WO2015160881A1

WO2015160881A1 - Centrifugal chiller system

Info

Publication number: WO2015160881A1
Application number: PCT/US2015/025859
Authority: WO
Inventors: William L. Kopko; Andrew M. Welch; Justin P. Kauffman; Satheesh Kulankara
Original assignee: Johnson Controls Technology Company
Priority date: 2014-04-15
Filing date: 2015-04-15
Publication date: 2015-10-22

Abstract

Chiller systems utilizing naturally occurring refrigerants. The refrigerants include methylene chloride and dichlorodifluoroethylene. The chiller systems utilizing these naturally occurring refrigerants have upgraded mechanical and electrical systems that are resistant to deterioration from the decomposition products of these refrigerants, permitting the use of environmentally friendly refrigerants such as methylene chloride and dichlorodifluoroethylene.

Description

CENTRIFUGAL CHILLER SYSTEM

FIELD OF THE INVENTION

[0001] The present invention is directed generally to chiller systems utilizing methylene chloride and dichlorodifluoroethylene as refrigerants, and more specifically to chiller systems having improved mechanical and electrical systems that permit the use of environmentally friendly refrigerants such as methylene chloride and dichlorodifluoroethylene.

BACKGROUND OF THE INVENTION

[0002] Chiller systems using centrifugal compressors in the refrigeration circuit, hereinafter referred to as centrifugal chiller systems, are commonly used to maintain temperature control, such as within structures. A major challenge facing centrifugal chiller systems has been the continually evolving refrigerant used for such systems. A major challenge with regard to refrigerants selected for use in refrigeration systems such as centrifugal refrigeration systems has been the environmental challenges posed by common refrigerants. For example, R1 1 and other chlorofluorocarbon refrigerants (CFCs) such as R22 have been phased out due to stratospheric ozone depletion. Such refrigerants are no longer available for new equipment placed into service in the United States.

[0003] Current refrigerants used in centrifugal chiller systems include refrigerants designated as R134a and R123, the R134a being the more commonly used. However, R123 is scheduled for phase out by 2030 under the Montreal Protocol. R134a also is becoming problematic, as it is facing regulations relating to its global warming potential due to its emission into the atmosphere.

[0004] Because of concerns for the environment and the effect of refrigerants on the environment, natural refrigerants are becoming the refrigerants of choice. Natural refrigerants include naturally occurring substances such as certain hydrocarbons (e.g. propane and iso-butane), C0₂, ammonia, water and even air. Naturally-occurring substances such as these are preferable over synthetic compositions for use as refrigerants since, unlike CFCs, natural mechanisms must exist for the removal of these naturally occurring substances from the environment. The use of these naturally occurring substances is not likely to result in any unanticipated consequences due to unexpected adverse effects on the environment since they have been in the environment for thousands if not millions of years. In addition, the ecosystems have adapted to the presence of these materials and any decomposition products over millions of years. Although new synthetic alternatives, such as R1234yf, R1234ze(E) and R1234zd(E) have been developed to address the environmental issues associated with CFCs and HFCs, the unanticipated consequences associated with these newly developed chemicals may remain unknown and unknowable for years to come, just as the anticipated consequences of CFCs and HFCs were unknown for many decades.

[0005] One potential long term issue with refrigerant is their decomposition products. Olefin-based refrigerants such as R-1234yf, R1234ze(E), R- 1336mzz(Z) and R1234zd(E) contain the CF₃ group, which means that trifluoroacetic acid is a decomposition product. While trifluoroacetic acid is known to occur naturally in the oceans, it could accumulate over time in other environments. Thus, there is an advantage to developing refrigerants without the CF₃ group that do not produce this decomposition product.

[0006] Although it is desirable to adapt naturally occurring chemicals for use as refrigerants, they must overcome problems associated with their use in modern compressors. Some of these issues make certain of these naturally occurring chemicals unsuitable for use in modern centrifugal compressors. Some of these substances have low molecular weights, making them unusable because of unrealistic high compressor tip speeds and/or mulitstage centrifugal compressors. The explosive nature of hyrdrocarbons in air eliminates them as a safe alternative. Butane and larger hydrocarbon molecules also experience problems with wet compression that reduce cycle efficiency.

[0007] Ammonia also undesirably is flammable and has a high acute toxicity. Ammonia is also incompatible with copper which limits its use with certain heat exchanger construction and has an extremely low molecular weight, severely limiting its use with centrifugal compressors.

[0008] Carbon dioxide is undesirable as it must be used under extremely high operating pressures that require extremely robust compressor construction, provide poor cycle efficiency and extremely high rotational speed requirements.

[0009] Water is undesirable as a refrigerant because it has extremely low vapor pressures and very low molecular weight, requiring movement of huge volumes of vapor with multistage compressors. Air is a gas under normal chiller operating conditions, so it cannot be used in a conventional centrifugal chiller. Other naturally occurring inorganic compounds such as sulfur dioxide are highly toxic, and some, such as hydrogen sulfide are highly flammable, both also being highly corrosive to the materials utilized in both centrifugal compressor construction and associated refrigeration equipment.

[0010] Certain chemicals that were utilized as refrigerants in the past were discontinued from use as a refrigerant because they weren't as efficient as the synthetic refrigerants developed as replacements. Further, these refrigerants became unworkable due to design changes in centrifugal compressor design as a result of the use of synthetic refrigerants.

[0011] One such naturally occurring substance is methylene chloride. It's natural sources include phytoplankton, mangrove swamps and biomass burning. Methylene chloride, CH₂CI₂ also known as R30 was used as a refrigerant in the first third of the last century. It was replaced by trichlorofluoromethane, CFCI3, as described in U.S. Patent 2,041 ,045. Various improvements in centrifugal compressor design have precluded the use of methylene chloride since its replacement in the 1920's and 1930's. These improvements in centrifugal compressors have made methylene chloride at best undesirable and at worst, unworkable. For example, many modern centrifugal systems use varnishes as a motor winding insulation and with magnetic bearings, and methylene chloride is incompatible with these varnishes. In addition, aluminum used in mechanical bearings is also incompatible with methylene chloride. Gasket materials and shaft seals also are susceptible to attack by methylene chloride. In addition, prolonged exposure to water can result in the formation of hydrochloric acid, which is very corrosive, and water leakage from water at higher pressure than the refrigerant poses a problem not only to the shell and tube heat exchangers commonly used in chillers, but also to other equipment within the refrigeration system exposed to the acid. Still another problem with the use of methylene choride which initially led to its replacement was its poor performance as compared to R-1 1 , especially in multistage direct-drive fixed speed compressors running at 2- pole speeds at maximum speeds of 3600 revolutions per minute (rpm) powered by 60 Hertz (Hz) utility lines.

[0012] What is needed are systems that can be designed for use with methylene chloride or other refrigerants that are naturally occurring substances and which will not result in unexpected consequences to the environment when refrigerant leakage inevitably occurs, but which still provide efficient refrigeration operation.

SUMMARY OF THE INVENTION

[0013] The present invention is directed to a chiller system that utilizes a natural refrigerant having a high critical temperature which improves high cycle efficiency, a freezing point sufficiently low to allow operation of a chiller with 5° C leaving water temperature, a low vapor pressure and a low condensing pressure. The refrigerant should produce vapor pressures at or below the vapor pressure of R123, a refrigerant currently being phased out due to its unfavorable environmental impact. The refrigerant desirably has a high molecular weight, greater than 60. High molecular weight refrigerant with low vapor pressures result in lower impeller rotational speeds but larger impeller diameters for a centrifugal compressor, simplifying compressor design. Desirably, the refrigerant should also have a condensing pressure no greater than that of R123, which is 19 psia. Low condensing pressures permit the use of non-code pressure vessels in the refrigeration system.

[0014] Natural refrigerants having a high critical temperature while also being safe to use in chiller systems can be used in suitably modified chiller systems. These natural refrigerants include methylene chloride and dichlorodifluoroethylene.

[0015] The chiller systems utilize the standard components of refrigeration systems. These standard components include a compressor compressing the natural refrigerant, the compressor in fluid communication with a condenser, which condenses the natural refrigerant, the condenser in fluid communication with and evaporator. An expansion valve is intermediate the condenser and the evaporator to lower the pressure of liquid refrigerant from the condenser. The condenser includes a heat exchanger to remove heat from the refrigerant during the condensation process. The evaporator includes a heat exchanger that cools water for the chiller system using evaporating liquid refrigerant in the form of a mist after passing through the expansion valve intermediate the condenser and the evaporator. The cooled water is maintained in the chiller for use in cooling the building space as needed. Although each of the components are generally found in all chiller systems, each of the components as well as interconnected piping must be modified in order to provide a system that accommodates the natural refrigerants. [0016] The chiller systems utilizing these natural refrigerants may must include a filter drier that can be used to remove moisture and acid from the refrigeration system. These natural refrigerants in the presence of water can decompose to form corrosive hydrochloric acid as well as hydrofluoric acid (for dichlorodifluoroeethylene). In addition, stabilizers may be added to the refrigerant to retard their deterioration.

[0017] The chiller systems also must be equipped with water detectors and isolation valves. As the refrigeration systems operating with these refrigerants operate below standard atmospheric pressure while the chiller systems operate at or above atmospheric pressure, in the event of a leak, water is likely to leak into the refrigerant. Once detected, the shut-off valves are activated to isolate the leak and prevent further contamination of the system.

[0018] The compressors in the chiller systems using these natural refrigerants are single stage or two stage centrifugal compressors. The compressors must be modified to minimize corrosion concerns. Thus compressor components and heat exchanger components should eliminate materials subject to attack by these natural refrigerants. Compressor mechanical bearings, whether of the roller, ball of fluid film type, should not include aluminum or its alloys, but rather should be fabricated from steel or ceramic materials. Aluminum, a common electrical and heat exchange material, as well as zinc should be removed from not only the compressor and heat exchanger components. Thus these materials also should be replaced in compressor motors and magnetic bearings. The replacement of aluminum and zinc would include its removal from sealing and joining material. Ideally potting material used in compressor motor must be compatible with methylene chloride and dichlorodifluoroethylene. Aluminum rotor bars in compressor motors must be replaced with copper rotor bars. Standard motor winding material must be replaced with glass insulation. The compressor impeller determined by its imperviousness to reaction with the natural refrigerants. [0019] All seals and gaskets in the chiller system also must be compatible with the natural refrigerants. Thus all seals and gaskets ideally comprise polytetrafluoroethylene (PTFE), Kapton®, epoxy or a similar material. Kapton® is a DuPont tradmark for a polyimide having a chemical name poly (4,4'-oxydiphenylene-pyromellitimide).

[0020] In order to improve compressor efficiency using these natural refrigerants, wet compression is utilized. Wet compression injects a small amount of liquid into the compressor suction. Additionally, the rotational speed of the impeller is operated at a speed of greater than 3600 rpm, so that the overall tip speed of the impeller is reduced. In order to produce such rotational speeds, the compressor drive, or power supply, must supply power at a frequency above 120 Hz design conditions, although other motors that can provide speeds greater than 3600 rpm may also be used.

[0021] Tubing for the heat exchangers in both the evaporator and the condenser not only should not include aluminum, but may be increased in size from the standard 5/8 inch diameter tubes to tubes in the size range of ¾- 1 inch, increasing the available surface area of each tube. The evaporator may be of the flooded design of or having a spray design, using a pump that eliminates the risk of refrigerant leakage through shaft seals.

[0022] As used herein, a high critical temperature means a critical temperature above 210° C. As used herein, the term natural refrigerant means a material that is naturally occurring, but can be adapted for use as a refrigerant in a refrigeration system, the material having natural mechanisms for removal from the environment with little or no environmental impact as a result of its decomposition. As used herein, a low vapor pressure is a vapor pressure below 6 psia. As used herein, a low condensing pressure is a condensing pressure below about 15 psia. [0023] Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a chart depicting relative cycle efficiencies and cooling capacity of possible refrigerants.

[0025] Figure 2 depicts a refrigerant circuit showing the components utilized in the present invention.

[0026] Figure 3 depicts a optional condenser design incorporating a subcooler for use in the refrigeration circuit of Figure 1.

DETAILED DESCRI PTION OF THE INVENTION

[0027] Global environmental issues are forcing the re-evaluation of refrigerant selections. The present invention is directed to a chiller system that has been modified to efficiently utilize natural refrigerants. Stability and flammability issues create additional constraints on the selection of compounds for use as refrigerants.

[0028] A group of compounds that are interesting for use as refrigerants are based on halogenated derivatives of ethylene (also called ethene, CH2=CH2). The double bond should give a very short atmospheric life, which results very low global warming potential and negligible ozone depletion potential for chlorine-containing compounds. Of particular interest in this group is 1 ,2-dichloro 1 ,2-difluoroethylene, CCIF=CCIF. Tests show that trichloroethylene is weakly flammable in air, while perchloroethylene (CCI₂=CCI₂) is nonflammable. In general substitution of fluorine for chlorine increases flammability, so only fully halogenated compounds are likely to be nonflammable. Even then, tetrafluoroethylene (CF₂=CF₂) is flammable, polymerizes easily, so only fully halogenated compound chlorine-containing compounds are likely to be stable and nonflammable.

[0029] In general, compounds with a CF₂= group have serious toxicity problems. For example CF₂=CFCI is considered highly toxic and also has flammability and stability issues. This constraint also eliminates CCI₂=CF₂ as a likely option. However, 1 ,2-dichloro 1 ,2-difluoroethylene is likely to have reasonably low toxicity, nonflammability or reduced flammibility and other physical properties that make it suitable for use as a refrigerant in advanced chiller systems. Dichlorodifluoroethylene has a critical temperature of 221⁰ C. This compound exists as two isomers, a cis- and a trans- isomer, the cis- form having a boiling point of 21 ° C and the transform having a boiling point of 22° C, these boiling points making it suitable for use as a refrigerant in most chiller applications. The cis- isomer has a liquid-to-solid transition temperature of -119.6° C, while the trans- isomer has a liquid-to-solid transition temperature of -93.3° C. The compound may also referred to as R11 12 using standard refrigerant numbering and nomenclature.

[0030] The two dichlorodifluoroethylene isomers have suitable boiling points for use as a refrigerant for most chiller applications. While either of these isomers may be used in a chiller application alone, mixtures of these two isomers are commercially available. Since the isomers have very close to the same boiling point, substantially azeotropic mixtures of the two isomers that are desirable as refrigerant in centrifugal chillers may be used. However, of the two isomers, the trans isomer is likely to be more stable and less flammable. The simple structure of dichlorodifluoroethylene should give high cycle efficiency and reduce the cost of the refrigerant. Its high molecular weight is desirable, which should allow lowering of compressor speed, which simplifies compressor design and allows the easy use of the direct-drive induction motors. As used herein a substantially azeotropic mixture is one having a temperature glide of no more than 1.0° C (1.0° K). A substantially azeotropic mixture, one having a temperature glide of 1.0° C (1.0° K) between or among the constituents, can normally neglect the effects of separation during such phase changes. In order to achieve this temperature glide, the boiling points of the components of the mixture, here the isomers of dichorodifluoroethylene, cannot be disparate.

[0031] One natural refrigerant of interest for use in advanced chiller applications is methylene chloride, also identified as CH₂CI₂. Methylene chloride is a liquid at room temperature with a boiling point of 40 °C. It is a common industrial chemical used as a solvent, as a paint stripper, in decaffeinating coffee, in the production of pharmaceuticals, as a foam- blowing agent and many other applications. A review of history shows that it was once used as a refrigerant in centrifugal chillers and home refrigerators in the 1920s and 1930s, but its use as a refrigerant was discontinued as more efficient refrigerants were developed. Today, it is used as a common industrial chemical as a solvent, as a paint stripper, in decaffeinating coffee, in the production of pharmaceuticals, as a foam- blowing agent and many other applications. Recent atmospheric science shows that there are large natural sources for methylene chloride.

[0032] Multiple studies have identified large natural sources of methylene chloride in the environment. Analysis of air trapped in Antarctic ice shows an atmospheric concentration of approximately 1.4 parts per trillion that predates significant industrial production of methylene chloride. As shown in Table 1 , this concentration corresponds to natural emissions of approximately 50,000 tons/year. The mass of the substance in Table 1 is calculated based on the measured atmospheric concentration and the mass of the atmosphere. The emission rate is the mass in the atmosphere divided by the atmospheric life. Emissions sources are mainly from biomass burning and oceans with mangrove swamps being a minor source. [0033] Table 1 also shows that pre-1940 natural emissions of methylene chloride are roughly two orders of magnitude higher than emissions of R123 based on atmospheric concentrations measured in 2003. Operating in a vacuum combined with the use of high efficiency purge systems, the use of methylene chloride as a refrigerant substitute for R123 would result in chiller emissions that are lower than that from R123 chillers. Thus methylene chloride refrigerant emissions from a well maintained chiller should only result in a small fraction of natural emissions for a reasonable usage scenario based on refrigerant handling procedures similar to that for R123 in modern chiller equipment.

Table 1

Comparison of Chiller Emissions to Natural Emissions of Methylene Chloride

[0034] The likely emissions from refrigerant use are also orders of magnitude lower than existing emissions from other uses. Worldwide industrial emissions are on the order of 650,000 tons/year, which is roughly 1000 times higher than the likely refrigerant emission rate. The very low emission from chillers means that refrigerant use should not cause a significant increase in the atmospheric concentrations of methylene chloride. [0035] Methylene chloride has favorable properties in terms of global environmental effects. The global warming potential (GWP) is 9 compared to carbon dioxide for a 100-year time horizon and its atmospheric life is 0.4 years. R30 is not regulated under the Montreal Protocol. The US EPA recognizes it as a non-ozone-depleting substitute in foam blowing and solvents.

[0036] Figure 1 and Table 2 compare cycle efficiency and cooling capacity for various refrigerants. The analysis is for an ideal cycle with 6° C evaporating temperature, 36° C condensing temperature, 5 K (5° C) liquid subcooling, and no suction superheat. The performance uses refrigerant properties from NIST REFPROP.

Table 2: Comparison of Refrigerant Properties

[0037] The general trend is that low pressure refrigerants have better cycle efficiency and reduced cooling capacity with methylene chloride providing the highest cycle efficiency of any refrigerant in the analysis. Based on the cooling capacity (kJ/m³) data in Table 1 , a centrifugal compressor using methylene chloride as a refrigerant is expected to have a size comparable to one using R123 as a refrigerant.

[0038] For the purpose of the comparison, carbon dioxide is included even though the condensing temperature exceeds the critical point. For this case, the performance is based on the adjusting the high side pressure to give the best cycle efficiency instead of a saturation condition.

[0039] Commercially available chillers use various types of enhanced tubes in evaporators and condensers. These enhanced tube designs are proprietary to chiller tube manufacturers and general correlations to estimate the heat transfer performance of such enhanced tubes are not available. In the absence of generalized correlations for chiller tubes, transport properties of the refrigerants are compared below in Table 3 to infer heat transfer performance of methylene chloride relative to other refrigerants.

[0040] Table 3 summarizes transport properties of several refrigerants that have relative capacity and efficiency close to that of methylene chloride along with R134a. The data in Table 2 are at 20 °C saturated conditions for all refrigerants. Properties for all refrigerants except methylene chloride are from NIST REFPROP. The properties of methylene chloride are derived from ASHRAE (1993) literature and Dow (1998). [0041] Table 3 Transport Properties at 20 °C

[0042] From Table 2 it can be seen that methylene chloride has the highest liquid thermal conductivity among the listed refrigerants. The liquid viscosity of methylene chloride is lower than that of R123 and R1233zd(E) but higher than that of R134a, butane and pentane. The latent heat of R30 is higher than that of R123, R134a and R1233zd(E) but lower than that of butane and pentane. Based on the transport property data from Table 2, the heat transformer performance of methylene chloride is expected to be equal or better than that of R123 or R1233zd(E). Higher pressure refrigerants, such as R134a and butane, are expected to have equal or better heat transfer performance than methylene chloride. There is no clear difference in projected heat transfer between methylene chloride and pentane.

[0043] The very low vapor pressure of methylene chloride favors shell-side evaporation and condensation. Methylene chloride has an advantage that its low vapor pressures exempt vessels from pressure vessel code requirements. On the other hand, refrigerant pressure drop would limit performance of methylene chloride in direct-expansion heat exchangers.

[0044] Materials compatibility issues can also affect heat exchanger design. Methylene chloride has a clear advantage compared to ammonia since methylene chloride can use enhanced copper tubes that are incompatible with ammonia. A disadvantage for methylene chloride is a possible compatibility problem with aluminum that limits material selections for heat exchangers and other components.

[0045] Handling requirements for refrigerants can be complicated and depend on laws and regulations that can vary regionally or locally. To simplify the analysis, we limited the review of methylene chloride handling requirements to the United States and China. These two countries have the two largest markets for centrifugal chillers, and a review in the US and China also provides a good starting point in assessing handling requirements in other parts of the world.

[0046] In the US, current ASHRAE Standard 34 classification for methylene chloride is B1 , which corresponds to higher toxicity with no flame propagation. Methylene chloride previously had a classification of B2, but the classification was recently revised as indicated, to B1. Extensive flammability testing of methylene chloride done in the 1970s showed no flame propagation in air at atmospheric pressure for temperatures below 100° C. ASHRAE Standard 34 (ASHRAE 2103a) requirement for no flame propagation is based on measurements at 60° C. As with other refrigerants elevated pressures, extreme temperatures, additions of flammable components, etc. can create flammable mixtures in air. Methylene chloride may burn and/or decompose in contact with an open flame or with a high-temperature surface because of a continuous supply of heat that drives the reactions. Flammable limits are sometimes shown for methylene chloride based on conditions that differ from those used by the ASHRAE classification system.

[0047] Methylene chloride has a long history as an industrial chemical, and there are many studies on its long-term toxicity in humans and in animals. While some animal studies have shown an increase in cancer rates for long-term exposures at high concentrations, other studies in workers and animals show no increase. While methylene chloride may pose a cancer risk, that risk is negligible because of differences between animals and humans and because of the intermittent nature of exposure to methylene chloride.

[0048] Methylene chloride handling requirements required by OSHA are generally consistent with practices required by ASHRAE Standard 15 (ASHRAE 2013b). For example, OSHA requires regular monitoring of methylene chloride concentration if it is expected to exceed 12.5 ppm. ASHRAE Standard 15 (ASHRAE 2013b) requires continuous monitoring in equipment rooms with an alarm at 10 ppm; the ASHRAE Standard 15 requirement is more restrictive that OSHA requirement. Clean-up and fluid transfer procedures are generally similar for methylene chloride and R123.

[0049] Table 3 summarizes a comparison of data related to handling issues for methylene chloride and other refrigerants. In terms of acute exposure from accidental release, methylene chloride is similar to R123 since IDLH values and the leak scenarios are similar. While not shown in the table, CFC1 1 and CFC1 13 also have a similar IDLH value of 2000 ppm and have demonstrated history of safe use in chillers.

[0050] Table 3: Comparison of Data Related to Handling Refrigerants

Lower none none 16.7% 1.2% none none

Flammable

Limit (LFL)

in Air

[0051] In Table 3, for R123, there are no recognized IDLH or PEL values, so one-hour exposure limit and AEL (Acceptable Exposure Limit) values from DuPont (2012) are used instead. IDLH values from CDC (2014). PEL values are from OSHA (2014). LFL values for ammonia and pentane are from ASHRAE (2013a).

[0052] Comparisons with R123 show that extremely low equipment room exposures are likely with appropriate handling. Standards for equipment room concentrations of R123 are 1 ppm or lower based on measurements in the early 1990s. Unlike R123, the entire refrigeration cycle for R30 is normally below atmospheric pressure, which means that air leaks in and is vented outside the building through the purge system rather than refrigerant leaking into the equipment room. This analysis indicates that normal equipment room concentrations should be orders of magnitude lower than allowable work place exposure limits for methylene chloride.

[0053] Pentane or other hydrocarbons have significant fire risks that are not present with methylene chloride and other nonflammable refrigerants. Under current US safety rules, hydrocabons are prohibited from commercial air conditioning applications even when used in water chillers.

[0054] Handling issues for methylene chloride generally compare favorably to those for ammonia. The acute toxicity for methylene chloride is much less than that for ammonia. In addition, a leak in an ammonia system can result in the immediate release of large amounts so ammonia as a vapor, while methylene chloride is a liquid at room temperature. Ammonia also has flammability issues that do not exist with methylene chloride. These advantages gave methylene chloride a competitive advantage over ammonia in chillers before the introduction of CFCs and would favor methylene chloride as a natural refrigerant in modern chillers.

[0055] In China, methylene chloride is mainly used as solvent, extractant, cleaning agent, foaming agent, release agent and the raw material of R32 production. China recognizes that methylene chloride is flammable under open fire or high heating condition, and its thermal decomposition can emit highly toxic phosgene in a manner similar to that for R22. When using or exposed to methylene chloride, smoking, eating and drinking are prohibited at the site. Impervious gloves for hand protection and safety glasses for eye protection are necessary. Two key emergency treatment and handling methods are for first aid measures, removing contaminated clothing and using soapy water and clean water to wash skin thoroughly at least 15 min if skin exposure occurred, and washing eye using flowing clean water or saline if eye exposure occurred. Charging and storage requirements for methylene chloride requires keeping ignition sources away and avoiding contact with alkali metals, alkaline earth metals and nitric acid. Due to the B1 classification by ASHRAE, requirements for handling methylene chloride as a refrigerant in chillers are expected to be similar to those for R123.

[0056] Methylene chloride currently lacks a refrigerant classification in Chinese standards, but ongoing revisions should resolve this issue shortly. The Chinese refrigerant classification standard lacks a classification for methylene chloride. An earlier version of the Chinese refrigerant classification listed methylene chloride as a B2 refrigerant, but a revision is expected in 2015 that should provide a classification as a B1 refrigerant.

[0057] A basic embodiment of a chiller system adapted to use either methylene chloride or dichlorodifluoroethylene as a refrigerant is depicted in Figure 2. A motor 10 drives a compressor 12 which discharges refrigerant vapor 14 to a condenser 16. A cooling fluid 18 cools refrigerant vapor in the condenser to produce refrigerant liquid 20. The cooling fluid is in fluid communication with a heat exchanger, not shown. Refrigerant liquid 20 flows through an expansion device 24 to an evaporator 26 which boils to cool a heat-transfer fluid 28. The heat transfer fluid 28, usually water or brine, is in fluid communication with a chiller storage, which supplies cooled water regions of an area, for example, in a building, requiring cooling on demand. Refrigerant vapor 30 then leaves evaporator and enters the suction end of the compressor 12. An optional variable frequency drive 32 supplies power to the motor 10. The optional variable frequency drive may be controlled from a control panel 34.

[0058] Variable speed drive systems available in centrifugal compressor systems permit centrifugal compressors to rotate at speeds greater than 3600 rpm, such as by using a 4-pole motor. In past designs, the rotational speed of the impeller has been dictated by the frequency of the three speed motors. However, variable speed drive systems provide frequencies above 120 Hz, so that the impellers can rotate at speeds above 3600 rpm. In the past, the limitation with compressor speeds at 3600 rpm required a system with multiple stages (three of more) in order to achieve the required cooling capacity using methylene chloride. In past designs, the CFC refrigerants also provided increased cooling capacity because of higher evaporating pressures. However, the ability to rotate the compressors at speeds greater than 3600 rpm allows the use of methylene chloride and dichlorodifluoroethylene in two stage centrifugal compressor systems and preferably in single stage centrifugal compressor systems utilizing smaller compressor sizes. Alternatively, speeds greater than 3600 rpm may be achieved using one or multiple gears to step up the speed to the compressor impeller. The geared configuration could use a hermetic motor or an air-cooled motor with a shaft seal. Other motor designs include permanent magnet motors, switched reluctance motors and other types of induction motors.

[0059] Liquid injection improves the efficiency of methylene chloride and dichlorodifluoroethylene used as a refrigerant in centrifugal compressor chiller systems. Liquid injection into the compressor routes a small amount of condensed refrigerant from the high pressure side of the refrigeration circuit and injects it into the suction of the compressor with the refrigerant vapor 30 and reduces superheat. This liquid injection is referred to as wet compression and the amount of condensed refrigerant liquid is throttled from the high pressure side of the compressor to the compressor inlet reduces the refrigerant gas quality from 100% saturated vapor to 95% quality. As a result, discharge temperature of the refrigerant is reduced, with may improve refrigerant stability. Liquid injection also allows for the use of impellers having a slightly larger diameter, increasing the ideal diameter from about 8.6 inches without wet compression to about 9 inches with wet compression. While wet compression is typically accomplished to reduce noise to the centrifugal compressor, for the natural refrigerants used with the chiller systems of the present invention, the efficiency of the system is improved by about 3.0%, the calculated efficiency being 2.9%. The amount of condensed refrigerant liquid throttled from the high pressure side to the compressor inlet will vary based on compressor capacity, with larger compressors throttling more condensed refrigerant than smaller centrifugal compressors. Compressor speed also may impact the amount of condensed liquid refrigerant throttled to the suction inlet, such as for compressors having variable speed capability. The amount of condensed refrigerant liquid throttled may be up to 5% of the total amount of refrigerant, by mass, at the compressor inlet. The use of methylene chloride advantageously provides liquid carryover which improves calculated cycle efficiency, so the size and pressure drop of mist eliminators may be reduced. While liquid injection into the compressor suction may also improve performance, the theoretical optimum corresponds to a small amount (~20°F) of discharge superheat, which may also serve as basis of system control. The optimum injection may differ from the theoretical optimum and the amount may be optimized based on test data. Droplet size and amount of liquid injected should be limited to reduce risk of compressor damage. [0060] With regard to the compressor impeller, the generation of hydrochloric acid from the decomposition of methylene chloride (as well as phosgene gas) and the generation of hydrochloric acid and hydrofluoric acid from the decomposition of dichlorodifluoroethylene has presented survivability problems for prior art steel impellers or light weight aluminum impellers, as both are subject to attack by both hydrochloric acid and hydrofluoric acid, both of which are extremely corrosive. There are several solutions to this problem that were not available with centrifugal chillers that previously used methylene chloride as a refrigerant. One of these solutions is to utilize stainless steel impellers to limit attack by hydrochloric acid and hydrofluoric acid. Preferably, these stainless steels are cast stainless steels and are austenitic stainless steels, preferably in the 300 series of austenitic stainless steels. Since even stainless steels may be subject to attack by these acids formed by decomposition of methylene chloride and dichlorodifluoroethylene in the presence of water, the effective life of these stainless steel castings can be further extended by coating them with a thin layer of material that is impervious to penetration by these acids. Of course, these coatings can also be applied to aluminum impellers as well. These coatings include Teflon^® (Polytetrafluoroethylene- PTFE), epoxy and other polymers.

[0061] Hydrochloric acid formed by the decomposition of methylene chloride hydrochloric acid and hydrofluoric acid formed by the decomposition of dichlorodifluoroethylene in the presence of water circulate through the system and have the potential of attacking various components in the system other than the impeller of the centrifugal compressor that include aluminum, zinc, alloys of aluminum and zinc and/or ferritic steel. Thus, mechanical bearings and portions of the motor are also subject to attack by these acids. Preferably, the mechanical bearings are designed to replace steel or aluminum portions with stainless steel, so that the bearings are stainless steel bearings. Alternatively, ceramic bearings may be used as replacements for metallic bearings. The motor housing may be replaced with a stainless steel motor housing or a housing having a coating of material resistant to attack by these acids. This is particularly desirable when refrigerant is also used as a coolant for the motor. Aluminum rotor bars and aluminum wiring used in the windings of the motor are replaced with copper rotor bars and copper wire windings. The decomposition products of methylene chloride and dichlorodifluoroethylene also attacks the potting used over the stator windings in motors. KAPTON®, a polyimide film available from Du Pont may be used as insulation for wire. In addition, varnishes used for pottin and which may be subject to attack by the refrigerant or its decomposition products preferably are replaced with acid-resistant potting compound, such as glass potting or epoxy.

[0062] Aluminum has also been used in heat exchanger shells and tubes. Once again, aluminum must be eliminated from the heat exchanger shells and tubes in contact with the refrigerant. For the shells, the shells are replaced with stainless steel shells. Alternatively, the shells may be coated with a coating such as epoxy or PTFE that protects the substrate from attack from the decomposition products of methylene chloride or dichlorodifluoroethylene. Aluminum tubes should be replaced with copper tubes.

[0063] The use of methylene chloride and dichlorodifluoroethylene also permits other modifications in heat exchanger design. The standard tube diameter for tubes utilized in shell/tube-type heat exchangers is 5/8 inch. Increasing the size of the tubes used in these shell/tube-type heat exchangers to sizes in the range of ¾-1 inch will provide more surface area for heat exchange, which is desirable since the mass flow and density of methylene chloride and dichlorodifluoroethylene is reduced compared to other CFC refrigerants. The cycle comparisons for methylene chloride as compared to other CFC refrigerants is set forth in Table 4. In addition, the tubes may be provided with enhanced surfaces on both the shell side and the water/tube side for better efficiency and improved capacity, again to improve heat exchange characteristics with these refrigerants as compared to CFC refrigerants which have better mass flow and higher density. These enhanced surface features include spiral internal enhancements and/or high performance nucleate boiling surfaces.

[0064] In addition to use of each of the fluids alone, mixtures of these fluids with each other or with other fluids of similar vapor pressure may be suitable as substantially azeotropic refrigerant blends. Other components prefereably have a short atmospheric life such as hydrofluoro-olefins (HFOs) and hydrochlorofluoro-olefins (HCFOs). Especially desirable fluids include R-1336mzz(Z), R-1233zd(E), R1336mzz(E), R-143, isomers of R- 245. Hydrocarbons may also be used, but flammability is a concern. Shortlived HFCs such as R-143, R-245ea, R-245eb and R-245ca are also alternatives.

Table 4: CYCLE COMPARISONS

R30 with 4% liquid

R123 R134a R1233zd(E) R30 injection

Pevap (psia) 6.07 51.70 8.83 3.60

Pcond (psia) 19.3 130.6 27.0 12.51

Tdisjsen (F) 96.0 101.9 96.0 155.1 109.7

Speed of Sound (ft/s) 414.2 481.3 432.9 592.2

Density (Ibm/ft3) 0.176 1.090 0.220 0.058

Omega 1.12 0.89 1.14 1.14

Theta (assumed) 0.07 0.07 0.07 0.07

Mass Flow (Ibm/min) 757.7 733.4 682.3 359.2 374.2

Diameter (inches) 18.9 6.9 15.7 19.0

Impeller RPM 7550 19910 9920 10720

kW/Ton 0.408 0.427 0.411 0.403 0.392

COP 8.623 8.240 8.552 8.731 8.965

COP compared to

R123 1.000 0.956 0.992 1.012 1.040 Ideal cycle with following conditions: ETP = 42° F, CTP = 96° F, SC = 8° F, SH = 0° F

[0065] The use of methylene chloride and dichlorodifluoroethylene also allow for changes to the evaporator configuration. The evaporator preferably is a flooded-type evaporator, eliminating the need for a pump for the refrigerant. Of course, a pump is subject to attack by the decomposition products of methylene chloride and dichlorodifluoroethylene, and its elimination reduces maintenance and possible shutdown due to pump leakage. In another embodiment a spray design may be utilized. Although an evaporator of this configuration may utilize a refrigerant pump that eliminates the risk of leakage from a shaft seal. Such pumps include magnetically coupled centrifugal refrigeration pumps, air-lift pumps using flash gas as the driving fluid and hermetically sealed motor-driven pumps. In another preferred embodiment, the evaporator may be of the falling film type, which also uses a pump to pump refrigerant over the tubes in a tube bundle. An "airlift" pump or eductor may be used to deliver liquid refrigerant to the distributor in the evaporator.

[0066] In one embodiment, shells are substantially circular to minimize material and labor cost compared to conventional rectangular shell constructions. In one embodiment, shells have greater widths than heights to allow centrifugal compressors to be mounted vertically to minimize refrigerant pressure drops into and out of the compressor. In one embodiment, a shell includes concave shell sides, top, and bottom and reinforcing ribs extending outwardly, creating a rectangular outside boundary. In one embodiment, an evaporator shell comprises a controlled orifice or valve for the flow into the distributor at the top of the evaporator.

[0067] Because the decomposition products of the refrigerants methylene chloride and dichlorodifluoroethylene in the presence of water pose operational problems for the centrifugal compressor chiller system, several remedial systems can be built into the chiller system. One of these systems includes a water detection system that detects the presence of water in the refrigerant. The water detection system includes a series of sensors placed on the refrigerant side of the chiller system. Because the refrigerants operate below standard pressure of 15 psia, while the water side of the system operates above standard pressure, any leakage between the water side of the system and the refrigerant side of the system, such as at the condenser and the evaporator, will result in water leaking into the refrigerant side of the system. These sensors can measure the electrical conductivity or dielectric properties of the fluid flowing on the refrigerant side. A change in the measurements of the refrigerant is an indication of water in the refrigerant. In response to leakage the chiller may be shut down or the leak can be isolated by various water-side shut-off valves within the system. The leak can then be repaired. In addition, the remedial systems may include a filter drier for the refrigerant or stabilizers added to the refrigerant to remove moisture or acid from the refrigerant in the system. Sensors in the equipment room can also be used to detect leakage of methylene chloride or dichlorodifluoroethylene refrigerant into the equipment room. In one embodiment, in case of refrigerant leakage, a metal pan can be used to collect the liquid refrigerant. In another embodiment, in case of refrigerant leakage, concrete pan with an impervious coating can be used to collect the liquid refrigerant, which is a liquid at ambient temperature and pressure.

[0068] In one embodiment, the expansion device is an actuated butterfly valve controlled in response condenser liquid level. In one embodiment, condenser level control is preferred if a subcooler is used. Alternatively, a fixed orifice can be used to reduce cost and refrigerant charge requirements for systems without a subcooler.

[0069] Because of the corrosive nature of the methylene chloride, dichlorodifluoroethylene and their decomposition products with certain materials, materials in contact with refrigerant used for sealing and joining should be compatible with the refrigerant. Welding and brazing are a good option in many cases. Compatible gasket materials for flanges and o-rings include a variety of fluoropolymers marketed under various brand names, such as Chemraz® and Kalrez®. Teflon® (PTFE) is another compatible gasket material. In some cases less expensive materials may be used if some swelling does not create operational problems.

[0070] Figure 3 shows an optional condenser design 302 with a subcooler 304. As further shown in Figure 3, the subcooler 304 has a non-circular shell 308 that is welded to the outside of the condenser shell 306, and the ends of the subcooler shell are welded to the condenser tube sheet 310. This setup reduces refrigerant charge requirements while improving performance with additional subcooling. This subcooler design is especially desirable with non-code vessels that can be used with methylene chloride. [0071] While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

CLAIMS We claim:

1. A centrifugal chiller system comprising:

an electrically driven centrifugal compressor with an impeller having a rotational speed greater than 3600 rpm, the compressor further comprising an impeller driven by an electric motor within a compressor housing and bearings selected from the group consisting of electromagnetic bearings, ceramic bearings and stainless steel bearings;

a condenser in fluid communication with the compressor;

an evaporator in fluid communication with the condenser and the compressor;

an expansion device intermediate between the condenser and the evaporator and in fluid communication with both the condenser and the evaporator;

a chiller in heat exchange communication with the evaporator with a chiller fluid, the chiller fluid being circulated from the chiller to the evaporator where it is cooled;

a refrigerant circulating through a refrigerant circulation system that further including piping fluidly connecting the compressor to, the condenser, the condenser to the evaporator and the evaporator to the compressor, the refrigerant further comprising a nonflammable, naturally occurring organic compound with a molecular weight greater than about 60 and a condensing pressure no greater than 19 psia;

water detectors in at least the condenser and the evaporator, detecting water leakage into the refrigerant circulating through the condenser and the evaporator;

shut-off valves isolating chiller fluid in the chiller when water leakage is detected in the refrigerant; wherein the evaporator is selected from the group consisting of a flooded type, a falling film type and a spray type;

wherein the compressor further includes

an electric motor characterized by a substantial absence of aluminum, zinc and aluminum zinc alloys, the electric motor further comprising copper windings and copper rotor bars; and

an impeller characterized by a material resistant to attack by decomposition products of the refrigerant; and

wherein the refrigerant circulation system is substantially free of contact with aluminum, zinc, alloys of aluminum and zinc and ferritic steel.

2. The centrifugal chiller system of claim 1 wherein the compressor electric motor is potted with insulation selected from the group consisting of glass insulation, epoxy and polyimide.

3. The centrifugal chiller system of claim 1 wherein the centrifugal compressor further includes a liquid refrigerant injection system throttling up to 5% liquid refrigerant into a suction port of the centrifugal compressor.

4. The centrifugal chiller system of claim 1 wherein the naturally occurring refrigerant has a molar ideal-gas heat capacity less than about 100 kJ/kgmol/K at 20° C and a vapor pressure of at least 0.05 bar at 0° C, a freezing point of at least 0° C and a vapor pressure of less than about 1 bar at 35° C.

5. The centrifugal chiller system of claim 3 wherein the chiller system further comprises a single stage centrifugal compressor.

6. The centrifugal chiller system of claim 4 wherein the naturally occurring refrigerant is selected from the group consisting of methylene chloride and dichlorodifluoroethylene.

7. The centrifugal chiller system of claim 6 wherein when naturally occurring refrigerant is dichlorodifluoroethylene, the refrigerant further comprises cis- dichlorodifluoroethylene having a boiling point of 21 ° C and trans- dichlorodifluoroethylene having a boiling point of 22° C and substantially azeotropic combinations of cis-dichlorodifluoroethylene and trans- dichlorodifluoroethylene.

8. The centrifugal chiller system of claim 6 further including an HFO, and HFC or an HFCO in combination with the naturally occurring refrigerant, the combination forming a substantially azeotropic refrigerant blend.

9. The centrifugal chiller system of claim 8 wherein the HFO, HFC or the HFCO is at least one refrigerant selected from the group consisting of R- 1336mzz(Z), R-1233zd(E), R1336mzz(E), R-143, isomers of R-245, R- 245ea, R-245eb and R-245ca.

10. The centrifugal chiller system of claim 1 wherein the system further includes seals and gaskets compatible with the natural refrigerant in the chiller system.

1 1. The centrifugal chiller system of claim 10 wherein the seals and gaskets comprise a material selected from the group consisting of PTFE, Kapton®, epoxy and combinations thereof.

12. The centrifugal chiller system of claim 1 wherein one or both the evaporator and the condenser include tubes having a diameter or ¾ to 1 ".

13. The centrifugal chiller system of claim 1 wherein the system further includes a filter drier that removes water and acid from the refrigerant.

14. A method of limiting the global environmental effects of a refrigerant comprising:

selecting a refrigerant fluid that comprises a nonflammable naturally occurring organic compound with a molecular weight greater than about 60 and a molar ideal-gas heat capacity less than about 100 kJ/kgmol/K at 20°C and a vapor pressure of at least 0.05 bar at 0°C and a freezing point of at least 0°C.

15. The method of claim 14 further including the step of providing a centrifugal chiller system that includes circulating the selected refrigerant fluid to a centrifugal compressor in fluid communication with a condenser, which is in fluid communication with an evaporator which is in fluid communication with the centrifugal compressor, the evaporator also in fluid communication with a chiller using water or a water solution.

16. The method of claim 15 wherein the step of providing a chiller system further includes providing a single stage or a two stage centrifugal compressor.

17. The method of claim 16 wherein the step of providing a single stage or a two stage compressor further includes providing a variable speed drive operating at frequencies of at least 120 Hz while rotating each compressor impeller at a rotational speed above 3600 rpm.

18. The method of claim 16 wherein the step of providing a chiller system further includes providing liquid injection into the suction side of the centrifugal compressor.

19. The method of claim 16 wherein the nonflammable naturally occurring organic compound has a vapor pressure of less than about 1 bar at 35°C.

20. The method of claim 16 wherein the nonflammable naturally occurring organic compound comprises an organic compound selected from the group consisting of methylene chloride and dichlorodifluoroethylene.

21. The centrifugal chiller system of claim 20 further including an HFO, and HFC or an HFCO in combination with the naturally occurring refrigerant, the combination forming a substantially azeotropic refrigerant blend.

22. The centrifugal chiller system of claim 21 wherein the HFO, HFC or the HFCO is at least one refrigerant selected from the group consisting of R- 1336mzz(Z), R-1233zd(E), R1336mzz(E), R-143, isomers of R-245, R- 245ea, R-245eb and R-245ca.