Perishable cargo are placed in reefer containers

Food provisions for the crew have to be placed in cold rooms

Some gas carriers use refrigeration to cool the cargo

Refrigeration is needed on board a cargo ship for storing of food provisions. Food provisions are stored in cold rooms. The cold rooms are demarcated for various temperatures. 

The rooms for storing meat, and fish are set at minus 25 degree C, while those used for vegetables are set at 10 degree C. When the proper temperatures are maintained, the food can last very long. 

This is required if the ship were to travel for long voyages lasting several months. The lack of fresh food it can be demoralizing for the seamen at sea. It is the Chief Engineer's job to make sure that the refrigeration system works well all the time.

The refrigeration system on board ship is not much different from land installations, using refrigerant like R-12, compressor, evaporator, and condenser. 

One important difference is the use of seawater to remove the heat. The condenser is a shell and tube type of heat exchanger with its own seawater pump. Individual thermostatic expansion valves and solenoid valves are installed for each cold room to cater for the different temperature requirements. 

Naturally, a passenger ship, or a reefer ship will have a much larger refrigeration system.