The absorption processes in refrigeration

The absorption processes

In the refrigerator process, there is a step where the refrigerant leaves the evaporator at low temperature and low pressure and is transformed into high temperature, high pressure steam. This allows cooling water to use higher temperature.

For a compression chiller, this step is done by the compressor. The refrigerant compression is expensive. Operation, installation, control equipment, the compressor itself - everything is expensive.

There are other methods as well. In the absorption process, the vapor coming from the evaporator in an auxiliary liquid - the absorbent - absorbed (dissolved). Subsequently, a pump increases the pressure of the liquid solution. Increasing the pressure of a fluid through a pump is a simpler and cheaper process than vapor compression. Another advantage of the absorption process is the quiet operation. A pump makes considerably less noise than a compressor. At higher pressure, the refrigerant and the absorbent are separated by distillation or simple evaporation. It creates - just like the compression refrigeration process - a high-pressure refrigerant vapor, which is then liquefied and relaxed as in a normal cycle.

During distillation, low-grade heat is added in a generator (expeller). This heat replaces the mechanical energy during vapor compression. After expelling the high pressure refrigerant vapor, the regenerated absorbent is returned to the absorber.

There are mainly two types of absorption systems: ammonia / water and water / lithium bromide.

Ammonia / water is a high pressure system (~ 24 bar), suitable for compact installations. Ammonia is the refrigerant and water is the absorbent. There are no particular corrosion problems except that copper and zinc should not be used. Normally, stainless steel is used.

Water / lithium bromide is a low pressure system (vacuum - 1 bar). Water is the refrigerant and lithium bromide is the absorbent. The concentrated lithium bromide solution may be corrosive to common structural material and special precautions must be taken. Due to the large volume of steam, the system becomes quite bulky.

applications

The economics of the two processes depend on the available energy. If cheap electric power is available, the compression chiller could be more economical. If low-level waste heat is available at a low price or for free, the absorption cycle could be the cheaper solution.

There are basically three types of applications for absorption processes where plate heat exchangers can be used. Typical of most is that the heat for the generator must be virtually free and that the heat exchanger play a crucial role in the efficiency of the system.

◆   waste heat utilization

A dairy, a slaughterhouse, a fish processing factory, etc. require electricity, steam / heat and cold water / cold.

The electricity is generated in a diesel engine or a gas turbine, the resulting combustion gas produces steam. Part of the steam or possibly the combustion gas is then used for the absorption system.

Industrial plate heat exchangers (all fully or cassette welded) are used at all points of the sorption plant.

◆   Chemical industry

sometimes produce large amounts of excess heat that can be used to produce cold water. Full or cassette-welded PHEs are used.

◆ Residential    limi - / heat pumps

This is a new development. In many countries, especially in East Asia, there is a demand for housing heating during the winter and air conditioning during the summer.

In contrast to more northerly states, which have peak energy consumption in winter, electric energy consumption is high throughout the year. The natural gas network has a consumption peak in winter. The growth of air conditioning units in recent years has led to an overloading of the electricity grid in the summer, while the natural gas grid is underutilized. All living room air conditioners require quiet units. Absorption A / C heat pumps are thus an excellent alternative. However, units based on S & THEs become too heavy (~ 250 kg) to be of practical use. These are sold as white goods, such as refrigerators, stoves, etc., and delivered by a van of two people. The solution could be heat pumps / air conditioners, based on heat exchangers soldered with nickel (or other ammonia resistant material). The weight can then be halved. The further development of such systems is necessary.

Was: Alfa Laval