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Summer Research Fellowship Programme of India's Science Academies

Compressor selection model for CO2 based cascaded refrigeration system

Ali Asad

Department of Mechanical Engineerig, Aligarh Muslim University, Aligarh, UP 202002

Guided by:

Prof Pradip Dutta

Department of Mechanical Engineering, Indian Institute of Science, Bengaluru, Karnataka 560012

Prof K. Srinivasan

Department of Mechanical Engineering, Indian Institute of Science, Bengaluru, Karnataka 560012

Abstract

A cascaded refrigeration system is a very energy efficient method to achieve refrigeration at evaporator temperatures below -20 0C with an added advantage of getting refrigeration at different levels of evaporator temperatures. A comparative study is done on various refrigerants working with various compressor models with the help of compressor selection software for different operating conditions and the cooling capacities required. Cascading possibilities for different refrigerants are studied based on fixed cooling loads. Further, the viability of using a trans-critical and subcritical CO2 cascade is explored using different compressor selection matrices. All collected and calculated data is used to establish the most-suited model for compressor selection based on a user-defined criteria such as higher COP, volumetric and isentropic efficiencies or easier maintenance or lesser energy consumption and emission.

Keywords: Refrigerants, Cascaded Refrigeration System, Compressor selection, Sub-critical and Trans-crirtical CO2

Abbreviations

VCRSVapor Compression Refrigeration System
LTCLow Temperature Cycle
HTCHigh Temperature Cycle
RCompression Ratio
GWPGlobal Warming Potential
GHGGreen House Gas i.e. CO2

INTRODUCTION

Compressors are the heart of any refrigeration and air conditioning plant and justly the costliest component. Hence, a great deal of effort is put into the selection of the perfect compressor model for the given requirements. Most of the compressor manufacturers provide their selection software to accomplish this task. It is surprisingly noted that none of these manufactures provide parameters such as volumetric and isentropic efficiency which are important as well as decisive factors in compressor selection. Thus there is a need for development of a selection model which takes the above stated parameters into account.

Compression Cycle

Single stage compression cycle can be shown on p-v curve (​Fig 1​​). Process 4-1 is intake, 1-2 is compression and 2-3 is discharge. Specific volumes v3, (v1-v3), and (v1-v4) are clearance volume, swept volume and actual intake volume respectively. Also process 1-2 is isentropic, hence the compression ratio is given by

R=p2p1=(v1v2)γ R=\frac{p_2}{p_1}=(\frac{v_1}{v_2})^\gamma
comp.png
    p-v diagram of single stage compression process

    Volumetric Efficiency

    It is defined as the ratio of actual volume of gas intake to the idle intake volume or swept volume of the piston. Thus

    ηv=v1v4v1v3 \eta_v=\frac{v_1-v_4}{v_1-v_3}

    or

    ηv=1ε(R(1γ)1) \eta_v=1-\varepsilon(R^{(\frac1\gamma)}-1)

    where, ε= clearance ratio which is ratio of clearance volume to swept volume and is given by

    ε=vcvs=v3v1v3 \varepsilon=\frac{v_c}{v_s}=\frac{v_3}{v_1-v_3}

    Further, a correction is done to take into account for leakage of refrigerant into the crankcase. This correction takes into account a standard operating conditions and subsequent calculation gives correction for present operating condition.

    ηv=1ε(R(1γ)1)0.02C \eta_v=1-\varepsilon(R^{(\frac1\gamma)}-1)-0.02C

    Where

    C=Square root of pressure difference of evaporator and condenser or gas cooler at present operating conditionSquare root of pressure difference of evaporator and condenser or gas cooler at standard operating condition=PcPePcPe C=\frac{\text{Square root of pressure difference of evaporator and condens}\text{e}\text{r or gas cooler at present operating condition}}{\text{Square root of pressure difference of evaporator and }\text{condenser}\text{ or gas cooler at }\text{standard}\text{ operating con}\text{dition}}=\frac{\sqrt[{}]{P_c-P_e}}{\sqrt[{}]{{P'}_c-{P'}_e}}

    Vapor Compression Refrigeration System

    Thermodynamic analysis of the considered vapor compression refrigeration system can be done using the p-h curve (​​Fig 2​​),

    Process 1’-2’ is isentropic compression, process 2’-3 is condensation, process 3-4 is isenthalpic expansion, process 4-1 is useful evaporation and process 1-1’ is superheating before suction to insure only gaseous intake.

    j.png
      VCRS cycle 

      Isentropic Efficiency

      It is the ratio of idle compressor work to the actual compressor energy input.

      ηs=m˙a(h2h1)Actualworkinput(KW)×3600 \eta_s=\frac{{\dot m}_a(h_2-h_{1'})}{Actual\;work\;input(KW)\times3600}

      Where, ṁa=actual mass flow rate (Kg/h)

      h2,h1=specific enthalpies at points 2 and 1’ (KJ/Kg) h_2,h_{1'}=\text{specific enthalpies at points 2 and 1'}\text{ (KJ/Kg)}

      Cascaded refrigeration systems

      Cascaded refrigeration systems are frequently employed to achieve refrigeration at evaporator temperatures below -20 0C in a very energy efficient manner with the added advantage of getting refrigeration at different levels of evaporator temperatures. A cascaded system has two cycles namely Low Temperature Cycle (LTC) and High Temperature Cycle (HTC) running at the least evaporation temperature and the highest condenser or gas cooler exit temperature respectively. The evaporator temperature of HTC has to be lower than the condensing temperature of LTC so that heat transfer from condensor of LTC to evaporator of HTC can be facilitated. These cascaded systems can be categorized in two groups which are Inter Cascade and Intra Cascade systems.

      Inter Cascade Systems

      In this system, different refrigerants are used in the LTC and HTC and the thermodynamic cycle representing it is shown in ​Fig 3​​. There are different state curves for different refrigerants. Heat transfer takes from LTC refrigerant to HTC refrigerant so a refrigerant with higher heat capacity is favorable in HTC. Process 1-2 is isentropic compression prior to suction superheat in LTC, process 2-3 is condensation in LTC, process 3-4 is isenthalpic expansion in LTC, process 4-1 is evapoaration in LTC. And process 5-6 is isentropic compression in HTC, process 6-7 is condensation in HTC, process 7-8 is isenthalpic expansion in HTC and process 8-5 is evaporation in HTC.

      Intra Cascade Systems

      In this system, the same refrigerant is used in the LTC and HTC and the thermodynamic cycle representing it is shown in ​Fig 4​ along with each process. A single state curve is present representig a single refrigerant both in HTC and LTC.

      default
        P-h curve of two different refrigerants cascaded in LTC and HTC
        default
          P-h curve of same refrigerant (trans-critical and subcritical CO2) cascaded in LTC and HTC

          Emissions

          Taking into account the present concerns of global warming and climate change, it is necessary to have an idea of emisions from compressors. As compressors use refrigerants which pose dangers for our environment, their emisssions need to be calculated and checked. Also, compresssors require a high amount of electrical energy to run on, which may be from a polluting source such as thermal power plants.

          Direct Emissions

          These are emissions in the form of direct leakage of refrigerants from the the compressor system mostly from the cylinder through the sides of piston rings or from other loosely packed components. We usually consider about 3-5% of the total refrigerant charge to have leaked anually. Now, to compare this leaked refrigerant with GHG we multiply the leakage (in Ton) to GWP. The higher the GWP the more dangerous the refrigegrant is for the environment. Therefore, need is to develop and use refrigerants with lower GWP.

          Indirect Emissions

          These emissions occur due to the nature of the source of electrical power. A source is considered to be emitting a certain amount of GHG per KWh of energy that it produces. By convention, it is considered that for each KWh of electrical energy produced there is 1 Kg of CO2 (GHG) produced in developing countries like India. Hence, it is evident that to avoid such emissions we need to select a more energy efficient model of compressors for a given cooling load.

          METHODOLOGY

          Refrigerants considered: R134a, R404A, R407C, R410A

          Operating conditions:

          Operating Conditions
          Sr no.RefrigerantEvaporator temperaturesCondenser Temperatures
          1R134a-10, -5, 045, 40, 35
          2R404A-10, -5, 045, 40, 35
          3R407C-15, -10, -545, 40, 35
          4R410A-10, -5, 0, 545, 40, 35
          5R744 (Carbon Dioxide) subcritical-20-5, 0, 5
          6R744 (Carbon Dioxide) trans-critical0,545 (gas cooler exit temp at 110 bar)

          Suction superheat = 10 K

          Useful superheat = 0 K

          Operating power: 400V at 50Hz

          Compressor manufacturer studied: BITZER

          Models Considered:

          Compresoor  models considered

          Refrigerants:

          R134a

          R404A

          R407C

          R410A

          2 CYLINDERS

          2KES-05Y

          2KES-05Y

          2KES-05Y

          2JES-07Y

          2JES-07Y

          2JES-07Y

          2JES-07Y

          2DES-2Y

          2DES-2Y

          2DES-2Y

          2DES-2Y

          4 CYLINDERS

          4FES-3Y

          4FES-3Y

          4FES-3Y

          4FDC-5Y

          4EES-6Y

          4EES-6Y

          4EES-6Y

          4EDC-6Y

          4EDC-6Y

          4CES-9Y

          4CES-9Y

          4CES-9Y

          4DDC-7Y

          4DDC-7Y

          4VES-10Y

          4VES-10Y

          4VES-10Y

          4CDC-9Y

          4CDC-9Y

          4TES-12Y

          4TES-12Y

          4TES-12Y

          4VDC-10Y

          4VDC-10Y

          4PES-15Y

          4PES-15Y

          4PES-15Y

          4TDC-12Y

          4TDC-12Y

          4JE-22Y

          4JE-22Y

          4JE-22Y

          4PDC-15Y

          4PDC-15Y

          4HE-25Y

          4HE-25Y

          4HE-25Y

          4NDC -20Y

          4NDC -20Y

          6 CYLINDERS

          6JE-22Y

          6JE-25Y

          6JE-25Y

          6GE-40Y

          6GE-40Y

          6GE-40Y

          6GE-40Y

          6FE-50Y

          6FE-50Y

          6FE-50Y

          6FE-50Y

          8 CYLINDERS

          8GE-50Y

          8GE-60Y

          8GE-60Y

          8FE-70Y

          8FE-70Y

          8FE-70Y

          8FE-70Y
          Compressor models considered for carbon dioxide based refrigeration systems
          RefrigerantR744 (Carbon Dioxide) SubcriticalR744 (Carbon Dioxide) trans-critical
          2 CYLINDERS2NSL-05K2MTE-5K
          2MSL-07K2KTE-7K
          2KSL-1K-
          2JSL-2K-
          2HSL-3K-
          2GSL-3K-
          2FSL-4K-
          2ESL-4K-
          2DSL-5K-
          2CSL-6K-
          4 CYLINDERS4FSL-7K4PTC-7K
          4ESL-9K4MTC-10K
          4DSL-10K4JTC-15K
          4CSL-12K4HTC-20K
          4VSL-15K4FTC-30K
          4TSL-20K-
          4PSL-25K-
          4NSL-30K-
          6 CYLINDERS-6FTE-50K
          -6DTE-50K

          1. Thermodynamic properties such as pressure (bar), density (Kg/m3), enthalpy (KJ/Kg) and entropy (KJ/Kg-K) at various operating conditions of evaporator temperature and condenser temperatures for refrigerants R134a, R404A, R407C, R410A and R744 (CO2) were taken from the REFPROP[1] software provided by NIST.

          2. BITZER (compressor manufacturer’s) selection software[2] was run and measures of displacement volume (m3/h), cooling capacity (KW), compressor work (KW), condenser capacity (KW), discharge temperature (0C) and mass flow rate (Kg/h) were taken on a spreadsheet in Microsoft Excel[3].

          3. Using the above collected data, compression ratio (R), idle mass flow rate (Kg/h), idle compressor work (KW), volumetric efficiency (ηv), isentropic efficiency (ηs), idle COP and actual COP were calculated.

          4. The results were clubbed on the basis of number of cylinders of the compressor for each refrigerant and graphs between efficiencies and compression ratio for each refrigerant were plotted.

          5. Plots between volumetric efficiency and compression ratio as well as volumetric efficiency and compression ratio for all refrigerants for same compressor models or compressors with comparable displacement volume were plotted.

          6. Emissions both direct and indirect were calculated considering an average 8 hours of daily usage. GWP value for each refrigerant is used to compare the direct emission with that of the GHG.

          7. Possibilities of cascaded refrigerant cycles were analyzed for maximum COP, based on criteria of operating conditions and minimum volumetric efficiency of 80%. Both cascades, among the same refrigerant (intra cascade) and among different refrigerants (inter cascade), were analyzed.

          8. Cascading was done based on fixed cooling capacity of LTC and HTC as well as by varying the number of compressor units required when running at a fixed operating condition which is, evaporation and condesnation temperatures of -200C and 100C respectively for LTC and evaporator and gas exit temperature at 110 bar of 50C and 450C respectively for HTC. Subcritical CO2 in LTC and trans-critical CO2 in HTC were considered. Trans-critical compressor models were used, working in subcritical mode for LTC and trans-critical mode for HTC. After this, parameters such as isentropic and volumetric efficiencies, COP for both HTC and LTC, total number of compressor units and total power consumption are compared.

          g1.PNG
            Isentropic and volumetric efficiencies vs Compression ratio for R134a
            g2.PNG
              Isentropic and volumetric efficiencies vs Compression ratio for R404A
              g3.PNG
                Isentropic and volumetric efficiencies vs Compression ratio for R407C
                g4_1.PNG
                  Isentropic and volumetric efficiencies vs Compression ratio for R410A
                  G8.PNG
                    Isentropic and volumetric efficiencies vs Compression ratio for R744
                    g5_1.PNG
                      Isentropic efficiency vs Compression ratio for various refrigerants for compressors with comparable swept volume
                      g6_1.PNG
                        Volumetric efficiency vs Compression ratio for various refrigerants for compressors with comparable swept volume
                        g7_1.PNG
                          Coefficient of Performance COP for various refrigerants for compressors with comparable swept volume
                          g9.PNG
                            Selection options based on total power consumed
                            g10.PNG
                              limits of operation for various refrigerants

                              RESULTS AND DISCUSSION

                              1. The graphs plotted ( Fig 5-9​​) confirmed to the usual characteristics of the theoretical curves i.e. volumetric efficiency decreased with compression ratio and isentropic efficiency first increased, reached some maxima and then decreased with compression ratio. These results ensured the validity of the data collected.

                              2. Amongst all cylinder variants, 4-cylinder compressors show a higher volumetric as well as isentropic efficiency for all refrigerants for a given compression ratio.

                              3. For the same compressor model or compressors with comparable displacement volume, R410A gives maximum isentropic efficiency for a given compression ratio followed by R404A, R134a and R407C as shown in ​​Fig 10​​.

                              4. For the same compressor model or compressors with comparable displacement volume R134a gives maximum volumetric efficiency for a given compression ratio followed by R404A, R410A and R407C as shown in ​​Fig 11​​.

                              5. For a fixed compressor model or compressors with comparable displacement volume and at a fixed operating condition of Tcond = 45 0C and Tevap = -10 0C. The COP for R134a is the maximum followed by R407C, R410A and R404A which has minimum COP among all compared refrigerants. COPs of R134a and R407C are almost equal as shown in ​​Fig 12​​.

                              6. A cooling capacity of 250W was fixed and compressor work for a given operating condition of Tcond = 45 0C and Tevap = -10 0C for each compressor model and each refrigerant was calculated and it was found that for all refrigerants the 4-cylinder variant of compressors have the least compressor work. Also, R134a required the least compressor work of only 103.88 KW whereas R404A required the highest compressor work of 117.12 KW for each of their respective optimum compressor variants.

                              7. Carbon Dioxide or Green House Gas (GHG) emissions both direct and indirect were calculated for each refrigerant operating at the stated operating conditions. As indirect emission is in direct relation with power consumption for unit cooling capacity thus, R404 with the least COP has the highest indirect carbon dioxide (GHG) emission. Also with GWP of 3922, R404A has the highest direct emission followed by R410A (GWP = 2088), R407C (GWP = 1774) and R134a (GWP = 1430)[4]. As GWP of CO2 is 1, its direct emission poses the least concern.

                              8. Both inter cascading and intra cascading of refrigerants other than CO2 is not viable. In case of intra cascading, these refrigerants do not provide a wider opearting range of evaporator and condensation temperatures (as shown in ​​Fig 14​​) and might require extra motor windings at such conditions. While in the case of inter cascading, there is a maintenance issue as all these refrigerants have similar kind of packaging. It becomes very confusing for the prevailing semi-skilled maintenace personnel and if by any chance there is a mixup the whole plant would be at risk of failure. In such cases CO2 has an advantage as it has a completely different packaging which is easily distinguished from others.

                              9. A cascade of subcritical CO2 in LTC and other considered refrigerants in HTC using compressors of comparable swept volume resulted in R134a in HTC taking up the least total power for a given combined cooling load whereas R410A in HTC required the maximum power. Thus R134a in HTC should be cascaded with CO2 in LTC in order to obtain an energy efficient system.

                              10. The cascade of subcritical CO2 in LTC and trans-critical CO2 in HTC and the conditions mentioned in point no 8 of the methodology, we obtain the following results-

                              • For a combined cooling load of 300 KW (50 KW in LTC and 250 KW in HTC) and total space of constraint of 8 compressor units, least power consumption of 196.48 KW is obtained when four 4MTC-7K compressor models in LTC and four 6DTE-60K compressor models in HTC are used.
                              • For a combined cooling load of 491 KW (81 KW in LTC and 410 KW in HTC[5]) and total space of contraint of 14 compressor units, least power consumption of 328.08 KW is obtained when six 2KTE-5K compressor models in LTC and eight 6FTE-50K compressor models in HTC are used as shown in ​Fig 13​.

                              11. The results obtained in point no 10 provide a selection model based on the least power consumption. This can be easily extended for a selection of compressors based on many other parameters already calculated in the spreadsheet such as volumetric and isentropic effiencies, COP, number of compressor units and also cooling loads can be varied. All these variations result in a matrix which may provide a basis for selection based on one's own requirements.

                              CONCLUSIONS

                              • With the present environmental scenario, it is almost certain that the refrigerants currently being used will have to be replaced with a greener alternative and governments all around the globe are enforcing laws to replace the considered refrigerants.
                              • Carbon Dioxide posesses properties well suited to be an alternate refrigerant. It is non toxic and has a GWP value of 1 as compared to that of the considered refrigerants whose GWP value is of the order of 1000. It also can be exploited in its transcritical state for HTC of an intra cascaded refrigeration system.
                              • Provision of a selection model which gives a matrix of criteria for selection of the desired set of compressor models for both HTC and LTC is of great help. Users can refer to software based on such models and can easily verify the selections done by the manufacturer's software or they can modify it according to their specific requirements.

                              FUTURE SCOPE

                              • Only a single manufacturer is studied in the present project. The selection model should be made robust by including more data from various manufactures.
                              • Not much variation of operating conditions is studied which can be done in future. This would again help to enrich the model with added data sets for a variety of operating conditions and a more stringent selection can be made.
                              • The model can be used to develop an interactive software where users can enter data from manufacturers' selection software and get a comparative result of all parameters which can then further be considered for selection.

                              REFERENCES

                              https://www.nist.gov/srd/refprop NIST Standard Reference Database 23, Version 9.0
                              https://www.bitzer.de/websoftware/Default.aspx?cid=1530774758485
                              Microsoft Excel 2016, userid: syedaliasad2010@gmail.com

                              ACKNOWLEDGEMENTS

                              I would like to extend my heartiest gratitude to respected Prof. Pradip Dutta, Department of Mechanical Engineering, IISc. Bangalore and Prof K. Srinivasan, Department of Mechanical Engineering, IISc. Bangalore for their sizable subvention and visionary guidance at each and every step of the project. It was an extremely enlightening experience to work under truly devoted and ingenious scholars at one of the most reputed institutes of India.

                              Also, I would be hideous if I forget to express my gratitude to all the colleagues whom I interacted with, in these 8 weeks. The thought of approaching the end of this small beautiful journey is filling my heart with sorrow. I would have liked to gain more from all the aforementioned delightful personnel. It would be grateful of me to lend my hand in future for any further task in the project and even experiment on it.

                              I also thank Academies of Science for their support throughout the programme and for providing me with such a golden opportunity. It would have not been possible without this program.

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