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ME340A: Prof. Sameer Khandekarhttp://home.iitk.ac.in/~samkhan

Department of Mechanical EngineeringIndian Institute of Technology Kanpur208016 Kanpur India

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ME340A: Refrigeration and Air ConditioningInstructor: Prof. Sameer KhandekarTel: 7038; e-mail: [email protected]

Department of Mechanical EngineeringIndian Institute of Technology KanpurKanpur 208016 India

Vapor Absorption Refrigeration Systems

Sameer KhandekarSir M. Visvesvaraya Chair ProfessorDepartment of Mechanical EngineeringIndian Institute of Technology KanpurKanpur (UP) 208016 INDIAWebpage: home.iitk.ac.in/~samkhan/

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In this lecture…

◉ What is an absorption system

◉ Construction

◉ Operation

◉ Comparison with vapor compression system



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Vapor absorption refrigeration system

◉ An absorption refrigerator is a refrigerator that uses a heat source (e.g., solar energy, a fossil-fuel flame, waste heat from factories, or district heating systems) to provide the energy needed for the cooling process.

◉ In this system mechanical compression process of vapor compression cycle is replaced by a thermal compression process.

◉ The thermal compression is achieved by the following process:

○ Absorbing a fluid vapor (e.g., say: ammonia) into another carrier liquid (e.g., say water).

○ Pumping this solution to a high pressure cycle by a simple pump

○ Producing vapors from the solution by heating (thus, cooling)

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Features

◉ ARS are often classified as heat-driven systems. They are used especially, when there is a source of inexpensive thermal energy at a temperature of 100 to 200°C.

◉ The principle can be used to air-condition buildings using available waste heat from a source. ARS are primarily used in large commercial, industrial installations or for storage in recreational vehicles.

◉ Some examples include geothermal energy, solar energy, and waste heat from cogeneration or process steam plants, and even natural gas when it is at a relatively low price.

◉ For example, using waste heat from a gas turbine makes it very efficient because it first produces electricity, then hot water, and finally, air-conditioning (called cogeneration/trigeneration).



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Vapor absorption refrigeration cycle

Heating

Condenser

Thermal Compression

Liquid pump

Water return via rectifier

Heat of absorption

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Another layout to represent the ARS



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Requirements of the system

◉ Solubility requirement: The refrigerant should have very high solubility in the absorbent material (usually much higher than the Roult’s law applicability limits), so that a strong solution is possible.

◉ Boiling point requirement: There should be a large difference in the normal boiling point of the two substances, of the order of 150 C to 200 C. Higher the better. The absorbent should have negligible vapor pressure at the generator temperature. Thus almost absorbent free refrigerant is boiled off from the generator and the absorbent alone returns to the absorber.

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Ammonia absorption refrigeration cycle.

Ammonia Absorption Refrigeration system

◉ ARS involves the absorption of a refrigerant by a transport medium.

◉ Most widely used system is NH3 – H2O system, where NH3 serves as refrigerant and H2O as transport medium.

◉ Other systems include water–lithium bromide and water–lithium chloride systems, where water serves as the refrigerant. These systems are limited to applications such as A-C where the minimum temperature is above the freezing point of water.

Thermal Compression



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Advantages of ammonia systems

◉ Ideally fits into the concept of Integrated Energy Systems such as Cogeneration involving combined generation, heat, refrigeration and power (CHRP Plants) on various fuels like bio-mas, coal, Natural Gas, Heavy Oil, Solar, geothermal, etc.

◉ Excellent for waste heat utilization

◉ Earns carbon credits, reduces taxes, promotes sustainable development.

◉ Uses best eco-friendly refrigerant – ammonia

◉ Wide operational range + 5 C to – 55 C

◉ Low maintenance cost – no moving parts

◉ Can operate well for over 25 years

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Equivalent thermodynamic model



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Determination of COP of ARS

◉ The COP of actual absorption refrigeration systems is usually less than 1.

◉ Air-conditioning systems based on absorption refrigeration, called absorption chillers, perform best when the heat source can supply heat at a high temperature with little temperature drop.

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Discussion on COP

◉ The COP depends on the main three operating temperatures:○ Temperature of the heat source (High temperature)○ Temperature of Condenser/ Absorber (They can be different)○ Temperature of the refrigeration space (Low temperature)

◉ Hot source should be as high as possible

◉ Condenser/Absorber should be low

◉ Refrigeration temperature should be high.

If Condenser and Absorber temperatures are different, then maximum COP is given by

𝑂𝑃 =𝑇 − 𝑇

𝑇

𝑇

𝑇 − 𝑇



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Device Construction

◉ Like a standard vapor compression system, a vapor absorption system also consists of a condenser, an expansion valve and an evaporator.

◉ These three devices have exactly the same function as in the vapor compression system

◉ Instead of a mechanical compressor, the vapor absorption system achieves the compression by the application of heat –thermal compression.

◉ Hence, to achieve this it has a refrigerant absorber, a liquid pump, a heat exchanger (thermal compression), an analyzer and a rectifier (for separating the solvent from the refrigerant).

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Operational steps

◉ Dry ammonia vapor at low pressure passes in to the absorber from the evaporator.

◉ In the absorber the dry ammonia vapor is dissolved in cold water and strong solution of ammonia is formed.

◉ Heat evolved (heat of absorption) during the absorption of ammonia in water is removed by circulating cold water through the coils kept in the absorber.

◉ The highly concentrated ammonia solution (known as Aqua Ammonia) is then pumped by a liquid pump to the generator through a heat exchanger.



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Operational steps

◉ In the heat exchanger, strong ammonia solution is heated by the hot weak solution returning from the generator to the absorber.

◉ Regeneration: The refrigerant-saturated liquid is heated by some available means (e.g. steam coils, gas, electricity or solar heating) , causing the refrigerant to evaporate out.

◉ The hot gaseous refrigerant passes through a heat exchanger, transferring its heat outside the system (such as to surrounding ambient-temperature air), and condenses. The condensed (liquid) refrigerant supplies the evaporation phase.

◉ The boiling point of ammonia is less than that of water, hence the vapors leaving the generator are mainly of ammonia.

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Operational steps

◉ The weak ammonia solution is left in the generator is called weak aqua.

◉ This weak solution (rich in water and depleted with ammonia) is returned to the absorber through the heat exchanger.

◉ Ammonia vapor leaving the generator may contain some water vapor.

◉ If this water vapor is allowed to enter the condenser and the expansion valve, it may freeze there resulting in chocked flow.

◉ Hence, analyzer and rectifiers are incorporated in the system before condenser.



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Operational steps

◉ The ammonia vapor from the generator passes through a series of trays in the analyzer and ammonia is separated from water vapor.

◉ The separated water vapor is returned to the generator.

◉ Then the ammonia vapor passes through a rectifier.

◉ The rectifier resembles a condenser and water vapor still present in ammonia vapor condenses and the condensate is returned to analyzer.

◉ Then, at last, virtually pure ammonia vapor passes through the condenser.

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Operational steps

◉ The latent heat of ammonia vapor is rejected to the cooling water circulated through the condenser and the ammonia vapor is condensed to liquid ammonia.

◉ The high pressure liquid ammonia is throttled by an expansion valve or throttle valve to low pressure liquid.

◉ This reduces the high temperature of the liquid ammonia to a low value and liquid ammonia partly evaporates (some flashing).

◉ Then this two phase mixture (mostly liquid) is led to the evaporator.

◉ In the evaporator the liquid fully vaporizes.



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Operational steps

◉ The latent heat of evaporation is obtained from the brine (as a secondary refrigerant) or other objects/material which is being cooled.

◉ The low pressure ammonia vapor leaving the evaporator again enters the absorber and the cycle is completed.

◉ This cycle is repeated again to provide the refrigerating effect.

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Comparison: Vapor compression and absorption systems

◉ Compared with vapor-compression systems, ARS have one major advantage: A liquid is compressed instead of a vapor and as a result the work input is very small (on the order of one percent of the heat supplied to the generator) and often neglected in the cycle analysis.

◉ ARS are much more expensive than the vapor-compression refrigeration systems. They are more complex and occupy more space, they are much less efficient thus requiring much larger cooling towers to reject the waste heat, and they are more difficult to service since they are less common.

◉ Therefore, ARS should be considered only when the unit cost of thermal energy is low and is projected to remain low relative to electricity.



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S. No. Vapor Compression System Vapor Absorption System

1. This system has more wear and tear and produces more noise due to the moving parts of the compressor.

Only moving part in this system is an aqua pump. Hence the quieter in operation and less wear and tear

2. Electric power is needed to drive the system

Waste, Low grade heat/Solar power can be used. No need of electric power

3. COP is more COP is less

4. At partial loads performance is poor. At partial loads performance is not affected.

5. Mechanical energy is supplied through a compressor

Heat energy is utilized

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S. No. Vapor Compression System Vapor Absorption System

7. Charging of the refrigerating to the system is easy

Charging of refrigerant is difficult

6. Energy supplied is ¼ to ½ of the refrigerating effect (less)

Energy supplied is about one and half times the refrigerating effect (more)

8. Preventive measure is needed, since liquid refrigerant accumulated in the cylinder may damage the cylinder

Liquid refrigerant has no bad effect on the system.



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Actual systems

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Lithium bromide – Air conditioning system



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Solar heating based absorption chiller

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Laboratory trainer kit1 condenser, 2 evaporator with heater, 3 absorber, 4 tank, 5 gas burner, 6 boiler with bubble pump to separate the ammonia, 7 displays and controls



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Electrolux ammonia refrigerator

Front view (1933 Model)

Rear view

Old advert of domestic Electrolux fridge

(Introduced in 1925)

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Any questions ?

You can find me at

◉ [email protected]

Thanks!

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