A reefer ship is a cargo vessel that specializes in carriage of cargo that requires to be maintained at a temperature other than ambient temperature. Reefer ships can carry any frozen or cooled cargo including meat, fish, fruits, vegetables. The temperature range is -30 to +12 deg C depending on the type of the cargo.
A reefer ship design incorporates more pipelines than those on a tanker; these lines are refrigerant lines which lead to each cargo hold. Usually this refrigerant is secondary cooling element and is brine. These brine lines lead to a cooling battery pair located in each deck. Each such brine line feeds a bank of cooling coils per battery, which cools the forced air flow generated by cooling fans over each such coil.
A reefer ship design incorporates more pipelines than those on a tanker; these lines are refrigerant lines which lead to each cargo hold. Usually this refrigerant is secondary cooling element and is brine. These brine lines lead to a cooling battery pair located in each deck. Each such brine line feeds a bank of cooling coils per battery, which cools the forced air flow generated by cooling fans over each such coil.
Secondary Refrigerants
The primary refrigerants can be very searching, that is they can escape through minute clearances, so it is essential to keep the number of possible leakage points to a minimum. Secondary refrigerants are usually liquids, and are used to transfer heat from the substance being cooled to a heat exchanger, where the heat is absorbed by a primary refrigerant. Secondary Refrigerants like brine are used where the installation is large and complex (reefer ships), to avoid circulation of expensive primary refrigerants in large quantities. In this case, the primary refrigerant is circulated to the evaporator in the secondary refrigerant, which is then transferred to the space to be cooled.
What are the advantages of secondary refrigerant ?
1. Low initial cost2. Low maintenance cost
3. Suitable for large refrigeration plant
4. Easily produced on board by mixing CaCl2 and distilled water.
5. Easily store as a salt on board
BRINE: Brine is made by mixing 250 grams of calcium chloride in 1 liter of fresh water. As the percentage of calcium chloride is increased the freezing point of the resultant brine will decrease. Sodium dichromate or lime may be added to maintain brine in alkaline condition in order to prevent corrosion.
Common salt brine may be used under emergencies as a replacement. Sea water can not be used as brine as it is highly corrosive and has scale forming properties. Also sea water does not have sufficiently low freezing point.
ETHYLENE GLYCOL:Systems have been designed in which brine is replaced by one of the Glycols, for example ethylene glycol. The glycols have an advantage of being non-corrosive and may be used at much lower temperature than brine.
TRICHOLROETHYLENE: Trichloroethylene has also being used as a secondary refrigerant, but has a disadvantage of being toxic and a solvent of many synthetic rubbers and other materials normally used as jointing.
Refrigeration system of a Reefer ship using Brine as Secondary refrigerant
This system of refrigeration using brine as secondary refrigerant is used on reefer ships for cooling cargo holds. Vapour compression system using Primary Refrigerants cools the brine at the evaporator coil. Brine gets cooled to about -150C and is circulated to the refer holds. By controlling the flow rate of brine through each hold at the return valves, we can control the temperature of any hold.
A circulating fan maintains the air circulation over cooling coils in the cargo holds.
A circulating fan maintains the air circulation over cooling coils in the cargo holds.
Cargo Refrigeration
 |
| Central Refrigeration system |
Refrigerated cargo vessel typically requires a system that provides cooling at various temperature. The arrangements installed could be in three parts i.e the installation of the cooling unit, the main system, brine circuit system with the flow of air to cool the charge in the charging space.
In the figure, a cooling system is shown. The coolant flow through the cooler which is divided into four units, with their own expansion valves for each of them. The four units are used for controlling the amount of the evaporator surfaces, depending on the degree of charge at a time of charging is required and the capacitor, which gives greater flexibility in the system. The large oil separator is a feature of the screw compressor systems and illustrates the return oil circuit.
Each circuit has its own primary refrigerant evaporator in the brine cooler (as shown in the figure) resulting in a completely independent gas system. Probably there will be three of these systems in a freight ship or a container.
Because they are completely independent, any method could be adapted controlling the output of brine at various temperature. Each temperature of the brine is identified by a color and has its own circulating pump. Cold brine is then fed to the cooling air space and controlled by the temperature of the air to leave the cooler.In the figure, a cooling system is shown. The coolant flow through the cooler which is divided into four units, with their own expansion valves for each of them. The four units are used for controlling the amount of the evaporator surfaces, depending on the degree of charge at a time of charging is required and the capacitor, which gives greater flexibility in the system. The large oil separator is a feature of the screw compressor systems and illustrates the return oil circuit.
Each circuit has its own primary refrigerant evaporator in the brine cooler (as shown in the figure) resulting in a completely independent gas system. Probably there will be three of these systems in a freight ship or a container.
The refrigerant in the loader space for the airflow over and through the front charge before returning. A fan arrangement and ducts direct the air to the cooling room and under load. The load is applied to the cooling air to the stand stacked by the load.
#Image source credit: www.nafsgreen.gr
# Various books, study material and other online sources has been refereed prior to writing this article but no part is copied or produced from any of the source but explained same thing in better detailed way.
Author Amit
Posts
Post a Comment