Design of lithium bromide absorption refrigerating machines

Lithium bromide absorption refrigerating machines are becoming widely used both in Russia and abroad.

Such machines are divided into the following types based on the thermal energy:

• How-water units;
• Direct burning of gas or liquid fuels;
• Units built on the basis of the heat recovery of the combustion products.

The building sphere in Russia deals mostly with the use of one-step absorption refrigerating units driven by hot water. One-step operation means units which operate by means of the one-step steam generation. The use of such units is explained by their prices, availability of boiler plants and by the use of combined cooling, heating and power. In the process of generating decentralized electrical and thermal energy in the course of cogeneration (based on gas reciprocating or gas turbine units) the issue of excess heat disposal is solved by the use of lithium bromide absorption refrigerating machines. Such machines shall also be cooled as any other refrigerating units but the low potential thermal energy (temperature range 30-35°C). Lithium bromide absorption refrigerating machines operating on direct burning of gas or liquid fuels shall be considered as hazardous facilities with high safety specifications. Directly fired lithium bromide refrigerating machines are more thermodynamically efficient due to the missing intermediary circuit of the heat carrier compared to the hot water driven units.

 There are more complicated solutions for the use of lithium bromide refrigerating machines equipped with vapor compression refrigerating machines. All the above mentioned solutions are aimed mostly for decreasing electricity consumption necessary to generate cooling for industrial, public and administrative buildings. Secondary energy resources of high potential make it possible to use absorption refrigerating machines. In case hot water or gas are used as main energy resources or there are some problems with electricity then this equipment may be used.

Minuses of the lithium bromide refrigerating machines are the missing of open cooling towers and necessity of a constant feeding that results in replacement of the heat carrier due to the increase of diluted salts and the necessity to install the open cooling towers higher than the hydro kits. Another disadvantage is pallets and extended contact surface what leads to a heavy weight of the wet cooling towers.

One of the system advantages is in a significantly low power consumption, an extended life-time (when properly operated and maintained up to 30-40 years) compared to vapor compression units and the key advantage is the simultaneous generation of electrical power, thermal power and cooling. 

Lithium bromide absorption refrigerating machines principle of work

Distilled water is used as a refrigerant in such machines. It is widely known that each pure substance which is under the critical point and each condensing or boiling temperature correspond to the fixed saturated pressure (the pressure when liquid transits to vapor or vice versa). The same concerns the fact that the fixed saturated temperature corresponds to each saturated pressure. That’s why by reducing vapor pressure one may reduce liquid boiling temperature or vapor condensation temperature. The type of the process – boiling or condensation – depends on the initial condition of the environment as well as on thermal feeding or dissipation.

As the boiling process requires heat to be delivered to liquid that corresponds to the value of the phase transition then heat shall be abstracted to condensate vapor. In order to have the processes running under different temperatures, it is necessary to provide different pressure values as of boiling and condensation. However, absorption refrigerating machines use the aqueous solution of lithium bromide salts. The choice of water and lithium bromide is based on the dissolubility as well as on the significant temperature difference for water and lithium bromide saturation under equal pressure. The aqueous solution of lithium bromide salts makes the solution transfer from one system to another easier. Lithium bromide absorption refrigerating machines use the technology where the working agent and refrigerant firstly transit the generator, then the aqueous solution of lithium bromide is delivered to the absorber passing the solution exchanger. Steam vapor that formed inside the generator transits to the condensation unit. Then the refrigerating agent condensation process takes place by means of heat extraction. The refrigerating agent is then pushed to the evaporator and boils under the lower saturated pressure. Steam vapor generated in the evaporator is sent to the absorber which is followed by the process of absorption of the steam vapor by the aqueous solution of the lithium bromide salts and the cycle is then closed. The aqueous solution of lithium bromide salts is carried out by pumps.

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