Absorption refrigerator

An absorption chiller (short AKM ) is a refrigerating machine, in contrast to compression refrigeration machine is in the compression by a temperature- influenced solution of the refrigerant. This is also known as thermal compressor. The refrigerant is absorbed in a solvent circuit at low temperature in a second material and desorbed at higher temperatures. In the process, the temperature dependence of the physical solubility of two materials is used. Is a prerequisite for the process that the two substances are soluble in each other in the temperature interval used in any ratio.

• 5.1 Description
• 5.2 Areas of application

Development

The absorption refrigeration cycle is considered the oldest known industrial process for producing refrigeration and originally the desire for deep freezing was the main reason to deal in 1755 with the development of heat pumps. In the first experiments of William Cullen, a physician and chemist, water was frozen with the aid of vacuum. A continuously operating whole process has not been developed.

It was not until 22 years later, in 1777, the principles of absorption were discovered and understood. John Leslie developed in 1810 an absorption refrigeration system with the refrigerant water and the absorbent sulfuric acid. The first reliably functioning refrigerator, with substantial portions of cold steam engine was built in 1834 by Jacob Perkins with a mechanically operated compressor. This progress has been filed in his patent No. 6662 " Apparatus for Producing Cold and Cooling Fluids". The explosive refrigerant diethyl ether ( ethyl ether ) hindered the further development, so that after his death a steep rise in the economic interest of this invention.

1840 was built by John Leslie -based Perkins patent working ice machine. Then one based on sulfuric acid and water as the working fluid ice machine was launched commercially in 1850 by Edmond Carré. In the further development of the working fluid pair was replaced by ammonia and water and written down in his patent in 1859 by Carré. In this and the following patents Ferdinand Carré described to a periodically functioning machine for very small services and for other machines with great achievements. These patents laid the foundation for further development and were the first industrially significant.

William Thomson was able to prove in 1852 that chillers can be used for cooling rather than heating. In its publication "Heating Machine" has been demonstrated that in an engine-driven heat pump less primary energy must be expended than in a direct heating. Another pioneer in the field of absorption machines was Charles Tellier, the 1864 system with its methyl ether built. These developments were continued until 1927 until the first refrigerator in Germany came on the market.

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

Today, in almost every camper an absorption refrigerator is built to be independent of electrical supply. In these refrigerators, the required heat by burning fuel, typically propane or butane produced. Most devices also allow optional operation with electric cartridge heaters are available for both AC power and on-board power. Furthermore, these refrigerators used in hotel rooms, but supplied here only with electric power, and guarantee the required silence during continuous cooling.

Since a few years, absorption heat pumps are available up to several megawatts for domestic and industrial use in the range of a few kilowatts. These are available in different designs for different applications. Pure heat pumps are used for heating, cooling, or a combination of both.

Basic processes

The force required to vaporize a liquid temperature decreases with decreasing pressure, ie The refrigerant is evaporated at low temperatures due to the low absolute pressure in the evaporator. In the case of LiBr absorption chiller water is used as a refrigerant is sprayed onto a coil at a near- evacuated container, and evaporated at about 3 ° C. The required for vaporization heat of vaporization is the building chilled water withdrawn in the coil, which is available as usable refrigeration available. The evaporation process was stopped when the saturation pressure of the refrigerant in the evaporator would be achieved. Therefore, the refrigerant vapor must be continuously removed in a second step. Here is made of the properties of certain salt solutions advantage to bind refrigerant vapor from the air, ie to absorb. Hence the name " absorption chiller ". This step takes place in the so-called absorber. Also, the absorption process would stop when the salt solution is saturated with refrigerant. Therefore, the refrigerant must be removed in the third step of the salt solution permanently. For this, the refrigerant -enriched salt solution is pumped to the generator so-called expeller or in which the moisture by heat - is boiled ( 80 ° 120 ° C) and evaporated again - but at a significantly higher temperature and pressure level. The concentrated salt solution is recycled back to the absorber in a closed circuit. The refrigerant vapor from the expeller is cooled down in the final step in the condenser by means of cooling water from a cooling circuit ( for example with cooling tower) again to the initial level, and then condensed to the evaporator.

Heat ratio

In refrigeration systems, the cooling capacity is evaluated in relation to the energy expended. With an absorption chiller is used, the ratio of heat, which is defined as the ratio of the cooling capacity of the evaporator for heating the expeller:

In compression refrigeration systems (in short, KKM ) is the coefficient of performance EER ( energy efficiency ratio), formerly COP ( coefficient of performance ) was used.

A sole juxtaposition of heat ratio of the AKM and the COP EER a KKM makes little sense, since in the AKM for the most part waste heat is used and exergiereicher in the KKM electricity or gas. The difference in value of the introduced in the process energy ( exergy ) must be considered.

The coefficient of performance EER refers to the electric power spent in a specific operating usually 100%. Their value changes with a shift of the operating point, eg the external temperature, and therefore KKM consider with more meaningful figure of merit which is also the part-load and falling outside temperatures used (Integrated Part Load Value ( short IPLV ) and Non-Standard Part Load Value ( short NPLV ) to AHRI Standard 550/590 (Air -Conditioning, Heating and Refrigeration Institute ( AHRI short ) ); European Seasonal EnergyEfficiency ratio ( short ESEER ) Eurovent ).

Ammonia-water absorption refrigeration system

In the ammonia-water absorption refrigeration system is the ammonia refrigerant, and water is used as solvent. The advantage of this combination is the use of natural substances that are very inexpensive. It is designed depending on the version of the system with or without inhibitor (eg sodium dichromate ) in the solvent circuit to prevent corrosion. The ratio of the thermal conditioning depends on the evaporation temperature, in the range of 0.65 ( evaporation temperature = 0 ° C ) and 0.3 (evaporation temperature = -50 ° C).

Areas of application

Numerically, the most common being small ammonia-water absorption refrigeration systems as a refrigerator in the camping area or hotel refrigerators. Large plants are traditionally found in the freeze-drying and in the chemical industry. The systems are primarily designed as single units custom, and the cooling capacities are in the MW range. By utilizing the waste heat eg gas engines to generate cold by means of an absorption refrigeration system was developed with the power, cooling and power, a new application area for the plants. The cooling capacity is significantly lower than for larger plants. It pre- conditioning modules are therefore built to be delivered ready for connection in one or more built-in frame.

Industrial absorption chillers

In the solvent evaporator of the absorption refrigeration system, the expeller, there is an ammonia - water mixture. This solution is indirectly heated directly by steam or by an oil or a gas burner here. The advantage is that any source of heat, which can provide the required evaporation temperature, is suitable. The solution is evaporated at temperatures of 170 ° C and pressures of 10 bar. From the boiling curve for the ammonia - water mixture boiling for this condition, a ratio of 5 percent - are determined % ammonia content in the water of the evaporator. One speaks here of the poor hot solution. The vapors are fed to the distillation column ( rectifying column as a further development of a simple Destillatiosnanlage ) TS, which is arranged above the evaporator. The column consists of an elongated upright cylindrical vessel which is equipped with superposed bell or tunnel trays. On the trays is a liquid layer, which runs over a weir on the tray below, and is fed back into the evaporator at the bottom of the column. Counter-current the gas rises in the column to the top. The openings of the gas-conducting baffles are designed so that the gas phase bubbles through the liquid standing on the ground and an energy - and mass exchange takes place. On each divided base, there is a temperature-dependent equilibrium between the liquid and condensed in the bottom of the vapor phase. There is a concentration of low-boiling phase to the column head instead of down.

Rich solution is given in the middle part of the column as a feed. The column part lying below the feeds is the stripping section and the overlying part of the amplifier. At the column head, the ammonia in the gas phase is greatly enriched with a residual content of about 0.2 wt - % of water. The vapors are fed to the condenser. The liquefied ammonia is stored in the high- pressure accumulator. A partial flow of the liquefied distillate as back flow ( reflux - liquid ammonia ) returned to the top tray of the column. The number of required separation trays can be determined with a McCabe - Thiele diagram. To optimize the cold gas from the cold consumers is connected to a heat exchanger 3 is supplied to cool the supply of the liquid refrigerant. The further process is analogous to the compression chiller.

By absorbing heat to the cooling equipment, the refrigerant ammonia is evaporated. The vapors are passed through the suction line after the heat exchange with the liquid flow to Absorbstionskammer. As absorbent poor cooled solution is used from the expeller. The lean solution is injected into the absorber and the absorber is cooled with cooling water to dissipate the heat of solution. The cold solution has to take the effort of ammonia to saturation. At 40 ° C and 0.5 bar (abs) the rich solution can reach a weight proportion of 15% ammonia in the absorber. The waste in absorber cold rich solution is pumped through the above-mentioned solvent heat exchanger 1, where it is heated and passed through a level control in the expeller.

The refrigeration consumer circuit is not shown in the image. The subcooled liquid ammonia can be injected into evaporators on thermostatic control valves and absorb through evaporation heat from the space to be cooled. But most pump systems are used, since a complex regulation of the ammonia mass flow can be omitted in the evaporator flow. The liquid ammonia is passed through a level control valve (high pressure float or level- controlled valve ) into a separator. About refrigerant pumps the ammonia flooded evaporator is fed. The operation is referred to as flooding, since only a portion of the liquid refrigerant is vaporized and both ammonia gas and ammonia fluid is returned to the separator. The separator is used for buffering the refrigerant due to level changes that are characterized by varying cooling requirements result ( refrigerant migration in non-operated vaporizers, change of the specific volume with temperature change ). Another function of the separator is to separate the liquid from the gas phase. The gaseous ammonia flows through the suction pipe into the absorber, in which the absorption of the ammonia gas into the cold solution and poor dissipation of the heat of solution just this transport gas is maintained.

There are many variations to the arrangement described herein of the refrigerant circuit of an ammonia-water absorption refrigeration system. For example, can be used for cooling the top of the column instead of ammonia, a reflux condenser at the head of the rectification column ( dephlegmator ), which is supplied with the cold rich solution from the absorber, or with cooling water.

The evaporation temperature of the absorption refrigeration system is limited by the available heating and cooling water temperatures. To heat engineering optimization and application of heat at a low temperature level as possible, it is possible to carry out the absorption process on the drive side in two stages. The equipment costs are significantly higher, since two absorber and desorber are necessary. The first expeller, however, can be heated with significantly lower temperatures.

Ammonia absorption refrigeration systems are used in particular at low evaporating temperatures, as in the plants, in contrast to compression refrigeration systems no oil is introduced into the refrigeration cycle. Due to the viscosity gradient at low temperatures the discharge of oil from the low points of the consumer circuits in compression refrigeration systems is problematic.

The absorption refrigeration system in h - diagram

The h - Graph showing depending on the mixing ratio of two phases, the boiling and dew line and the corresponding enthalpies. In the diagram for ammonia and water, the states of an ammonia-water refrigeration system can be entered ( see diagram).

The point 8 describes the boiling state in the evaporator ( expeller ): p = 10 bar / T = 170 ° C. The state of the vapor above the solution is 7 to 8 at an isothermal enrichment of the vapor in the rectification column to the head of the point 9, in the sketched example, a top temperature of 50 ° C is added. In the vapor at the column head, the ammonia is at 99.5 - % enriched. The states of the individual trays are not shown here. In dependence on the temperature of the observed ground and the adoption of an equilibrium between the vapor and liquid states of the two phases are isothermal. The vapor line is at the predetermined pressure of 10 bar to the vapor state, and the boiling state of the liquid.

In point 9, the isotherm is formed and on the boiling point is obtained the 9 *. The state change between 9 * and 8, the returning fluid in the column represents the above soils effluent is heated by the gas counter-current again. Concentrated in the top of the column, ammonia gas is liquefied 9 isobaric: state 1, the refrigerant is routed to the low-pressure separator and adiabatic relaxed ( in the flow image not shown). A subset evaporated during the expansion ( condition 2 ), and for cooling, the subcooled liquid 12 can be used by being pumped to the cold evaporator in refrigerators. The ammonia gas from the separator 2 is absorbed in the absorber of the poor cold solution. The energy released in the heat of absorption can be read from the corresponding enthalpies 2-6. The point 6 in the diagram describes the state in the absorber (here, p = 0.3 bar, T = 25 ° C); the ammonia concentration is here 20 wt - %. The rich solution is heated in the solvent heat exchanger from the hot weak solution flowing to the absorber and the rich solution is then fed to the expeller.

Small plants

One advantage of small systems (camping fridges, mini bar in hotels ) is in silent mode, as can be eliminated by additionally introduced hydrogen on mechanical solvent pumps. The necessary flow of the solvent water is ensured by a vapor bubble pump, which operates on the principle of the lift pump. This compounds an already much lower compared to a compressor required for a compression chiller noise emission is obtained, and a completely silent operation allows. In general, the heat required for the operation of these systems is provided by electric current either with on-board power of 12 volts or AC power. Self-sufficient operation is possible by the burning of fuel gases, such as the usual in camping propane, butane or mixtures thereof. The required thermal capacity is usual refrigerators to about 120 watts. The efficiency (COP ) of these small plants is approximately 0.2.

Boiling temperatures

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

Water - lithium bromide absorption chillers (short AKM )

In addition to the fluid pair ammonia / water and lithium bromide / water is common, although in this case water is the refrigerant. This also has the lowest chilled water outlet temperature is limited to about 5 ° C. Absorption refrigeration systems with the combination of substances are therefore usually used in the climate field and process cooling. There is a two-stage designs and substantially. Single- AKM -fired hot water ( 70 .. 120 ° C) or steam ( max. 1.5 bar) and have a heat ratio of about 0.7. Two-stage AKM can with hot water (up to about 180 ° C ), steam -fired (up to about 8 bar ), oil, natural gas or gas from cogeneration and gas turbines. The heat ratio is used in this version of 1.0 ... 1.3. The investments would therefore be used only economically viable when waste heat is available (preferably free or very inexpensive ) or heat generated by solar energy available.

Directly heated LiBr absorption chillers have cooling capacities from 10 kW to 5,300 kW. Single-stage systems are offered as standard products in cooling capacity from 15 kW to 5,300 kW. There are also special to 23 MW.

The advantage of LiBr absorption refrigeration systems is the low Austreibertemperatur and the safety of the use of water as a refrigerant. Since the cooling takes place in the vacuum range, bursting is prevented by excess pressure when the heating is hedged. Another advantage lies in the widely separated boiling points of the pairs of substances LiBr and water. This has the effect that is generated during desorption in pure expeller water steam. In contrast, results from the ammonia-water absorption refrigeration system in the generator next to the refrigerant vapor and water vapor. In a complex, downstream process, the refrigerant must therefore be concentrated and the water vapor are removed (see rectification).

For LiBr absorption chillers also the technical work of the pump much less significant falls ( about a factor of 500).

Description

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 omitted, since lithium has virtually no vapor pressure at the driven temperatures and thus is not volatile. The company Carrier uses two cylindrical containers which are equipped according to the function with partitions, coils and nozzle assemblies. The upper container provides the expeller AT represents the weak solution is heated indirectly by heat transfer through a pipe snake and the water evaporates. On the right side of the chamber coils are installed, the flow through cooling water KüW. The evaporated and not laden with salt water condenses in the right chamber sump.

The lower chamber houses the evaporator part VD and the absorber section AB. The low water content and thus LiBr - rich solution 1 from the expeller is cooled and controlled via a nozzle in the left part of the lower chamber 2 is injected through a solution heat exchanger WT1. The salt-rich fine disgerierte solution strives to absorb the water vapor in the chamber. The resulting solution is transferred to heat the cooling water through cooling coils. In the left bottom of the chamber, the water-rich solution 3 is passed through the solution pump through the heat exchanger WT1, preheated and promoted again in the top Austreiberkammer.

In the lower chamber there is a strong negative pressure of about 2 mbar, which corresponds to a water- saturated steam temperature of 6 ° C. In the right part of the chamber water is pumped in a circle 6 and atomized. In the absorption caused by the negative pressure water evaporates at a temperature of 6 ° C. The heat of vaporization does the cold water CW, which is run in coils in the atomization of the water. The water is thus the refrigerant that escapes the cold water heat.

Areas of application

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

• Chilled water for air conditioning or process cooling waste heat from engines or gas turbines (combined heat, cooling and power )
• Reducing the summer peak electricity consumption for air conditioning of buildings, which is caused by electrically driven compression chillers, and instead use natural gas or fuel oil and solar energy via collectors as an energy source
• Chilled water for air conditioning and process cooling from waste heat from CHP, gas turbines, power plants, waste incineration plants and district heating (steam or hot water ) in summer when heat is produced as a "waste product".

Other methods

A new, largely unknown alternative is the use of ionic liquids as absorption medium. These are often characterized by a high activity towards the water, but have advantages over lithium bromide the advantage that they can not crystallize as "liquid salts" in expelling. Furthermore, they are less corrosive than lithium.

Swell

• Prof. Dr. -Ing. F. Ziegler ( 2000): Cooling with Sorptionskaltwassersätzen, plumbing and heating, Issue 7/2000, p.42 -47
• Prof. Dr. -Ing. Reichelt (2000), Where is the cooling technology in Germany and worldwide ( PDF, 52 kB)
• Walther Pohlmann, eds Walter Maake and Hans -Jürgen Eckert: Paperback refrigeration, ISBN 3-7880-7092-7
• Jürgen Lang Reck, Vaals ( NL), cooling with ammonia and water - building blocks for absorption refrigeration, Refrigeration and Air Conditioning 11/1999