Vapour Absorption Refrigeration Systems Based On ......Raoult’s law predicts a vapour pressure of 6.47 bar, whereas the measured vapour pressure is 3.029 bar. The vapour pressure

www.saks

hiedu

catio

n.com

Vapour Absorption Refrigeration Systems Based On Ammonia- Water Pair

Nagendra M

CBM Engineer, Hindusthan Zink .Ltd

The specific objectives of this lesson are to: 1. Introduce ammonia-water based vapour absorption refrigeration systems. 2. Discuss the properties of ammonia-water mixtures and introduce pressure temperature- concentration (p-T-ξ) and enthalpy-temperature-concentration (h-T- ξ) charts. 3. Analyze some basic steady flow processes using ammonia-water mixtures such as adiabatic and non-adiabatic mixing, throttling of solution streams and the concept of rectification. 1.1. Introduction

In vapour absorption refrigeration systems based on ammonia-water pair, ammonia is the refrigerant and water is the absorbent. These systems are more adaptable than systems based on water-lithium bromide as they can be utilized for both below zero (refrigeration) too over 0 0C (cooling) applications. On the other hand, these systems are more unpredictable in outline and operation because of the littler boiling point temperature difference between the refrigerant and absorbent (around 1330C). Because of the littler boiling point temperature difference the vapour produced in the generator consists of both ammonia and additionally water. If water is allowed to circulate with ammonia in the refrigerant circuit, then:

i. Heat transfer in condenser and evaporator becomes non-isothermal ii. Evaporator temperature increases iii. Evaporation will not be complete iv. Water may get accumulated in the evaporator leading to malfunctioning of the plant iv. Circulation ratio increases

Since all the above effects are detrimental to the performance of the system, it is necessary to minimize the concentration of water vapour in ammonia at the inlet to the condenser. This requires additional components, namely a rectification column and a dephlegmator between generator and absorber, which increases the design complexity and cost and also reduces the system COP compared to water-lithium bromide system. 1.2. Properties of ammonia-water solutions 1.2.1. Composition

Similar to water-lithium bromide solutions, the composition of ammonia-water solution is also expressed either in mass fraction (ξ) or mole fraction (x). However, for ammonia-water solutions, the mass and mole fractions are defined in terms of ammonia. For example the mass fraction ξ is defined as the ratio of mass of ammonia to the total mass of solution, i.e.,

www.sakshieducation.com


www.saks

hiedu

catio

n.com

where mA and mW are the mass of ammonia and water in solution, respectively. Similarly, the mole fraction of ammonia-water solution is defined as:

where n A and nW are the number of moles of ammonia and water in solution, respectively. The number of moles of ammonia and water can easily be obtained from their respective masses in solution and molecular weights, thus;

where MA (= 17.0 kg/kmol) and MW (= 18.0 kg/kmol) are the molecular weights of ammonia and water respectively. 1.2.2. Vapour pressure of ammonia-water solutions

Liquid ammonia and water are totally miscible in all proportions, henceforth can frame solutions of all concentrations from 0 to 1, at normal temperatures. The impact of ammonia in water is to bring down the vapour weight of water; comparatively the impact of water in ammonia is to bring down ammonia's vapour weight. Along these lines the aggregate weight over ammonia water solutions is comprised of incomplete weight of ammonia and fractional weight of water vapour, and is always in between the saturation pressures of unadulterated ammonia and water. If Raoult’s law is applied to ammonia-water mixtures, then the total pressure at any temperature, Ptotal is given by:

Ptotal = xPA + (1− x)PW

where x is the liquid phase mole fraction of ammonia, PA and PW are the saturation pressures of pure ammonia and pure water at that temperature. However, similar to water-lithium bromide solutions, ammonia-water solutions also deviate from ideal solution behaviour predicted by Raoult’s law in a negative manner, i.e., at a given temperature of the solution the actual vapour pressure will be less than that predicted by Raoult’s law (activity coefficient is much smaller than 1.0). For example, at a mass fraction of 0.4 and temperature of 400C, Raoult’s law predicts a vapour pressure of 6.47 bar, whereas the measured vapour pressure is 3.029 bar. The vapour pressure data of ammonia-water solutions is also available in the form of Dühring and other P-T-ξ plots. 1.2.3. Composition of ammonia-water vapour

Since the vapour above ammonia-water fluid consists of both ammonia and water vapour, it is crucial to recognize the composition in fluid phase and composition in vapour phase. The superscripts L and V will be utilized to recognize fluid and vapour phase



www.saks

hiedu

catio

n.com

compositvapour pgotten bcomposit

Bammoniadiverse tone can phase ma

Fig.1.1. V

1.2.4. BF

the mixtuAt first Presentlyexpandedcertain tebubble shpressure.as indica

O

continuoufluid dropdew poingoes intoliquids, texperienctemperatutemperatu

tions. Accorphase mass fby acceptingtion goes am

Based on tea-water mixttemperaturesget the com

ass fraction (

Vapour-liqu

Bubble poiigure 1.2 sh

ure is kept up(State 1) th

y warmth isd consistentlemperature hows up is c Further warted in the fig

On the off chusly, as morplet vaporiznt temperatuo superheatedthe temperatces vaporizaure coast, ures. On th

rdingly ξ L rfraction. In g perfect s

miss from tha

est estimatioture in vapous. Figure 1.1mposition of(ξ L ) and im

id equilibriu

int and dewhows a barrelp constant w

he barrel cos supplied tly, the massthe first vapcalled as bubrming resultgure (State 2

hance that wre fluid is co

zes. The tempure (Tdew). Ad vapour (Stture of the aation. This iswhich is th

he off chanc

remains for spite of theolution con

at anticipated

ons, graphs ur phase in h1 shows the f mixture in

mmersed temp

um chart for

w point fol containing

with the assisonsists of suto the frams fraction ofpour bubblebble point (=ts in increme2).

arming is coonverted intoperature at w

At the point tate 3). It ouammonia-was to say that he differencce that this

fluid phase e fact that thnduct, it is d by perfect

have been harmony wit

constructiovapour pha

perature of u

ammonia-w

or ammonimixture of

stance of a frub cooled s

mework and f the solutioe shows up.=Tbubble) of tent in tempe

ontinued furto vapour, lawhich the lawhen warm

ught to be noater mixture

the phase chce betweenprocedure i

mass fractiohe vapour p

watched thmixture equ

created forth a solution

on of such aase from knunadulterated

water solution

ia-water mammonia an

free-gliding psolution of

the temperon stays con The tempethe solution erature and f

ther, then thastly at a speast fluid dropming is conti

ticed that noincrements hange metho

n the dew is rehashed

on and ξ V

hase compohat the gen

uations.

r getting coof ammonia

an outline utnown estimad ammonia o

n

mixtures nd water. Thpiston with fammonia wrature of thnstant at ξ1 erature at wat that conc

formation of

he temperatuecific temperplet dissipateinued furtheot at all like continuousl

odology is dpoint and with distin

remains foosition can bnuine vapou

omposition oa and water tilizing whictions of fluior pressure.

he pressure ofixed weight

water mixturhe solution

at first. At which the fircentration anf more vapou

ure incremenrature the laes is called aer the mixturunadulterate

ly as the fluidescribed by bubble poin

nctive startin

or be ur

of at ch id

on ts. re. is a

rst nd ur

nts ast as re ed id a

nt ng



www.saks

hiedu

catio

n.com

concentrasame pre

O

ammoniatemperatupoints byTemperademonstrthe loci oimmersedpressure.both fluidbe gotten

F

pressuresfirst withis the temis the tem

Fig.1.2: A

Fig.1.3: E

ations beginessure, divers

Obviously wa) the bubbures (bubbley a curve anature versus rated in Fig.of all the ded fluid line The regiond and vapou

n if the inves

For instances, P1 and P2h superheatedmperature at mperature at

A simple exp

Equilibrium te

nning from 0se estimation

when the coble and dewe point and

nd all the deconcentrati

1.3. The locew points arand the dew

n between thur coincide istigation is c

e, Figure 1.42. The samed vapour anwhich the fiwhich the la

periment illu

emperature-co

(unadulterans of bubble

oncentration w points agd dew point)ew points byion curve foci of all the bre known as w point line he bubble anin balance. Dompleted wi

4 shows thee results cand after that birst fluid droast vapour bu

ustrating the

oncentration c

ated water) toe and dew po

is 0 (unadgree. Presen) against co

y another cuor ammoniabubble pointhe dew pois the soake

nd dew pointDistinctive bith diverse p

e bubble ann likewise bebegin coolin

oplet frames ubble conden

e principle of

curve for NH

o 1 (immacuoints will be

dulterated wntly in the oncentration urve, then wea-water mixtnts are calledoint line. Theed vapour lint lines is thebubble pointpressures.

nd dew poine gotten if ong it. For thifrom the vapnses.

f bubble and

3-H2O at a co

ulate ammonacquired.

water) or 1 event that and join a

e would gettures at tha

d as bubble pe bubble pone for the me two phase t and dew po

nt lines for one begins this situation, tpour and the

d dew points

onstant pressu

nia) and at th

(immaculawe plot th

all the bubbt the harmonat pressure apoint line anint line is th

mixture at thregion wher

oint lines wi

two separahe analysis the dew poine bubble poin

ure

he

te he le

ny as nd he at re ill

ate at nt nt



www.saks

hiedu

catio

n.com

Nand wateconcentrasaturatedvapour wliquid anwill be ξvapour wshown in

O

(ammoniexample,the conceof indivi(mtotal) an

where m From the

Fig.1.4:

Now since ther will be cation of su

d liquid existwill be diffend vapour wiξ2 L (intersewill be ξ2 V

n the figure.

Obviously theia) and the li, as shown ientration of tidual compond ammonia

2 L and m2 V a

e above equa

Bubble poin

he process isonserved. T

uperheated vts in equiliberent. For exill be same aection of conV (intersecti

e vapour foriquid remainin Fig.1.3, ththe last liqui

onents is alw(mA) mass a

are the mass ations it can b

nt and dew p

s carried outThe concentrvapour. Howbrium with sxample, at pas they are instant tempon of const

rmed initiallyning will be he concentraid droplet wways conserat state 2 as:

of liquid andbe easily sho

point curves

t in a closedration of subwever, in tsaturated vappoint 2 in Fin equilibriuperature lintant temper

y will be richrich in high

ation of the ill be ξ1 L

rved, we can

d vapour at sown that:

at two differ

d system, thb cooled liqthe two-phapour, the coFig.1.3, the um, hence, the with bubbrature line

her in the lowh boiling poifirst vapour

L. Since the tn write mas

state 2, respe

rent pressure

he mass of bquid will bease region ioncentration

temperaturehe concentrable point linwith dew p

w boiling point substancebubble will

total mass asss balance f

ectively.

es

both ammone same as thin which thof liquid an

e of saturateation of liquine) and that opoint line) a

oint substance (water). Fol be ξ1 V ans well as mafor total mas

ia he he nd ed id of as

ce or nd ss ss



www.saks

hiedu

catio

n.com

The above equation is called as the mixing rule or lever rule for the binary mixtures such as ammonia and water. It implies that the fraction of liquid and vapour in the two-phase mixture is inversely proportional to the distance between the mixture condition 2 and the saturated liquid and vapour states 2 L and 2 V, respectively. 1.2.5. Enthalpy of ammonia-water mixtures Liquid phase: The enthalpy of ammonia-water solution in liquid phase, hL is calculated in a manner similar to that of water-lithium bromide solutions, i.e., by the equation:

Where ξL is the liquid phase mass fraction of ammonia, hAL and h WL are liquid phase enthalpies of pure ammonia and water respectively. Δh mix is the heat of mixing, which is negative (exothermic) similar to water-lithium bromide mixtures. Using the above equation one can calculate the specific enthalpy of ammonia water solutions at any concentration and temperature provided the heat of mixing is known from measurements.

Thus enthalpy charts for solution are plotted as a field of isotherms against mass fraction by taking suitable reference values for enthalpy of ammonia and water. Since pressure does not have a significant effect on liquid enthalpy (except at critical point), normally pressure lines are not shown on typical solution enthalpy charts. Also enthalpy of sub cooled liquid is generally assumed to be equal to the saturated enthalpy at that temperature without loss of much accuracy. Vapour phase:

Evaluation of enthalpy of a mixture of vapours of ammonia and water is more

complicated compared to liquid phase enthalpy. This is due to the dependence of vapour enthalpy on both temperature and pressure. However, to simplify the problem, it is generally assumed that ammonia and water vapour mix without any heat of mixing. Then the enthalpy of the vapour mixture, hV is given by:

Where ξ.V is the vapour phase mass fraction of ammonia and the specific enthalpies of ammonia vapour and water vapour respectively at the temperature of the mixture. However, since vapour enthalpies depend on temperature as well as pressure, one has to evaluate the vapour enthalpy at suitable pressure, which is not equal to the total pressure. An approximate, but practically useful method is to evaluate the vapour enthalpies of ammonia and water at pressures, PA and PW given by

where y is the vapour phase mole fraction of ammonia and Ptotal is the total pressure. It should be noted that PA and PW are equal to the partial pressures of ammonia and water only if they behave as ideal gases. However since ammonia and water vapour may not approach the ideal gas behaviour at all temperatures and pressures, in general PA and PW are not equal to the



www.saks

hiedu

catio

n.com

partial pressures. Using these method enthalpies of ammonia-water mixtures in vapour phase have been obtained as functions of temperature and mass fraction. 1.2.6. The complete enthalpy-composition diagram for ammonia-water mixtures:

Normally, charts of enthalpy-temperature-mass fraction are available which give both liquid phase as well as vapour enthalpy of mixtures. Figure 1.5 shows one such chart. Figure 1.6 shows the enthalpy-synthesis diagram at a constant weight P. In the figure point a represents to the state of saturated liquid mixture at a temperature T with a liquid phase mass fraction of ξL. The liquid phase enthalpy corresponding to this condition is given by h L. The organization and enthalpy of vapour mixture in harmony with the liquid mixture at temperature T and weight P are obtained by drawing a vertical line from an up to the auxiliary line and then drawing a horizontal line to one side from the convergence of the vertical line with the auxiliary line.

The crossing point of this horizontal line with the dew point line a' gives the vapour

phase mass fraction ξV and the vapour phase enthalpy h V as demonstrated in the figure. The isotherm T in the two-phase district is obtained by joining focuses on and an' as indicated in the figure. Point b in the figure lies in the two-phase district. The particular enthalpy of this point h b is given by:

where ψb is the quality or dryness fraction of the two-phase mixture at b. Since points a, a’ and b are co-linear, the dryness fraction ψb is given by:

In actual enthalpy-composition diagrams the isotherms are not shown in two-phase region as a different set of them exist for each pressure. It is important to note that it is not possible to fix the state of the mixture (sub cooled, saturated, two-phase or superheated) just from temperature and mass fraction alone, though one can calculate enthalpy of the mixture from temperature and mass fraction. This is due to the reason that at a given mass fraction and temperature, depending upon the pressure the point can be sub cooled or saturated or superheated. For example, a liquid mixture with a mass fraction of 0.4 and temperature of 80oC has an enthalpy of 210 kJ/kg, and it will be in sub cooled condition if the pressure is 4.29 bar and saturated if the pressure is 8.75 bar.



www.saks

hiedu

catio

n.com

F

Determ

Aphase disThe trialsystem foweight Pas demonand then

Fig.1.6: Enth

mination ofA trial-and-erstrict in the el-and-error sfor finding thx, liquid phanstrated in thdrawing a h

Fig.1.5: h-T

halpy-comp

f temperatrror system event that itsstrategy canhe temperatuase mass frahe figure byhorizontal li

T-ξ chart f

osition diagr

ture of mihas to be uts enthalpy, l

n be graphicure of pointaction ξx andy drawing a vne from the

for ammonia

ram of NH3-

ixture in twilized to focliquid phase cal or numet x in the twd enthalpy hvertical line convergenc

a-water solut

-H2O at a co

wo-phase cus the temp

mass fractioerical. Figurwo-phase reghx. To start w

from point ce point an"

tion

nstant pressu

region: erature of a

on and weighre 1.7 showgion which iwith, point ax up to the up to the de

ure P

point in twoht are knowns a graphicis at a knowan' is obtaineauxiliary linew point lin

o-n. al

wn ed ne

ne,



www.saks

hiedu

catio

n.com

the convepoint b' horizontathrough xcan be obpassing t

T

bubble pocompositvalues ofξV. ThisξL are ob

F 1.3 Ba a)Adiabare mixedas:

ergence of wis obtained al line from x. This procbtained fromhrough x, i.e

To start withoint line. Thtion charts uf h L and ξL as procedure ibtained then

Fig.1.7: A gr

asic stead

batic mixid adiabatica

which gives aby drawingb" up to theedure is rep

m the equatioe.,

h guess valuhen saturatedusing the gueare obtainedis repeated t

n the tempera

raphical met

dy-flow pr

ing of twolly as shown

a'. Then a str a vertical le dew point eated until mon, which n

ues of h L and vapour proess values ofd. Then theseill the value

ature is read

thod for find

rocesses

o streams:n in Fig.1.8,

raight line a'line up to thline to get

meeting is obneeds to be s

nd ξL are asoperties h V anf h L and ξL.e new valuess converge. from the ent

ding tempera

with bin

: When two one can writ

'-x-an is drawhe auxiliary b'. Then linebtained. Numsatisfied for

ssumed by tnd ξV are ob. Then usings are used toOnce the cothalpy comp

ature of liqui

ary mixt

streams of te mass and

wn as demonline and th

e b'-x-b is dmerically theach end of

taking somebtained from

g the above eo obtain nextonverged valposition char

d-vapour mi

tures

ammonia-wenergy bala

nstrated. Nexhen drawing drawn passine temperaturf the isotherm

e point on thm the enthalpequation, net set of h V anlues of h L anrt.

ixture

water solutionance equation

xt a

ng re m

he py w

nd nd

ns ns



www.saks

hiedu

catio

n.com

From the

F

phase regimplies th1 and 2. using theconsists o(T3) havethe vapou

b) Mixiwith heatFor examsaturatedsolution

e above equa

igure 1.9 shgion on the hat some vaThe enthalp

e equations of saturated e to be obtaiur in the mix

ing of twot transfer takmple, Fig.1.d solution of

that is rich

ations, the m

Fig.1.8: Ad

hows the adienthalpy-co

aporization hpy and comgiven abovliquid and v

ined by trialxture at 3 is t

o streams wkes place in.10 shows t

f refrigerant- in refrigera

mass fraction

diabatic mix

iabatic mixinomposition dhas occurred

mposition of e. Howevervapour. Thel-and-error mthen given b

with heat tn absorber anthe mixing absorbent (sant (state 3)

and enthalpy

xing of two s

ng process wdiagram. Th

d during adiathe two-pha

r, since this e dryness framethod by apby:

transfer: Thnd generatorof saturate

state 2) in th). Since the

y of the mix

solution strea

with the mixhe mixture sabatic mixingase mixture is in two-p

action and tepplying mix

he process or of absorpti

ed refrigerane absorber. T

e process is

xture at 3 are

ams

xture state 3state in two-g of the twoat 3 can be

phase regionemperature oxing rules. T

of mixing ofion refrigerant vapour (The resulting

exothermic

e given by:

3 lying in tw-phase regio

o inlet streame obtained b

n, the mixturof the mixturhe fraction o

f two streamation systemstate 1) witg mixture is c, heat (Q)

wo on ms by re re of

ms ms.

th a is



www.saks

hiedu

catio

n.com

released written a

From the

Thus wit(Q/m3) bwould lie c) Throtakes plashows thare conse

during this s:

Fig.1

e above equa

th heat transbelow the ste above the s

ottling prce in the sol

he throttling erved during

process. M

1.9: Adiabat

ations, the en

sfer from thetate which wstate without

ocess: Throlution expanprocess on

g this process

Mass and en

tic mixing of

nthalpy of th

e mixing chawould resultt heat transfe

ottling or ission valve oenthalpy-cos, and there i

ergy balanc

f two stream

he mixture at

amber, the et without heer if heat is t

senthalpic exof the absorpomposition dis neither wo

ξ1 =ξ

h1 =h

ce equations

ms on h-T-ξ

t 3 is given b

exit state lieeat transfer transferred to

xpansion of ption refrigerdiagram. Sinork nor heat

ξ2 h2

for this pr

diagram

by:

s at a vertic(point 3’).To the mixing

f ammonia-wration systemnce both mas

transfer, we

rocess can b

al distance oThe exit poing chamber.

water solutiom. Figure 1.1ss and energe obtain:

be

of nt

on 11 gy



www.saks

hiedu

catio

n.com

H

diagram flashing, pressure Taking pthe vapou

d) Heatarrangem(HX A) imixture t

Hence the inlas shown inthe exit coP2 then the

point 2 as in ur fraction a

ting and cment whereinin such a wathen flows i

Fig.1.10: M

let and outlen the figure. ondition may

exit temperthe two-phand exit temp

Fig.1.11:

cooling prn an initiallyay that the einto an adiab

Mixing of tw

et states, poinHowever, a

y be a mixturature T2 wi

ase region coperature T2 b

Throttling o

rocess–Cony sub cooled exit conditiobatic separa

wo streams w

nts 1 and 2 aas there is poure of saturll be much

orrespondingby trial-and-e

of ammonia-

ncept of rsolution (sta

on 2 lies in tator (SEP A)

with heat tran

are identical ossibility of rated liquid lower than

g to the outleerror method

water soluti

rectificatioate 1) is heatthe two-phas) where the

nsfer

on enthalpyvapour geneand vapourthe inlet tem

et pressure Pd as discusse

on

on: Figure 1ted in a heatse region. Thsaturated li

y-compositioeration due tr at the outlmperature T

P2, one can ged earlier.

1.12 shows at exchanger Ahis two-phasquid (state 3

on to et

T1. et

an A se 3)



www.saks

hiedu

catio

n.com

and saturstate 5 inmixture i(state 6) takes pla

Fi Now mas

rated vapourn another heis then fed toand saturatce at a const

g.1.12: Hea

ss and energ

r (state 4) areat exchangeo another aded vapour (tant pressure

ating and co

gy balances a

re separated.er B (HX B

diabatic sepa(state 7) aree and is a ste

oling of NH

are applied to

The saturat) by rejectinrator B (SEPseparated. I

eady-flow pr

H3-H2O solut

o each of the

ted vapour ang heat 4Q5.P B), where It is assume

rocess.

tion – conce

e component

at state 4 is t The resultiagain the sa

ed that the e

ept of rectifi

ts as shown

then cooled tng two-phas

aturated liquientire proces

fication

below:

to se id ss



www.saks

hiedu

catio

n.com

Similar equations can be obtained for heat exchanger B and separator B. The entire process is also shown on enthalpy-composition diagram in Fig.1.12.

It may be noticed that from the above arrangement comprising of heating, cooling and separation, one finally obtains a vapour at state 7 that is rich in ammonia. That is the combination of heat exchangers with separators is equivalent to the methodology of rectification. Heat exchanger A plays the part of generator, while heat exchanger B plays the part of dephlegmator. To enhance the procedure of rectification in actual vapour absorption refrigeration frameworks, a rectifying column is presented between the generator and dephlegmator. In the rectifying column, the vapour from the separator an interacts with the saturated liquid originating from separator B.

Therefore, there will be heat and mass transfer between the vapour and liquid and

finally the vapour turns out at a much higher concentration of ammonia. The practical ammonia-water based vapour absorption refrigeration framework incorporating rectifying column.



Documents

Vapour Absorption Refrigeration Systems Based On ......Raoult’s law predicts a vapour pressure of 6.47 bar, whereas the measured vapour pressure is 3.029 bar. The vapour pressure