.

Before going into the detailed analysis of such a system, let us first understand the basic features of these two types of refrigeration systems. For a two-stage ammonia system, there is a low-stage or booster compressor and a high-stage compressor, both operating on a common refrigerant – ammonia. The vapour compression is carried out in two stages. The booster or low-stage compressor discharge is introduced to the suction of the high-stage compressor via an inter-stage cooler (which cools the booster discharge gas by evaporation of high pressure liquid). A typical simple system with a flash type inter-cooler p - h (pressure - enthalpy) diagram is shown in Figure 1.

Fig.01
In a cascade refrigeration system there can be more than one refrigerant depending on the application or requirement of the plant. In such a cascade system, each refrigerant circuit is separate. For the present application CO2 will be used as a refrigerant for the low temperature circuit and ammonia will be used for the high temperature circuit.

The condenser of the CO2 circuit will act as the evaporator of the NH3 circuit (generally known as cascade cooler or cascade condenser). Thus there will be no inter-stage cooler. For better understanding please refer Figure 2 which shows a schematic arrangement for a CO2 / NH3 cascade system.

Now for a detailed techno-economic analysis and comparison, let us consider a typical refrigeration system for food processing or similar application - for which the basic requirements are :

* Process temperature : (–)40 °C
* Evaporating temperature : (–)45 °C (for low-stage or low temperature cycle)
* Condensing temperature : (+)40 °C
* Capacity of plant required : 100 TR (351.63 kW)

Fig.02

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Please note that these parameters form a basis for comparison so that we can make all calculations and evaluate both the systems based on these common system parameters. Similarly for the compressor performance, the same pressure drop or temperature penalty for the suction / discharge line and the same suction gas super heating has been considered for both the systems. Also a specific make of screw or reciprocating compressor has been considered for analysis of operating parameters.

For the two-stage ammonia system we will consider operation of the low-stage compressor at a saturated evaporating temperature (–)45°C and saturated condensing temperature (–)10°C. Whereas, for the highstage compressor, operation has been considered at a saturated evaporating temperature of (–)10°C and saturated condensing temperature of (+)40 °C.

For the CO2 / NH3 cascade system we will consider operation of the low temperature circuit CO2 compressor at a saturated evaporating temperature of (–)45°C and saturated condensing temperature of (–)5°C. Whereas for the high temperature circuit, NH3 compressor operation will be considered at a saturated evaporating temperature of (–)10°C and saturated condensing temperature of (+)40 °C. This overlap of refrigerant temperatures in the cascade condenser is a “must” for such systems.

Fig.03

A typical simple CO2 / NH3 cascade system p - h (pressure - enthalpy) diagram is shown in Figure 3.

With these system parameters, important plant and relevant operating performance parameters are calculated and analysed . The evaluation of these parameters is made for both the options of screw as well as reciprocating compressors. A detailed analysis and comparison of all these basic operating and performance parameters is given in Table 1 (for Screw Compressors) and in Table 2 (for Reciprocating Compressors).

Also, for better understanding, the comparison of these performance-related parameters is shown in graphical form in Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9.

Let us now critically examine these operating performance parameters for both the refrigeration systems, for comparison and analysis, keeping Table 1 & Table 2 as a reference. Columns 3 & 4 show the value of parameters for the low-stage ammonia of the two-stage system and low temperature CO2 circuit of the cascade system respectively. Similarly columns 6 & 7 show the value of parameters for the high-stage ammonia of the two-stage system and high temperature NH3 circuit of the cascade system respectively.

For analysis / comparison of these two systems, the following important operating / performance parameters are considered :

* Capacity of compressor required
* Coefficient of performance (COP)
* Compressor shaft power
* Oil cooler load & oil flow (for screw compressor)
* Volumetric efficiency of compressor
* Adiabatic compression efficiency of compressor
* Condenser capacity & water flow rate required
* Discharge vapour temperature
* Compression ratio
* Suction mass & volume flow rate required
* Compressor swept volume required
* Number of compressors required
* Total number of compressors required (for low + high stage)
* Compressor head / side cover cooling medium required (for recip compressor)

Fig.04

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From a study of Table 1 and Table 2 we can conclude the following advantages / benefits of the CO2 / NH3 cascade system over a two-stage ammonia system:

1. Compressor size (or compressor swept volume) required for the CO2 low-stage side is appreciably smaller as compared to the low-stage ammonia.

CO2 has a much lower vapour specific volume at low temperatures compared to NH3 . This is approximately 97% less at a saturated vapour of (–)45°C. Basically, compressors are selected based on volume flow rate requirement of a particular plant. Greater the vapour volume flow rate requirement - larger the compressor that is required. Hence with this advantage the compressor size for the low-stage is drastically reduced. This effect will have an added advantage when considering superheated suction vapour, as the difference of suction vapour specific volume (for CO2 & NH3 ) will be more for superheated vapour than saturated vapour.

As per Table 1, we require only one CO2 low-stage screw compressor against nine similar capacity NH3 lowstage screw compressors. Similarly, from Table 2 we require only one CO2 low-stage reciprocating compressor against eleven similar capacity NH3 low-stage reciprocating compressors.

Fig.05

The major contributing factor for the initial cost of a refrigeration plant is the compressor (approximately 15 to 25%) and thus the CO2 low-stage system will appreciably reduce the initial plant cost . Each screw compressor is to be taken as a module consisting of compressor and its independent items such as motor, oil cooler, oil separator, suction / discharge valves, suction strainer, coupling, capacity control arrangement, interconnected piping, instruments & controls, cabling, base frame and structural items. Thus when the number of screw compressors is reduced from nine to one, there will be a huge reduction in cost of all such related items.

A related saving is the smaller plant room required and the lower cost of installation.

With the CO2 low-stage system requirement of only 302.79 m3/h compressor swept volume we can easily consider the option of adopting a reciprocating compressor; which in case of ammonia may not be a viable option-as the swept volume required is very high (3420.32 cu.m/hr.). A refrigeration plant with reciprocating compressors will be much cheaper (approximately 10 to 15%) as compared to a plant with screw compressors. Thus we can clearly conclude that with the option of reciprocating compressor, instead of screw compressor, there will be an appreciable saving in initial plant cost.

2. The compression ratio required for the low-stage is much lower for CO2 . It is approximately 44 to 49% less compared to the ammonia booster stage.

The advantages of a lower compression ratio are better volumetric efficiency, lower discharge gas temperature and higher adiabatic compression efficiency. All these advantages of a lower compression ratio will have a greater effect on a reciprocating compressor compared to a screw compressor and this is clear from Table 1 and Table 2.

As the discharge gas temperature is much lower for the low side CO2 compressor, the chances of oil decomposition and its related operating problems are absent. Because of this lower discharge gas temperature and appreciably lower compression ratio, we have the option to adopt a reciprocating compressor for the lowstage CO2 compressor. In the case of ammonia, it is almost impossible to consider a reciprocating compressor for such a high compression ratio and discharge gas temperature.

Fig.06

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As the compressor discharge gas temperature is appreciably lower for the CO2 low-stage screw compressor (59°C as compared to 71.50°C for ammonia) the oil cooler load will also be less for CO2 low-stage screw compressor (can be seen in Table 1). Thus the size of the oil cooler will be smaller, oil circulation rate will be lower, total oil required for the compressor will be less and the oil pump capacity and its power consumption will also be reduced. All these will result in reduction of initial plant cost and operating cost for the CO2 lowstage side of a cascade system.

Similarly, because of low discharge gas temperature (62.40°C as compared to 112.90°C for ammonia) no external water head and side cover cooling is required for a CO2 low-stage reciprocating compressor. In the case of ammonia low-stage reciprocating compressor, a water head / side cover cooling is a "must" under these operating conditions. Thus no water piping with valves, fittings and supports is required for the CO2 low stage reciprocating compressor and this will reduce the cooling water pump power consumption. All of which would result in a reduction of initial as well as operating cost.

With a low discharge gas temperature there is less chance of oil decomposition with its related problems and failure of the discharge valve plate of the compressor. Thus, less service and maintenance is required for such plants with CO2 .
















Fig.07
Table 1 : Comparison of operating parameters between two-stage NH3 and CO2 / NH3 cascade system for Screw compressors
Sr. No. Parameter Low stage of
2-stage system Low temp.
circuit of cascade Remarks High stage of
2-stage system High temp.
circuit of cascade
1 Refrigerant
Ammonia
Carbon dioxide
Ammonia Ammonia
2 Refrigerant designation R 717 R 744
R 717
R 717
3 Sat. evap. / cond. temp. in °C. (-)45 / (-) 10 (-)45 / (-) 5 (-)10 / (+) 40
(-)10 / (+) 40
4 Type of compressor Screw Screw Screw Screw
5 Suction pressure in bar abs. 0.50 8.30 For R717 less than atm. pressure
2.90 2.90
6 Discharge pressure in bar abs. 3.60 30.5 87.05% less for R717 15.70 15.70
7 Capacity of comp. reqd. in TR 100 100 133.45 131.55
8 Capacity of comp. reqd. in kW 351.63 351.63 469.23 462.55
9 COP (comp. cap. / power) 2.99 3.17 6.02% more for R744 3.31 3.31
10 Comp. shaft power in kW 117.60 110.92 5.68% less for R744
141.76 139.74
11 Oil cooler load in kW 53.32 5.13 90.38% less for R744
74.62 73.56
12 Oil flow reqd. in LPM 104.15 36.35 65.09% less for R744 88.29 87.04
13 Volumetric effy. of comp. in % 88.80 88.50 Nearly same since screw comp. 91.40 91.40
14 Adiabatic (compression) effy. in % 65.80 78.50 19.30 % more for R744 80.30 80.30
15 Condenser capacity reqd. in kW N.A.
N.A.
610.99
602.30
16 Condenser water flow rate in LPM N.A. N.A. 2189.39 2158.24
17 Discharge vapour temp. °C 71.50
59
12.50°C less for R744
82.30 82.30
18 Compression ratio 7.2
3.67
49.03% less for R744 5.41 5.41
19 Suction mass flow in kg/ hr. 1026.49
5145.41
80.05 % more for R744 1588.68
1566.07
20 Suction vol. flow rate cu.m./ hr. 2166.58
249.09
88.50% less for R744 687.25 677.47
21 Suction line size in mm NB 150
65
Appreciable lower size for R744 100 100
22 Suctn. line thermal insulation thk. in mm 150 125 Less insultn. reqd. for R744 75 75
23 Wet return line size reqd. in mm NB 200 80 Appreciable lower size for R744 N.A. N.A.
24 Wet return line insulation thk. in mm 200
125 Less insultn. reqd. for R744 N.A.
N.A.
25 Discharge line size reqd. in mm NB 100 50 Appreciable lower size for R744 65 65
26 Comp. swept vol. reqd. in cu.m./hr. 2439.84 281.46 88.46% less for R744 751.91 741.21
27 No. of comp. (@282 cu.m/hr. each) reqd. 9 1 8 nos. additional comp. reqd. for R717 3 3
28 Total power for comps. (low+high) in kW 250.66 kW for cascade system
29 Total no. of comps. reqd. (low+high) 4 nos. for cascade system

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3. The COP (coefficient of performance) for the CO2 low stage compressor is much higher compared to the ammonia compressor for the required operating conditions.

Thus, there is a reduction in compressor power consumption, both for the screw as well as the reciprocating compressor (5.68% less for screw compressor and 18.18% for reciprocating compressor). Based on the total plant capacity we will require a lower kW rating motor for CO2 low-stage compressor (screw or reciprocating). This advantage of lower power consumption has an effect on the entire plant life operating cost. We are all aware of the present-day crisis of electric power and its ever-increasing price all over the world. Therefore, the lower power requirement of a CO2 low-stage compressor for such a cascade system has high potential in reducing operating cost for end users in future refrigeration plants.

The high-side ammonia compressor capacity required will be lower for a cascade system (as compared to the high stage of a conventional two-stage ammonia system). This is because of lower low-stage compressor power for CO2 . From Table 1 & Table 2 this is 1.42% less for a screw compressor and 5.03% less for a reciprocating compressor.

Similarly, the power consumption of the high-stage compressor will be less (1.42% for a screw compressor and 5.03% for a reciprocating compressor) for a cascade system compared to a two-stage ammonia system.

Thus the total power consumption of compressors (low-stage + high-stage) for cascade system is less (3.35% for screw and 11.13% for reciprocating) for a CO2 / NH3 cascade system compared to a two-stage ammonia system.

We can also conclude that the condenser capacity required is lower (1.42 % for screw and 5.03% for reciprocating) for a CO2 / NH3 cascade system compared to a two-stage ammonia system. Thus the condenser size will be smaller, water flow rate across the condenser will be less, pipeline size and valves etc. shall be smaller, cooling water pump capacity and power consumption for such a pump will be less, cooling tower capacity required also will be less, cooling tower fan capacity and power consumption by the fan motor will also be marginally less. All these will result in a further reduction in initial as the well as operating cost of plant.

Fig.08

Table 2 : Comparison of operating parameters between two-stage cascade system for Reciprocating compressors
Sr. No. Parameter Low stage of
2-stage system Low temp.
circuit of cascade Remarks High stage of
2-stage system High temp.
circuit of cascade
1 Refrigerant
Ammonia
Carbon dioxide
Ammonia Ammonia
2 Refrigerant designation R 717 R 744
R 717
R 717
3 Sat. evap. / cond. temp. in °C. (-)45 / (-) 10 (-)45 / (-) 5 (-)10 / (+) 40
(-)10 / (+) 40
4 Type of compressor Reciprocating Reciprocating Reciprocating Reciprocating
5 Suction pressure in bar abs. 0.53 8.16 For R717 less than atm. pressure
2.85 2.85
6 Discharge pressure in bar abs. 3.60 30.84 83.33% less for R717 15.77 15.77
7 Capacity of comp. reqd. in TR 100 100 133.31 131.35
8 Capacity of comp. reqd. in kW 351.63 351.63 486.35 461.86
9 COP (comp. cap. / power) 2.61 3.19 18.18% more for R744
3.13 3.13
10 Comp. shaft power in kW 134.72 110.23 18.18% less for R744 155.38 147.56
11 Comp. head / side cover cooling both water both water 90.38% less for R744 both water both water
12 Volumetric effy. of comp. in % 63 82 68 68
13 Condenser capacity reqd. in kW N.A.
N.A.
641.74
609.42
14 Condenser water flow rate in LPM N.A. N.A. 2299.56 2183.75
15 Discharge vapour temp. °C 112.90 62.40 50.50°C less for R744
137.20 137.20
16 Compression ratio 6.79 3.78 44.30% less for R744 5.53 5.53
17 Suction mass flow in kg/ hr. 1022.57 5134.13 80.08 % more for R744 1651.00 1567.87
18 Suction vol. flow rate cu.m./ hr. 2154.8 248.29 88.48% less for R744 719.84 683.60
19 Suction line size in mm NB 150 65
Appreciable lower size for R744 100 100
20 Suctn. line thermal insulation thk. in mm 150 125 Less insultn. reqd. for R744 75 75
21 Wet return line size reqd. in mm NB 200 80 Appreciable lower size for R744 N.A. N.A.
22 Wet return line insulation thk. in mm 200
125 Less insultn. reqd. for R744 N.A.
N.A.
23 Discharge line size reqd. in mm NB 100 50 Appreciable lower size for R744 65 65
24 Comp. swept vol. reqd. in cu.m./hr. 3420.32 302.79 91.15% less for R744 1058.59 1005.29
25 No. of comp. (@282 cu.m/hr. each) reqd. 11 1 10 nos. additional comp. reqd. for R717 3 3
26 Total power for comps. (low+high) in kW 257.80 kW for cascade system
27 Total no. of comps. reqd. (low+high) 4 nos. for cascade system

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4. The low-stage compressor suction pressure is higher for CO2 , higher than atmospheric pressure; thus there is no chance of entry of air from a low side leakage and its related operating problems. In the case of an ammonia plant this is a common problem in low temperature applications. Hence, for a two-stage ammonia system, a costly automatic air purger with its controls and piping is always used to get rid of this problem of entry of non-condensable air in the system from the low side.

So a costly automatic air purger with its controls, instruments, valves, piping, and thermal insulation, can be totally eliminated for the CO2 plant. This will have an appreciable effect on reducing the initial cost of the plant. Also there is no chance of accumulation of noncondensable air in the system causing high condensing pressure which increases compressor power requirement for the compressor.

Fig.09

5. CO2 suction vapour specific volume is much lower compared to ammonia ; hence for a similar capacity plant the suction line size will be smaller.

From Table 1 and Table 2 we find that for 100 TR (351.63 kW) plant with (–)45 °C evaporation temperature the suction line size will be 65 mm NB for carbon dioxide as compared to 150 mm NB for ammonia. Since the suction line size is smaller the thermal insulation requirement will be also less (insulation thickness with EPS 125 mm for CO2 as compared to 150 mm for NH3 ).

As the suction line size is smaller for CO2 , hence suction valves, strainer, fittings etc. will also be of smaller size. All these items for such low temperature application require low temperature carbon steel (LTCS) or suitable grade Stainless Steel (S.S.) material, which are extremely costly compared to general carbon steel materials. Thus with CO2 refrigerant in the low temperature side of a cascade system there is a significant reduction of initial plant cost and installation cost.

6. The vapour volume flow rate for CO2 at suction temperature is appreciably lower compared to NH3 for the same capacity and temperature. Hence the accumulator / liquid separator used for separating the suction vapour from the liquid to avoid liquid carry over to the compressors can be much smaller. This vessel also calls for LTCS or special grade S.S. material; hence the advantage of a smaller size accumulator will also result in an appreciable reduction in initial plant cost.

The thermal insulation of the accumulator will also be reduced compared to a similar capacity ammonia accumulator. This will result in further reduction in initial cost of plant as well as installation cost.

With a similar logic for pumped re-circulation system the wet return line size and its thermal insulation requirement will also be less compared to ammonia.

7. The discharge condition CO2 vapour has lower specific volume compared to ammonia. Hence the discharge line size for a CO2 plant will be smaller compared to similar capacity ammonia. This will also result in advantages of lower piping, fittings and smaller size valves resulting in a further reduction in the overall plant cost.

8. Because of lower suction / wet return lines, lower size discharge line and a smaller accumulator, the total first charge of refrigerant for such a CO2 / NH3 cascade system will be smaller than a conventional two-stage NH3 system.

The estimated total initial refrigerant charge requirement will be 60 to 70% less as compared to a two-stage ammonia plant.

CO2 is approximately 37% cheaper than ammonia. Thus there will be an additional benefit in future cost saving while replenishing the refrigerant.

9. CO2 gas is non-toxic and non-flammable. Hence carbon dioxide can be used in direct contact with food items.

CO2 is also odourless and it is a better and a safer refrigerant for food processing or other industries. Also, it is environment-friendly and not lethal like ammonia for human inhaling. Hence in food processing industries, where customers object to ammonia because of possible leakage in the food processing area, they can safely decide to go for carbon dioxide, by adopting the CO2 / NH3 cascade system.

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10. CO2 compressors require special synthetic lubricating oil with food grade quality and this is 80% more costly compared to standard lubricating oil required for ammonia compressor. But the requirement of such compressor oil for CO2 is lower because of less number of compressors or smaller compressors and smaller oil cooler. In the case of a two-stage ammonia system, for both the stages (booster & high stage) we need to use a better quality oil suitable for low temperature ammonia service, as both the stages are interconnected . Whereas standard oil for ammonia for (–)10°C temperature can be used for the cascade system ammonia side (since both circuits are separate and independent).
Table3 : Comparison of important properties
Sr. No. Parameter Ammonia Carbon dioxide
1 Chemical formula
NH3
CO2
2 Molecular weight 17 44
3 Refrigerant designation R 717 R 744
4 Critical temp. °C 133 31.06
5 Critical pressure Bar abs. 113 73.84
6 Type Inorganic Compound
7 Boiling point °C at std. atm. pressure( NBP) (-) 33.30
(-)78.30
8 Safety group B2 (evidence of
toxicity identified,
lower flammability limit)
A1 (toxicity
not identified, No
Flame propagation)
9 Sp. Heat at const. press.( Cp) kJ / kg .K
2.1269 0.8709
10 Sp. Heat at const. vol.( Cv) kJ / kg .K 1.6705 0.6783
11 Ratio of Cp / Cv 1.2732 1.2839
12 Gas constant (R) J / kg .K 487 189
13 Flammability Flammable with 16 to
25% by vol. In air Not flammable in air
14 Health hazard Injurious / lethal for
0.5 to1% conc. for
0.5 Hr. exposure Not injurious or lethal
15 ODP factor (ozone depletion potential)
Nil
Nil
16 Sat. press. at (-)45°C sat temp. bar abs. 0.545 8.336
17 Sat. press. at (-)5°C sat temp. bar abs. 3.548 30.47
18 Sp. vol. sat. vap.at (-)45°C temp. cu.m / kg 2.00458 0.0459
19 Sp. vol. sat. liq..at (-)5°C L / kg 1.5495 1.0447
20 Consideration for food contact Direct contact with
food not permitted Can have direct
contact with food
21 Odour Pungent smell Odourless

Thus the overall cost of the first charge of compressor oil (low temperature side CO2 compressor oil and high temperature side ammonia compressor standard oil) will be less as compared to a two-stage ammonia plant.

Please also refer to Table 3 for a comparison between CO2 and NH3 as refrigerants for various important properties and parameters; this shows that as a refrigerant, carbon dioxide can be considered a better refrigerant compared to ammonia. In fact, it was being used long before we became familiar with CFC, HFC, ammonia or Hydro carbons as refrigerants.

But CO2 cannot be used on the high-stage side of the plant, as condensing pressure at 40 °C temperature will be much higher than ammonia. This calls for a condenser design pressure which is extremely high and not economically viable. Hence it is used only in the low temperature side. Whereas ammonia is used on the high temperature side of the cycle, with its benefit of a lower condensing pressure. Thus by having these two refrigerants in a cascade refrigeration system we can make the plant design most cost-effective and optimum, taking advantage of the properties of both the refrigerants, CO2 & NH3 .

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However, like any other system, the CO2 / NH3 cascade system has some disadvantages as compared to a two-stage ammonia refrigeration system. The major disadvantages of such a cascade refrigeration system are:

1. For carbon dioxide the saturated pressure is much higher (more than 75 bar) when liquid refrigerant is warmed to ambient temperature (say 40°C). This condition would require that all the components in the low temperature circuit be suitable for such high pressure, which is economically not viable.

To make the plant viable a suitable volume fade-out vessel is provided on the CO2 circuit (in between the condenser and the chiller) to permit the liquid refrigerant to be warmed to room temperature. When the plant is shut down for a long period, such a situation may occur. A fade-out vessel is simply an empty vessel that is open to the cascade refrigerant CO2 . This is designed with suitable volume so that when the system is shut down and temperature rises the liquid can expand to vapour without exceeding a reasonable limiting pressure. Lower the limiting pressure, larger is the volume required of the fade-out vessel. Thus we can have, say 40°C equalising temperature which has enough room to expand to vapour at a pressure not higher than 32 bar absolute. This is additional equipment required for cascade systems
2. In the case of liquid overfeed refrigeration system, the CO2 liquid pump capacity required is 2.5 to 3.5 times higher than an NH3 pump for similar operating parameters. Thus, liquid line sizes for such a pump suction and discharge will be higher compared to ammonia.
3. The CO2 side vessel and exchanger design pressure is higher compared to the booster ammonia side vessel and heat exchanger.

In spite of the above mentioned disadvantages the CO2 / NH3 cascade system can be considered a better and more cost-effective proposal for a system requiring less than (–)40 °C evaporation.
Conclusion

Since compressors and lubricating oils suitable for CO2 are already available in the market and since a CO2 / NH3 combination in a cascade system performs better and at a lower cost, both initial and operating, than the conventional two-stage NH3 system there is every possibility of such systems becoming the standard for all food processing and industrial applications in the future.

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