Vapour Absorption (V-A) Cycle MCQ Quiz in मराठी - Objective Question with Answer for Vapour Absorption (V-A) Cycle - मोफत PDF डाउनलोड करा

Concept:

vapour absorption system:

• In the vapour absorption system, the energy input is given in the form of the heat.
• While in the vapour compression system the energy input is given in the form of the mechanical work from the electric motor run by the electricity.
• Waste Heat or free energy is effectively used in the vapour absorption refrigeration cycle
• Thus, the Vapour absorption system works on low-grade thermal energy such as waste heat or solar energy.
• The compressor of the vapour compression cycle requires large quantities of power for its operation, in case of the vapour absorption system refrigeration system, the pump requires a very small amount of power.
• vapour absorption system has lower COP than VCRS.

​

In the vapour absorption cycle, the compressor is replaced by 

• Absorber 
• Pump
• Generator or desorber

Therefore the vapour-absorption refrigeration cycle one moving part, i.e. the liquid pump. 

The coefficient of performance of the vapour-absorption refrigeration cycle is much lesser than the vapour-compression cycle because the desired effect achieved in the vapour-absorption refrigeration cycle is very low. 

Basic Vapour Absorption Refrigeration System

• The basic absorption cycle employs two fluids, the absorbate or refrigerant, and the absorbent
• The most common fluids are Water/Ammonia as the refrigerant and lithium bromide/ water as the absorbent
• These fluids are separated and recombined in the absorption cycle
• In the absorption cycle, the low-pressure refrigerant vapour is absorbed into the absorbent releasing a large amount of heat (Absorber pressure is equal to evaporator pressure)
• The liquid refrigerant/absorbent solution is pumped to a high-operating pressure generator using significantly less electricity than that for compressing the refrigerant for an electric chiller
• Heat is added at the high-pressure generator from a gas burner, steam, hot water or hot gases
• The added heat causes the refrigerant to desorb from the absorbent and vaporize
• The vapours flow to a condenser, where heat is rejected and condense to a high-pressure liquid
• The liquid is then throttled through an expansion valve to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling
• The remaining liquid absorbent, in the generator, passes through a valve, where its pressure is reduced, and then is recombined with the low-pressure refrigerant vapours returning from the evaporator so the cycle can be repeated

Explanation:

From the P-T diagram of vapour absorption cycle, it is clear that evaporator and absorber have the same pressure.

Now, let's have a look at the whole cycle.

In the vapour absorption cycle the compressor is replaced by 

• Absorber 
• Pump
• Generator or desorber

Therefore the vapour-absorption refrigeration cycle one moving part, i.e. the liquid pump. 

The coefficient of performance of the vapour-absorption refrigeration cycle is much lesser than the vapour-compression cycle because the desired effect achieved in the vapour-absorption refrigeration cycle is very low. 

Basic Vapour Absorption Refrigeration System

• The basic absorption cycle employs two fluids, the absorbate or refrigerant, and the absorbent
• The most common fluids are Water/Ammonia as the refrigerant and lithium bromide/ water as the absorbent
• These fluids are separated and recombined in the absorption cycle
• In the absorption cycle, the low-pressure refrigerant vapour is absorbed into the absorbent releasing a large amount of heat (Absorber pressure is equal to evaporator pressure)
• The liquid refrigerant/absorbent solution is pumped to a high-operating pressure generator using significantly less electricity than that for compressing the refrigerant for an electric chiller
• Heat is added at the high-pressure generator from a gas burner, steam, hot water or hot gases
• The added heat causes the refrigerant to desorb from the absorbent and vaporize
• The vapours flow to a condenser, where heat is rejected and condense to a high-pressure liquid
• The liquid is then throttled through an expansion valve to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling
• The remaining liquid absorbent, in the generator, passes through a valve, where its pressure is reduced, and then is recombined with the low-pressure refrigerant vapours returning from the evaporator so the cycle can be repeated.

Explanation:

In Vapor Absorption Refrigeration Systems most commonly these two pairs are used.

• Water-Lithium Bromide (H₂O – Li-Br) system for above 0°C applications such as air conditioning. Here water is the refrigerant and lithium bromide as the absorbent.
• Ammonia-Water (NH₃ – H₂O) system for refrigeration applications with ammonia as refrigerant and water as absorbent.

Important Points

• In the vapor absorption refrigeration system (VARS), the components used are Evaporator, Absorber, Pump, Generator, Condenser, Expander.
• In the vapor compression refrigeration system (VCRS), the components used are Evaporator, Compressor, Condenser, and Expander.
• Thus generator, absorber, and pump are absent in VCRS.
• In VARS less mechanical work is used compared to VCRS.
• In the vapor absorption system, the compressor is replaced with the absorber, pump, and generator.

Concept:

Vapor absorption system:

Basic Vapour Absorption Refrigeration System

• The basic absorption cycle employs two fluids, the absorbate or refrigerant, and the absorbent
• The most common fluids are Water/Ammonia as the refrigerant and lithium bromide/ water as the absorbent
• These fluids are separated and recombined in the absorption cycle
• In the absorption cycle, the low-pressure refrigerant vapour is absorbed into the absorbent releasing a large amount of heat (Absorber pressure is equal to evaporator pressure)
• The liquid refrigerant/absorbent solution is pumped to a high-operating pressure generator using significantly less electricity than that for compressing the refrigerant for an electric chiller
• Heat is added at the high-pressure generator from a gas burner, steam, hot water or hot gases
• The added heat causes the refrigerant to desorb from the absorbent and vaporize
• The vapours flow to a condenser, where heat is rejected and condense to a high-pressure liquid
• The liquid is then throttled through an expansion valve to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling
• The remaining liquid absorbent, in the generator, passes through a valve, where its pressure is reduced, and then is recombined with the low-pressure refrigerant vapours returning from the evaporator so the cycle can be repeated.
• From the given diagram, the total heat is, 
• Q_generator + Q_evaporator = Q_absorber + Q_condensor

​

In the vapour absorption cycle the compressor is replaced by 

• Absorber 
• Pump
• Generator or desorber

Concept:

Vapour Absorption Refrigeration System:

• In a vapour absorption refrigeration system, the Coefficient of Performance (C.O.P.) is the ratio of the refrigeration effect to the net heat input required.

• The maximum C.O.P. of a refrigeration system operating between three temperature levels can be calculated using the formula:

\({\bf{COP}} = \frac{{{{\bf{T}}_{\bf{E}}}\;\left( {{{\bf{T}}_{\bf{G}}} - {{\bf{T}}_0}} \right)}}{{{{\bf{T}}_{\bf{G}}}\;\left( {{{\bf{T}}_0} - {{\bf{T}}_{\bf{E}}}} \right)}}\)

Where, TE = Evaporator temperature, TG = Generator temperature, T0 = Condenser and absorber temperature.

and the pump work is neglected.

Calculation:

Given:

TE = - 5°C + 273  = 268 K, TG = =100°C+273=373K, T0 = 20°C + 273  = 293 K.

\({\bf{COP}} = \frac{{{{\bf{T}}_{\bf{E}}}\;\left( {{{\bf{T}}_{\bf{G}}} - {{\bf{T}}_0}} \right)}}{{{{\bf{T}}_{\bf{G}}}\;\left( {{{\bf{T}}_0} - {{\bf{T}}_{\bf{E}}}} \right)}}\)

\({\rm{COP}} = \frac{{268\;\left( {373 - 293} \right)}}{{373\;\left( {293 - 268} \right)}}\)

∴ COP = 2.3

Important Points

• The C.O.P. of a refrigeration system depends on the temperatures of the heat source, heat sink, and the refrigerated space.
• Higher temperature differences between these points generally result in a lower C.O.P.

Additional Information

Other types of refrigeration systems include:

(1) Vapour Compression Refrigeration System:

• Uses mechanical energy to drive a compressor that circulates refrigerant through the system.
• Commonly used in household refrigerators and air conditioners.

(2) Thermoelectric Refrigeration System:

• Uses the Peltier effect to create a heat flux between the junction of two different types of materials.
• Often used in small cooling devices and portable coolers.

(3) Gas Refrigeration System:

• Uses gas cycles for refrigeration, commonly employing gases like helium or hydrogen.
• Often used in cryogenics and scientific research applications.

Explanation:

Aqua-ammonia Absorption System:

• An aqua-ammonia absorption system consists of an absorber, a pump, a generator and a pressure-reducing valve to replace the compressor of the vapour compression system.
• The other components of the system are the condenser, expansion valve and evaporator as in the vapour compression system.
• Ammonia is used as a refrigerant while water is used as an absorbent.
• Liquid ammonia (normally a mixture of liquid and vapour) from the expansion valve enters the evaporator, either it absorbs heat from the evaporator space or it cools the secondary refrigerant in a heat exchanger.
• Normally these units have a large cooling capacity of the order of 80 TR and above.
• In such units, liquid ammonia absorbs heat from the secondary refrigerant which would be used as a medium to cool the space or products in the refrigerated space.

Some parts involved in ammonia absorption plant are listed below:

• Heat Exchanger:

  • The location of the heat exchanger between the generator and absorbers is ideal. The strong solution pumped from the absorber to the generator must be heated; and the weak solution from the generator to the absorber must be cooled. The heat exchanger between the two streams, therefore, reduce both the cost of heating the generator and the cost of cooling the absorber.

• Analyzer:
  • The analyzer is a direct contact heat exchanger consisting of series of tray mounted above the generator. Its function is to remove partly some of the unwanted water particle associated with ammonia vapour going to the condenser The water water vapours if allowed to enter the condenser may enter the expansion valve Where they will freeze and choke cl the pipe line.
• Rectifier:
  • The final reduction (elimination) of the percentage of water vapour occurs in the rectifier, a water cooled heat exchanger which condenses water vapour (and some ammonia) and returns it to the generators. The net refrigerating effect of such a machine is the heat extracted in the evaporator. The total energy supplied for operating the machine is the sum of the work done by the liquid pump and the heat supplied in the generator.

• Low-pressure ammonia vapour enters the absorber.
• This vapour is allowed to be mixed and absorbed in the absorber with a weak solution of aqua ammonia flowing from the generator under gravity through a pressure-reducing valve.
• The water has the ability to absorb very large quantities of ammonia vapour and the solution thus formed is known as aqua-ammonia.
• The absorption of ammonia vapour in water lowers the pressure in the absorber which in turn draws more ammonia vapour from the evaporator and thus raises the temperature of the solution.
• Some form of cooling arrangement (Usually water cooling) is employed in the absorber to remove the heat of the solution evolved in it.
• This is necessary in order to increase the absorption capacity of water because, at higher temperatures, water absorbs less Ammonia vapour.
• The strong solution thus formed in the absorber is pumped to the generator by a liquid pump.
• The pump increases the pressure of the solution up to 10 bar.
• The strong solution of ammonia in the generator is heated by some external source such as gas or steam.
• During the heating process, ammonia vapour is driven off the solution at high pressure leaving behind the hot weak ammonia solution in the generator.
• This weak ammonia solution flows back to the absorber at low pressure after passing through the pressure-reducing valve.
• The high-pressure ammonia vapour from the generator is condensed in the condenser to high-pressure liquid ammonia.
• This liquid ammonia is passed to the expansion valve through the receiver and then to the evaporator.
• This completes the aqua-ammonia absorption cycle.
• The heat required for the operation of the generator can be supplied by burning kerosene using solar energy or waste heat from the process industry in the case of industrial applications.
• The electrical energy required for the operation of the aqua pump in this system is extremely small compared to the electrical energy needed for the compressor of a vapour compression cycle.
• The basic difference here is that the aqua pump handles the liquid ammonia while the compressor has to work with the refrigerant vapour of high specific volume.

Concept:

Vapor absorption system:

• In the vapor absorption system, the energy input is given in the form of the heat.
• While in the vapour compression system the energy input is given in the form of the mechanical work from the electric motor run by the electricity.
• Waste Heat or free energy is effectively used in the vapour absorption refrigeration cycle
• Thus, the Vapour absorption system works on low-grade thermal energy such as waste heat or solar energy.

​

In the vapour absorption cycle the compressor is replaced by 

• Absorber 
• Pump
• Generator or desorber

Therefore the vapour-absorption refrigeration cycle one moving part, i.e. the liquid pump. 

The coefficient of performance of the vapour-absorption refrigeration cycle is much lesser than the vapour-compression cycle because the desired effect achieved in the vapour-absorption refrigeration cycle is very low. 

Basic Vapour Absorption Refrigeration System

• The basic absorption cycle employs two fluids, the absorbate or refrigerant, and the absorbent
• The most common fluids are Water/Ammonia as the refrigerant and lithium bromide/ water as the absorbent
• These fluids are separated and recombined in the absorption cycle
• In the absorption cycle, the low-pressure refrigerant vapour is absorbed into the absorbent releasing a large amount of heat (Absorber pressure is equal to evaporator pressure)
• The liquid refrigerant/absorbent solution is pumped to a high-operating pressure generator using significantly less electricity than that for compressing the refrigerant for an electric chiller
• Heat is added at the high-pressure generator from a gas burner, steam, hot water or hot gases
• The added heat causes the refrigerant to desorb from the absorbent and vaporize
• The vapours flow to a condenser, where heat is rejected and condense to a high-pressure liquid
• The liquid is then throttled through an expansion valve to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling
• The remaining liquid absorbent, in the generator, passes through a valve, where its pressure is reduced, and then is recombined with the low-pressure refrigerant vapours returning from the evaporator so the cycle can be repeated

Explanation:

Aqua-ammonia Absorption System:

• An aqua-ammonia absorption system consists of an absorber, a pump, a generator and a pressure-reducing valve to replace the compressor of the vapour compression system.
• The other components of the system are the condenser, expansion valve and evaporator as in the vapour compression system.
• Ammonia is used as a refrigerant while water is used as an absorbent.
• Liquid ammonia (normally a mixture of liquid and vapour) from the expansion valve enters the evaporator, either it absorbs heat from the evaporator space or it cools the secondary refrigerant in a heat exchanger.
• In aqua-ammonia absorption refrigeration system, incomplete rectification leads to the accumulation of water in the condenser.
• Normally these units have a large cooling capacity of the order of 80 TR and above.
• In such units, liquid ammonia absorbs heat from the secondary refrigerant which would be used as a medium to cool the space or products in the refrigerated space.

​

• Low-pressure ammonia vapour enters the absorber.
• This vapour is allowed to be mixed and absorbed in the absorber with a weak solution of aqua ammonia flowing from the generator under gravity through a pressure-reducing valve.
• The water has the ability to absorb very large quantities of ammonia vapour and the solution thus formed is known as aqua-ammonia.
• The absorption of ammonia vapour in water lowers the pressure in the absorber which in turn draws more ammonia vapour from the evaporator and thus raises the temperature of the solution.
• Some form of cooling arrangement (Usually water cooling) is employed in the absorber to remove the heat of the solution evolved in it.
• This is necessary in order to increase the absorption capacity of water because, at higher temperatures, water absorbs less Ammonia vapour.
• The strong solution thus formed in the absorber is pumped to the generator by a liquid pump.
• The pump increases the pressure of the solution up to 10 bar.
• The strong solution of ammonia in the generator is heated by some external source such as gas or steam.
• During the heating process, ammonia vapour is driven off the solution at high pressure leaving behind the hot weak ammonia solution in the generator.
• This weak ammonia solution flows back to the absorber at low pressure after passing through the pressure-reducing valve.
• The high-pressure ammonia vapour from the generator is condensed in the condenser to high-pressure liquid ammonia.
• This liquid ammonia is passed to the expansion valve through the receiver and then to the evaporator.
• This completes the aqua-ammonia absorption cycle.
• The heat required for the operation of the generator can be supplied by burning kerosene using solar energy or waste heat from the process industry in the case of industrial applications.
• The electrical energy required for the operation of the aqua pump in this system is extremely small compared to the electrical energy needed for the compressor of a vapour compression cycle.
• The basic difference here is that the aqua pump handles the liquid ammonia while the compressor has to work with the refrigerant vapour of high specific volume.

Coefficient of performance (COP) of vapour absorption refrigeration system:

• COP = \((\frac{T_E}{T_C~-~T_E})~\times~(\frac{T_G~-~T_C}{T_G})\)

Explanation:

Concept:

\(C.O{.P_{VARS}} = {{\rm{\eta }}_{{\rm{carnot}}}} \times {\rm{C.O}}{{\rm{.P}}_{{\rm{carnot}}}} \)

\(C.O.P_{VARS}= \frac{{{{\rm{T}}_{\rm{g}}} - {{\rm{T}}_{\rm{o}}}}}{{{{\rm{T}}_{\rm{g}}}}} \times \frac{{{{\rm{T}}_{\rm{e}}}}}{{{{\rm{T}}_{\rm{o}}} - {{\rm{T}}_{\rm{e}}}}}\)

where, T_g = Generator Temperature; To = Ambient Temperature; Te = Evaporator Temperature

Calculation:​

Given:

T_o = 30° C + 273 = 303 K, Te = 10° C + 273 = 283 K,

COP_VARS = 2, and Tg = ??

\(C.O.P_{VARS}= \frac{{{{\rm{T}}_{\rm{g}}} - {{\rm{T}}_{\rm{o}}}}}{{{{\rm{T}}_{\rm{g}}}}} \times \frac{{{{\rm{T}}_{\rm{e}}}}}{{{{\rm{T}}_{\rm{o}}} - {{\rm{T}}_{\rm{e}}}}}\)

\(2 = \frac{{{{\rm{T}}_{\rm{g}}} - 303}}{{{{\rm{T}}_{\rm{g}}}}} \times \frac{{283}}{{303 - 283}}\)

∴ T_g = 352.87 = 352.87 - 273 = 79.87 = 80° C​.