Ammonia refrigeration systems operate in a similar manner to fluorocarbon systems, but have several key differences. While you will not usually find an ammonia-based system inside a home (ammonia is a very toxic substance, and the refrigerators are very expensive), they are used in factories that need large refrigeration devices that can cool substances very quickly.
A refrigeration system is based on a type of refrigerant gas, which is constantly run through the system to gather and disperse heat. These gases are made of many different substances--most household refrigerants are actually a synthetic mixture designed for efficiency, but ammonia-based version simply use ammonia. Whatever the type of gas, it is passed through various devices, including a compressor, condenser, expansion device and evaporator.
Ammonia as a Refrigerant
Each part of a refrigerator is designed to change the state of the gas in some way. By changing the state of the gas, the system also changes its temperature and how much heat it carries. The compressor, for instance, makes the gas hot and raises its pressure, enabling it to hold larger amounts of heat. The condenser changes the gas to a liquid, allowing it to lose some of its heat in the process, while the expansion device turns the liquid back into a cold gas, releasing most of the heat it held. The evaporator cools the gas into a chilled vapor that is ready to be circulated back through the system. This is how all refrigerators remove heat from their compartments and disperse it.
Ammonia refrigerators in factory settings need to cool very quickly. While household refrigerators take a few minutes to begin cooling after they are started, this delay is not acceptable in a manufacturing environment. To begin refrigeration immediately, the ammonia is distributed among pressure vessels that separate liquid ammonia from the gas, store the refrigerant and send it to different parts of the system when it is needed. By contrast, a hydrocarbon refrigerator has no pressure vessels.
Ammonia-based refrigerators do not need to constantly recombine oils into their systems to function properly, so oils are quickly drained rather than dissolved back into the gas. Ammonia systems are also able to deal with the accidental collection of water in some pipes, something hydrocarbon systems cannot work with. The water will still need to be drained, but ammonia can function even with water present. Of course, these positive factors also come with some drawbacks. Ammonia is a very caustic chemical and ammonia systems must be made with steel or nickel. No copper or copper-based piping can be used.