Absorption chiller

Absorption chiller with 1.4 MW cooling capacity on a transporter

An absorption refrigeration machine (AKM for short) is a refrigeration machine in which, in contrast to the compression refrigeration machine , the compression takes place by means of a temperature-influenced solution of the refrigerant . This is also known as a thermal compressor . The refrigerant is absorbed in a second substance in a solvent cycle at low temperatures and desorbed at higher temperatures . The process uses the temperature dependency of the physical solubility of two substances. The prerequisite for the process is that the two substances are soluble in each other in the temperature range used.

Basic processes

Schematic diagram of an absorption refrigeration machine.

Like any chiller, an absorption chiller is a heat pump . H. a device that transports heat from an (inner) area with temperature to an (outer) area with temperature , where : The purpose of the chiller is to maintain the low temperature in the inner area (e.g. inside a refrigerator or a building to be cooled) or to reduce it even further. According to the second law of thermodynamics , this is only possible by supplying energy from outside. ${\displaystyle T_{1}}$ ${\displaystyle T_{2}}$ ${\displaystyle T_{1}\leq T_{2}}$ ${\displaystyle T_{1}}$

In the refrigeration machine, a refrigerant circulates in a closed circuit between the inner and outer areas. In the inner area, refrigerant is evaporated in the so-called evaporator, thereby extracting heat from the environment and lowering the temperature . The gaseous refrigerant is directed to the outer area and liquefied there again. In the case of compression chillers, liquefaction occurs by increasing the pressure, whereas in absorption chillers, on the other hand, it is through salt solutions that have the ability to bind refrigerant vapor, i.e. H. to absorb. ${\displaystyle T_{1}}$

One of the most popular types of absorption chillers is the lithium bromide absorption chiller. It uses water as a refrigerant. In an almost evacuated evaporator , it is sprayed onto a pipe coil and evaporated at approx. 3 ° C. This makes use of the fact that the evaporation temperature also decreases with falling pressure.

The water vapor is absorbed by a LiBr solution in the so-called absorber . The absorption process would stop as soon as the saline solution was saturated with refrigerant. Therefore, in the third step, the refrigerant must be permanently removed from the salt solution. For this purpose, the salt solution enriched with refrigerant is pumped into the so-called generator or expeller , in which the refrigerant is boiled through the action of heat (approx. 80 to 120 ° C) and evaporated again - albeit at a significantly higher temperature and pressure level. The resulting fluids (the concentrated salt solution and the refrigerant vapor) are then separated from one another.

The evaporated refrigerant is cooled in the condenser . As a result, it condenses and can then be fed back to the evaporator. This completes the refrigerant cycle. The concentrated salt solution is returned to the absorber in a second separate circuit.

In compression refrigeration machines, the energy supply necessary to increase the temperature gradient is usually provided by electrical energy, i. H. the mechanical energy for compression is supplied by electrically operated compressors. In the case of absorption chillers, however, mainly thermal energy is supplied. Absorption chillers are usually less efficient than compression chillers, but they are advantageous wherever heat is available cheaply or free of charge, for example in the form of waste heat.

development

The absorption refrigeration cycle is considered to be the oldest known technical process for generating refrigeration and originally the desire for deep freezing was the decisive reason for the development of heat pumps in 1755. In the first experiments by William Cullen , a physician and chemist, water was frozen with the aid of a vacuum. A continuous overall process was not developed.

It was not until 22 years later, in 1777, that the principles of absorption were discovered and understood. In 1810, John Leslie developed an absorption refrigeration system with the refrigerant water and the absorbent sulfuric acid. The first reliably working refrigerator, with essential parts of the cold steam engine, was built by Jacob Perkins in 1834 with a mechanical compressor. These advances were filed in his patent No. 6662 Apparatur for Producing Cold and Cooling Fluids . The extremely flammable refrigerant diethyl ether hindered further development, so that only after his death did economic interest in this invention rise sharply.

In 1840 John Leslie built a working ice machine based on Perkins' patent specification. As a result, in 1850 Edmond Carré produced an ice machine based on sulfuric acid and water as a working medium. As a further development, Carré replaced the pair of working fluids with ammonia and water and wrote it down in his patent in 1859. In this and the following patents, Ferdinand Carré described, on the one hand, periodically working machines for very small capacities and, on the other hand, machines with large capacities. These patents laid the foundation for further developments and were the first industrially significant.

William Thomson was able to prove in 1852 that chillers can be used for heating rather than cooling. In his publication Heating Machine , he demonstrated that a motor-driven heat pump consumes less primary energy than direct heating. Another pioneer in the field of absorption machines was Charles Tellier , who built his system with dimethyl ether in 1864 . These developments continued until 1927 when the first refrigerator came onto the market in Germany.

The company Carrier Corporation began in 1940 with research on a lithium bromide / water absorption chiller and led 1945 the first major development. These units were designed for a capacity of 100 to 700 tons and worked with low-pressure steam as a heat source.

Nowadays, an absorption refrigerator is built into almost every mobile home and caravan in order to be independent of an electrical supply. In these refrigerators, the necessary heating is generated by burning fuels, usually propane or butane gas. Most devices also allow the optional operation with electrical heating cartridges, which are available for both mains power and on-board power. Furthermore, these refrigerators are also used in hotel rooms, but here they are only supplied with electrical energy, and guarantee the desired noiselessness with continuous cooling.

For a few years now, absorption heat pumps have been available for domestic and industrial use in the range from a few kilowatts to several megawatts. These are available in different designs for different areas of application. Pure heat pumps are used for heating, cooling or a combination of both.

Heat ratio

In the case of refrigeration systems, the refrigeration capacity is assessed in relation to the energy used. In an AKM, the heat ratio is used, which is defined as the quotient of the cooling capacity at the evaporator to the heating capacity at the expeller : ${\displaystyle \zeta }$ ${\displaystyle {\dot {Q}}_{0}}$ ${\displaystyle {\dot {Q}}_{A}}$

${\displaystyle \zeta ={\frac {{\dot {Q}}_{0}}{{\dot {Q}}_{A}}}}$

Compression refrigeration systems (KKM for short) use the EER ( energy efficiency ratio ), formerly COP ( coefficient of performance ).

A mere comparison of the heat ratio of the AKM and the coefficient of performance EER of a KKM does not make sense, since the AKM mostly uses waste heat and the KKM uses exergy-rich electricity or gas. The different values ​​of the energies introduced into the process (exergy) must be taken into account. ${\displaystyle \zeta }$

The coefficient of performance EER refers to the electrical power used at a certain operating point, usually 100%. Their value changes when the operating point is shifted, e.g. B. the external temperature, therefore KKM are also used with more meaningful performance figures that also take into account the part load and falling outside temperatures ( Integrated Part Load Value (IPLV for short) and Non-Standard Part Load Value (NPLV for short) according to AHRI Standard 550/590 ( Air Conditioning, Heating and Refrigeration Institute (AHRI for short); European Seasonal Energy Efficiency Ratio (ESEER for short) according to Eurovent).

Ammonia-water absorption refrigeration system

In the ammonia-water absorption chiller ( diffusion absorption chiller ), ammonia is the refrigerant and water is used as the solvent. The advantage of this combination is the use of natural materials that are very inexpensive. Depending on the design of the system, it is designed with or without an inhibitor (e.g. sodium dichromate ) in the solvent circuit in order to avoid corrosion. The heat ratio of the systems depends on the evaporation temperature and is in the range of 0.65 (evaporation temperature = 0 ° C) and 0.3 (evaporation temperature = −50 ° C) for single-stage devices. ${\displaystyle \zeta }$

Areas of application

The most widespread in terms of numbers are small ammonia-water absorption refrigeration systems as refrigerators in camping areas or hotel refrigerators. Large systems are traditionally found in the freeze drying and chemical industries. The systems are primarily designed as individual systems, and the cooling capacities are in the MW range. By using the waste heat z. B. from gas engines to generate cold by means of an absorption refrigeration system, a new area of ​​application for the systems was opened up with the power-refrigeration coupling. The cooling capacity is significantly lower than that of the large systems. For this reason, prefabricated system modules are built that are delivered ready for connection in one or more installation frames.

Industrial absorption chillers

Industrial directly heated absorption refrigeration system with evaporation temperatures down to −60 ° C (for description see text )

In the solvent evaporator of the absorption refrigeration system, the expeller, there is an ammonia-water mixture. This solution is heated indirectly by steam or directly by an oil or, in this case, a gas burner . The advantage is that any heat source that can provide the required evaporation temperature is suitable. The solution evaporates at temperatures of 170 ° C. and pressures of 10 bar. From the boiling curve for the ammonia-water mixture, a ratio of 5% by weight of ammonia in the water of the evaporator can be determined for this boiling state. One speaks here of the poor hot solution. The vapors are fed to the distillative separation column ( rectification column as a further development of a simple distillation plant) TS , which is arranged above the evaporator. The column consists of an elongated, upright cylindrical vessel equipped with bell or tunnel trays arranged one above the other. There is a layer of liquid on the trays, which runs off via a weir onto the tray below and is returned to the evaporator at the bottom of the column. The gas rises in countercurrent to the top of the column. The openings in the gas-carrying internals are designed so that the gas phase bubbles through the liquid on the floor and an exchange of energy and substances takes place. The temperature-dependent equilibrium between the liquid condensed on the tray and the vapor phase exists on each partition. There is a concentration of the lower-boiling phase towards the top of the column.

Rich solution is added as feed in the middle part of the column. The column part lying below the feed is the stripping part and the one above it is the amplifier part. At the top of the column, the ammonia is heavily enriched in the gas phase with a residual proportion of approx. 0.2% by weight water. The vapors are fed to the liquefier ( condenser ). The liquefied ammonia is stored in the high pressure receiver. A partial flow of the liquefied distillate is returned to the upper tray of the column as a return flow (reflux - liquid ammonia). The number of dividers required can be determined using a McCabe-Thiele diagram. For optimization, the cold gas from the connected cold consumers is fed to a heat exchanger 3 in order to cool the flow of the liquid refrigerant. The rest of the process is analogous to the compression refrigeration machine.

The refrigerant ammonia is evaporated through the absorption of heat by the cold consumers . After the heat exchange with the liquid flow, the vapors are conducted to the absorption chamber via the suction line. The poor, cooled solution from the expeller is used as the absorbent. The poor solution is injected into the absorber and the absorber is cooled with cooling water in order to dissipate the heat of the solution. The cold solution tends to absorb ammonia until it is saturated. At 40 ° C and 0.5 bar (abs) the rich solution in the absorber can reach an ammonia weight fraction of 15%. The cold, rich solution obtained in the absorber is pumped through the above-mentioned solvent heat exchanger 1, heated there and passed into the expeller via a level control.

The refrigeration consumer circuit is not shown in the picture. The supercooled liquid ammonia can be injected into evaporators via thermostatic control valves and absorb heat from the room to be cooled through evaporation. Mostly, however, pump systems are used, since a complex regulation of the ammonia mass flow in the evaporator flow can be dispensed with here. The liquid ammonia is fed into a separator via a level control valve (high pressure float or level controlled valve). The ammonia is fed to flooded evaporators via refrigerant pumps. The operating mode is referred to as flooded, as only part of the liquid refrigerant evaporates and both ammonia gas and ammonia liquid are returned to the separator. The separator is used to buffer the refrigerant due to level changes that result from changing refrigeration requirements (refrigerant shift in non-operated evaporators, change in the specific volume when the temperature changes). Another function of the separator is to separate the liquid from the gas phase. The gaseous ammonia flows through the suction line into the absorber, in which this gas transport is maintained through the absorption of the ammonia gas in the poor cold solution and removal of the heat of the solution.

There are many variants of the arrangement of the refrigeration circuit of an ammonia-water absorption refrigeration system described here. For example, instead of the ammonia reflux, a liquefier in the head of the rectification column ( dephlegmator ) can be used to cool the top of the column , to which the cold, rich solution from the absorber or cooling water is applied.

The evaporation temperature of the absorption refrigeration system is limited by the existing heating medium and cooling water temperatures. For thermal optimization and the use of heat at the lowest possible temperature level, it is possible to carry out the absorption process on the drive side in two stages. The outlay on equipment is significantly higher because two absorbers and expellers are required. The first expeller stage can, however, be heated at significantly lower temperatures.

Ammonia absorption refrigeration systems are used in particular at low evaporation temperatures, since in the systems, in contrast to compression refrigeration systems, no oil is introduced into the refrigeration circuit. Due to the viscosity gradient at low temperatures, the removal of oil from the low points of the consumer circuits in compression refrigeration systems is problematic.

The absorption refrigeration system in the h-ξ diagram

h- diagram and representation of an ammonia-water absorption refrigeration system ( see text for description ) ${\displaystyle \xi }$

The h- diagram shows the boiling and dew line and the associated enthalpies depending on the mixing ratio of two phases . The states of an ammonia-water cooling system can be entered in the diagram for ammonia and water (see diagram). ${\displaystyle \xi }$

Point 8 describes the boiling state in the evaporator (expeller): p = 10 bar / T = 170 ° C. The state of the steam above solution 7 is on an isotherm to 8 . The enrichment of the steam in the rectification column up to the top is outlined in point 9 . In the example, a head temperature of 50 ° C is entered. The ammonia is enriched to 99.5% by weight in the vapors at the top of the column. The states on the individual column trays are not shown here. Depending on the temperature of the soil under consideration and the assumption of an equilibrium between vapor and liquid, the states of the two phases lie on an isotherm. At the specified pressure of 10 bar, the dew line shows the vapor state and the boiling line shows the state of the liquid.

The isotherm is formed at point 9 and point 9 * is obtained on the boiling line . The change in state between 9 * and 8 represents the liquid flowing back in the column. The liquid running off over the trays is heated up again by the gas countercurrent. The ammonia gas 9 concentrated in the top of the column is liquefied isobarically: state 1 . The refrigerant is led to the low-pressure separator and expanded adiabatically (no longer shown in the flow diagram). A partial amount evaporates during the relaxation (state 2 ) and the supercooled liquid 12 can be used for cold generation by being pumped to the cold evaporators in the cold rooms. The ammonia gas from the separator 2 is absorbed by the poor cold solution in the absorber. The heat released during absorption can be read from the associated enthalpies between 2 and 6 . Point 6 in the diagram describes the state in the absorber (here: p = 0.3 bar; T = 25 ° C); the ammonia concentration here is 20% by weight. The rich solution is heated in the solvent heat exchanger by the hot poor solution that flows to the absorber and the rich solution is then fed back to the expeller.

Small systems

One advantage of small systems (camping fridges, minibars in hotels) is the silent operation, since mechanical solvent pumps can be dispensed with thanks to the additional hydrogen. The necessary flow of the solvent water is ensured by a vapor bubble pump , which works on the principle of the mammoth pump . As a result, compared to a compressor required for a compression refrigeration machine, very much lower noise emissions are achieved and completely silent operation is made possible. As a rule, the heat required to operate these systems is provided by electrical power either with on-board power of 12 volts or mains power. Self-sufficient operation is possible by burning fuel gases such as propane , butane , mixtures thereof or petroleum , which are common in camping . The required thermal output for conventional refrigerators is around 120 watts. The efficiency (COP) of these small systems is around 0.2.

Boiling temperatures

• Ammonia: −33.33 ° C / 1 bar
• Water: 100 ° C / 1 bar

Water lithium bromide absorption refrigeration machines (AKM for short)

Lithium bromide absorption heat pump with an output of 14 MW

In addition to the material pair ammonia / water also is lithium bromide / water use, although here water is the refrigerant. This also limits the lowest cold water outlet temperature to approx. 5 ° C. Absorption refrigeration systems with the combination of substances are therefore usually used in air conditioning and in process cooling. There are essentially one and two-stage versions. Single-stage AKM are fired with hot water (70… 120 ° C) or steam (max. 1.5 bar) and have a heat ratio of approx. 0.7. Two-stage AKM can be heated with hot water (up to approx. 180 ° C), steam (up to approx. 8 bar ) or fueled with oil, natural gas or with exhaust gas from combined heat and power plants (CHP) or gas turbines. The heat ratio in this design is 1.0… 1.3. The systems are therefore only used economically if waste heat is available (if possible free of charge or very cheaply) or solar-generated heat is available. ${\displaystyle \zeta }$

Directly heated LiBr absorption refrigeration systems have cooling capacities from 10 kW to 5,300 kW. Single-stage systems are offered as series products in the cooling capacity range from 15 kW to 5,300 kW. There are also special designs up to 23 MW.

The advantage of the LiBr absorption refrigeration systems is the low expulsion temperature and the harmlessness of using water as a refrigerant. Since the cold is generated in the negative pressure range, bursting due to overpressure is impossible if the heating is secured. Another advantage lies in the widely differing boiling temperatures of the material pairs LiBr and water. As a result, pure water vapor is generated in the expeller during desorption. In contrast to this, in the case of the ammonia-water absorption refrigeration system, in addition to the refrigerant vapor, water vapor is also produced in the expeller. In a complex, downstream process, the refrigerant must therefore be concentrated and the water vapor removed (see rectification).

In the case of LiBr absorption refrigeration systems, the technical work of the pump is also much less important (approx. By a factor of 500).

description

Water-lithium bromide absorption refrigeration system, Carrier type

The water-lithium bromide absorption refrigeration system contains the same components as the ammonia-water absorption refrigeration system with the exception of the rectification column. This can be dispensed with because lithium bromide has practically no vapor pressure at the temperatures used and is therefore non-volatile. The company Carrier uses two cylindrical tanks, which are equipped with dividing walls, pipe coils and nozzles according to their function. The upper container represents the expeller AT . The poor solution is heated indirectly by supplying heat via a pipe coil and the water evaporates. In the right area of ​​the chamber pipe coils are installed through which cooling water KüW flows. The evaporated and not loaded with salt water condenses in the right chamber sump.

The lower container accommodates the evaporator part VD and the absorber part AB . The water-poor and thus LiBr-rich solution 1 from the expeller is cooled via a solution heat exchanger WT1 and injected in a controlled manner via a nozzle in the left part of the lower chamber 2 . The salt-rich, finely-dispersed solution tries to absorb the water vapor in the chamber. The resulting heat of solution is transferred to the cooling water via cooling coils. In the left sump of the chamber, the water-rich solution 3 is passed through the heat exchanger WT1 via the solvent pump , preheated and conveyed back into the upper expeller chamber .

In the lower chamber there is a strong negative pressure of around 2 mbar, which corresponds to a saturated water vapor temperature of 6 ° C. In the right part of the chamber, water is pumped in a circle 6 and atomized. With the negative pressure caused by the absorption, the water evaporates at a temperature of 6 ° C. The heat of evaporation is supplied by the cold water KW , which is guided in pipe coils in the atomization area of ​​the water. The water is therefore the refrigerant that extracts the heat from the cold water.

Areas of application

Water-lithium bromide absorption refrigeration systems are used in the following areas:

• Cold water generation for air conditioning or process cooling from the waste heat from engines or gas turbines ( combined heat, power and cooling )
• Reduction of the summer peak electricity consumption for building air conditioning, which is caused by electrically operated compression refrigeration machines, and instead use of natural gas or heating oil and solar energy via collectors as an energy source
• Cold water generation for air conditioning and process cooling from waste heat from CHP units, gas turbines, power plants , waste incineration plants and district heating (steam or hot water) in summer when heat is created as a "waste product".

Further procedures

A new, largely still unknown alternative is the use of ionic liquids as absorption media. These are often characterized by a high activity towards water, but have the advantage over lithium bromide that they cannot crystallize out as “liquid salts” when driven out. They are also less corrosive than lithium bromide.

See also

• Diffusion absorption refrigerator
• Adsorption chiller
• Chiller

literature

• F. Ziegler: Cooling with sorption chillers, sanitary and heating technology. Issue 7/2000, pp. 42-47.
• J. Reichelt: Where is refrigeration technology in Germany and worldwide? 2000, pp. 4-9. (PDF; 52 kB)
• Walter Maake, Hans-Jürgen Eckert: Pohlmann-Taschenbuch der Kältetechnik , 1978, ISBN 3-7880-7092-7
• Jürgen Langreck: Refrigeration with ammonia and water - building blocks for absorption refrigeration systems. In: The refrigeration and air conditioning technology. 11/1999