Vapour absorption cycle

by Manoj.P.J Assistant Prof(Mech)

module2

Vapour compression refrigeration – working principle and essential components- simplevapour compression refrigeration cycle – representation of cycle on T-S and p-h charts – thermodynamic analysis - effect of operating parameters on the performance – liquid vapour regenerative heat exchanger - actual cycle - numerical Problems. Multi pressure systems – multi compression and multi evaporator systems – inter cooling – flash inter cooling and flash gas removal. Vapour Absorption System – working principle - calculation of max COP – description and working of NH3 – water and Li Br –water systems - comparison with vapour compression system. Three fluid absorption system. Refrigerants and their properties – nomenclature of refrigerants - selection of refrigerants – environmental aspects.


Vapour absorption refrigeration system

Heat operated refrigeration cycleAqua ammonia refrigeration system



Compressor work can be reduced if vapour is dissolved in a suitable liquid before compression.

At the exit of evaporator (state1) the vapour is dissolved in a weak ammonia solution in the absorber


During this state heat must be removed to increase the absorption rate.

The strong solution is pumped to generator pressure.

Before reaching the generator the solution is first heated in the heat exchanger.

Further heated in the generator.


Since the saturation temperature of ammonia is less than water, ammonia vapour leaves the generator first

and the left out weak solution goes back the absorber via heat exchanger and expansion valve.

The heating source in the generator may be waste heat or by seam coil.


Energy balance= Qg +QL +Wp = Qh +Qa



Practical ammonia absorption system



Analyzer and rectifier remove water vapour completely

Other wise it may freeze at expansion valve. sub-cooler using evapourator out let

improve refrigeration effect


Comparison of absorption with compression system.

Absorption system-bulky-High capital cost-Use waste heat - can go well above 1000 TONS (vapour

compression generally below 100 TON -inertia)

COP is low(but using waste heat)



Lithium Bromide absorption refrigeration system



Water itself work as refrigerant.Lithium Bromide act as absorbent.The vapour pressure of an aqua solution of

Lithium bromide is very lowThere fore it has high affinity towards water

vapour.So if water and Lithium Bromide solution are

placed adjacent to each other, in a closed evacuated system the water will evaporate.


The weak LB solution is pumped to generator.The heating is done at generator and steam get

separated and goes to condenser.The strong LB solution goes to absorber and mix

with stem from evapourator.The steam entered in condenser get condensed

by liberating heat out side and converted to water

This water throttle to evapourator pressure. There again evaporation of water at low

pressure takes place cooling effect.


comparison

L B solution absorber

1. pure water vapour leaves the generator since Lithium Bromide salt does not evaporate. No need of analyser and rectifier

Ammonia absorber1. ammonia vapour

may contain water vapour and there fore analyser & rectifier are needed.


2. working pressure are below atmosphere. No fear of bursting, toxicity. Pressure limit between evapourator and generator are low

2. pressure limits are higher.


3. pump work is less due to lower pressure limits. Some times pump can be eliminated with the proper placing of components.

less vibration

3. pump work is higher.


4. temperature below 2 degree C not possible. Larger volume flow rate.

4. temperature can be much less than 2 degree C.


Three fluid absorption system

(Electrolux refrigerator)Ammonia , hydrogen and water



Ammonia evaporate in the presents of lighter gas

In the generator the ammonia is separated due to heating .

Passed through separator, rectifier and analyser to remove water content.

Passed through condenser become liquid ammonia.

Liquid ammonia is mixed with hydrogen and passed through evapourator.


Ammonia liquid get evaporated in the presence of lighter hydrogen gas produce refrigeration effect.

The mixture (ammonia and hydrogen) is passed to through the absorber through a heat exchanger. The hot hydrogen coming from absorber is cooled by the cold mixture coming from evaporator.

The mixture is comes contact with the weak ammonia liquid comes from generator.

the weak liquid ammonia liquid absorb ammonia vapour liberating hydrogen.


During absorption, the heat is liberated and hydrogen absorb this heat and become hot.

This hot hydrogen is cooled in the heat exchanger.

The strong solution from the absorber is passed to the generator through liquid heat exchanger where the strong solution is heated and weak solution is cooled.

No need of pump and expansion valve.Uniform pressure though out the system.


Permitting the refrigerant to evaporate at the low temperature corresponding to it’s partial pressure of ammonia (because it is a mixture of ammonia and hydrogen).

In the condenser side , there exists only the refrigerant (ammonia) , which is subjected to the total pressure of the system so that it is condensed by normal cooling water.

the solution is circulated by thermal action alone.

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Vapour absorption cycle