The simplified lithium bromate (LiBr) and water (H2O) absorption cycle consists of a condenser, an evaporator, an absorber, a pump, and a steam generator.
In these cycles there may also be a heat exchanger, many times called an economizer, which transfers heat between the hot and cold absorber solution streams, thus recycling energy.
The goal of this system is to absorb heat from the evaporator, so in fact you are removing heat from the outside by cooling the environment. In this type of cycle the absorber and steam generator together replace the compressor in steam compression cycles.
The special feature of this cycle is that it requires very work labor because the pumping process is done with a liquid. (For a reversible process and in steady state and with the changes in kinetic and potential energy, the work is equal to -∫ and the specific volume of the liquid is much smaller than that of the steam.)
Water in this case acts as the coolant, and instead lithium bromate is the absorber, due to the fact that it is a hygroscopic substance, i.e., a substance capable of readily absorbing water molecules present in the surrounding environment.
Some of the advantages of this cycle are:
– the cost of installation of the system and the cost of operation,
– the fact that the high heat of vaporization of water allows high cooling capacity to be obtained with relatively small fluid flow rates: the most important saving comes from the regenerative exchanger between the rich and poor mixture.
As disadvantages, there are:
– the low coefficient of performance (COP) compared to vapor compression refrigeration cycles, which discourages its use in cases of direct combustion;
– the need for larger cooling towers;
– the corrosive potential of lithium bromide, which is why corrosion inhibitor additives are used;
– the use of water as a cooling fluid limits the temperature in the evaporator to values above 5°C.
Milena Vilar França
DSc degree in Mechanical Engineering
Engineering Dept., Unilab Srl
ASHRAE, ASHRAE Handbook – Fundamentals, American Soceity of Heating, Refrigerating, and Air Conditioning Engineers, Atlanta, 2017.
ASHRAE, ASHRAE Handbook – Refrigeration, American Soceity of Heating, Refrigerating, and Air Conditioning Engineers, Atlanta, 2018.
E. Fornasieri, September 1982, Air conditioning, heating, refrigeration: Lithium bromide absorption refrigeration machines.
Stoecker, Wilbert F. and Jones, Jerold W. 1985. Refrigeração e Ar Condicionado. s.l.: McGraw-Hill do Brasil, 1985.
National Institute of Standards and Technology website https://webbook.nist.gov/cgi/cbook.cgi?ID=C7550358&Mask=2 (lithium bromide).