How Ammonia Refrigeration Systems Work
I often see questions about this and wonder what sort of answer someone is expecting. Almost all industrial refrigeration use ammonia as a refrigerant. Although there is a renewed interest in CO2 refrigeration systems, ammonia is the dominant refrigerant by choice. When compared to any other mechanical refrigeration system the basic fundamentals apply.
The four basic components of a mechanical refrigeration system consist of the following:
- Compressor (a means of increasing the refrigerant vapor to a high pressure)
- Evaporator (heat absorption device)
- Throttling device (expands liquid from a higher pressure to a two-phase mixture at lower pressure)
- Condenser (heat rejection device)
In any mechanical refrigeration system you will find these four basic components.
A fifth component is also required to allow the four items listed above to work together. This is the piping, which connects the four components into a system. Otherwise you would just have parts and pieces that do nothing. If the four basic components are properly selected with either; the piping not being installed correctly or an improper pipe diameter chosen, the performance of the system will be less than expected.
Anyone with a basic understanding of a residential refrigerator or air conditioning system already knows how an ammonia refrigeration operates.
The compressor increases the pressure of the vapor to a pressure sufficient for condensing the vapor into a liquid based on the available heat sink temperature. The heat sink temperature is either water, air, or a combination of both. Cooler heat sink temperature’s will allow lower condensing pressures. Conversely, high heat sink temperature’s require the condensing pressure to be even greater.
Once the vapor is condensed into a liquid the liquid flows to the throttling device. The liquid is expanded from the condensing pressure to the lower pressure desired in the evaporator. This pressure reduction of liquid creates a two-phase mixture of liquid and vapor. The vapor is called flash gas, which is the energy expended to cool down the high pressure liquid to a low pressure liquid.
In some system arrangements the flash gas and cold liquid are fed directly into the evaporator. This is what occurs in direct expansion liquid feed systems. This liquid flow control could be accomplished by a capillary tube or thermostatic expansion valve.
In other configurations the flash gas is separated and the cold liquid is fed solely into the evaporators. This occurs in liquid overfeed systems or gravity flooded evaporators. The flash gas and the vapor formed inside of the evaporator are returned to the compressor to increase the pressure for condensing to occur once again.
In the condenser the high pressure, high temperature vapor is desuperheated and condensed. All of this heat is removed by circulating water, air or both over the condenser heat transfer surface.
The process is continuous for as long as cooling is required to maintain the temperature of the space.
In short, the biggest difference between ammonia refrigeration systems and a residential refrigerator is based on this: various types of heat exchangers and liquid feed systems are used with a different refrigerant.
There are several more slight difference though. In an ammonia refrigeration system the components are a lot larger and ammonia stinks (to some).