vertical tubular absorbers for ammonia-salt absorption refrigeration

63 Pages·2007·55.69 MB·English

VERTICAL TUBULAR ABSORBERS FOR
AMMONIA-SALT ABSORPTION REFRIGERATION
Proefschrift
ter verkrijging van de graad van doctor in de
technische wetenschappen aan de Technische Hogeschool Delft,
op gezag van de Rector Magnificus,
prof.dr. J.M. Dirken
in het openbaar te verdedigen
ten overstaan van het College van Dekanen op
dinsdag 26 maart 1985 om 14.00 uur.
door
CARLOS ALBERTO INFANTE FERREIRA
geboren te Moura (Portugal)
engenheiro mecanico
werktuigkundig ingenieur
TR diss
1428
Dit proefschrift is goedgekeurd door de promotoren
PROF.IR. A.L STOLK
PROF. J.M. SMITH, M.Sc.
My advisors. Prof. ir. A. L. StolK and
Prof. J. H. Smith, M. Sc., generously
gave their time in the orientation of
my study and I gratefully acknowledge
their assistance.
I wish to extend my sincere .thanks to
Mrs. Angelina de Wit from Qualitype
Services for having typed this disser­
tation and to Mr. Jaap Keuvelaar for
having drawn the illustrations.
This work was greatly assisted by the
help and friendship of graduate students
and technical staff of the Laboratory for
Refrigeration and Indoor Climate Techno­
logy. I want to express my sincere gra­
titude to all those who contributed to
the achievement of this dissertation.
CONTENTS
SAMENVATTING
1. INTRODUCTION
1.1. Absorptiorl refrigeratian systei
1.2. l-lethods oi- Improving the perfa: ce of absorption refrigeration machines 2
1.3. Framework of the investigation
SELECTION OE POSSIBLE ABSORBENT-REFRIGERANT SYSTEMS
2.1. Introduction
2.2. Qualitative review of possible systems 5
2.2.1. Absorbent-refrigerant systems with water as refrigerant 5
2.2.2. Absorbent-refrigerant systems with ammonia as refrigerant
2.2.3. Absorbent-refrigerant systems with methanol as refrigerant
2.2.4. Absorbent-refrigerant systems with methylamine as refrigerant
2.2.5. Absorbent-refrigerant systems with halagenated organic compounds as refrigerant
2.2.6. Potential absorbent-refrigerant systems
2.3. Quantitative selection 0
2.3.1. Introduction 8
2.3.2. Engineering parameters for a single stage absorption refrigeration cycle
2.3.3. Data required for comparison of refrigerant-absorbent systems
2.3.4. Results and conclusions 12
2.3.4.1. Processes with water cooling 12
2.3.4.2. Processes with air cooling 13
"2.3.4.3. Pumping factor, pumping work and heat transfer rate in recuperator and
precDOler 14
2.3.5. Comparison with other studies 1E
2.4. Main conclusions of chapter 2 19
AMMONIA-SALT ABSORPTION REFRIGERATION MACHINES 21
3.1. Introduction 21
3.2. Simulation model 22
3.3. Physical properties data 24
3.4. Experimental absorption refrigeration machine for the system NH3-LÜMO3 25
3.4.1. Flaw sheet and operation 25
3.4.2. Components and control devices 27
3.4.3. Secondary circuits 28
3.4.4. Data acquisition 26
3.4.5. Ammonia-salt solution 29
3.4.6. Experimental component characteristics 29
3.4.7. Evaluation of data 30
3.E. Comparison of simulation model with experimental data 30
3.5.1. Introduction
3.5.2. Effect of the strong solution mass flaw rate 31
3.5.3. Effect of the heating medium temperature 32
3.5.4. Effect of the cooling water temperature 33
3.5.5. Effect of the absorber cooling water flow rate 33
3.5.6. General conclusions 34
3.6. Effect of component design on system performance 34
3.6.1 . Component size 34
3.6.2. Water side heat transfer coefficient in the absorber 38
MOMENTUM, HEAT AND MASS TRANSFER IN VERTICAL TUBULAR ABSORBERS . 41
1.1. Introduction 41
4.2. Absorber configuration . 41
4.3. Hydrodynamics of vertical tubular absorbers 43
4.3.1. Flow patterns 43
4.3.1.1. Transition between churn flow and slug flow 45
4.3.1.2. Transition between slug flow and bubble flow 46
4.3.2. Churn flow characteristics 46
4.3.3. Slug flow characteristics 47
4.3.3.1 . Bubble shape 47
4.3.3.2. Gubble rise velocity 40
4.3.3.3. Film thickness 49
4.3.3.4. Bubble length, separation distance and bubble frequencies
4.3.3.5. Liquid mixing characteristics 52
4.3.4. Bubble flow characteristics 52
4.4. Heat transfer in vertical tubular absorbers 54
4.4.1. Churn flow region 54
4.4.2. Slug flow region 55
4.4.3. Bubble flow region 55
4.5. Mass transfer in vertical tubular absorbers
4.5.1. Churn flow region
4.5.2. Slug flow region 56
4.5.3. Bubble flow region 57
5. MACROSCOPIC ABSORBER MODEL AND ITS EXPERIMENTAL VALIDATION 59
5.1. Introduction ABSTRACT
5.2. Design equations for vertical tubular absorbers BO
5.3. Experimental procedure 61
5.3.1 . Introduction 61
5.3.2. Range of variables En
5.3.3. Test facility 62
5.4. Comparison with experimental data 66
5.4.1. Accuracy of the calculated outlet conditions 66
5.4.1.1. Absorption height 66
5.4.1.2. Outlet temperature of the liquid phase 67 A method is proposed for an evaluation and comparison of different refrigerant-
5.4.2. Accuracy of the variation of the gas phase mass Flow rate with absorption height 69 absorbent systems. Only limited data is required for this analysis and this is
5.5. Correlation of the experimental data mostly available in the literature. The method is used for comparison of eleven
5.5.1. Accuracy of the calculated outlet conditions
5.5.2. Effect of the different parameters on mass transfer promising systems. From these ammonia-sodium thiocyanate and ammonia-lithium
5.6. Design of vertical tubular absorbers 75 nitrate have the best performance. The practical feasibility of the ammonia-
5.6.1. Introduction 75
5.5.2. Effect of mass flew rate of cooling medium lithium nitrate system is verified in an experimental set-up.
5.5.5. Effect of tube diameter 79
5.6.4. Effect of cooling water temperature
5.7. Comparison with the wetted-wall absorber 79 A simulation model for an ammonia-salt absorption refrigeration machine is de­
5.3. Performance prediction of vertical tubular absorbers S3 veloped which allows performance prediction. Basic data required for calcula­
5.8.1. Introduction 93 tions are the evaporating, heating medium, and cooling water temperatures, the
5.6.2. Validation of the absorber model B6
heating medium, cooling water to absorber and to condenser, and weak solution
6. DIFFERENTIAL ABSORBER MODEL AND ITS EXPERIMENTAL VALIDATION B9 mass flow rates and the geometry of the components. The internal and external
6.1. Introduction
6.2. Governing momentum, energy and mass diffusion equations 31 irreversibilities of the components are determined from heat and mass transfer
6.2.1. Momentum equation 91 relations. The effects of strong solution mass flow rate, heating medium tempe­
6.2.2. Energy and mass diffusion equations 92
6.2.3. Boundary conditions associated with the energy and moss equations 93 rature, cooling water temperature and cooling water flow rate to the absorber
6.2.4. Method of solution 94 on the performance of an ammonia-lithium nitrate absorption refrigeration ma­
6.3. Literature review 94 chine are experimentally verified. These experimental data allow for a valida­
6.4. Comparison with Nusselt's theory and penetration theory 96
6.4.1. Introduction 96 tion of the simulation model. The use of the simulation model for component size
6.4.2. Comparison with Nusselt's solution of the energy equation optimization is illustrated.
6.4.3. Comparison with the penetration theory
6.4.4. Effect of bubble nose 99
The hydrodynamics of vertical tubular absorbers are studied so that physical
5.5. Temperature profiles 102
models can be developed for the heat and mass transfer processes in the differ­
7. GENERAL CONCLUSIONS 105 ent flow regions encountered. A macroscopic model is developed for the absorber
NOMENCLATURE 107 which, in combination with the phenomenological description of heat and mass
transfer processes, allows the prediction of the absorber performance. A large
REFERENCES 109
number of experiments in vertical tubular absorbers with the systems ammonia-
CURRICULUM VITAE 116 sodium thiocyanate and ammonia-lithium nitrate allows for a validation of the
macroscopic absorber model. Four different absorber tubes have been tested with
internal diameters of 10, 15, 20, and 25 mm. The accuracy of the macroscopic ab­
sorber model in combination with an empirical approach for mass transfer predic­
tion is shown to be smaller. The use of the model for design purposes is il­
lustrated. The validity of the model for a multi-tube vertical absorber is ex­
perimentally verified for a 30 tube ammonia-lithium nitrate absorber.
An analysis of the simultaneous processes of momentum, heat and mass transfer
along a Taylor gas bubble departing from differential equations is presented.
This allows for a comparison with Nusselt's theory and the penetration theory
used in the macroscopic absorber model. Conclusions may also be drawn for the
effect of the bubble nose on mass transfer. Temperature profiles during absorp­
tion in these films can be analysed.
iii
SAMENVATTING
BELLENABSORBERS VOOR AMMONIAK-ZOUT ABSORPTIEKOELING
Een methode wordt voorgesteld voor het evalueren an vergelijken van verschillen­
de koudemiddel-absorbent stofparen. Deze methode volstaat met een geringe hoe­
veelheid stafeigenschappen die meestal te vinden zijn in de literatuur. Elf ver­
schillende stofparen worden vergeleken. Van de vergeleken stofparen hebben arnmo-
nlaK-natrium thiocianaat en ammoniak-lithium nitraat de beste eigenschappen. De
praktische geschiktheid van het stofpaar ammoniak-lithium nitraat wordt beves­
tigd met behulp van een experimentele opstelling.
Een simulatiemodel wordt ontwikkeld voor een ammoniak-zout absorptiekoelmachine
dat een voorspelling van de machineprestaties toelaat. De rekenmethode eist de
volgende gegevens: verdampings-, verwarmingsmedium- en koelingwatertemperatuur,
verwarmingsmedium-, absorber koelwater-, condenser koelwater- en arme oplossing-
massastroom en de geometrie van de komponenten. De interne en externe onomkeer-
baarheden van de komponenten worden bepaald uit warmte- en stofoverdrachtsrela­
ties. De invloeden van massastroom rijk oplossing, verwarmingsmediumtemperatuur,
koelwatertemperatuur en massastroom koelwater voor de absorber op de prestaties
van een ammoniak-lithium nitraat absorptiekoelmachine worden experimenteel be­
paald. De experimentele gegevens maken een validatie van het simulatiemodel mo­
gelijk. Het gebruik van het simulatiemodel voor bepaling van optimale komponen­
ten geometrie wordt geïllustreerd.
De hydrodynamica van bellenabsorbers wordt bestudeerd zodat fysische modellen
ontwikkeld kunnen worden voor de warmte- en stofoverdrachtsprocessen in de ver­
schillende stromingsgebieden. Een macroscopisch model is ontwikkeld voor de ab­
sorber dat, gecombineerd met een fenomenologische beschrijving van warmte- en
stafoverdrachtsprocessen, een voorspelling van de absorber werking mogelijk
maakt. Een groot aantal experimenten met bellenabsorbers met de stofparen ammo-
niak-natrium thiocianaat en ammoniak-lithium nitraat staan een validatie van het
macroscopisch absorber model toe. Vier verschillende absorber pijpen worden ge­
test met inwendige diameters 10, 15, 20 en 25 mm. De nauwkeurigheid van het ma­
croscopisch absorber model gecombineerd met een empirische aanpak voor stofover-
drachtsvaorspelling blijkt minder te zijn dan van het eerder genoemde model.
Hel gebruik van het model voor ontwerpdoeleinden wordt geïllustreerd. De geldig­
heid van het model voor een meer-pijpen bellenabsorber is experimenteel aange­
toond voor een 30 pijpen ammoniak-lithium nitraat absorber.
Een studie wordt gepresenteerd van de gelijktijdige processen van impuls-,
warmte- en stofoverdracht langs een "slug", uitgaande van partiële differenti­
aalvergelijkingen. Deze studie staat een vergelijking toe met de theorie van
Nusselt en met de penetratietheorie die gebruikt worden in het macroscopisch
absorber model. Er kunnen conclusies getrokken worden over de invloed van het
bovenste gedeelte van de bel op de stofoverdracht. Temperatuurprofielen geduren­
de absorptie in deze films worden bestudeerd.
1, INTRODUCTION
1.1. ABSORPTION REFRIGERATION SYSTEMS
Since the "oil crisis" in the 1970's, efforts have been developed leading to the
application of low temperature energy for cooling and heating purposes [Hodgett
and Oelert [1962]). The eventual large-scale replacement of fossil fuels de­
pends on their somewhat unpredictable costs. Currently, fossil fuel costs have
stabilized; however, this may change at any time and make low temperature ener­
gy more attractive from the economical standpoint.
AISD in the refrigeration world attention has been paid to the use of alterna­
tive energy sources (see LSEQ (1977], and StolK [1976;1980b,C;1981j19B2j].
Stalk (1976,1980b) compared the most suitable systems for solar-driven refrige­
ration machines and concluded that flat plate collectors combined with an ab­
sorption refrigeration machine are appropriate systems for the near future. Host
absorption system developments are due to solar cooling applications for air
conditioning purposes (see Bonnin et al. (1980), Duffie and Beekman (1980),
Hanna and Wilkinson (1982), Kreider and Kreith (1975), McVeigh (1977) end Newton
(1981)) but also for refrigeration purposes (see Haverhals (1982] and McNeils
(1982)). Heat pump developments also contributed to intensive absorption system
studies (see Hanna and Wilkinson (1982) and Hodgett (1982)).
Hasselt (1981) published the low grade energy potential of the Netherlands for
available fluid temperatures above 110°C. The equivalent of 5 milliard cubic
meter natural gas is available at this high energy level. At lower energy level
much more low grade energy is available possibly at a much lower price. The
availability of low grade energy in the process industry is often combined with
refrigeration needs, defining a second application field for heat driven refri­
geration machines.
Absorption systems present very simple refrigeration capacity storage facili­
ties and can easily be adapted for the use of intermittent heat energy (see
Holldorf (19B0)) . Recently the absorption systems for upgrading of industrial
waste heat (heat transformers) attained the practical stage of utilization
[Sana et al. (1983)) .
Absorption refrigeration machines as such have relatively low values for the
coefficient of performance. Bonnin et al . (1980) and Keizer (197Ö) compared
solar absorption with solar vapor compression cooling methods. Keizer obtained
similar overall performance for each while Bonnin et al. showed a slightly
favourable efficiency for the absorption cooling. As described by Bonnin et al. ,
the solar vapor compression cooling system has the disadvantage of larger ini­
tial and maintenance costs.
After a comparison of absorption systems with mechanical vapor compression sys­
tems, Threlkeld (1970) concluded that a mechanical compression system has lit­
tle thermodynamic superiority over an absorption system as a result of the need
to convert thermal into mechanical energy at a low temperature for the compres­
sion system.
The present study aims to contribute to the improvement of absorption refrige­
ration systems.
1.2. METHODS OF IMPROVING THE PERFORMANCE OF ABSORPTION REFRIGERATION MACHINES 1947 presented an absorber calculation method which included the simultaneous
effects of heat and mass transfer, the method proposed by Niebergall (1981) is
mostly used for absorber design (see Stolk (1G8Ga)). This method only takes
There are, for given working conditions, three possibilities of improving the
heat transfer into account and uses a logarithmic temperature difference to de­
performance of absorption systems:
termine the required cooling area.
a. To identify the most favorable absorbent-refrigerant mixtures The need far a detailed absorber design model has been expressed by Briggs
from a thermodynamic standpoint, (1971), Keizer (1982), Yaron in the discussion of a paper by Trommelmans et al.
b. To improve the efficiency of the system components. (1981) and a working group on the research priorities for absorption heat pumps
c. To consider system nodifications to the standard cycle, for residential and small commercial applications (1982), among others. Friggs
proposed a mathematical model of the absorber in which all the transfer resis­
Ammonia-water and water-lithium bromide are the only absorbent-refrigerant mix­
tance is assumed to occur as heat transfer resistance between the main body of
tures which have found extensive commercial use. However, there are systems that
the absorbing liquid and the coolant. The effect of mass transfer on the ab­
possess better thermodynamic properties than NH3-H2O and H20-LiBr and probably
sorption process was named but not taken into account. A first attempt leading
would lead to better system performance (see ASHRAE's Handbook of Fundamentals
to design criteria for vertical tubular absorbers, in which the simultaneous
(1981), Buffington (1949), Hainsworth (1944a,b), and Niebergall (1949)). Stabi­
processes of heat and mass transfer are taken into account, has been presented
lity, corrosion and thermodynamic properties information on most of these sys­
by Infante Ferreira (1981a) and later by Keizer (1932). The need for a more
tems is very sketchy. This is probably the reason why absorption cycle manufac­
widely applicable and accurate absorber model remained. The second part of this
turers choose the conventional systems with proven technical feasibility.
study will be dedicated to the search of absorber optimization methods.
he first part of this study presents a qualitative and quantitative selection
of possible systems. The practical feasibility of one of the resulting systems
[ammonia-lithium nitrate) is experimentally tested. Point c will not be considered in the scope of the present work. For some possi­
ble system modifications you are referred to Altenkirch (1954), Keizer (1982),
Venkatesh (1981) presents a study on the effect of the component efficiency on Niebergall (1981), Phillips (1976), and Wilbur and Mitchell (1975).
the performance of an ammonia-water absorption refrigeration machine. Some re­
sults are shown in fig. 1. Infante Ferreira (1981e) has shown that the same
1.3. FRAMEWORK OF THE INVESTIGATION
trends apply for the ammonia-salt systems. From fig. 1 it can be concluded that
Based on points a and b of last section, this study can be divided into two dis­
tinct parts. The first part (chapters 2 and 3) includes studies of the absorp­
COP tion cycle while the second part (chapters 4, 5 and 6) concerns the vertical
tubular absorbers.
0.8-
In chapter 2, after a qualitative review of possible absorbent-refrigerant sys­
tems, a basis is presented for a quantitative selection of these systems. From
this selection two promising systems for refrigeration processes result: ammonia-
lithium nitrate and ammonia-sodium thiocyanate. A comparison with other studies
shows a reasonable agreement.
Chapter 3 presents a theoretical and experimental study of the system ammonia-
lithium nitrate. A simulation model is introduced which, given the available
recuperator cooling water flow rate and temperature, heating medium flow rate and tempera­
ture, and evaporating temperature, allows calculation of temperatures, concen­
trations and flow rates at different points of an absorption refrigeration ma­
chine. After the description of the experimental set-up, a comparison is made
between simulation model (with the same geometry as the set-up) and the experi­
mental data. The model is used to predict the effect of component size on the
0.8 1.0
Efficiency performance of the cycle.
Fig. 1 - Ammonia-water system performance as a function Chapter 4 starts with a motivation for the selection of vertical tubular absor­
of component efficiency. bers for absorption cycles. It further gives a physical description of the pro­
cesses occuring in this type of absorbers. After the hydrodynamics of the pro­
absorber and liquid heat exchanger (recuperator) efficiency have large effect cess has been quantified, literature heat and mass transfer relations are pre­
on performance while precooler and generator efficiency have small effects. sented for the different flow regions encountered in the absorber.
Many studies have been published on liquid heat exchanger design and optimiza­
tion (see Kays and London (1955)). A good liquid heat exchanger design will In chapter 5 a macroscopic model for vertical tubular absorbers is presented.
thus be less of a problem. An experimental set-up to check the accuracy of this model is described. The
Briggs (1971), Ballard and Stolk in the discussion of a paper by Heigaard Knudsen experimental data, obtained for the systems ammonia-lithium nitrate and ammonia-
(1980) and later Keizer C1982) pointed out the large effect of the absorber ef­ sodium thiocyanate, are compared with the model predictions. A statistical cor­
ficiency on the performance of the system. Although Ruhemann (1947b) already in relation of the experimental data is also shown. The macroscopic absorber model
2 3
is then used to predict the effect of some parameters on the absorber perfor­
2, SELECTION OF POSSIBLE ABSORBENT-REFRIGERANT SYSTEMS
mance and to compare the vertical tubular absorber with the wetted-wall absor­
ber. Finally the model results are compared with the experimental data obtained
with the 3G parallel tubes absorber of the set-up described in chapter 3.
Chapter 6 presents a differential absorber model and is meant to verify the ac­
curacy of the heat and mass transfer assumptions made in the slug flow region
2.1. INTRODUCTION
of the vertical tubular absorber. Characteristic temperature profiles are shown.
The most relevant conclusions from this investigation are summarized in chapter Absorption refrigeration machines have relatively low values for the coefficient
of performance. Extensive searches have been carried out in the past to identify
7.
refrigerants, absorbents, and their combinations that could overcome the limita­
tions of absorption systems. These have resulted in significant performance im­
provements .
Mainsworth [1944aj in 19 44 presented an informal survey of the more important
refrigerant and absorber developments. Hainsworth [1944b] also presented a list
of the desirable properties and characteristics of the absorption system,. The
desirable properties of the refrigerant are common to all refrigeration systems
and much information about these requirements can be found in the literature.
Kuprianoff et al. (1956] and Niebergail [1949), for instance, give extensive
qualitative considerations about the desirable properties of the refrigerant.
Niebergail [1949,1901] presents extensive considerations about the desirable
properties and characteristics of the absorption system. The chief requirements
are also listed in ASHRAE's Handbook of Fundamentals (1981J . Although not named
by Hainsworth, safety is nowadays an important requirement. Fluids must be sub­
stantially nontoxic and nonflammable if they are in an occupied dwelling.
Industrial process refrigeration is less critical in this respect.
Buffington [1949] presented qualitative requirements for the screening tests of
absorbent-refrigerant combinations which might conceivably be considered. His
paper was the basis for a series of research work in absorbent-refrigerant com­
binations. In section 2.2. the most relevant publications for each refrigerant
will be screened. From this literature review, a qualitative selection of a num­
ber of refrigerant-absorbent combinations will be made. In the following sec­
tions of chapter 2 the basis for a quantitative comparison of these systems and
the results of this comparison will be presented.
2.2. QUALITATIVE REVIEW OF POSSIBLE SYSTEMS
2.2.1. Absorbent-refrigerant systems with water as refrigerant
Water may be an ideal refrigerant because of its high latent heat and safety
characteristics but its temperature range is limited,
Water-lithium bromide [LiBr] is a favorite candidate as working fluid in absorp­
tion systems being extensively used for air conditioning purposes. Since the
refrigerant turns to ice at 0°C, the pair cannot be used for low temperature
refrigeration. Lithium bromide crystallizes at moderate concentrations. When the
absorber Is air cooled, these concentrations tend to be reached; thus, the pair
Is usually limited to applications in which the absorber is water cooled. Rush
[196B] and Weil [1968] proposed the use of biBr-biSCN in water to overcome this
disadvantage. This refrigerant-absorbent pair has not yet found commercial use.
Recently the system water-lithium bromide-ethylene glycol reached the develop­
mental stage in a form of a solar-powered residential air conditioner prototype
of Carrier Corporation (Macriss [1982)].
2.2.2. Absorbent-refrigerant systems with ammonia as refrigerant 2.2.4. Absorbent-refrigerant systems with methylamine as refrigerant
Ammonia is one of the best refrigerants from a thermodynamic standpoint. The In order to reduce the working pressure of ammonia, some investigators assessed
ammonia-water refrigeration system has found extensive commercial use. Keizer its replacement by monomethylamine. Miebergall (1981) presents a review of the
(19B2) presents research done with this system at the Refrigeration Laboratory studies on the system methylamine-water. The main disadvantage of this system
of the Delft University of Technology. is that the saturation points of hath components are very close (-6.9/100°C)
In 1966, Roberson et al . (1966] studied the solubilities of ammonia in nonaque- and so rectifier losses are large. Roberson et al. (1966) compared the solubili­
ous solvents. On a weight basis, sodium thiocyanate (NaSCN) solution was the ties of methylamines in organic solvents. They found that on a weight basis the
best salt system followed by the solutions containing both sodium iodide (Mai) best system is monomethylamine (NH2CH3)-1,4 butanediol (tetramethylene glycol).
and sodium thiocyanate (NaSCN). However the system NHa-NaSCN-Nal has the defi­ Biermann (1978) studied the solubilities of monomethylamine in salts. The lithi­
nite advantage that possible salting out effects are of concern only at lower um thiocyanate (LiSCN) solution and a mixture of 2 moles of lithium thiocyanate
temperatures and concentrations. From the systems of ammonia with organic sol­ and 1 mole of sodium thiocyanate (NaSCN) seem to be very good salt systems. Un­
vents, on a weight basis 1,4 butanediol [tetramethylene glycol) was best and on fortunately both systems are reported to be chemically unstable (Bierrnann (1976,
a molar basis tetraethylene glycol. Tyagi and Shankar (1976) presented a graph­ 1981)).
ical comparison of the coefficient of performance CGF' (defined as refrigerant
effect/energy supplied) of different systems of ammonia with organic solvents 2.2.5. Absorbent-refrigerant systems with halogenated organic compounds as
as a function of generator temperature. It can be concluded from their graphical refrigerant
comparison that the system NI 13 -1,4 butanediol (TMG) has the best performance,
the values of the coefficient of performance of this system being at least 0.05
In 1959, Eiseman (1959) presented the advantages of the use of halogenated or­
larger than the values for the system NH -tetraethylene glycol (TEG) .
ganic compounds as the refrigerant in absorption refrigeration machines. After
a comparison of refrigerants and absorbents, Eiseman concluded that refrigerant
In 1937, Gensch (1937) presented an absorption refrigeration machine working 22 has relatively good thermodynamic and physical properties and is excellent
with the system ammonia-lithium nitrate (LiNO.3) , Chinnappa (1961) extended the in chemical stability. Dimethylether of tetraethylene glycol (DMETEG) has most
equilibrium and physical data for this system as given by Gensch. More data on desirable solubility properties. It has also a favorably low vapour pressure
this system are available in the reports by Bouman (1971), Hermans [1969a], and and viscosity and is reported to be low in toxicity. Furthermore, it has good
Smeulers (1971). This absorption system has been in the research program of the chemical stability. From the systems working with halogenated organic compounds,
Refrigeration Laboratory of the Delft University of Technology during some years the system R22-DMETEG was preferred. In 196D, Albright et al. (1960) studied
(1968-1971). Some reports have been published on this system: Bouman (1971), thirty-four binary systems with halogenated organic compounds and concluded
Hermans (1969a,b), rleijs (1969), Smeulers (1971), Smeulers and Kool (1971), and that the DrlETEG was the best solvent of those studied for refrigerants 11, 21,
Vellekoop (1971). The research program was stopped when difficulties were en­ and 22. In 1959, Hastrangelo (1959) presented stability data of R22 and other
countered in the absorber. chlorofluorohydrocarbons in DHETEG. In 1965, Kriebel and Löffler (1965) pre­
Eggers-Lura et al. (1975) made a theoretical comparison between the systems sented extended experimental thermodynamic properties of R22-DF1ETEG solutions.
ammonia-water, ammonia-sodium thiocyanate, and ammonia-lithium nitrate to be In 1961, Thieme and Albright (1961) presented two new promising solvents for the
used in an intermittent absorption refrigeration machine. These authors con­ halogenated organic compounds. On a weight basis diethyl formamide (DEF) and
cluded that the system ammonia-lithium nitrate, for the conditions studied, dimethyl formamide (DNF) are better solvents for R22 than DrlETEG. These authors
gives the largest values for the coefficient of performance of the system. also present equilibrium data for the systems R22-DEF and R22-DMF among others.
After Yarcn et al. (1978), the system R22-DF1F should lead to values of the COP
2.2.3. Absorbent-refrigerant systems with methanol as refrigerant larger than the values obtained for the system R22-DMETEG.
The system R133a-EFTE (ethyltetrahydrofurfuryl ether) is under development for
Methanol, like water and ammonia, has a high latent heat. an absorption heating/cooling heat pump In the U.S.A. (see Allen (1962), Hanna
Based on the qualitative requirements from Buffington (1949), in 1965 Aker et and Wilkinson (1982), and Phillips (1982)).
al. (1965) investigated various salt-alcohol systems for the absorption refri­
geration machine. Based on equilibrium data, these authors concluded that the 2.2.6. Potential absorbent-refrigerant systems
systems containing methanol plus mixtures of about 2 moles of lithium bromide
and 1 mole of zinc bromide are of definitive interest. The system CH3DH-LiBr- From the literature review in sections 2.2.1. to 2.2.5., a selection can be made
ZnBr2 presents two advantages over the system CH30H-LiBr: of the most promising systems for each refrigerant. Only this restricted number
of systems will then quantitatively be compared. The selected systems are shown
a. The possibility of salting out in the absorption refrigeration
in table 1. Table 1 also presents the sources for the equilibrium data used in
unit becomes smaller.
further calculations in this chapter. Although the two methylamine systems (9
b. The viscosity is moderate when compared with the viscosity of
and 10) have shown to be chemically unstable, they were included in the analysis
the concentrated methanol-lithium bromide mixtures.
because they seem to be promising systems. From the results in section 2.3.4.
The major continuing work on the CH3DH-LiBr-ZnBr2 system concerns absorption it appears worthwhile to search additives which would improve the chemical sta­
heat pump research (see Hodgett (1932) and Iedema (1984)). Alloush and Gosney bility of these systems.
(1983) present an experimental absorption refrigerator working with this system. When not available in the literature, specific heat and density data have been
Grosman et al. (1983) analyse the theoretical possibilities of the system for taken from Perry and Chilton (1973). The values used were then weighted values
refrigeration applications. The chemical stability of the CH30H-LiBr-ZnBr2 sys­ between refrigerant and pure absorbent.
tem limits the maximum solution temperature ta 120-130°C (see Koebel and
Aegerter (1981)).
B
Table 1 - Possible systems for the absorption refrigeration machine.
No. Refrigerant Absorbent Souroe5
CkJ/kg]
1 Water hL,G Lithium bromide LiBr 2257 [■lnfleely [ 1 <P 7 3 1 -C: QrC ('t cond' u
Ammonia MH3 Water H20 1369 Bosnjakovic (1937]
Lithium nitrate LiNO Chinnappa (19611. Gen.fich (1937)
Sodium thiocyanate NaSCN Blytas et al.(19621.Sargent et al.(1963J
Sodium iodide - 2 Nal
-sodium thiocyanate 1 NaSCN Roberson et al. (1966)
6 Tetraethylene glyr.nl TEG Robereon et al. (13661
? 1,4 butanetiiol TMG Roberson et al. i19661 RECUPERATOR
'6 Methanol CH30H Lithium bromide - 2 LiBr 1 190
-zinc bromide 1 ZnBr., AKer et al. ( 1965)
"■lonomethylamine NH?CH„ Lithium thiocyanate- 2 LiSCN
"Sodium thiocyanate 1 NaSCN Biermann (1978)
10 Lithium thiocyanate LiSCN Biermann [19733. dacriea et al. (1979]
11 Chloradifluaromethane CHC1F Dimethylether of l<\/ ABSORBER^-^ °A 'Vond'
tetraethylene glycol DF1ETEG Kriebel and Löfflsr [1965]
heat of vaporization of pure refrigera Fig- 2 - Schematic diagram of a single stage absorption refrigeration cycle.
1 n p
2.3. QUANTITATIVE SELECTION
2.3.1. Introduction
In a practical absorption system the most important engineering parameters which
are usually considered and which affect performance, size, and cast of an ab­
sorption refrigeration unit are as follows (see Iedema (1982,1984), Maoriss
[1976], Nansoori and Patel [1979], and Niebergall (1949)]:
a. Coefficient of performance CCOP): ratio of useful cooling produced
to energy input to the absorption refrigeration machine. gen 1/T
b. Pumping factor [f); the ratio of the mass flow rate of solution
pumped from low pressure to high pressure through the liquid pump Fig. u - Ln p-l/T diagram showing absorption cycle
per unit mass of refrigerant produced in the generator.
c. Pumping work CWn) : the pumping work of the rich solution from low
pressure in the absorber to the high pressure in the generator.
d. Heat transfer rate (Qrgc) '• rate of heat transferred in the liquid-
liquid heat exchanger (recuperator) between absorber and generator.
e. Heat transfer rate in precooler (Qpre^ : he-at. transferred in the pre-
cooler to provide for cooling the refrigerant liquid between the tem­
perature of condenser and evaporator.
f. Heat removed from rectifier CÖRE^ : ^or the systems with an absorbent
with high volatility (from the systems compared only the system
ammonia-water requires a rectifier).
The importance and relevance of heat and mass transfer will be discussed in
chapter 3.
Fig. 4 - Enthalpy-concentration diagram showing absorption cycle.