vapor absorption refrigeration system

Vapor Absorption

Refrigeration System

(VARS)

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Faculty Name: Prof. Ajaypalsinh G. BaradBranch: MechanicalSemester: 6th

Name of Subject: Refrigeration and Air Conditioning

Outline

• Introduction

• Desirable characteristics of refrigerant

• Selection of pair,

• Practical H2O -NH3 cycle,

• LiBr – H2O system and its working,

• Electrolux refrigeration system

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Introduction:

• The vapour absorption refrigeration system is one of

the oldest methods of producing refrigeration effect.

• An ammonia is the commonly used refrigerant in a

vapour absorption system.

• The vapour absorption system uses heat energy,

instead of mechanical energy as in vapour

compression systems, in order to change the

conditions of the refrigerant.

• This system may be used in both the domestic and

large industrial refrigerating plants.

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• The compressor is replaced by an absorber, a pump, agenerator and a pressure reducing valve.

• In this system, the vapour refrigerant from theevaporator is drawn into an absorber where it isabsorbed by the weak solution of the refrigerantforming a strong solution.

• This strong solution is pumped to the generator whereit is heated by some external sources.

• During the heating process, the vapour refrigerant isdriven off by the solution and enters into thecondenser where it is liquefied. The liquid refrigerantthen flows into the evaporator and thus the cycle iscompleted.

Introduction:

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Introduction:

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Solubility requirement:

• The refrigerant should have proper solubility in the

absorbent so that a strong solution, highly rich in the

refrigerant, is formed in the absorber by the absorption

of the refrigerant vapour.

Boiling point requirement:

• There should be a large difference, about 200oC, in the

boiling points of the two substances, thus absorbent

free refrigerant is boiled off from the generator.

Thermodynamic Requirement of Refrigerant –

Absorbent Mixture:

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Some other Properties required:

• The refrigerant should have high affinity for the

absorber at low temp and less affinity at high temp.

• It should have low freezing point.

• It should have good thermal and chemical stability

• Irreversible chemical reactions of all kinds are to be

avoided.

Thermodynamic Requirement of Refrigerant –

Absorbent Mixture:

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• Ammonia – Water

• Ammonia – Sodium Thiocynate

• Ammonia – Lithium Nitrate

• Ammonia – Calcium Chloride

• Water – Lithium Bromide

• Methylene Choride – Dimethyl Ether tetra ethylene

glycol

Selection of Pairs:

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Simple Ammonia - Water VAR:

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Practical Ammonia - Water VAR:

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Advantages:

• Feasible to separate refrigerant from absorbent, as water is

more volatile compared to Li-Br.

• Operating pressures are low so pumping cost is low.

• Wall thickness of the system less compared to VCR.

• Water as refrigerant is non toxic, so can be directly used

for chilled coil.

• Can be built up to 1,00,000 TR capacity plants.

• Operation and maintenance cost is very low.

Lithium Bromide Absorption System:

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Disadvantages:

• Evaporation temperature must be kept above the freezing

point of water and hence the temperature of chilled water

cannot be less than 5oC.

• Li-Br solution is corrosive. So inhibitors need to be added

to the system to prevent the metal parts from corrosion.

Lithium Bromide Absorption System:

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Introduction:

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Introduction:

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Introduction:

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• Silent operation

• Cost effective

• Balanced pressure

• No need of valve

• No lubrication problem

• Less maintenance

Advantage of Three-Fluid

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• VAR uses heat energy, VCR uses mechanical energy

• Operation is simple in VAR as less moving parts

• Less wear, tear and noise.

• No bad effect of liquid refrigerant leaving the evaporator

• Law COP

• More component, bulky system

• Charging of refrigerant is difficult

Comparison of VAR and VCR

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