asme case study

8/11/2019 ASME Case Study

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VAPOUR ABSORPTION

LiBr-Water

REFRIGERATION SYSTEM


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INTRODUCTION

The energy and global warming crises have drawn

renewed interests to thermally driven cooling systems from

the air conditioning and process cooling fraternities

A vapour absorption refrigeration system is a heat operated

unit which uses refrigerant (water) that is alternately

absorbed by and liberated from the absorbent (LiBr).

It uses low graded heat energy (Solar energy, Geothermal

energy) instead of mechanical energy.


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WHY LiBr water system is preferred

(i) it can be thermally driven by gas, solar energy, andgeothermal energy as well as waste heat, which help to

substantially reduce carbon dioxide emission;

(ii) its use of water as a refrigerant;

(iii) it is quiet, durable and cheap to maintain, being nearly

void of high speed moving parts;

(iv) its vacuumed operation renders it amenable to scale up

applications


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Howit works???

Basic Components

Generator

Condenser

Evaporator

Absorber


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Function1. Generator-

The purpose of the generator is to deliver the refrigerant

vapour to the rest of the system

a high-temperature energy source , typically steam or hot

water, flows through tubes that are immersed in a dilute

solution of refrigerant and absorbent.

2. Condenser-

The purpose of condenser is to condense the refrigerant

vapours.

Inside the condenser, cooling water flows through tubes andthe hot refrigerant vapour fills the surrounding space which

converts into condensed refrigerant liquid.


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Function

3. Evaporator-

The purpose of evaporator is to cool the circulating water.

The evaporator contains a bundle of tubes that carry thesystem water to be cooled/chilled.

High pressure liquid condensate (refrigerant) is throttled downto the evaporator pressure (typically around 6.5 mm Hgabsolute).

4. Absorber-

Inside the absorber, the refrigerant vapour is absorbed by the

lithium bromide solution. As the refrigerant vapour is absorbed, it condenses from a

vapour to a liquid, releasing the heat it acquired in theevaporator.


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Thermodynamic representation


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Assumption As the refrigerant vapour is absorbed, it condenses from a

vapour to a liquid, releasing the heat it acquired in the

evaporator.

Specific enthalpy of superheated refrigerant at inlet of

condenser from generator is equal to the specific enthalpy of

saturated refrigerant at the generator temp.

Solution leaving the absorber and generator are saturated atthe unit temp.

Absorber and generation pressure are equal to the evaporator

and condenser respectively

Solution entering generator at generator pressure.


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MATHEMATICAL MODELLING BASED ON ABOVE

ASSUMPTION


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Model Validation


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FEASIBILITY AND COST COMPARISON DATA

Note- 20 S = 1 POUND


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Area of improvement in future

(i) develop advanced absorption cycles which could work at

low heat source temperature or recover more heat to improve

system performance;

(ii) improve the design of major components such as

generator and absorber to enhance their heat and masstransfer efficacy;

(iii) avoid crystallization problem;

(iv) develop new and reliable working pairs.


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Practical Problem with water-LiBr

refrigeration system

1. CRYSTALLIZATION To prevent crystallization the condenserpressure has to be maintained at certain level, irrespectiveof cooling water temperature. This can be done byregulating the flow rate of cooling water to the condenser.

-Additivesare also added in practical systems to inhibitcrystallization

2. AIR LEAKAGE - Since the entire system operates undervacuum, outside air leaks into the system. Hence an airpurging system is used in practical systems .

3. PRESSURE DROP - Since the operating pressures are verysmall and specific volume of vapour is very high, pressuredrops due to friction should be minimized. This is done byusing twin- and single-drum arrangements in commercialsystems.


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Conclusion

The heat input required for per KW cooling

capacity is 1.43 KW

COP obtained by this experiment variesfrom .35-.80 .


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Reference

Micallef , Micallef: Mathematical model of vapour absorption

refrigeration unit Xiaolin Wang and Hui T. Chua,Western Australia Geothermal

Centre of Excellence, School of Mechanical Engineering M050,

The University of Western Australia, 35 Stirling Highway,

Crawley, WA 6009, Australia

Laboratoire de Physique de la Matire Condense, Facult desSciences Ben M'Sik, Casablanca, B.P. 7955, Maroc

THANK YOU

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