Vapor absorption system

SIMPLE VAPOR-ABSORPTION REFRIGERATION SYSTEM

Manmeet Singh

INTRODUCTION

The major drawback of the vapour compression refrigeration

system is that it requires large volume of refrigerant vapour

which requires large mechanical power for its operation.

If some methods are used to reduce this volume before

compression, there would be considerable reduction in weight

of the system and power requirement for its operation.

Heat energy can be used instead of work for producing

refrigeration because it gives high COP of the system with

machine operated with supply of work energy.

Theabsorption

system differs fundamentally from vapour

in the method of employedfor

compression system only

compressing the refrigerant.

In the absorption system, the compressor is replaced by an

absorber, generator and a pump.

A French Scientist Ferdinand Canedeveloped the first

absorption refrigeration machine in early 1860.

Nowadays, units are developed upto 1500 tons capacity.

The units which are generally used for

air conditioning purposes are available from 100 tons

capacity .

PRINCIPLE OF ABSORPTIN SYSTEM

There is the peculiar property of some substances to have

affinity for another substances at some temperatureand

pressure conditions and less affinity at another conditions.

This idea for the working principle of a vapour absorption

system was generated by Michael Faraday in 1824.

He knew that silver chloride (AgCl)m a white powder , had a

property of absorbing large amount of ammonia gas at the

normal temperature and pressure.

Two chambers are combined with the help of a tube.

The white powder was kept inside the first chamber to which ammonia

gas was supplied and sealed.

The powder was heated up while other end was cooled using circulating

water.

Liquid ammonia was obtained in the cool end of the apparatus. After

stopping heat, it was observed that, the liquid ammonia instead of sitting

there, started boiling( bubbles produced) and vapour was reabsorbed by

the white powder.

Upon touching the boiling end , it was astonished to find that the vessel

was very cold.

He repeated the experiments and cooling was observed again.

This led to invention of the intermittent Vapour

absorption system having solid as an absorber.

REFRIGERANT

ABSORBER STATE OF ABSORBER

AMMONIA WATER LIQUID

AMMONIA SODIUMTHIOCYNATE SOLID

AMMONIA LITHIUM NITRATE SOLID

AMMONIA CALCIUM CHLORIDE SOLID

AMMONIA ISOBUTANE SOLID

WATER LITHIUM BROMIDE SOLID

WATER LITHIUM CHLORIDE SOLID

METHYLCHLORIDE

DIMETHYL ETHER OF TETRAETHYLENE GLYCOL

LIQUID

REFRIGERANT –ABSORBER PAIRS

SIMPLE ABSORPTION SYSTEM

SIMPLE VAPOR ABSORPTION REFRIGERATION SYSTEM

@Point 1-low temperature and low pressure

refrigerant vapour from evaporator at state 1 enters the absorber and is absorbed by solution weak in refrigerant (state 8) .The heat of absorption (Qa) is rejected to an external heat sink at T∞.


@Point 2-The solution, rich in refrigerant is

pumped to the generator pressure (Pg) by the solution pump (state 3). The pressurized solution gets heated up sensibly as it flows through the solution heat exchanger by extracting heat from hot solution coming from generator (state 4).


@Point 5-Heat is supplied to this solution from an

external heat source in the generator (Qg at Tg), as a result refrigerant vapour is generated (absorbent may also boil to give off vapour in case of ammonia-water systems) at state 5.

This high-pressure refrigerant vapour condenses in the condenser by rejecting heat of condensation to the external heat sink (Qc at T∞) and leaves the condenser as a high pressure liquid (state 9).


@Point 10- the high pressure refrigerant liquid is

throttled in the expansion device to evaporator pressure Pe from where it enters the evaporator, extracts heat from low temperature heat source (Qe at Te) and leaves the evaporator as vapour at state 1, completing a cycle.


@Point 6 - The hot solution that is weak in

refrigerant (state 6) leaves the generator at high temperature and is cooled sensibly by rejecting heat to the solution going to the generator in the solution heat exchanger (state 7). Then it is throttled to the evaporator pressure in the throttle valve (state 8), from where it enters the absorber to complete the cycle.

COP FOR IDEAL VAPOR ABSORPTION REFRIGERATION SYSTEM


Absorption system requires a relatively large amount of low-grade thermal energy at generator temperature to generate refrigerant vapour from the solution in generator. Thus while the energy input is in the form of mechanical energy in vapour compression refrigeration systems, it is mainly in the form of thermal energy in case of absorption systems.


The solution pump work is often negligible compared to the generator heat input. Thus the COPs for compression and absorption systems are given by:

COP FOR IDEAL VAPOR ABSORPTION REFRIGERATION SYSTEM Thus absorption systems are advantageous

where a large quantity of low-grade thermal energy is available freely at required temperature. However, it will be seen that for the refrigeration and heat rejection temperatures, the COP of vapour compression refrigeration system will be much higher than the COP of an absorption system as a high grade mechanical energy is used in the former, while a low-grade thermal energy is used in the latter.

COP FOR IDEAL VAPOR ABSORPTION REFRIGERATION SYSTEM However, comparing these systems based on

COPs is not fully justified, as mechanical energy is more expensive than thermal energy. Hence, sometimes the second law (or exergetic) efficiency is used to compare different refrigeration systems. It is seen that the second law (or exergetic) efficiency of absorption system is of the same order as that of a compression system.

MAXIMUM COP FOR IDEAL VAPOR ABSORPTION REFRIGERATION SYSTEM


From first law of thermodynamics,

Where Qe is the heat transferred to the absorption system at evaporator temperature Te, Qg is the heat transferred to the generator of the absorption system at temperature Tg, Qa+c is the heat transferred from the absorber and condenser of the absorption system at temperature To and Wp is the work input to the solution pump.


If we assume that heat rejection at the absorber and condenser takes place at same external heat sink temperature To, then a vapour absorption refrigeration system operates between three temperature levels, Tg, To and Te.


The maximum possible COP of an ideal VARS system is given by:


Thus the ideal COP is only a function of operating temperatures similar to Carnot system. It can be seen from the above expression that the ideal COP of VARS system is equal to the product of efficiency of a Carnot heat engine operating between Tg and To and COP of a Carnot refrigeration system operating between To and Te,


Thus an ideal vapour absorption refrigeration system can be considered to be a combined system consisting of a Carnot heat engine and a Carnot refrigerator as shown in Fig.14.4. Thus the COP of an ideal VARS increases as generator temperature (Tg) and evaporator temperature (Te) increase and heat rejection temperature (To) decreases. However, the COP of actual VARS will be much less than that of an ideal VARS due to various internal and external irreversibilities present in actual systems.

PRACTICAL VAPOR ABSORPTION REFRIGERATION SYSTEM

A practical VARS has 3 additional parts :

ANALYSERRECTIFIERHEAT EXCHANGER

PRACTICAL VAPOR – ABSORPTION REFRIGERATION SYSTEM

THREE FLUID VAPOR ABSORPTION SYSTEM- ELECTROLUX SYSTEM

1. Hydrogen enters the pipe with liquid ammonia (or lithium bromide solution)2. Ammonia and hydrogen enter the inner compartment of the refrigerator. An increase in volume causes a decrease in the partial pressure of the liquid ammonia. The ammonia evaporates, requiring energy to overcome the ΔHVap. The required energy is drawn from the interior of the refrigerator, thus cooling it.3. Ammonia and hydrogen return from the inner compartment, ammonia returns to absorber and dissolves in water. Hydrogen is free to rise upwards.4. Ammonia gas condensation (passive cooling).5. Hot ammonia (gas).6. Heat insulation and distillation of ammonia gas from water.7. Heat source (electric).8. Absorber vessel (water and ammonia solution).

ROLE OF HYDROGEN

ADVANTAGES THREE FLUID SYSTEM

DISADVANTAGES THREE FLUID SYSTEM

VAPOUR ABSORPTION REFRIGERATION SYSTEMS BASED ON WATER-LITHIUM BROMIDE PAIR

VARS BASED ON H2O – LIBR PAIR

Vapour absorption refrigeration systems using water-lithium bromide pair are extensively used in large capacity air conditioning systems.

In these systems water is used as refrigerant and a solution of lithium bromide in water is used as absorbent.

Since water is used as refrigerant, using these systems it is not possible to provide refrigeration at sub-zero temperatures. Hence it is used only in applications requiring refrigeration at temperatures above 0oC.

VARS BASED ON H2O – LIBR PAIR Hence these systems are used for air

conditioning applications. The analysis of this system is relatively easy as the vapour generated in the generator is almost pure refrigerant (water), unlike ammonia-water systems where both ammonia and water vapour are generated in the generator.

STEADY FLOW ANALYSIS OF WATER-LITHIUM BROMIDE SYSTEMS A steady flow analysis of the system is carried out with the following assumptions: i. Steady state and steady flow ii. Changes in potential and kinetic energies across each component are negligible iii. No pressure drops due to friction iv. Only pure refrigerant boils in the generator. The nomenclature followed is: m= mass flow rate of refrigerant, kg/s mss= mass flow rate of strong solution (rich in LiBr), kg/s mws= mass flow rate of weak solution (weak in LiBr), kg/s

Circulation ratio (λ) -defined as the ratio of strong solution flow rate to refrigerant flow rate.

It is given by:

λ = mss/m

@Condenserm1 = m2 = m3

Qc = m(h1 – h2)Pc = Psat (TC)where TC is the condenser temperature

@Expansion valve (refrigerant): m2 = m3 = mh2 = h3

@Evaporator: m3 = m4 = mQE = m(h4 – h3)PE = PSAT(TE)where TE

is the evaporator temperature

@AbsorberFrom total mass balance:m + mss = mws

but mss = λm,mws = (1+λ)m

QA=mh4+ λmh10

- (1+λ)mh5

@Solution pumpm5 = m6 = mws

Wp = mws(h6-h5) =(1+λ)m(h6-h5)

Even though the solution pump work is small it is still required in the selection of suitable pump.

@Generatorm7 = m8 +m1

Heat input to the generator is given by:QG=mh1+λmh8

-(1+λ)mh7

@Solution heat exchangerm6 = m7 = mws

m8 = m9 = mss

heat transfer rate in the solution heat exchanger, Q is given by:

QHX = (1+λ)m(h7-h6) =λm(h8-h9)

@Solution expansion valvem9 = m10 = mws

h9 = h10

The COP of the system is given by:

ADVANTAGES OVER VAPOUR COMPRESSION SYSTEM

As there is no moving parts in the entire system, the operation is essentially quiet and subjected to a very little wear, so that the maintenance cost is low.

The pump motor is quite small compared with the compressormotor. Vapour absorption system is used the thermal energy,

they can be used in places, where electric power is hard to obtain.

Absorption unit can be built in capacities well above 1000 tons Space requirement is less.

PROPERTIES OF GOOD ABSORBER

PROPERTIES OF GOOD REFRIGERANT

SAMPLE PROBLEM IN SIMPLE VARS9. The operating temperatures of a single stage vapour absorption refrigeration system are: generator: 90oC; condenser and absorber: 40oC; evaporator: 0oC. The system has a refrigeration capacity of 100 kW and the heat input to the system is 160 kW. The solution pump work is negligible. a) Find the COP of the system and the total heat rejection rate from the system. b) An inventor claims that by improving the design of all the components of the system he could reduce the heat input to the system to 80 kW while keeping the refrigeration capacity and operating temperatures same as before. Examine the validity of the claim.

SAMPLE PROBLEM IN SIMPLE VARS Solution:

Vapor absorption system

Engineering

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