Answer EKG Question 9

Q9. With Respect to Container Ship: –
A. Sketch a ship’s indirect refrigeration system arranged for cooling containers stowed in stacks in the hold;
B. Describe the refrigeration system sketched in (A) ;
C. State the advantages of the system described in (A) compared with containers with their own refrigeration self-contained units.

Answer: (A) sketch of ship's indirect refrigeration arranged for cooling container stowed in stacks in the hold.

 

B. Description of  the refrigeration system sketched above:-

Special porthole container ships are equipped with refrigeration units permanently installed below deck which supply the containers with the cold air they require. Originally, individual refrigeration ducts were used to supply cold air to up to 48 refrigerated containers and were arranged horizontally and vertically. Subsequently, only a vertical arrangement was used. This was better suited to on-board handling, enabling a clear division of containers and temperatures and producing smaller batches of cargo.

 Working principle of indirect refrigeration arrangement:-

•   In cooling systems permanently installed on the ship, heat is dissipated from the porthole containers via fixed ducts in the cooling system. The vertical cooling ducts are divided into two channels (supply air duct and return air duct).
•  The supply air duct is connected to the lower aperture in the end wall of the container via switchable couplings. The cold air flows through this duct into the container below the grating, through the cargo and then back over the cargo through the upper opening via the coupling to the return air duct.
•  The heated air is drawn off from the return air channel by the refrigeration duct fan and then conveyed back into the supply air channel by the fan.
• Figure shows an indirect system with an intermediate brine circuit which connects the cooling system which is located centrally in the engine room to the cold air system in the hold.
•  The supply air temperature is here controlled by adjusting the three-way valve in the brine circuit. 
•  Since the installed refrigeration units are relatively large, porthole containers may thus achieve a higher level of efficiency than integral containers. 
•  the arrangement of the refrigeration ducts with the couplings in the hold of a container ship on which up to seven refrigerated containers can be stacked and connected to a single refrigeration duct.
• Sometimes, porthole containers are also transported on deck. If this is done, a "clip-on unit" is attached to the container which supplies it with cold air. Clip-on units are also used to refrigerate the containers at terminals or when transporting them by truck.

Working principle of direct refrigeration arrangement:- 

 In contrast to portable containers, integral refrigerated containers are equipped with their own refrigeration unit. This normally relies on a three-phase electrical power supply. Cold air flows through and around the goods in the container. This air is blown in through the gratings in the floor and then drawn off again below the container ceiling. The circulating fans then force the air through the air cooler, which also acts as the evaporator in the cold circuit, and back through the gratings into the cargo. Integral containers are dominating the porthole type containers. Thus the direct cooling system is dominating over indirect cooling. Direct cool containers provide flexibility while loading of cargo on board while indirect cooled containers are required in load in a manner to keep similar cargo type together.