# Compressor selection model for CO2 based cascaded refrigeration system

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

Guided by:

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

 VCRS Vapor Compression Refrigeration System LTC Low Temperature Cycle HTC High Temperature Cycle R Compression Ratio GWP Global Warming Potential GHG Green 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

$R=\frac{p_2}{p_1}=(\frac{v_1}{v_2})^\gamma$
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

$\eta_v=\frac{v_1-v_4}{v_1-v_3}$

or

$\eta_v=1-\varepsilon(R^{(\frac1\gamma)}-1)$

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

$\varepsilon=\frac{v_c}{v_s}=\frac{v_3}{v_1-v_3}$

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.

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

Where

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

## 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.

VCRS cycle

## Isentropic Efficiency

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

$\eta_s=\frac{{\dot m}_a(h_2-h_{1'})}{Actual\;work\;input(KW)\times3600}$

Where, ${\stackrel{̇}{m}}_{a}=\text{actual mass flow rate (Kg}\text{/}\text{h}\text{)}$

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

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.

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.

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.

P-h curve of two different refrigerants cascaded in LTC and HTC
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. Refrigerant Evaporator temperatures Condenser Temperatures 1 R134a -10, -5, 0 45, 40, 35 2 R404A -10, -5, 0 45, 40, 35 3 R407C -15, -10, -5 45, 40, 35 4 R410A -10, -5, 0, 5 45, 40, 35 5 R744 (Carbon Dioxide) subcritical -20 -5, 0, 5 6 R744 (Carbon Dioxide) trans-critical 0,5 45 (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
 Refrigerant R744 (Carbon Dioxide) Subcritical R744 (Carbon Dioxide) trans-critical 2 CYLINDERS 2NSL-05K 2MTE-5K 2MSL-07K 2KTE-7K 2KSL-1K - 2JSL-2K - 2HSL-3K - 2GSL-3K - 2FSL-4K - 2ESL-4K - 2DSL-5K - 2CSL-6K - 4 CYLINDERS 4FSL-7K 4PTC-7K 4ESL-9K 4MTC-10K 4DSL-10K 4JTC-15K 4CSL-12K 4HTC-20K 4VSL-15K 4FTC-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.

Isentropic and volumetric efficiencies vs Compression ratio for R134a
Isentropic and volumetric efficiencies vs Compression ratio for R404A
Isentropic and volumetric efficiencies vs Compression ratio for R407C
Isentropic and volumetric efficiencies vs Compression ratio for R410A
Isentropic and volumetric efficiencies vs Compression ratio for R744
Isentropic efficiency vs Compression ratio for various refrigerants for compressors with comparable swept volume
Volumetric efficiency vs Compression ratio for various refrigerants for compressors with comparable swept volume
Coefficient of Performance COP for various refrigerants for compressors with comparable swept volume
Selection options based on total power consumed
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