ASHRAE Journal 09-00 Refrigerant Use in Europe by dandanhuanghuang


									ASHRAE Journal                                                                                             Refrigerants

Refrigerant Use
In Europe
By Horst Kruse, Dr.-Ing.                                                                         refrigerants into the atmosphere must be
Fellow ASHRAE                                                                                    prevented.

                                                                                                    In this respect, Denmark’s Energy and
                                                                                                 Environmental Ministry seems to have
          he environmental issues of ozone depletion and global warming                          the most stringent measures to phase out
          have considerably affected the refrigeration, air-conditioning and                     the application of HFCs by 2006.
                                                                                                    The worldwide refrigerant consump-
          heat pump industry over the last 10 years.                                             tion in 1991 was 484.2 tons/year. The com-
  The Montreal Protocol has caused the refrigeration industry to introduce                       mercial refrigeration sector was the third
                                                                                                 largest refrigerant consumer with 17%;
newly developed hydrofluorocarbons (HFCs) as substitutes for the chlorine-                       mobile air-conditioning with 31%; and
containing, ozone-depleting chlorofluorocarbons (CFCs). This process for                         unitary air-conditioning sectors with 28%.
                                                                                                    New systems consumed one-third of
new systems has been finalized in industrialized countries and is ongoing in                     refrigerants. Two-thirds were consumed
developing countries. Hydrochlorofluorocarbons (HCFCs), such as R-22,                            by after-market service. Leakages may
                                                                                                 have been an important reason for past
have much lower ozone-depleting potential (ODP) and will be phased out                           refrigerant consumption.
within the next 20 years worldwide. However, national regulations require a                         Because the old CFCs and HCFCs and
                                                                                                 the new HFCs all have a high GWP, it is
much earlier phase-out date, especially in Europe. Therefore, chlorine-free                      essential to reduce leakages and tighten
HFCs will replace chlorine-containing refrigerants in the marketplace.                           refrigerant systems.
                                                                                                    Another solution could be to use re-
   R-22 is the most important refrigerant    new equipment was permitted only until              frigerants with negligible GWP such as
used in the refrigeration and air-con-       Dec. 31, 1999.                                      ammonia, hydrocarbons or carbon diox-
ditioning industry worldwide. The               The search for suitable substitutes has          ide (CO2). Between these often-called
Montreal Protocol, as well as European       shown that no single replacement covers             natural fluids, ammonia has a zero GWP
Council Directive 3093/94, already con-      the wide application range of R-22 and              due to its short lifetime in the atmosphere.
trol the production and sale of R-22 and     possesses all the technical advantages              Hydrocarbons and CO2 have a small GWP.
other HCFCs. Both regulations provide a      this refrigerant offers. The main sub-                 Because refrigeration systems need
phase-out schedule for the stepwise re-      stitutes for R-22 are the zeotropic mixture         energy, they cause CO2 emissions from
duction of the permitted quantities of       R-407C and the near azeotropic mixture              fossil fuel power plants, which contrib-
HCFCs. However, the EC Directive has a       R-410A. Also, the hydrocarbon propane,              ute indirectly to the greenhouse effect.
much shorter phase-out schedule. The         R-290, may be a possible substitute.
European Parliament has proposed more           The Kyoto Protocol for the reduction             About the Author
stringent modifications. The next time the   of greenhouse gases, including HFCs (be-            H. Kruse, Dr.-Ing., is a professor at
European Council and the Parliament          cause of their inherent Global Warming              the Research Center for Refrigeration
meet, they will develop a final regulation   Potential [GWP]), is a further milestone            Technology and Heat Pumps, Hann-
concerning HCFCs.                            on the way to alternate refrigeration sys-          over, Germany. He is a member of
   In addition, some European countries      tems. In the Kyoto Protocol, the HFCs               ASHRAE Standing Standards Project
have national regulations. For example,      are nominated between six gases as one              Committee 34, Designation and Safety
Sweden prohibited the use of R-22 and        of the three industrial gases, including            Classification of Refrigerants, and is a
HCFCs in new equipment since 1998. In        perfluorocarbons and sulphurhexa-                   past member of the ASHRAE Program
Germany and Denmark, the use of R-22 in      fluoride. Therefore, emissions of the new           Committee.
16            ASHRAE Journal                              w w w. a s h r a e j o u r n a l . o r g                  September 2000

Figure 1: Direct (fluid) and indirect (energy) parts of TEWI.2

  The sum of both effects per system, the direct effect of refrig-
erant emissions and the indirect effect caused by the energy to
drive the refrigeration system is called Total Equivalent Warm-
ing Impact (TEWI). The relationship between the direct and the
indirect greenhouse effect is different for various applications
of refrigeration systems as shown in Figure 1.2

Comparison of R-22 and Its Substitutes
   A thermodynamically correct performance comparison for
refrigerants with different properties in each optimized system
is extremely difficult or nearly impossible.
   An optimized adjustment of compressors and other equipment
components to the different refrigerant properties is necessary.
After optimization and adjustments for all possible replacements
for R-22, an accurate comparison of performance and capacity
can be done. This ideal method of comparison is impossible in
practice, but TEWI can help with selecting refrigerants.
   In reality, the theoretical comparison of fluids normally is the
first partial step for an exact comparison, followed by refriger-
ant compressor performance tests, and practical measurements
at refrigerating installations.
                                                                      Figure 2: Experimental comparison of energy consump-
   For substitutes of R-22 and R-502, calorimeter tests have
                                                                      tion; direct and indirect parts of the TEWI of R-22 and
been performed at the Research Centre for Refrigeration and
Heat Pumps, Hannover, Germany, with the substitute HFCs
R-404A, R-407A, R-407B, R-407C, R-507 and R-410A, as well as
with R-290. The most important experimental result is the             of flammability by the refrigerant emissions. This can be over-
energy consumption of the substitutes as compared to R-22.            come partly by designing indirect systems with secondary fluids
Figure 2 shows that only R-410A and R-290 have an energy              for the use of hydrocarbons, but then the energy consumption
efficiency comparable to R-22, whereas the other fluids show a        of such systems would be higher compared to systems with
5% to 15% higher energy consumption3 such as the CFC R-502            direct evaporation.
that formerly was used in commercial refrigeration.                      In conclusion, from the experimental comparison of R-22,
   R-410A, especially, shows an advantageous energy efficiency        substitutes R-410A would be the best choice between the non-
at low evaporation temperatures, that is even better than R-290.      flammable fluids, from an energy and emission viewpoint. No
In the higher temperature range, R-290 has a small advantage.         large differences exist between the other options, R-407A,
The flammable R-290 has a negligible GWP, whereas R-410A is           R-407B, R-407C, R-404A and R-507. The last two options show
non-flammable, with a similar low GWP to R-22.                        the highest TEWI coming from the GWP of their mixtures com-
   Considering the emissions of CO2 from the power plant that is      ponent R-143a.
caused by the energy consumption of the refrigeration system,            For refrigeration and air-conditioning systems with high leak-
both R-410A and propane are slightly better in the indirect effect    age rates, such as in commercial refrigeration, mobile air-condi-
caused by the energy consumption in comparison to R-22.3 The          tioning, unitary air-conditioning and heat pump systems, the
results in Figure 2 are based on a yearly leakage rate of 20% and     refrigerant used influences (via its GWP and COP) the TEWI
annual operating hours of 2,000 or 3,000 hours, respectively.         number of the respective system. These relations were pre-
   Taking into account the direct effect caused by refrigerant        sented in the first AFEAS/DOE Study in 1991 and refined in
emissions, R-410A still has a slight advantage as compared to         1997 in the so-called AFEAS/DOE TEWI 3 Project that also
R-22. R-290 is better under these assumptions, but implies a risk     covered refrigeration systems in Europe.
September 2000                                                                               ASHRAE Journal                        17
ASHRAE Journal
Commercial Refrigeration
   The TEWI 3 study for European super-
market systems was performed with differ-
ent refrigerants for current and future sys-
tems, such as the commonly used indirect
cooling systems with a secondary refriger-
ant loop, and decentralized systems as fa-
vored in the United States. The decentral-
ized systems are small, water-cooled, and
acoustically insulated systems used by su-
permarkets. They provide refrigeration via
short liquid and suction lines to the cabi-
nets.4 The TEWI was calculated for those
systems using reasonable assumptions for
European conditions and for the two tem-
perature levels in a supermarket. The results Figure 3: TEWI of low-temperature refrigeration in Europe.4
in Figure 3 show a large TEWI for the con-
ventional direct-compound systems with various refrigerants tributed direct expansion supermarket systems. Therefore, an
as compared to indirect cooling or decentralized systems for important field of future research and development is to find
low-temperature application.                                       suitable secondary refrigerants with low additional energy con-
   In conclusion, conventional supermarket refrigeration sys- sumption, as caused by the pumping power requirement for the
tems with direct expansion and a large refrigerant charge and coolant and the additional temperature differences in the pri-
long refrigerant lines, which have a potential to cause reason- mary/secondary refrigerant heat exchangers.
able amounts of leakage, cannot be a future solution, although       For various conventional secondary coolants available on
leakages can be decreased to a certain amount. Therefore, the the market, the pumping power has been calculated for pipes at
choice for future refrigeration systems in supermarkets is either 35´1.5 per meter length.5,6 The heat transfer behavior of the
indirect systems with secondary refrigerants or direct decen- coolants was evaluated by a characteristical number F.5, 6
tralized water-cooled HFC systems.                                   The fluid properties viscosity h, density r, specific heat ca-
   Another possibility would be the application of refrigerants pacity c and heat conductivity l are important parameters for
with very low GWP in direct systems. Ammonia and hydrocar- the pumping power as well as for the heat transfer characteris-
bons show dangerous local behavior because of their flamma- tic. The results for low-temperature cooling circuits in super-
bility or toxicity and are not suited for direct expansion systems markets are shown in Figure 4.
inside public areas of supermarkets. Therefore, for these refrig-    From Figure 4, it is evident that for the low-cooling applica-
erants, indirect systems have to be used. For direct systems, tion the pumping power for hydrous salt solutions is nearly the
only CO2 can be used, which presents other problems that are same and in the order of the old calcium chloride (CaCl2) brine
discussed later.                                                   systems. The synthetic fluids need much higher pumping power.
                                                                   This is valid also for the pumping power of the higher alcohols,
Indirect Refrigeration Systems                                     especially propyleneglycol.
Theoretical Consideration                                            Concerning heat transfer properties, which also influence
   As can be seen from the results of the TEWI 3 Project (Fig- system energy consumption, the resulting lower evaporation
ure 3), indirect refrigerant systems have a lower TEWI, but temperatures in the primary refrigerant cause the synthetic flu-
cause higher energy consumption than conventional or dis- ids and the glycols to show a less favorable behavior than the

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18            ASHRAE Journal                              w w w. a s h r a e j o u r n a l . o r g         September 2000
ASHRAE Journal

     U.S. Refrigerant Issues
 By Jim Lavelle                                                        uled for a ban in new equipment in 2010 and production will
 Member ASHRAE                                                         stop in 2020. This timing will allow industry to solve some of
                                                                       the remaining issues that need to be addressed, particularly
 The phaseout of CFCs prompted a quick response within the             large refrigerated storage applications that have always used
 U.S. supermarket industry. Because installed systems typi-            R-22.
 cally had high leakage rates and required large quantities of            Research and development efforts for the next generation
 service refrigerant, many companies developed refrigerant             of equipment are taking into consideration the reduction of
 management plans that included reducing leakage and im-               TEWI for the application. These efforts are not looking sim-
 proving maintenance; planned retrofits of some equipment;             ply at choice of refrigerant, however, since TEWI is influ-
 and planned renovation schedules for other equipment. Re-             enced by more than just the GWP of the gas. The focus is
 covered and reclaimed refrigerant has been used to maintain           again on providing the best engineering solution, taking into
 existing systems until the scheduled renovation.                      account reliability, simplicity of design (which minimizes first
    Conversations with some supermarket equipment execu-               cost, cost of install, ease of repair, etc.), and safety.
 tives have produced the following historical account and crys-           To this end the priority of TEWI reduction efforts has been
 tal ball predictions regarding the choice of equipment/refrig-        as follows:
 erant type and the influence of global warming legislation on            1) Reduce Leakage in Traditional Systems – do not use
 the decision-making process.                                          a fixed value of 30% leakage for the industry, but design
                                                                       equipment and improve installation practices to reduce this
 New Equipment Selection after CFC Phaseout                            assumption;
    New equipment installations have used R-22, R-404A (or                2) Reduce Charge Size in Traditional Systems – design the
 R-507), and to a lesser extent R-134a. The decision to move           system, piping layout, system components, etc., to hold fewer
 from R-12 and R-502 to these products was a natural progres-          pounds of refrigerant, thereby lessening the impact of a leak;
 sion of sound engineering and business practices:                        3) Use Lower GWP Gas; and
    • R-22 equipment had been available for some time, however            4) New Technology – use indirect systems, non-fluoro-
 it was not as attractive a choice given the benefits of R-12 and      carbon refrigerants, etc., to eliminate the use of high GWP
 R-502. When these products went away, R-22 was taken “off-            refrigerants.
 the-shelf” and re-applied to supermarkets. The technology to             In general basic engineering, benefits would not be sacri-
 prevent overheating is inherently more expensive, but is a reli-      ficed for incremental reductions in TEWI. For example, it may
 able choice given the “untested” HFC technology.                      not be worth the added expense, complication, and potential
    • R-404A was developed as the HFC blend with the closest           safety risk to use a secondary loop, hydrocarbon-based sys-
 properties to R-502. This allowed the equipment manufactur-           tem if there is only a slight improvement in TEWI over a tradi-
 ers to quickly engineer new low-temp HFC equipment based              tional HFC-based, reduced leakage system. The definition of
 on existing R-502 designs and production capabilities. R-507          how much improvement is worthwhile is yet to be decided.
 is very similar (chemically) to R-404A, but it has slightly higher
 pressures and capacity. (Most equipment built today is rated          Long-Term Future of HFCs
 for both products.)                                                      While there may be future regulation designed to reduce
    • R-134a has successfully replaced R-12 in both automotive         impact on global warming, most industries in the United States
 AC and small hermetic refrigeration systems (such as                  are hoping that this will not include a phaseout of HFC refrig-
 appliances and water coolers). Some supermarket chains ad-            erants. For practical reasons already described, and the safety
 opted R-134a in their medium- to high-temperature applica-            implications of using ammonia, hydrocarbons, or very high-
 tions, however others chose to use R-22 or even R-404A/507            pressure products like CO2 in public buildings, the incremen-
 for these systems.                                                    tal benefits of using these working fluids would need to be
    Throughout this transition phase, little consideration was         justified to the U.S. legal system in the least before HFC prod-
 given to the global warming impact of the gas or TEWI for the         ucts could be removed from the equipment inventory.
 system, only for replacing the traditional products as quickly
 and reliably as possible.                                                Jim Lavelle is the technical sales manager for National
                                                                       Refrigerants, Philadelphia. He is a member of ASHRAE Tech-
 HCFC Phaseout: Life after R-22                                        nical Committee 3.1, Refrigerants and Secondary Coolants,
    U.S. industry, EPA, and the Montreal Protocol representa-          and has served on Standard Project Committee 34, Designa-
 tives support the current timeframe for HCFC phaseout, which          tion and Safety Classification of Refrigerants, SPC 15, Safety
 includes reduced CAP values and restrictions on end-use               Code for Mechanical Refrigeration, and Guideline Project
 through the year 2030. Within this framework, R-22 is sched-          Committee 6, Format for Information on Refrigerants.

20           ASHRAE Journal                                 w w w. a s h r a e j o u r n a l . o r g            September 2000
Figure 4: Pumping power and heat transfer characteristic
of secondary refrigerants (low cooling).
hydrous salt solutions. In low-temperature applications,
HYCOOL 50 and Tyfoxit F40 are the best fluids followed by
CaCl2 (Figure 4).
  In comparison to CaCl2 brine, there is not a secondary refrig-
eration on the market that has better pumping power require-
ments and heat transfer capabilities, which will decrease the
energy disadvantages of indirect cooling systems. With these
kind of fluids, the sensible heat is used for heat transport, which
means that large quantities of liquid have to be circulated. Spe-
cial problems occur in the low-temperature range where high
viscosity increases the pumping power.                                Advertisement in the print edition was formally in this
Experimental Investigations
  A German Federal government research project evaluated
the suitability of indirect cooling systems for supermarkets,
together with new fluids. A subcontractor to the system manu-
facturer measured the energy consumption and compared it to
conventional supermarkets with dry expansion evaporation.
During one year, one conventional supermarket has been
measured—first with the new refrigerant R-404A and then with
R-407C—and compared for both cases to find the energy
advantages of the two fluids. The second fluid shows a certain
temperature glide that could benefit energy consumption. The
result was that within both years of measurement with the
refrigerants, no remarkable advantage or disadvantage of one
of the two fluids could be identified. Therefore, the different
GWPs of the refrigerants, when assuming the same leakage
rate, shows a benefit for R-407C that also can be seen in the
TEWI 3 Project for low temperatures (Figure 3).
  To find the best solution for future supermarkets concerning
the greenhouse effect, an ammonia supermarket with indirect
September 2000                                                                            ASHRAE Journal                        21
ASHRAE Journal

                                                                                                                           Figure 6:
                                                                                                                           costs of
Figure 5: Energy Consumption of an NH3 and a R-404A

cooling was built by the same manufacturer and measured by
the subcontractor for another government research project. The
system was compared as far as possible with the afore-men-
tioned conventional supermarket with R-404A and R-407C, re-          pumping power requirements and better heat transfer charac-
spectively, for the same degree days. The comparison led to          teristics —especially at low temperatures. This can be achieved
15% higher energy consumption of the indirect system that            by applying fluids that change phase from the liquid to the
could be decreased by certain optimization measures to around        gaseous state in this region of temperatures. Refrigerants also
10% (Figure 5). Another supermarket system manufacturer con-         could be applied as a secondary coolant. Synthetic refrigerants
firmed this statement by measurements showing higher energy          have the well-known environmental drawbacks concerning
consumption of 18% for Tyfoxit and 11% for HYCOOL.7                  GWP, and natural refrigerants, with exception of CO2, have the
   More or less the same results can be found in the theoretical     local safety problems. Only CO2 is a possible candidate for
evaluation of the TEWI 3 Project, leading to the conclusion          such a secondary coolant. It has a much better heat transfer
that although a much lower TEWI can be achieved by indirect          behavior than the conventional single-phase fluids and also a
cooling systems as compared to conventional systems with             very low viscosity that is one to two magnitudes of order lower
HFCs, the energy consumption is always higher for indirect           than that of the previously mentioned conventional coolants.
systems (although using the energy-efficient refrigerant am-            The high heat of vaporization leads to a very low mass flow
monia). This mainly is due to the high viscosity of the second-      of CO2 coolant resulting in very small pipe diameters6 as com-
ary coolant in the low-temperature part of the supermarket sys-      pared to either direct evaporation systems with R-22 or single-
tems where pumping power depends strongly on the very high           phase fluids like Tyfoxit (which is one of the bests fluids of that
viscosity at low-cooling temperatures. Therefore, the conclu-        family). A disadvantage is the relatively high pressure of CO2
sion is that especially for the low-temperature cabinets in su-      within the secondary circuit, but this does not cause any prin-
permarket systems, another secondary coolant must be found           cipal problems.
to decrease the energy consumption due to the extremely high            The application of CO2 as a secondary refrigerant was first
pumping power needed.                                                mentioned in 1992 by Forbes Pearson. A pilot supermarket sys-
                                                                     tem was built in Scotland to investigate this system.9 Already in
Improvement of Indirect Systems                                      1992, such a system was built in the laboratory and investi-
  To improve such a system, another possibility would be to          gated together with solvents for CO2 to decrease the pressure
use a secondary coolant with phase change that has lower             inside the system. Regrettably, all solvents for CO2 are flam-

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22            ASHRAE Journal                              w w w. a s h r a e j o u r n a l . o r g             September 2000
mable fluids and therefore the high pressures have to be ac-        Such a system has been theoretically and experimentally inves-
cepted.10 In 1996, a Swedish company first built an ammonia         tigated and evaluated by Neksa.14
system with indirect CO2 cooling in the low-temperature circuit        The results show that such systems with conventional direct
in a commercial supermarket in Lund, Sweden.11                      expansion of CO2 in the cabinets show no direct emission prob-
                                                                    lems concerning global warming. However, the energy con-
Direct Cooling                                                      sumption of the transcritical cycle is one-third higher than for a
  Is a CO2 system with direct evaporation in the low-temperature    R-22 system.
region a more competitive system such as the secondary coolant         Certain improvements can be expected if using a two-stage
application of CO2, as far as costs and energy are concerned?       CO2 system, subcritical in the lower and transcritical in the higher
  Theoretical investigation of both systems was done for com-       temperature stage.
parison by Ferreira et al.12 They came to the conclusion that
both systems are nearly equivalent when comparing energy            Conclusion
consumption and yearly costs (Figure 6).                              So, in conclusion, two ways exist to fulfill the requirements of
  One important disadvantage for the cascade system is that         the Kyoto Protocol concerning greenhouse gas emissions.
the cascade cooler has to have a rather low-temperature level         1) Tighten the refrigerant systems to reduce the leakage of
so that it will not exceed the pressures for conventional com-      HFC refrigerants.
pressors in the low-temperature cycle. This means that the            2) Use fluids with very low GWP.
evaporation temperature for the high-temperature cycle must           Indirect refrigeration systems reduce leaks remarkably and
be much too low for cooling purposes, which leads to higher         are necessary for using locally dangerous refrigerants like hy-
heat transfer losses in the high-temperature cycle.                 drocarbons or ammonia in supermarkets. However, they show a
  One possible solution could be to use especially designed         higher energy consumption that can be reduced when using
CO2-compressors for higher pressures in which case it could be      CO2 as a secondary coolant.
possible to use the transcritical CO2 cycle for the high-tempera-     Since liquid chillers for indirect systems can be made at the
ture application in supermarkets. Special CO2 compressors have      manufacturer’s site with great accuracy and tightness control,
been designed by a compressor manufacturer for this purpose.13      the choice of the refrigerant between the environmentally or

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September 2000                                                                              ASHRAE Journal                          23
ASHRAE Journal
locally dangerous types of the synthetical or natural fluids,            brauchs indirekter Systeme durch Auswahl von Kälteträgern.” Ki Luft-
respectively, is not important from the aspects of the environ-          und Kältetechnik (10):472.
mental problems of refrigeration systems. This choice can be                6. Enkemann, T. 1996. Kälteträger für Anlagen mit indirekter Kühlung,
made between synthetic or natural fluids from the viewpoint of           Übersicht, FKW-Seminar XVIII: Aktuelle Entwicklungen in der
                                                                         Kältetechnik in Supermärkten/Gewerbekälte, Hannover, December.
costs, which in one case are more the costs of the fluids, and in
                                                                            7. Arneg SpA: Future Supermarket Systems under the Aspect of
the other case are the costs of the protection equipment against         Environmental Issues. 1999. Marsango, Italy.
local danger. Because of costs, it might be advantageous to                 8. Haaf, S. 1998. “Ammoniak-Kälteanlagen für Supermärkte.” KK-
apply a local safe natural fluid like CO2 in direct expansion sys-       Die Kälte & Klimatechnik 8(51):520.
tems. The supercritical heat rejection for CO2 leads to lower               9. Pearson, F. 1993. “Development of improved secondary refriger-
energy-cycle performance that can be compensated partly by               ants.” Paper presented at The Institute of Refrigeration, London.
better component efficiencies, and cycle configurations.                    10. Enkemann, T. and M. Arnemann. 1994. “Investigation of CO2
                                                                         as a secondary refrigerant.” Proc. Int. Conf. IIR, New applications of
References                                                               natural working fluids in refrigeration and air conditioning, Hannover,
  1. Montreal Protocol 1991 Assessment, Report of the Refrigeration,     p. 721.
Air Conditioning and Heat Pumps Technical Options Committee, UNEP,          11. Rolfsman, L. 1996. “CO2 and NH3 in the supermarket ICA-
December.                                                                focus.” Proc. Int. Conf. IIR, Applications for natural refrigerants,
  2. 1991. DOE/AFEAS GW Project: Energy and Global Warming               Aarhus, Danmark, p. 219.
Impacts of CFC Alternative Technologies, Executive Summary, Oak             12. Infante Ferreira, C.A. and R.A. Boukens. 1996. “Carbon dioxide
Ridge National Laboratory, December.                                     – secondary coolant or refrigerant for cascade systems?” Proc. Int. Conf.
  3. Arnemann, M., D. Gebhardt, H. Kruse. 1995. “Experimentelle          IIR, Applications for natural refrigerants, Aarhus, Danmark, p. 185.
Bewertung neuer Kältemittelgemische als Ersatz für R-22 und R502.”          13. Dorin, 19. 1998. International Trade Fair for Refrigeration and
DIE KÄLTE und Klimatechnik (2):66.                                       Air Conditioning, IKK, Nürnberg, Germany, October.
  4. 1997. DOE/AFEAS TEWI-III Report: Energy and Global Warm-               14. Neksa, P., S. Girotto and P.A. Schiefloe. 1998. “Commercial
ing Impacts of HFC Refrigerants and Emerging Technologies: TEWI          refrigeration using CO2 as refrigerant – system design and experimen-
Phase 3, Oak Ridge National Laboratory, March.                           tal results.” Proc. Int. Conf. IIR, Natural working fluids ’98, Oslo,
  5. Kruse, H. and T. Enkemann. 1997. “Minderung des Energiever-         Norway, p. 227.

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24             ASHRAE Journal                                 w w w. a s h r a e j o u r n a l . o r g                       September 2000
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