Research on working fluids for refrigeration, air-conditioning, and hea pump systems

              Rev .Acad .Canar
.
Cienc . , VI (Nums. 2,3 y 4), 25-45 (1994)
RESEARCH ON WORKING FLUIDS FOR REFRIGERATION, AIR
CONDITIONING, AND HEAT PUMP SYSTEMS
H. Kruse, M. Burke
Institute of Refrigeration
University of Hannover
INTRODUCTION
Research on the working fluids for refrigeration and air-conditioning and heat pump systems
has been conducted at the Institute of Refrigeration (IKW) at the University of Hannover,
Germany, for more than 15 years. This research is mainly concerned with the interaction
between refrigerants and lubricants in those systems and only to a minor extent with
refrigerants, because the IKW is working in the field of applied research. The investigation on
properties of pure refrigerants and refrigerant blends is the domain of scientists in the field of
thermodynamics at various universities, e.g. the University of Las Palmas de Gran Canaria
(ULPGC). Up to the late seventies, the research has been carried out mainly with
chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) like R12, R22, and R502
in the mixture with mineral oils. At that time, two facts were the starting point for refrigerant
oil research at the IKW in Hannover:
Energy Conservation
The oil crisis in 1974 had initiated in the industrialized countries research for energy
conservation, namely for domestic heating systems by the development of heat pumps.
Those heat pumps had to compete with conventional heating systems with lower initial
costs. In order to compensate for the higher capital costs of heat pumps in an adequate
time, energetic improvements in comparison to conventional heat pump systems had to
be achieved . For this reason, research at the University of Hannover was started in
order to apply the Lorenz cycle with gliding temperatures for energy saving using
zeotrope binary refrigerant mixtures [1]. The knowledge of the behaviour of those
refrigerant mixtures like R12/R114 and R22/R114 in combination with lubricants were
not known until that time when only some minor investigations on oil/refrigerant had
been done on blends of R22 and R12 outside their azeotropic point. Further on, the
higher working temperatures in heat pumps as compared to refrigeration systems asked
for more thermal stability of the lubricants and required for that purpose special
developments of synthetic oils [2].
Also the gradual shortage of mineral oils with adequate low temperature behaviour,
namely more of the paraffinic than the naphthenic type, had led to the development of
synthetic lubricants like alkylbenzenes (AB), polyalphaolefins (PAO) and polyglycols
(PG) during that period. The behaviour of these new synthetic oils together with the
conventional refrigerants and especially together with their mixtures, was mostly
unknown at that time, when the research of oil/refrigerant systems at the University of
Hannover started. This research led to two Ph.D. theses by Schroeder [3] and Hesse
[4] dealing mainly with this problem in order to find a way for predicting
oil/refrigerant properties by using thermodynamic relations, instead of what up to that
time was only possible by empirical equations.
25
Environmental Effects
Ozone Depletion
Another impact on the oil/refrigerant research was the theory of ozone depletion by
Molina and Rowland [5] issued in the same year of the oil crisis 1974 but
acknowledged in the scientific and political world only in 1987 when the "Montreal
Protocol on Substances that Deplete the Ozone Layer of the Earth" was signed. This
agreement led to a new direction in the refrigerant oil research. Instead of
chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC) new
hydrofluorocarbon (HFC) refrigerants as listed in table 1 without chlorine together
with new lubricants of the polyglycol- or ester-type had to be applied in refrigeration
and air-conditioning systems. The enforcement of the Montreal Protocol during the
follow-up conferences in London 1990 and in Copenhagen 1992 accelerated the
research towards ozone benign working fluids and led to a further Ph.D. thesis by
Arnemann [6], who did experimental investigation with polyglycol/R134a systems as
well as theoretical work in order to predict the behaviour of pure and mixed
refrigerants with new lubricants theoretically. In parallel, measurements on those
fluids were done extensively and published by Burke and Kruse [7, 8, 9].
With R134a a suitable alternative for the CFC R12 was found. So far there is no pure
fluid from the HFC group known as a possible substitute for the HCFC R22 and the
CFC R502. The presently most favoured alternatives are mixtures containing the
HFCs R32, R125, R143a, and R134a [9]. To fmd a way to estimate the properties of
binary and ternary refrigerant blends with oil will be the challenge in the future.
Refrigerant No. Chemical
Formular
Molecular
Mass
T. T„ Pc,
[kg/kmol] [°C] [°C] [bar]
Chlordifluoromethane R22 CHCIF2 86,480 -40,8 96,0 49,7
Blend R502 CHCIF2/ 111,640 -45,4 82,2 40,7
R22/R115 : 48.8/51 2 CCIF2CF3
Difluoromethane R32 CH2F2 52,020 -51,8 78,4 58,3
Pentafluoroethane R125 CHF2CF3 120,020 -48,1 66,3 36,3
Trifluoroethane R143a CH3CF3 84,040 -47,8 73,1 37,9
Tetratfluoroethane R134a CH2FCF3 102,030 -26,2 101,1 40,7
Propane R290 C3H8 44,094 -42,0 96,7 42,5
Annmonia R717 NH3 17,030 -33,3 133,0 114,2
Table 1 -Properties of R12, R22, R502 and some of their possible alternatives
Global Warming, Greenhouse Effect
During the last years, another environmental problem, the global warming effect
appeared on the horizon and influenced the development of refrigeration systems. This
has led to the reappearance of old refrigerants like ammonia (NH3), hydrocarbons
(HCs), and now under development carbondioxide(C02), which require again
lubricants under the aspect of modern, technologically well developed refrigeration and
air-conditioning systems. Therefore, special lubricants for ammonia have been
26
detected in a research project at the IKW [10], which allow the application of ammonia
in small refrigeration systems. These lubricants are now already on the market.
On the other hand, the market gain of hydrocarbons in refrigerators first in Germany
and now in Europe has shown the importance of the interaction of lubricant and
refrigerant. Especially concerning the lubrication behaviour of natural refrigerants
together with new oils will further on be the challenge the lubricant/refrigerant
research for technically well developed refrigeration and air-conditioning systems.
Therefore, in the following, the general problems of the working fluids in refrigeration
and air-conditioning systems will be described first and the results of the research on
those working fluids at the IKW in Hannover will be briefly discussed later.
LUBRICANTS IN REFRIGERATION SYSTEMS
In contrast to the refrigerant, the lubricant, which is termed refrigeration oil, is needed only in
the compressor of the refrigeration system. There, its primary job is to lubricate the bearings
and other gliding areas inside the compressor. Besides, it provides for better sealing between
the piston and the cylinder or heat transport out of the compressor. The migration of oil from
the compressor into the refrigerant cycle can be reduced by an oil separator but not completely
prevented. Driven by the refrigerant flow, the oil has to pass t!ie cycle as balast and return to
the compressor.
On its way through the refrigeration cycle, the lubricant must to withstand great fluctuations in
temperature. The primary requirement for a refrigeration oil is a high thermal and chemical
stability. Both, carbonization, chemical reactions with the refrigerant or other system materials
at high temperatures as well as flocculations at low temperatures can reduce the life span of a
refrigeration system dramatically.
For the so-called CFC and HCFC refrigerants, mineral oils, semi-synthetic and fully synthetic
lubricants proved to be useful. Mineral cmIs are classified as paraffins, naphthenes, aromatics,
and olefins. The available synthetic lubricants were mainly products on the basis of alkyl
benzenes (AB) and polyalphaolefins (PAO). Only in rare cases, silicone or silicate oil or
polyglycol lubricants (PG) were used.
The conversion of refrigeration systems from CFC to HFC refrigerants was accompanied by the
conversion of the refrigeration system to new lubricants, as the previously used ones are not
sufficiently miscible with the new refrigerants [7]. By the separation of lubricant and liquid
refrigerant, which is termed miscibility gap of the oil/refrigerant system, the performance of the
refrigeration setup can be influenced considerably.
Oil and Refrigerant in the Refrigeration System
An insufficient miscibility of lubricant and refrigerant can cause problems as is exemplified in
the diagram of a refrigeration system in figure 1. On the right, in the scheme of a refrigerator
system, there is the "oil cycle" consisting of the compressor and the oil separator. As
mentioned before, a small fraction of the lubricant migrates into the refrigerant cycle and has to
be transported back from there to the compressor in order to supply it with the necessary
lubricant quantity.
27
Oil Separator
lOOOOO) Condenser
Flooded Evaporation
-J Ccncenlrate R717
Concentrate Oi
i''r^s2^^3
Figure 1 - Oil transport in a refrigerant cycle [8]
A good miscibility of oil and refrigerant has an advantageous effect on the oil-return from the
refrigerant cycle to the compressor. Especially at low temperatures as they exist in the
evaporator of a refrigeration system, in comparision to the pure oil the liquid viscosity of the
oil/refrigerant mixture decreases considerably with an increasing refrigerant fraction. The
flowability of the mixture grows correspondingly so that the lubricant, which is flowable
because of its refrigerant fraction, can be transported by the refrigerant gas flow. In this
condition, the oil can be returned to the compressor without any additional construction devices.
In the compressor, where the highest temperatures exist, a sufficient viscosity of the lubricant
has to be guaranteed in order to ensure its lubrication. A good miscibility of oil and refrigerant
results in a reduction of the lubricant's viscosity caused by the dissolved refrigerant. This fact
has to be taken into consideration for the design of the compressor and the selection of the
refrigeration oil.
In case of immiscibility with the refrigerant, oil separation may appear in the oil sump of the
compressor and in apparatuses such as condenser, receiver or evaporator. Especially the
evaporator may become an oil trap because of the increasing fluid viscosity with decreasing
temperature. To realize a dry evaporation in the case of immiscibility, the pure oil has to be
very low viscous in order to allow its transportation by the circulating refrigerant. Otherwise,
the oil will remain in the evaporator. The oil-covered heat exchanger tubes will diminish the
heat transfer and cause a pressure drop. In such refrigeration systems normally the principle of
the flooded evaporation is applied. In a flooded evaporator, liquid refrigerant is always
present. In the case of a miscibility gap, two liquid phases occur as shown in figure 1. If the
oil has a lower density than the refrigerant—which is the case for CFC and HFC refrigerants in
the relevant temperature range-, its evaporation is additionally hampered by the oil-rich liquid
phase floating on top. These are unfavourable conditions.
28
The flooded evaporation is commonly used in large R22 and ammonia systems, where the oils
are not miscible with ammonia are only partly miscible with R22 [11]. The return of the
separated oil is facilitated in that way that in the case of ammonia the density is lower than the
density of oil. By constructive devices or by draining the evaporator's sump during
maintenance works, the refrigeration oil is removed from the evaporator and refilled to the
compressor. In the case of R22 the floating oil on top of the refrigerant liquid surface is sucked
from there by special oil return lines to the compressor.
Such a procedure is only possible for high capacity refrigeration systems with costly oil
separating systems, which are constantly maintained by trained personnel. For low capacity
systems this procedure is not economical. In these systems well soluble oil-refrigerant systems
are applied to enable a dry evaporation. To find an oil that provides both solubility with the
refrigerant at low temperatures and sufficient viscosity at high temperatures even in the mixture
with the refrigerant is very important for most applications in refrigeration.
THE MISCIBILITY GAP
Depending on temperature and pressure, not all liquids are fully miscible in each other. The
separation of a system into two separate liquid phases as shown in figure 2, is called a
miscibility gap. In the case of an oil/refrigerant mixture, there are two liquid phases, a more
oil-containing liquid of and a more refrigerant-containing liquid B. They are divided by a phase
boundary. The separation is caused by the different densities of the two liquid phases. Above
the liquids is a vapour phase, which for oil/refrigerant mixtures in regard to the tremendous
differences of oil and refrigerant in vapour pressure consists nearly of pure refrigerant so that
its amount of oil can be neglected.
".| liquid a[
p.l • const.
phase boundary
gas / liquid
.phase boundary
liquid / liquid
Miscibility Gap Miscibility Diagram
Figure 2 - Miscibility gap and miscibility diagram
In the right diagram of figure 2, the miscibility cur\'e of the mixture is plotted as a function of
temperature and mass fraction oil. The miscibility curve separates the scope of complete
homogeneous solubility from the scope of the solubility gap, in which two liquids appear. The
side of the miscibility curve, where the immiscibility starts here, is marked by a hatching.
Figure 3 shows a total of five possible versions of miscibility curves. The curves of type A to
D have been determined both in past studies [12] and in the here presented results. Type E, a
completely encompassed miscibility gap, for example occurs in the nicotine-water system.
29
Typ dI ^
Figure 3 - Different forms of miscibility curves
Results of Experimental Investigations on Miscibility
Investigations made by Hesse [4] of the refrigerant mixture of R22 and R114 with an alkyl
benzene (Al) of the viscosity class ISO 32, whose results are reflected in figure 4, led to a
considerable decrease of the separation temperature of binary refrigerant blends in contrast to
the pure substances. The investigated, binary refrigerant blends possess a clearly more
favourable miscibility with this oil in comparison to their pure components.
Temperature [ °C ]
R22/R114 0.00
1 R22/R114 0,25
2 R22/R114 0,50
3 R22/R114 0,75
— -R22/R114 1,00
0,1 0.2 0,3 0.4 0,5 0.6 0.7 0,8 0,9
Massfraction Oil [
-
1
Figure 4 - Miscibility of R22 and Rl 14 with the alkyl benzene lubricant Al
In a subsequent research project [7], the miscibility of the refrigerants R23, R134a and R152a
as well as those of the binary blends R23/R152a and R134a/R152a with various oils were
experimentally examined. Like R13, R23 is a low temperature refrigerant, whose normal
boiling point is at -82.1°C and whose critical temperature at 25.6°C is quite low [13].
30
Temperature I "C
Masiiraclion RiS7 ;~'n.„i
0.00 (B23)
0,5
10 (RiSii)
-' '-'' '"-" "• " 3
0,1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Massfraction Oil [
-
]
Figure 5 - Miscibility of R23 and R152a with an allvyl benzene lubricant A2
In contrast to the system R22/R114, the refrigerant blend R23/R152a shows another oil
behaviour. Investigations of the solubility of this system with an alkyl benzene refrigeration oil
A2 of viscosity ISO 46 of led to exactly the opposite effect as can be seen in figure 5. It shows
the miscibility curve of the pure refrigerants as well as of a blend consisting in equal fractions
of R23 and R152a with this oil. While R23 is only soluble with A2 for oil-rich compositions,
R152a also has soluble compositions on the oil-poor side. With the refrigerant blend, the oil
A2 shows an even more unfavourable solubility behaviour than the pure refrigerants.
While the examples of R22/R1 14 and R23/R152a show these unexpected results, the miscibility
curves of most systems composed of oil and a refrigerant blend run inbetween those of the
systems containing pure refrigerants and oil.
Within an ongoing research project of the European Community [14] at the IKW, the behaviour
of the earlier mentioned refrigerants R32, R125, and R134a is investigated with a new but
already commercially available lubricant (E8). This is a polyolester-type lubricant of the
viscosity ISO 32, which is used in compressors for supermarket refrigeration.
The HFC R32 has the main disadvantage to be tlammable, R125 of these three refrigerants has
the highest contribution to the greenhouse warming effect, caused by its high direct greenhouse
warming potential (GWP) and its high energy consumption as a refrigerant. As shown in table
1, R134a with a normal boiling point of -26.2 °C is not suitable for a typical R22 or R502
application with evaporation temperatures down to -40°C.
Besides the pure refrigerants, the oil miscibility of a ternary refrigerant blend with mass
fractions of 30% R32, 30% R125, and 40% R134a was determined by experiments.
The solubility behaviour of the three oil/pure refrigerant mixtures and the oil/refrigerant blend
mixture is shown in figure 6. The investigations were carried out in the temperature range
between -80°C and +80°C. The system E8/R32 forms a misciblitity gap at temperatures
below -18°C and above 4-70°C. The oil E8 proved completely soluble with R125 for low
temperatures down to -80°C. Only for temperatures above +65 °C, that is near the critical
31
temperature of R125, separations of the liquid could be observed. The separation temperatures
of the third binary mixture E8/R134a are below -10° C and thereby far below the application
temperature of the refrigerant R134a. As with R134a, no miscibility gaps were identified for
the ternary refrigerant blend with the oil E8 at high temperatures. For low temperatures these
appear below -65 °C.
Temperature [ °C
Sepa ration Temp.
1
' E8/R32
. E8/ni34a
^' E6/R125
o E8/R32/R125/R134a
^ 1 1—I—I—I—I—I
1
0,1 0,2 0.3 0,4 0,5 0.6 0.7 0,8 0.9 1
Mass Fraction Oil [
-
]
Figure 6 - Miscibility of R32, R125 and R134a with an ester-type lubricant E8
Since refrigerant blends, such as R32/R125/R134a are looked at as R502 substitutes for the
application in a supermarket refrigeration system with evaporation temperatures of about -40°C,
an oil/refrigerant mixture with this blend is sufficiently miscible.
Besides those HFC blends as substitutes for R502 and R22 in low temperature refrigeration,
ammonia is an alternative refrigerant for R22, which contributes neither to the depletion of the
ozone layer nor to the global warming of the atmosphere. Ammonia is energetically and
volumetrically favourable as it has among other properties a large specific heat of vaporization
and a high volumetric refrigerating capacity. Due to its favourable energetic behaviour, the
indirect contribution to the greenhouse warming effect is kept at a minimum. Moreover,
ammonia is inexpensive and available in sufficient quantities.
The nonferrous metals normally used in refrigerating systems, e.g. copper and brass are as well
as the majority of the jointing materials are not compatible with ammonia. Further
disadvantages are the flammability and the toxicity of ammonia. Ammonia is classified as a gas
which is not easily inflammable, and w^hose explosion hazard in air is relatively small. The
smell of ammonia is regarded as very disagreeable. Even smallest volumetric amounts in the
air are being preceived by human beings.
In spite of its local danger potential, the refrigerant ammonia has proved its reliability in large
industrial plants for decades. It is used there in absorption as well as in compression
refrigeration cycles. The commonly used oils in ammonia refrigeration systems, e.g. mineral
oils or polyalphaolefines, are miscible with ammonia only on a very limited basis, as shown in
32
figure 7 [11] with a logarithmic abscissa. Only in compositions with oil mass fractions far
below 1% or above 99%, these working fluids shown here are completely soluble. For
concentrations inbetween these critical values, this mixture has a miscibility gap and therefore
does not meet the requirements, which are necessary for a dry evaporation.
Temperature I* C 1
20
Separat ion Temp
M ner al 0.1 R7I7
kyl Bene -le / n22
. 10"^ 10"' 1 50 99
Mass Fraction of Refrigerant [ - 1
Figure 7 - Miscibility of a mineral oil with ammonia
This was one reason why ammonia could not be used economically in refrigeration systems of
smaller capacity in the past and was exactly the starting point, where a research project [10] at
the IKW was initiated, which comprised investigations of the miscibility of lubricants with
ammonia. The basic idea was that similar to the polar R12-substitute R134a, the polar
refrigerant ammonia could likewise be soluble with the new, polar, synthetic lubricants.
A total of five lubricants, one ester oil and four polyglycol oils, with regard to their miscibility
with ammonia were investigated within this research project. The polyglycols P3, P4, and P5
are base oils, P6 is an added version of the oil P3. Some data of these oils are contained in
table 2. Besides the kinematic viscosity, the pour point, and the principal solubility with water
and mineral oil, the information to what fractions they are composed of ethylene oxide (EO)
and of propylene oxide (PO) is additionally given for the polyglycols.
Table 2 -Investigated oils
Sign Type of Oil
(EO : PO)
. [10-^^]
40 / 100°C
Pour Point
[°C]
Solu
Water
Dility in
Min. Oil
E3 Polyol Ester 25 / 5 -54 no ?
P4
P3, P6
P5
PG (0:1)
PG (1:1)
PG (4:1)
50 / 9
70 / 14
55/11
-45
-50
no
yes
yes
yes
no
no
33
Temperature I °C 1
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,
Massfraction Oil I
-
]
Figure 8 - Miscibility of polyglycols with ammonia
The results of the miscibility investigations with polyalkylene glycols are plotted in figure 8.
Within the investigated temperature range between -80°C and 4-90°C, none of the investigated
oil/ammonia systems was completely miscible. The oil P4 consisting of pure propylene glycol,
already showed separation at temperatures between 4-20°C and +40°C. This oil, therefore,
does not constitute an improvement in comparision to conventional refrigeration oils.
The solubility limit of the systems consisting of oil P3 or P6 -both composed of equal fractions
of ethylene and propylene oxid-- and ammonia is shifted to clearly lower temperatures! For
compositions with less than 5% oil as they can exist in the evaporator of a refrigeration system,
the separation temperatures are clearly below -40°C. In refrigeration systems with evaporation
temperatures between -40°C and -50°C the miscibility of this oil could, therefore, be sufficient
for the necessary viscosity reduction in the evaporator and thereby enable the automatic oil
return to the compressor. The additives of the oil P6 did not have any impact on the miscibility
in this case.
For the mixture oil P5/ammonia, the miscibility curve is rather atypical. The mixture ratio EO
to PO of this lubricant is 4 to 1. For refrigerant-rich compositions, this mixture showed the
lowest separation temperatures. With a growing oil mass fraction, however, the separation
temperature also increased continuously. Compositions with oil mass fractions of more than
95% already showed miscibility gaps for temperatures above 0°C. This could particularly
cause problems for the oil transport through the suction line to the compressor in small capacity
refrigeration systems due to the high viscosity of the oil-rich liquid phase.
The investigations with the polyol ester E3 revealed on the one hand insolubility with ammonia
in the investigated temperature range and on the other hand chemical reactions. Therefore
ester-type lubricants are not suitable for applications with ammonia.
34
Theoretical Investigations on the Miscibility of Oil/Refrigerant Systems
The calculation on the miscibility of oil/refrigerant systems at the Institute of Refrigeration in
Hannover was started by Hesse [4]. For the mixture of oil A2/R22 as shown in figure 4.
Using a Lee-Kesler-Ploecker equation of state for the vapour phase and the UNIQUAC equation
for the liquid phase and binary interaction parameters, which were fitted to the results of the
vapour pressure measurement, the miscibility line was estimated as presented in Figure 9. The
shape of the curve was very similar to that of the experimentally determined miscibility curve.
Unfortunately, the separation temperatures of the calculated curve show deviations up to 30
Kelvin.
-20
Bereich
Oil /R22
V calc. , UNIQUAC
F experimental \^
; Miscibility Gap \\ 20 '.0 60
Oil Mass Fraction [%] -
Figure 9 - Comparison of the measured miscibility gap and the results calculated by
using the UNIQUAC model
The theoretical work was continued by Arnemann [6]. In addition to UNIQUAC, he included
the equations of Flory-Huggins, of Wilson and the equation of Redlich-Kwong-Soave (RKS) for
the calculation of the liquids in his investigations. The vapour phase was always calculated
using the RKS equation of state. In the first step, the empirical equations were fitted to the
vapour pressure data for the oil/refrigerant mixture. The results of the calculation are
illustrated in figure 10, where the separation temperatures are shown as a function of refrigerant
mass fraction. The results are similar to those of Hesse with similar shapes of the curves but a
significant deviation in separation temperature. Although the binary interaction parameters of
the Flory-Huggins equation were adapted as a linear and quadratic function of the temperature,
the results calculated by UNIQUAC, using constant interaction parameters, are closer to the
experimental data.
In the second step of this work, the calculations were fitted to the results of some miscibility
measurements. As presented in figure 11, the deviation between the calculated and the
experimental results decreased drastically and correspond very well for low refrigerant mass
fractions. Like in the first step, using the UNIQUAC equatioq results in the best data.
35
280
260
220
200
180
160
A Flory-Huggins (qua.)
D riory-Hug^ins (lin.)
O UNIQUAC (kons.)
• Exp.
Polynom
0.2 0.4 0.6 0.8 1.0
^K. / (g/g)
Figure 10 - Calculation on miscibility of alkyl benzene Al with R22 using
parameters fitted to the vapour pressure measurements
_^.^
1 ' 1 ' 1
^ D«>\
bd 200- DSiT
^^—
/ CP \ 180- / d:
t-
160- 1 1 1 1
V RKS
O UNIQUAC
D Flory — Huggins
• Exp.
Polynom
0.0 0.2 0.4 0.6 0.8 • 1.0
i^ / (g/g)
Figure 11 - Miscibility of alkyl benzene Al with R22 using parameters fitted to the
miscibility measurements
The results of the estimation on miscibility for mixtures of a polyglycol-type lubricant with
R134a illustrated in the figures 12 and 13 showed the same tendencies as seen before. For this
mixture, the immiscibility starts at the side above the curve for high temperatures. Only with
the UNIQUAC equation, it was possible to predict liquid separation using parameters that are
fitted to the vapour pressure, yield in high deviation to the measurements. Adapted to the
results of some miscibility measurements, it was possible to estimate separation temperatures
with a Flory-Huggins and RKS equation as well. For the lower mass fraction of refrigerant,
the prediction of the RKS differs rather strong from other equations.
When comparing the binary interaction parameters for both oil/refrigerant systems, no physical
relation could be found between the fits on the results of the vapour pressure measurements and
miscibility measurements. Finding satisfactory possibilities in order to predict the miscibility
behaviour of oil/refrigerant systemy will be one challenge of the future.
36
oou-
.. o .
340- \ i V /
320- ^^--^.
Oa • a
^ 300- - ^ &o • a
^ 280- .
£- & a» i
260- -
«>
240- -
*
220- .-«^, ,- , . «
0.0 0.2 0.4 0.6 0.
^K. / (g/g)
A UNIQUAC (konsl.)
O UNIQUAC (lin.)
UNIQUAC (quad.)
• Exp.
Polynom
1.0
Figure 12 - Calculation on the miscibility of a polyglycol with R134a using
parameters fitted to the vapour pressure measurements
^^
ODU- 1 1
\ '
;•!
^ 340-^ V
V
""—
^
7 ^^ V c# 6 \ 320- \ ^^*«^^-^_
c—
300-
1
V RKS
O UNIQUAC
D Flory-Huggins
• Exp.
Pol>'nom
0.0 0.2 0.4 0.6 0.8 1.0
^K. / (g/g)
Figure 13 - Calculation on miscibility of a polyglycol with R134a using parameters
fitted to the miscibility measurements
VAPOUR PRESSURE OF OIL/REFRIGERANT SYSTEMS
Results of Vapour Pressure Measurements
With increasing mass fraction of oil, the vapour pressure of the oil/refrigerant systems
decreases as shown in figure 14, where the vapour pressure of the ternary blend
R32/R125/R134a in the mixture with the ester-type oil E8 is shown as a function of temperature
with the mass fraction of oil as a parameter. This kind of diagram is for example helpful to
estimate the solved refrigerant in the oil in a crankcase of the compressor. For a given suction
pressure and oil temperature the composition of the oil refrigerant mixture can be determined.
37
Vapour Pressure 1 bar
30 -n-
-40 -20 20 40 60 80 100
Temperature [ °C ]
Figure 14 - Vapour pressure of the ternary refrigerant blend R32/R125/R134a
(30/30/40) with the ester-type oil E8
0.0B 2B .40 .60 .80
Oil Mass Fraction
Figure 15 - Vapour pressure of a polyglycol/R134a system
In figure 15, the vapour pressure of six isotherms of the polyglycol/R134a system is shown as a
function of mass fraction oil. In addition, the miscibility curve is included in this diagram. For
oil mass fractions lower than 40%, the decrease in vapour pressure in comparision to the pure
refrigerant is very small. With a higher oil mass fraction, the vapour pressure of the system
drops drastically down to the pressure of the pure oil, which is negligibly low in comparison to
the refrigerant.
38
Calculation of the vapour pressure of oil refrigerant systems
For the calculation of the vapour pressure simple, empirical polynoma equations were adapted
to the results of the measurements by Schroder [3]. First improvements were done by Hesse
[4] by the application of the Lee-Kesler-Plocker, Wilson and UNIQUAC equation for the
activity coefficients of the liquid. Using temperature dependent parameters, the average
deviation in comparison to the results of measurements was about 5%. These results could be
confirmed by Arnemann [6], who also included the Flory-Huggins equation and the RKS
equation of state for the liquid.
— UNIFAC u RKS
A 333 15 K
Q 273.15 K
• 253.15 K
0.0 0.2 0.4 0.6 0.8 1.0
Figure 16 - Calculation on vapour pressure of the R22/R114 blend using the
UNIFAC equation
ber '-' y Der *" x exp
Figure 17 - Calculation on the vapour pressure of the mixture R22/A1 using the
PSRK equation of state
39
Arnemann further on extended the prediction of the vapour pressure of oil/refrigerant systems
using the UNIFAC equation. The results for the refrigerant blend R22/R114 are illustrated in
figure 16, where the dew and boiling curves are given for three temperatures. The calculated
pressures in tendency are too small and show high deviations in comparison to the results of the
measurements. The average deviation for a temperature of 0°C is about 13%. For
oil/refrigerant systems, the deviation increased to values from 30 to 60%. Further
investigations at the IKW with an improved equation, a so-called Predictive Soave-Redlich-
Kwong equation [15] confirmed the results of Arnemann. As an example, the vapour liquid
data of, the A1/R22 mixture at a temperature of 20°C are illustrated in figure 17.
The estimation of the behaviour of HFCs was not possible because of the missing group
parameters for the fluorine containing components.
VISCOSITY
The liquid dynamic viscosity is determined at the IKW by using a specially designed falling ball
viscometer of the Hoppler type as illustrated in Figure 18. Driven by gravity, a ball glides
and/or rolls in an inclined glass tube filled with the test fluid. The dynamic viscosity of the
fluid is a function of the falling time the ball needs for a given distance, the differences in the
density of the ball and the fluid and an apparatus constant [8].
Ojiniiiiii
Inlet Fluid (•'"Vhi- ,
Pressure Transducer
Cooling/Heating
Fluid
Metal Bellow
Figure 18 - Falling ball viscosimeter
40
Results of Viscosity Measurements
In figure 19 the dynamic viscosity of ammonia in the mixture with the polyglycol P3 for
different concentrations is shown as a function of temperature. Besides some isobaric lines the
miscibility curve is enclosed in this diagram. With respect to increasing ammonia fractions, the
viscosity of the mixtures due to the mutual solubility of both fluids decreases drastically.
10000
1000
100
kin. Viscosity [ cSt
Oi fracton
( 1.00
0.897
^ 0.8
0.696
• 0.516
V 0.10
0.05
0.0
0,1
20 40
Temperature [°C ]
Figure 19 - Kinematic viscosity of oil P3 in the mixture with ammonia depending on
temperature and pressure
Pure oil at the temperature of 80°C has a viscosity of 20 cSt. The viscosity of a mixture
containing 10% ammonia at the same temperature is about 7 cSt, extrapolated for 20%
ammonia around 3 cSt and for 30% less than 2 cSt. On the other hand, the viscosity is
sufficiently low for pure oil concentrations at low temperature. These are good conditions for
an oil return driven by the refrigerant flow out of the evaporator into the compressor.
In contrast to ammonia and the new HFC-fluids, hydrocarbons show a high solubility with
mineral oils, even higher than the former CFC refrigerants. As presented in figure 20, the
reduction of viscosity with increasing mass fraction of propane is drastically higher than with
increasing mass fraction of R12. The question is, if this reduces the oil viscosity to such a
degree that it is necessary to increase the viscosity class when changing from the CFC R12 to a
hydrocarbon refrigerant.
In figure 21 the kinematic viscosity also for the isobaric curves of propane and R12 is presented
as the function of temperature. Concerning the higher vapour pressure of propane, the maxima
of the isobaric lines are slightly higher than for R12 in the mixture with the same oil. With
increasing pressure the deviation of both systems decreases.
41
^0 -20 20 40 60 80 °C 120
Temperature -*
Mineral Oil / Propane
100000
]
0--ivVs
10000
^=^-^ 1
i
1
-1—-4—1
-4-
V^^ :_l
J-
1
2CO0
1000-
I
.*--' 2i0n0n
S7:p4l&3""^^f"
=4=5
^ise^Jar-f- -rr 1
^ mXix- ?^
—
=—= ^
\ \ \ \ \\
\|\ \ \ V, -<1
oo
100- >s'^ ^;sk^ -^
> cnJ
=d—-^^ mi^,^—
o 50- S \ \i^\ ^ \ \^
\ N ^-^ \ \\\' s Wn\ \ N s ^ S
S 20- XV X^-MX.
^ 1
' ' ' V .
\ \ \ \ X
iii \ \ s \
\^ \ \ \ \ \
\ \ N S
\ \ . \ ss;
\ \ \ \ \ \\
\
N \
\ \ \ V-S^
\ \ ^. \ '' \ ^K
3 _____ __ _ N'ilh
20 40 60 80
Temperature -»
Mineral Oil / R12
°C 120
Figure 20 - Viscosity Behaviour of a Mineral Oil/Propane and Mineral Oil/R12
System With Increasing Refrigerant Mass Fraction [16]
Beside the isobaric curves, the viscosity lines for different mole fractions of a refrigerant are
included in this diagram. In contrast to the usually shown dependency on the mass fraction, the
reduction of viscosity for given mole fractions of the refrigerant seems to be nearly the same for
both systems. As is known from the thermodynamics, the vapour pressure of the mixture is
related to the mole fraction of the components and not to their mass fraction. Comparable
liquid viscosities of both systems are possible if the hydrocarbon and the CFC refrigerants have
a similar vapour pressure. Then the difference in the liquid viscosity of both oil/refrigerant
systems in the compressor crankcase can be expected to be very low.
42
100000
lO'mVs
10000
5000
; 1
: >:i 1 ^ : / ' \: x v \
1 ' i\ V ' s' oar ; ^ i !\
1 !
/ ^1 ^: ^v_>^'H !
/ . / : ^ / ^ 1^ ^^
1 1 / 7^ ' "^k J V
i
1 :/ i / ; '(C bsr pxl
1
1 / 1 ^/ \l/1 N<.|\
i
^ /^ \| h
i
1
1 / /i \ ,5bo.
i
/ / ^
/ n ^1 N _ 1 \ / N _
40 -20 20 40 60 "C TOO
Temperature -*
100000^Ny^-=k\ i 1 M !
'
1
10 *mVs
10000 - ^ 1
---s^
"TjTz^
5000J
-V^ =F^ 2000 .. X^^§ ^ •TT ^Sfe
t 500 ih^ \''\% 1^- !
i
'
^^^ —v^-^ -H-^1
—
.ti »vv^ \
*^ '' s^ 1 1 ^. ^ ^ 1 1 o
1^ 100
\
\ > \ ! 11 =,
> —
== \ \ ^4->-i S '
'
'
'-^
(J 50- -^"k \ i
! 1
\ /rCPNi k i 1 1
<u 20-
y ' \ ^N. \ I
1
—
1 ,h i\Li^N^J 1
.= 1 —
1
—
—U-'/-Ktvj -^JWI
1
^ i t s 'N^iVk
1 ! / \! !\ i v^ '^sl
1 / ' > ^
/ M N 1 >-
1
/ / "^
L.-.^ ' !'i
/ |\hJ5£:i
1
5 J
1 A, I>^
^ 1 / ^ 1 >
1/ i\i '-
1 :/ 1 1 l^baM
T l_ ! ! 1
i " ' 1^'M
20 JC 60
Temperature -»
100
Mineral Oil / Propane Mineral Oil / R12
Figure 21 - Viscosity-Pressure-Temperature Diagram of a Mineral Oil/Propane and a
Mineral Oil/R12 System With the Refrigerant Mole Fraction as Parameter
[16]
Calculation of viscosities
For the calculation of the liquid viscosity, various simple approximations are known, where the
viscosity of a mixture can be calculated as a function of its composition by mass, molee or
volume fraction. Arnemann [6] found out that the equations of Lederer [17], Wilson [18] and
Lees [19] showed the best results for the oil/refrigerant systems investigated. Beside the
dependence on the mixtures' composition these equations include a binary interaction parameter
that has to be fitted to the results of viscosity measurements. Arnemann showed that this
interaction parameter depends both on temperature and composition.
As the above mentioned equations are only suitable for pure binary refrigerant/oil systems,
Schroeder [3] and Hesse [4] tried to extend these equations to multicomponent oil/refrigerant
systems. Their investigations resulted in high deviations from the experimental data. The
estimation of the viscosity of an oil/refrigerant blend mixture could be improved in a way that
instead of the pure components the viscosity each of the oil/refrigerant mixtures was used for
the calculation.
43
SUMMARY
More than 15 years' work has been done at the Institute of Refrigeration of the University of
Hannover in the research of working fluids for refrigeration, air-conditioning and heat pump
systems. This research mainly included the determination of thermophysical properties of
oil/refrigerant mixtures, both experimental and theoretical. With regard to the CFC and HCFC
phase-out, alternatives are and will be introduced into the market. These alternatives include
both HFCs and natural fluids, such as hydrocarbons, ammonia, or carbondioxide. Beside pure
components for the substitution of R12, for many applications the future alternatives for R22
and R502 are considered to be refrigerant blends belonging to the HFC group. The
thermophysical properties of these blends and their mixture with a refrigeration oil are mostly
unknown and have to be determined by experiments in the near future. The experimental
measurements of properties, such as miscibility, density, vapour pressure, or viscosity are very
time consuming and, therefore, cost extensive. The scientific tasks in the near future will be to
improve the theoretical calculations in a way that the properties of oil/refrigerant systems can be
estimated over a wide range of temperature, pressure, and composition, based on the properties
of the pure components and the results of a few mixture measurements. This work is on a good
way for the establishment of equations to calculate the vapour pressure and the liquid viscosity
of oil/refrigerant systems. For the estimation on miscibility of oil/refrigerant systems some
additional work and further investigations are necessary to yield satisfactory results.
44
LITERATURE
[I] H. Kruse: Energy Saving by Using Suitable Refrigerants for Heal Pumps in Europe.
ASHRAE Meeting Chicago, International Da>*, February 1977
[2] H. Kruse, M. Schroeder: Fundamentals of Lubrication in Refrigeration Systems and
Heat Pumps. ASHRAE Annual Meeting Kansas City, USA, 1984
[3] M. Schroeder: Beitrag zur Bestimmung thermophysikalischer Eigenschaften von
Mischungen synthetischer Kaltemaschinenole mit Ein- und Zweistoffkaltemitteln.
Forschungsbericht Nr. 19 dcs Deutschen Kalte- und Klimatechnischen Vereins, Stuttgart
1986.
[4] U. Hesse: Experimentelle und theoretische Untersuchungen der Eigenschaften binarer
und ternarer 01-Kaltemittelgemische. Forschungsbericht Nr. 29 des Deutschen Kalte-und
Klimatechnischen Vereins, Stuttgart 1989.
[5] F.S. Rowland, M.J. Molina: ChloroHuoromethans in the Environment. Reviews of
Geophysics and Space Physics 1 (1974), pp. 1-35.
[6] M. Arnemann: Methoden zur Bestimmung thermophysikalischer Eigenschaften von 01-
Kaltemittel-Gemischen. Dissertation, Universitat Hannover 1993.
[7] H. Kruse, M. Burke: Untersuchungen zum Olverhalten der Kaltemittelgemische
R23/R152a und R134a/R152a. Forschungsbericht Nr. 35 des Deutschen Kalte- und
Klimatechnischen Vereins, Stuttgart 1992.
[8] M. Burke, H. Kruse: Solubility and Viscosity of New Oil/Ammonia Systems.
International Conference on Energy Efficiency in Refrigeration and Global Warming
Impact, University of Ghent, Belgium, 1993.
[9] M. Burke, S. Carre, H. Kruse: Oil Behaviour of the HFCs R32, R125, R134a and One
of Their Mixtures. IIR International Conference "CFCs - The Day After", Padova,
Italy, 1994.
[10] H. Kruse, M. Burke: Untersuchungen von Schmierstoffen auf Loslichkeit und
Anlagenverhalten mit Ammoniak. AbschluBbericht zum AIF-Forschungsvorhaben Nr.
161D, Hannover 1993.
[II] J.G. Romijn: An Oil-Free Refrigerant Plant. 17th International Congress of
Refrigeration, Vienna 1987.
[12] H.P. Jaeger: Empirische Methode zur Vorausberechnung thermodynamischer
Eigenschaften von 01-Kaltemittelgemischen. Dissertation, Universitat Braunschweig
1972.
[13] ASHRAE Handbook Fundamentals, American Society of Refrigeration and Air-
Conditioning Engineers, Inc., Atlanta 1993.
[14] EC-Joule II Project: Replacement of CFCs in Refrigeration Equipment by
' Environmentally Benign Alternatives. JOUII-CT92-0060.
[15] T. Holderbaum: Vorausberechnung von Dampf-Fliissig-Gleichgewichten mit einer
Gruppenzustandsgleichung. Fortschrittsbericht der VDI-Reihe 3 Nr. 243, VDI-Verlag
Dusseldorf 1991.
[16] Fuchs Mineralolwerke Gm'bH, Technische Mitteilungen Nr. 143 und Nr. 145.
[17] E.L. I -^.rer: Viscosity of Binary Mixtures. Nature 139 (1937), pp. 27-28.
[18] O.G. Wilson: Chart Method of Predicting Viscosity of Lubricating Oil Blends. Nat.
Petrol News 21 (1929) 21, pp. 87-92.
[19] C.H. Lees: On the Viscosities of Mixtures of Liquids and of Solutions. Phil. Mag. 1
(1901) 1, pp. 128-147.
45