REFRIGERANT COMPOSITION COMPRISING 1,1,2,2-TETRAFLUOROETHANE

Abstract: A composition comprising 1,1,2,2-tetrafluoroethane, useful as a refrigerant, particularly in small turbine compressors, is described. Improved efficiency and lower global warming potential are obtained when 1,1,2,2-tetrafluoroethane is used in small turbine centrifugal compressors.

Description: REFRIGERANT COMPOSITION COMPRISING 1 , 1,2,2-TETRAFLUOROETHANE FIELD OF INVENTION This invention relates to the use of 1 , 1 ,2,2-tetrafluoroethane and more particularly to the use of 1, 1,2,2-tetrafluoroethane as a refrigerant, aerosol propellant, cleaning agent, heat transfer medium, gaseous dielectric, fire extinguishing agent, and or a power cycle working fluid. More particularly, this invention relates to the use of 1,1,2,2- tetrafluoroethane as a highly effective and potentially environmentally safe refrigerant in refrigeration equipment that use centrifugal compression and in particular high speed small turbine centrifugal compression. BACKGROUND OF THE INVENTION Mechanical refrigeration is primarily an application of thermodynamics wherein a cooling medium, such as a refrigerant, goes through a cycle so that it can be recovered for reuse. Commonly used cycles include vapor-compression, absoφtion, steam-jet or steam-ejector, and air. The equipment used in a vapor-compression cycle includes an evaporator, a compressor, a condenser, a liquid storage receiver and an expansion valve. Liquid refrigerant enters the evaporator through an expansion valve, and the liquid refrigerant boils in the evaporator at a low temperature to form a gas to produce cooling. The low pressure gas enters a compressor where the gas is compressed to raise its pressure and temperature. The high pressure gaseous refrigerant then enters the condenser in which the refrigerant condenses and discharges its heat to the environment. A receiver collects the condensed high pressure liquid refrigerant, and the refrigerant goes to the expansion valve through which the liquid expands from the high pressure level in the condenser to the low pressure level in the evaporator. There are various types of compressors that may be used in refrigeration applications. Compressors can be generally classified as reciprocating, rotary, jet, centrifugal, or axial-flow, depending on the mechanical means to compress the fluid, or as positive-displacement or dynamic, depending on how the mechanical elements act on the fluid to be compressed. A centrifugal compressor uses rotating elements to accelerate the refrigerant radially, and typically includes an impeller and diffuser housed in a casing. Centrifugal compressors usually take fluid in at an impeller eye, or central inlet of a circulating impeller, and accelerate it radially outwardly. Some static pressure rise occurs in the impeller, but most ofthe pressure rise occurs in the diffuser section ofthe casing, where velocity is converted to static pressure. Each impeller-diffuser set is a stage ofthe compressor. Centrifugal compressors are built with from 1 to 12 or more stages, depending on the final pressure desired and the volume of refrigerant to be handled. The pressure ratio, or compression ratio, of a compressor is the ratio of absolute discharge pressure to the absolute inlet pressure. Pressure delivered by a centrifugal compressor is practically constant over a relatively wide range of capacities. Positive displacement compressors draw vapor into a chamber, and the chamber decreases in volume to compress the vapor. After being compressed, the vapor is forced from the chamber by further decreasing the volume ofthe chamber to zero or nearly zero. A positive displacement compressor can build up a pressure which is limited only by the volumetric efficiency and the strength ofthe parts to withstand the pressure. Unlike a positive displacement compressor, a centrifugal compressor depends entirely on the centrifugal force ofthe high speed impeller to compress the vapor passing through the impeller. There is no positive displacement, but rather what is called dynamic-compression. The pressure a centrifugal compressor can develop depends on the tip speed ofthe impeller. Tip speed is the speed ofthe impeller measured at its tip and is related to the diameter ofthe impeller and its revolutions per minute. The capacity ofthe centrifugal compressor is determined by the size ofthe passages through the impeller. This makes the size ofthe compressor more dependent on the pressure required than the capacity. Because of its high speed operation, a centrifugal compressor is fundamentally a high volume, low pressure machine. A centrifugal compressor works best with a low pressure refrigerant, such as trichlorofluoromethane (CFC-11) or 1,1,2- trichlorotrifluoroethane (CFC-113). Systems which require smaller equipment size often use chloro- 1,2,2-difluoromethane (CFC-12). Large centrifugal compressors typically operate at 3000 to 7000 revolutions per minute (φm). Small turbine centrifugal compressors are designed for high speeds, from about 40,000 to about 90,000 (φm), and have small impeller sizes, typically less than 0.15 meters. A two-stage impeller is common for many conditions. In operation, the discharge ofthe first stage impeller goes to the suction intake of a second impeller. Each stage can build up a compression ratio of about 4 to 1, that is, the absolute discharge pressure can be four times the absolute suction pressure. A proposed world- wide reduction in the production of fully halogenated chlorofluorocarbons such as CFC-11, CFC-12, and CFC-113 has developed a need for alternative, more environmentally acceptable products. Primarily, movement from CFC-12 has been toward 1,1,1,2-tetrafluoroethane (HFC- 134a). HFC- 134a has zero ozone depletion potential and lower global warming potential than CFC-12. SUMMARY OF THE INVENTION Accordingly, this invention relates to a refrigerant that may be used in centrifugal compressors, and particularly compressors designed for the refrigerant 1,1,1,2- tetrafluoroethane (HFC- 134a). This invention also relates to a refrigerant that has a lower global warming potential than HFC- 134a. Suφrisingly and unexpectedly it was found that the advantages and improvements discussed above, and others, are achieved by the use of a refrigerant containing 1, 1,2,2-tetrafluoroethane (HFC-134). It was found that HFC-134 can be used as a refrigerant in centrifugal compression refrigeration equipment designed for HFC- 134a while achieving improved operating performance versus HFC- 134a. The present invention is also particularly useful in small high speed turbine centrifugal compressors used in automobile and window air conditioning, heat pumps, as well as other applications. The present invention further relates to the discovery that 1,1,2,2- tetrafluoroethane may be used as an aerosol propellant, a cleaning agent, a heat transfer media, a gaseous dielectric, a fire extinguishing agent, and as a power cycle working fluid. DETAILED DESCRIPTION The present invention relates to the use of 1,1,2, 2-tetrafluoroethane (HFC- 134) as a refrigerant for use in centrifugal compression refrigeration equipment. As early as the 1970s with the initial emergence of a theory that the ozone was being depleted by chlorine atoms introduced to the atmosphere from the release of fully halogenated chlorofluorocarbons, it was known that the introduction of hydrogen into previously fully halogenated chlorofluorocarbons markedly reduced the chemical stability of these compounds. Hence, these now destabilized compounds would be expected to degrade in the atmosphere and not reach the stratosphere and the ozone layer Ozone Depletion Potential (ODP) is based on the ratio ofthe calculated ozone depletion in the stratosphere resulting from the emission of a compound compared to the ozone depletion potential resulting from the same rate of emission of CFC-1 1, which is set at 1 0. HFC- 134 does not contain any chlorine or bromine and therefore has an Ozone Depletion Potential (ODP) of 0 as compared with CFC-12 at 1 0 HFC-134 also has a shorter atmospheric lifetime than HFC- 134a and lower global warming potential referenced to CO2 as shown in Table 1 below TABLE 1 Lifetime (yrs.) GWP (500yr) HFC-134 11 9 370 HFC- 134a 14.0 420 There are three important considerations in selecting or designing a centrifugal compressor (a) the diameter ofthe impeller, which means the length from the end of one ofthe impeller blades to the end of an opposite blade, (b) the width ofthe passage in the impeller, and (c) the refrigerant The impeller and refrigerant must be selected in a combination that best suits a desired application The diameter ofthe impeller depends on the discharge pressure that must be achieved For a given rotative speed, a large impeller diameter provides a higher tip speed, which results in a higher pressure ratio. Tip speed means the tangential velocity of the refrigerant leaving the impeller. If a small turbine centrifugal compressor is driven by an electric motor operating at 40,000 φm, the impeller diameter needed for the 165.1 m/s tip speed of HFC-134a is about 0.0788 meters It is desirable to find a "close match" replacement for HFC 134a. By "close match", it is meant a refrigerant that may be used in equipment designed for HFC- 134a or that performs similarly to HFC- 134a To perform as well as HFC- 134a, a refrigerant must be such that when it is used, the impeller achieves a tip speed that is comparable to the tip speed of the impeller when HFC- 134a is used HFC- 134 provides a tip speed comparable to the tip speed of HFC- 134a when the refrigerants are used at the same operating conditions The liquid density ofthe refrigerant is another important design characteristic The liquid densities of HFC-134 and HFC-134a are 1.245 g/cc and 1 387 g/cc respectively at room temperature Also, the molecular weights of HFC- 134 and HFC- 134a, another important characteristic, are identical because they are structural isomers Suφrising and unexpectedly, it was found that HFC- 134 is a significantly more energy-efficient refrigerant than HFC- 134a By "significantly more" it is meant having improved efficiency of at least about 4% EXAMPLE 1 Tip Speed to Develop Pressure Tip speed can be estimated by making some fundamental relationships for refrigeration equipment that use centrifugal compressors. The torque an impeller ideally imparts to a gas is defined as T = m*(v2*r2-vl *rl) Equation 1 where T = torque, N*m m = mass rate of flow, kg/s v2 = tangential velocity of refrigerant leaving impeller, m/s r2 = radius of exit impeller, m vl = tangential velocity of refrigerant entering impeller, m/s rl = radius of inlet of impeller, m Assuming the refrigerant enters the impeller in an essentially radial direction, the tangential component ofthe velocity vl = 0, therefore T = m*v2*r2 Equation 2 The power required at the shaft is the product ofthe torque and the rotative speed P = T*w Equation 3 where P = power, W w = rotative speed, rev/s therefore, P = T*w = m*v2*r2*w Equation 4 At low refrigerant flow rates, the tip speed ofthe impeller and the tangential velocity ofthe refrigerant are nearly identical; therefore r2*w = v2 Equation 5 and P = m*v2*v2 Equation 6 Another expression for ideal power is the product ofthe mass rate of flow and the isentropic work of compression, P = m*Hi*( 1 OOOJ/kJ) Equation 7 where Hi = Difference in enthalpy ofthe refrigerant from a saturated vapor at the evaporating conditions to saturated condensing conditions, kJ/kg. Combining the two expressions Equation 6 and 7 produces, v2*v2 = 1000*Hi Equation 8 Although Equation 8 is based on some fundamental assumptions, it provides a good estimate ofthe tip speed ofthe impeller and provides an important way to compare tip speeds of refrigerants. Table 2 shows theoretical tip speeds that are calculated for HFC- 134, HFC- 134a and ammonia. The conditions assumed for this comparison are: Evaporator temperature 40.0°F (4.4°C) Condenser temperature 110.0°F (43.3 °C) Liquid subcool temperature 10.0°F (5.5°C) Return gas temperature 75.0°F (23.8°C) Compressor efficiency is 70% These are typical conditions under which small turbine centrifugal compressors perform. TABLE 2 Impeller Diameter Calculations at 40,000 φm Impell. Impell. Hi Hi*0.7 Hi*0.7 V2 Diam. Diam. (Btu/lb) (Btu lb) (KJ/kg) (m/s) (m) (in) HFC- 134 17.73 12.41 28.79 169.8 0.0810 3.19 HFC- 134a 16.79 11.75 27.26 165.1 0.0788 3.10 Ammonia 119.4 83.58 193.9 440.3 0.2102 8.28 Example 1 shows that HFC- 134 has an impeller diameter within about 3% of HFC- 134a. If another refrigerant such as ammonia were used in the equipment designed for HFC- 134a, the equipment would require an impeller diameter of 0.2102 meters. Therefore, ammonia could not be used in equipment designed for HFC- 134a because the impeller diameter of that equipment would need to increase to 0.2102 meters for the equipment to perform as well with ammonia as it performs with HFC- 134a. EXAMPLE 2 Refrigerant Performance Table 3 shows the performance of HFC- 134 and HFC- 134a at the following conditions: Evaporator temperature 40.0°F (4.4°C) Condenser temperature 110.0°F (43.3°C) Subcool temperature 10.0°F (5.5°C) Return gas temperature 75.0°F (23.8°C) Compressor efficiency is 70% Coefficient of Performance (COP) is intended to mean the ratio ofthe capacity to compressor work. It is a measure of refrigerant energy efficiency. TABLE 3 Evaporator Condenser Compressor Capacity Pressure Pressure Discharge BTU/Min Psia (kPa) Psia (kPa) Temp °F (°C) COP (kW) HFC-134 38.4 265 129.1 890 173.0 78.3 4.01 161.1 2.8 HFC-134a 49.7 343 161.1 1111 171.3 77.4 3.87 194.1 3.4 The data above show HFC- 134 is a more efficient refrigerant than HFC- 134a. The compound ofthe present invention could also be used as a cleaning agent, aerosol propellant, heat transfer medium, gaseous dielectric, fire extinguishing agent, displacement drying agent, and power cycle working fluid. A process for cleaning a solid surface includes treating said surface with an effective amount of 1 , 1 ,2,2-tetrafluoroethane. A process for preparing aerosol formulations includes combining active ingredients in an aerosol container with an effective amount of 1,1,2,2-tetrafluoroethane. A process for atomizing a fluid includes a step of using an effective amount of 1,1,2,2-tetrafluoroethane as an aerosol propellant. A process for transferring heat from a heat source to a heat sink using an effective amount of 1,1,2,2-tetrafluoroethane as a heat transfer medium. A process for electrically insulating includes a step of using an effective amount of 1,1,2,2-tetrafluoroethane as a gaseous dielectric. A process for suppressing a fire includes a step of using an effective amount of 1,1,2,2-tetrafluoroethane as a fire extinguishant. A process for delivering power includes a step of using an effective amount of 1,1,2,2-tetrafluoroethane as a power cycle working fluid. By "effective amount" it is meant the amount of HFC- 134 required to produce an efficient refrigerant. Generally about 2.5 lbs. to 80 lbs. of refrigerant is required for three ton to one hundred ton refrigerating units, respectively. ADDITIONAL COMPOUNDS Additives such as lubricants, corrosion inhibitors, surfactants, stabilizers, dyes and other appropriate materials may be added to the compositions ofthe invention for a variety of puφoses provided they do not have an adverse influence on the composition for its intended application.

Claims: CLAIMS What is claimed is: 1. A composition for use with a centrifugal compressor, said composition comprising an effective amount of 1 , 1,2,2-tetrafluoroethane. 2. The composition of Claim 1 wherein said composition is used as a refrigerant. 3. A refrigerant for use with a centrifugal compressor comprising 1,1,2,2- tetrafluoroethane. 4. The composition of Claim 1 wherein the compressor is a small turbine centrifugal compressor. 5. The composition of Claim 2 wherein the compressor is a small turbine centrifugal compressor.

Inventors: MINOR BARBARA HAVILAND, MINOR, BARBARA, HAVILAND
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1997
Main classification: C09K5
IPC codes: C09K3, C09K5, C11D7, F04D29