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By Hyeok Lee and Yong Su Kim, Ship & Ocean R&D Institute, Daewoo Shipbuilding & Marine Engineering Co., Ltd., Geoje City, Korea
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The amount of trade and variety of cargo that make use of large containers has greatly increased during the last two decades. Refrigerated, or reefer containers, which transport frozen or chilled food, need to keep their cargoes from perishing during long journeys. Container ships carry many reefer containers at a time, and the delivery time is longer than for trains and trucks, so a separate cooling system is required for the refrigeration unit. Air-cooled (ventilation) systems have been adopted in recent years, and are gradually replacing water-cooled systems because they offer reasonable performance at low cost. The regulation and classification procedures that are in place for the design and manufacturing of ships and ship systems are now being developed for reefer ventilation systems as well. In addition to meeting regulations, ship owners also want to understand the performance of the refrigeration cooling system, since it impacts the quality of the refrigerated cargo.
Pathlines for portside NEMS reefer containers
(colored by particle ID)
Temperature distribution on the outside of NEMS reefer containers
Reefer ventilation systems consist of ventilation ducts and fans. The ducts are located between the watertight bulkhead and container guide structure, and the fans are located at the ends of each duct. There are two types of reefer ventilation system in common use today, named for the method used to supply cool air: NEMS (Natural Exhaust-Mechanical Supply), and MENS (Mechanical Exhaust-Natural Supply).
Usually there are three to four cargo holds in a 4000TEU container ship (which holds about 4000 20 ft. containers). In a typical cargo hold, there is hot exhaust air from the condensers of the refrigeration units mixing with the air inside the hold and cool air from the outside. The resulting complex flow is one of the main reasons that it has been difficult to establish regulations for reefer ventilation systems. Daewoo Shipbuilding & Marine Engineering Co. Ltd. (DSME) has used FLUENT to analyze reefer ventilation systems since 2001. Their goal is to establish an objective method for evaluating the performance of, and making improvements to, these systems.
In a typical CFD analysis of a ventilation system, the performance of NEMS and MENS type systems are compared. Results have indicated that the average cargo hold temperature of a MENS system is lower than that of a NEMS system, but in the access area, the NEMS temperature is lower.
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Comparison of temperature (K) distributions on a plane of the NEMS test model, computed by CFD (left) and measured (right) |
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Comparison of the overall temperature distribution for the test model, operated using MENS (left) and NEMS (right); the MENS system produces lower temperatures |
The accuracy of the CFD model needs to be validated as well, and separate analyses have been done for this purpose, using data from sea trial records and the results of a simplified test rig. The test rig measurements revealed that MENS is more efficient than NEMS on the basis of temperature rise inside the cargo hold. A CFD analysis of the test rig was performed and it validated this result. In addition, through the CFD analysis, it was found that ventilation duct arrangement, in particular the location of each duct branch, is as important to the cooling performance as the type of ventilation system used. By verifying FLUENT's reliability through model tests, DSME has found FLUENT to be a dependable tool for reefer ventilation analysis. In the future, DSME plans to widen its CFD application area to cover more of the company's working procedures.
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