What is an Absorption System?

An absorption system is a means of refrigeration or air conditioning where cooling is achieved using a fluid or gas combination and a heat source. Unlike conventional compressor driven systems, absorption cooling utilizes a secondary absorbent fluid and heat from a propane or kerosene flame to create the pressure differential necessary to circulate the refrigerant. The rest of the basic refrigerant cooling cycle is shared by both systems, however. There are two basic types of absorption system: lithium bromide/water and ammonia/water. The absence of a compressor makes the absorption system appropriate for applications where mains power is unavailable or unreliable.

The main difference between compressor and absorption systems is the method used to pressurize and circulate the refrigerant. Absorption cooling appliances do not use a compressor but rather rely on the energy from an external heat source to drive the system. This heat source is typically a propane or kerosene flame, although solar heating is becoming more popular as an energy component. Apart from this fundamental departure, absorption refrigerators and air conditioners use the same basic principle of heat transfer as that of their compressor driven counterparts.

There are two basic types of absorption system, each working in a similar fashion but suitable for different temperature ranges. The first is the lithium bromide/water system for applications when temperatures of 32° F (0° C) and above are required. In these systems, the lithium bromide component is used as an absorbent and the water is the refrigerant agent. The second type of absorption system is the ammonia/water variant used in applications requiring temperatures of 32° F and below. This type of system utilizes the ammonia as a refrigerant and water as an absorbent.

Absorption systems consist of five basic stages: generator, separator, condenser, evaporator, and absorber. The refrigeration process starts in the generator where heat is applied to the refrigerant/absorbent mixture. Here the refrigerant is boiled off to form a gas and flows, with the absorbent, to the separator. The two elements are then removed from each other, and the refrigerant cycles through to the condenser. In the condenser, the refrigerant gas sheds through condensation and its heat regains its liquid state.

The liquid refrigerant then flows into the evaporator located in the room or refrigerator interior. Here it mixes with a pressurized charge of hydrogen gas which causes the refrigerant to absorb heat from the inside of the space and rapidly evaporate. From the evaporator, the refrigerant and hydrogen gas flow to the absorber where the absorbent that was originally separated is remixed with them. The absorbent forms a solution, and the refrigerant and the shed hydrogen gas travels back to the evaporator. It is this stage of the cycle which gives the absorption system its name.

Once this process is complete, the refrigerant/absorbent mixture reenter the generator to resume the cycle. The lack of a mains supply in absorption system coolers makes these systems ideal for campers, recreational vehicles (RVs), boats, and caravans. Locations where the mains power grid is unreliable are also good applications for this type of air conditioning and refrigeration. These systems also may make a lot of sense in today's increasingly conservation conscious environment.