Chillers are among the most important equipments in cooling that can be generally divided into two categories of compression chillers and absorption chillers. In general, compression chillers use electrical energy and absorption chillers use thermal energy as the main source to create cooling. They do.
Absorption refrigeration technology is an excellent method for central air conditioning in facilities that have additional boiler capacity and can provide the steam or hot water needed to operate the chiller. Absorption chillers can easily provide capacities between 25 and 1200 tons of refrigeration.
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Of course, it should be noted that some Japanese manufacturers have succeeded in producing absorption chillers with a capacity of 5,000 tons. In absorption systems, water is often used as a refrigerant, the heat required for the operation of these chillers is directly from natural gas. Or diesel is supplied.
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Indirect sources of heat in absorption chillers are high pressure and low pressure steam hot water, based on this, different manufacturers in the world offer three main types of absorption chillers , which are: direct flame, steam and hot water.
Types of absorption chillers
In a general classification, absorption chillers can be classified into two categories: water and ammonia absorption chillers and lithium absorption chillers. In fact, in each absorption refrigeration cycle, there is an absorbing fluid and a refrigerant fluid. This basis has been done.
In water and ammonia system, ammonia refrigerant is water-absorbing fluid, in lithium bromide system and water is water-cooling fluid and absorbent fluid is lithium bromide solution.
In addition to the pairs of refrigerants and adsorbents mentioned, some adsorption refrigeration cycles use other pairs as shown in Table (1), but according to the components of the system, other classifications can be provided, for example, the cycle can be Classified adsorption refrigeration into one-effect, two-effect and three-effect refrigeration cycles.
Today, single-effect and double-effect absorption refrigeration cycles are made on a very large scale and in various forms, and three-effect cycles are still under study.
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Refrigerants and chemicals used
An interesting point about absorption chillers is that they do not use ordinary refrigerants, instead they use water as a refrigerant that is either mixed with “ammonia solution” or “lithium bromide”, lithium bromide is more common. Because it is safer and non-toxic, so we’re looking at how water-type chillers work with lithium bromide.
1- Refrigerant and adsorbent pairs
- Absorbent / refrigerant / type of adsorbent
- H2O / LiBr / alkaline halide
- H2O / LiClO3 / alkaline halide
- H2O / CaCl2 / alkaline halide
2. Common technical terms in absorption chillers
The generator is usually located in the upper chamber of the absorption chillers and is responsible for concentrating the dilute lithium bromide solution and separating the refrigerant water.
The absorber is usually located in the lower shell of the absorption chillers and is responsible for absorbing the refrigerant vapor produced in the evaporator chamber.
The evaporator is usually located in the lower shell of the absorption chillers. The refrigerant evaporates in the evaporator due to the low chamber pressure (relative vacuum) and reduces the temperature of the cold ventilated water inside the evaporator tubes.
The condenser is usually located in the upper shells of the absorption chillers and is responsible for distilling the refrigerant evaporated by the generator.
Refrigerant vapor is distilled in contact with the tower’s water pipes and overflows into the evaporator pan. Absorbent solution
This solution is a solution of lithium bromide and water in the cycles of the present project.
The refrigerant in the absorption chillers of the present project is pure water (distilled water) which will have cooling properties due to the low pressure of the evaporator chamber due to evaporation.
Lithium bromide solution is a liquid in normal concentration.
However, if the initial concentration continues too long, the volume of tiny crystals that form in them will increase and may completely block the passage of the solution.
This phenomenon is called crystallization.
The parameter of performance coefficient in refrigeration devices, including absorption chillers, is an indicator of the efficiency of the device. Higher values of this parameter indicate the optimal consumption of thermal energy.
Before discussing absorption chillers, you need to understand the three basic concepts and why water and lithium bromide are used.
1- When we boil water, it changes from liquid to steam.
Water boils at different temperatures under different pressures.
If you increase the pressure, the water boils at higher temperatures, and if you reduce the pressure, the water boils at lower temperatures.
By changing the surrounding pressure inside the absorption chiller, water and lithium bromide are able to easily phase change between liquid and vapor, which is necessary to provide a cooling effect.
2- The second understandable concept is that lithium bromide is only a salt in liquid form and salt absorbs moisture.
So if we spray lithium bromide on a part of water vapor, the two are absorbed and mixed together. The GIF below illustrates this point.
3- The third thing to keep in mind is that water and lithium bromide can be mixed together, but if heated, they separate, so that the water evaporates and the lithium bromide settles.
Principles of work of absorption chiller
- The main components of the absorption chiller are the condenser and the generator, which make up most of the upper part of the chamber.
- Evaporators and adsorbents form most of the lower part of the chamber.
- There is also a heat exchanger to improve system efficiency.
The main components of an absorption chiller. Top to bottom: Condenser, generator, evaporator, absorber Left shape: Heat exchanger.
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First of all, a mixture of about 50% lithium and 40% water from the absorber is pumped throughout the heat exchanger and then enters the generator.
The generator part is the part that fills to form a tank for mixing lithium and forming water.
A heat source (hot water / steam) flows through a tube in the generator tank, causing the lithium to rise and the water to separate.
The water evaporates as steam and reaches the condenser, leaving behind lithium bromide.
Lithium bromide sinks due to the weight of the molecules.
This creates a liquid concentration of lithium bromide at the base of the generator and then flows down through the heat exchanger and is sprayed on the absorber where it can be mixed with water molecules.
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Meanwhile, water vapor in the condenser section, as it comes in contact with a cooling coil, turns into a liquid.
Water passes through a cooling tower through a sealed tube inside the condenser to remove the heat of water vapor to become a liquid.
This liquid water is then collected in a tray inside the condenser and flows through a tube to the evaporator.
The volumetric flow of water is controlled through a fixed hole.
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The evaporator is at a very low pressure near the vacuum condition, which causes the water temperature to drop due to the rapid pressure drop. So that the water temperature drops to about 4 (40).
The cold water line runs just like a conventional chiller across the evaporator, carrying all the unwanted heat from the fan coil and ساختمان units.
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It passes through a cooling coil in which cold water is sprayed from the condenser onto the surface to remove unwanted heat energy.
Cold water enters the evaporator coil at a temperature of about 12 (54) and transfers its thermal energy when the cold condensed water spray comes in contact with the cold water line pipe.
Remember that these two waters are never mixed together, they are always separated by a pipe wall.
As heat is transferred through the pipe wall to the condenser water outside the pipes, the condenser water evaporates due to the low pressure of the chamber.
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As this water evaporates, it carries with it unwanted heat energy.
The cold water circuit has already lost its heat and over time, at a temperature of about 7 (45) leaves the evaporator and is ready to be pumped around the building to collect more heat.
The other ring recirculates any water that has been lost from the pipe and has not been boiled.
This water is pumped to the top of the evaporator and sprayed again to evaporate it all.
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The water vapor or steam produced by the evaporator is then absorbed and drawn into a strong lithium bromide solution which is sprayed on the absorber.
It’s almost like a magnetic force, the gravity is strong enough that the water particles themselves flow directly to the absorber to accompany the lithium bromide.
This attraction between water particles and lithium bromide particles creates a vacuum in the chamber.
When these two fluids come in contact with each other, they produce little heat, and this, as well as the heat collected from the cold water ring, must be removed, which is why the cooling tower water ring passes through the absorber.
The water in the cooling tower also condenses the remaining vapor particles into liquid. A mixture of lithium bromide and water collected at the bottom is ready to be pumped back to the generator to repeat the cycle.
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3- Comparison of absorption and compression chillers
Suction chillers in some ways act like compression chillers, the most important of which are:
A. In the evaporator, the heat of the building ventilation water is used to evaporate a volatile refrigerant at low pressure.
B) Low pressure refrigerant gas is taken from the evaporator and high pressure refrigerant gas is sent to the condenser.
C – Refrigerant gas is distilled in the condenser.
D. The refrigerant is always in circulation in a cycle.
The main differences between absorption and compression chillers are:
A. Compression chillers use a compressor to circulate the refrigerant, while absorption chillers do not have a compressor and instead use the heat energy of different sources and change the concentration of the adsorbent solution. As the concentration changes, so does the pressure. Different chiller components change. This pressure difference causes the refrigerant to circulate in the system.
B. The generator and absorber in the absorption chillers have replaced the compressor in the compression chillers.
C. In absorption chillers, an adsorbent is used, which is usually water or lithium bromide salt.
D. Refrigerant in compression chillers is one of the types of chlorofluorocarbons or halochlorofluorocarbons, while in absorption chillers the refrigerant is usually water or ammonia.
E) Compression chillers supply their required energy from electrical energy, while the input energy to absorption chillers is supplied by hot water or steam entering the generator. The heat may have come from a hot air oven or boiler. Sometimes the heat of other processes is used, such as low-pressure steam or industrial hot water, heat extracted from the exhaust fumes of gas turbines, or low-pressure steam from the exhaust of steam turbines.
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The most important advantages of absorption chillers over compression chillers are:
A – Saving electricity consumption
As mentioned, absorption chillers use natural gas, diesel or waste heat as the main source of energy and their power consumption is very low, the amount of electricity consumption, comparisons and quantitative analysis will be mentioned in later chapters.
B- Saving on the cost of electricity services
The cost of installing an electrical network system in projects can be determined based on the maximum harvesting power.
An absorption chiller also reduces the cost of services because it consumes less electricity.
In most buildings, the installation of absorption chillers frees up electrical power for other uses.
C- Saving the cost of emergency electrical equipment
In buildings such as medical centers or computer rooms where the presence of emergency power systems is necessary to support cooling equipment, the use of absorption chillers will significantly save on the cost of this equipment.
D- Save on the initial cost required for the boilers
Some absorption chillers can be used as heaters in winters and provide hot water for heating systems with temperatures up to 203, if you use these chillers, not only will the cost of buying a boiler be reduced, but also economical. Significant atmospheric space will also be obtained.
E- Improving the efficiency of boilers in summer
Complexes such as hospitals, which require steam throughout the year for sterilization systems, autoclaves and other equipment, are equipped with large boilers, which operate mostly during the summer with low load.
Installation of steam absorption chillers in such cases will increase the load and steam consumption in summer, and as a result, the operation of boilers and their efficiency will be significantly improved.
And – Return on initial investment
Suction chillers reduce operating costs due to lower power requirements compared to compression chillers, if the difference between the price of a suction chiller and a condensing chiller also reduces capacity as an annual investment and savings from reduced costs. Considering energy as a return on investment, we can say with certainty that the return on investment spent on the installation of absorption chillers will be in very good condition.
G – Noise and sound vibrations
Vibration and noise due to the operation of absorption chillers is much less than compression chillers, the main source of noise and vibration in compression chillers is the compressor.
Suction chillers do not have a compressor and the only source of noise and vibration in them are small pumps that are used to circulate refrigerant and lithium bromide solution, the amount of noise and vibration of these small pumps can be ignored.
H. Elimination of environmental hazards caused by harmful refrigerants
Absorption chillers Unlike chillers density of any substance CFC or HCFC, which destroy the ozone layer, they do not use it for environmental risk do not constitute, absorption chillers, often using water as refrigerant use, a new chiller in Every circumstance is a twenty-year investment.
Permanent changes in the rules and regulations for the use of refrigerants make the use of natural refrigerants such as water in absorption chillers a very significant option.
Depending on the amount of greenhouse gas emissions and pollutants
Emissions of greenhouse gases (such as carbon dioxide), which have a significant effect on global warming, and pollutants (such as sulfur oxides, nitrogen oxides, and particulate matter) by absorption chillers are much lower than those of condensing chillers.
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