Repsol YPF

Absorption systems

An absorption system can be one of two different types:

a) Cooling units.
b) Mixed cooling and heating units (heat pump).

These systems are based on the absorption processes between substances that have a certain type of physico-chemical affinity between them and which also have very different boiling points, enabling them to be separated through the addition of heat to the mixture.

The most volatile substance acts as a refrigerant, whilst that which has the highest boiling point acts as an absorbent.

The appliances can be single-effect, with just one generation stage or one stage for separating the refrigerant and absorbent substances; or double-effect, with two generation stages, which provides a greater performace than the single effect.

The cycle carried out by the single-effect system is shown in the diagram below:

1-2: As heat is added to the heat generator, externally by a gas burner, the temperature of a mixture of refrigerant and absorbent substances, with a very high refrigerant (water) content, rises. As the temperature rises, the components separate: part of the refrigerant leaves the solution in vapour form and the solution, now with a low refrigerant content, passes into the absorber due to the difference in pressure.
2-3: The refrigerating vapour is cooled by a water or air condenser and passes to a liquid phase.
3-4: Following this, the refrigerant is subjected to expansion, which makes it pass through a heat exchanger, causing it to evaporate and cooling the water in the external refrigeration circuit. This water is used to cool the premises in which the air conditioning is required.
4-1: Once it has been converted into a vapour phase, the refrigerant is reabsorbed by the other component (absorbent) and the pressure of the liquid mixture is raised by pumping it towards the heat generator, thus closing the absorption cycle.

The double-effect cycle differs from the previous one in that the solution is heated in two stages, allowing better use to be made of the condensation heat: in the first stage, the high temperature stage, the solution receives the heat directly from the gas burner (generator) ; in the second stage, the low temperature stage, the refrigerating vapour that is formed in the high temperature zone is used to heat the solution that is in the low temperature part, where a lower pressure has been created and which therefore has a lower boiling point.

In general, systems that run on hot water are single-effect, whilst those with a direct flame, vapour or exhaust gases can be single- or double-effect. Single-effect systems have a refrigeration COP value of between 0.5 and 0.75, whilst double-effect systems have values closer to one unit. The most commonly used mixtures are those made up of lithium bromide and water and ammonia and water:

• Litihium bromide – water: the refrigerant is the water and the absorbent substance is the lithium bromide. These systems are characterised by the fact that water is used for the cooling process in the absorber and condenser.
• Water – ammonia: the refrigerant is the ammonia and the absorbing agent is the water. In these systems, the condensation can be by water or by air.

In lithium bromide-water systems, an evaporation tower needs to be used to lower the temperature of the water coolant, as it is impossible to lower the water temperature with outside air when it is at a higher temperature than the water (for example, in summer with temperatures of 35ºC). In view of this requirement, these systems are not recommended in areas with high temperatures and high atmospheric humidity, due to the corrosion and great investment that the evaporating towers entail.

In order for the system to work in heating mode, it is just necessary to open the bypass valve, which allows the refrigerating vapour to pass straight through, without passing through the condenser, at the same time as the flow of water coolant is intercepted. In the evaporator, the vapour heats a fluid, such as water, which is flowing through a section of tubing. This heated fluid is suitable for use in heating installations.

Air conditioning systems which incorporate absorption systems can take one of two different forms; either that of a modular unit, made up of an absorption system for cold production and a conventional gas boiler for heating, which therefore caters for the heating and cooling needs of the premises in one sole module, or that of an absorption unit whcih produces cold when the absorption cycle acts as a refrigeration cycle and generates heat when it acts as a heating cycle. In some cases, the use of an auxiliary boiler is required.