PERFORMANCE EVALUATION OF GAS COOLER/ CONDENSER FOR CO2 REFRIGERATION SYSTEMS

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1 PERFORMANCE EVALUATION OF GAS COOLER/ CONDENSER FOR CO2 REFRIGERATION SYSTEMS Yunting Ge (a) IDewa Santosa (a), Konstantinos Tsamos (a), Savvas Tassou (a) Nick Atkins (b), Simon Jones (b) (a) School of Engineering and Design, Brunel University, Uxbridge, Middlesex, UB8 3PH, UK, address: (b) GEA Searle, Product Development,

2 CONTENTS Background Test rig of CO2 refrigeration system Modeling and validation of CO2 gas cooler/condenser Sizing of CO2 gas cooler/condenser in refrigeration system Conclusions

3 BACKGROUND As a natural working fluid, CO2 has been widely used in refrigeration and heat pump systems. The high-side air cooled heat exchanger in a CO2 refrigeration can operate as either gas cooler or condenser depending on ambient conditions The optimal design of the CO2 gas cooler/condenser can greatly affect the performance of the refrigeration system and requires further investigation and optimisation.

4 TRANSCRITICAL CO 2 REFRIGERATION TEST FACILITIES

5 CO2 REFRIGERATION SYSTEM

6 Gas cooler specification GAS COOLER RIG Gas cooler test rig Length:1600 mm Air OFF Air ON Inlet header outlet header Height : 66 mm Fins Material : Aluminium Thickness : 0.16 mm Pithc: 14 Fins/inch Row wide: 32 Row deep: 3 Circuit: 4 Tube spacing: 25.4 and 22 Tubes Material : Copper Dia. : 8 mm Thickness : 1 mm

7 DETAILED INSTRUMENTATION AND MEASUREMENT OF GAS COOLER TEST RIG Air Outlet = Thermocouples position ΔP m Air OFF v Air ON Main Fan Ref. inlet header Gas cooler Ref. outlet header Heater m v v ΔP = Pressure transduser position = mass flow meter = velocity meter = Air pressure drop Air Inlet Ref. inlet Ref. outlet a. Scematic of gas cooler testing unit b. Position of themocouples on coil tubes in one circuit

8 MODELING OF CO2 GAS COOLER/CONDENSER (I: Distributed Method/Detailed Model) Refrigerant in Air Refrigerant out k i j= j K K 4 i= i j Z k 3 2 Z j 1 cross-section perpendicular to i Z i view from j direction

$supercritical Desuperheating region f sup Heat transfer area fraction f sup =1 Two phase region$

9 MODELING OF CO2 GAS COOLER/CONDENSER (II: Lumped Method/Simple Model) Condenser Model:three sections and refrigerant pressure is in subcritical Gas Cooler Model: one section and refrigerant pressure is in supercritical Desuperheating region f sup Heat transfer area fraction f sup =1 Two phase region Refrigerant Flow f tp Air Flow Refrigerant Flow Subcooling region f sub A Sample single circuit finned tube heat heat exchanger

10 MODEL EQUASTIONS FOR EACH SECTION OF CONDENSER AND GAS COOLER

11 MODEL VALIDATION-TWO TESTED HXS Inputs: Xin(1)=28 C Xin(2)=50% Xin(3)=0 C Xin(4)=0% Xin(5)=0% Xin(6)=2186 l/s Xin(7)=120 C Xin(8)=90bar Xin(9)=0.074kg/s Room air dry-bulb temperature Room air rel. humidity to air flow Fresh air dry-bulb temperature Fresh air relative humidity Percentage mass flow rate ratio of fresh air Total air mass flow rate Inlet refrigerant temperature Inlet refrigerant pressure Inlet refrigerant mass flow rate Refrigerant in Refrigerant out Inputs: Xin(1)=28 C Xin(2)=50% Xin(3)=0 C Xin(4)=0% Xin(5)=0% Xin(6)=2186 l/s Xin(7)=120 C Xin(8)=90bar Xin(9)=0.074kg/s Room air dry-bulb temperature Room air rel. humidity to air flow Fresh air dry-bulb temperature Fresh air relative humidity Percentage mass flow rate ratio of fresh air Total air mass flow rate Inlet refrigerant temperature Inlet refrigerant pressure Inlet refrigerant mass flow rate Refrigerant out Airflow Airflow Refrigerant in Parameters: Par(1)=3 Par(2)=3 Par(3)=32 Par(4)=4 Par(5)=1.6 m Par(6)=0.008 m Par(7)= m Par(8)=0.398 kw/m.k Par(9)= m Par(10)=453 fins/m Par(11)=0.236 kw/m.k Par(12)= m Par(13)=0.022 m Cross flow Total number of tubes parallel to air flow Total number of tubes perpendicular to air flow Number of pipe circuits Tube length Outside tube diameter Inside tube diameter Tube thermal conductivity Fin thickness Fin density Fin thermal conductivity Tube spacing perpendicular to air flow Tube spacing parallel to air flow Parameters: Par(1)=3 Par(2)=2 Par(3)=32 Par(4)=2 Par(5)=1.6 m Par(6)=0.008 m Par(7)= m Par(8)=0.398 kw/m.k Par(9)= m Par(10)=453 fins/m Par(11)=0.236 kw/m.k Par(12)= m Par(13)=0.022 m Cross flow Total number of tubes parallel to air flow Total number of tubes perpendicular to air flow Number of pipe circuits Tube length Outside tube diameter Inside tube diameter Tube thermal conductivity Fin thickness Fin density Fin thermal conductivity Tube spacing perpendicular to air flow Tube spacing parallel to air flow (Coil A-3 rows 4 circuits) Two tested gas cooler/condenser in the laboratory (Coil B-2 rows 2 circuit)

12 MODEL VALIDATION-TEST CONDITIONS HX Type Number of Cases Gas cooler mode Test ranges Tair_on Maflow Tref_in Pref_in Mref_in C l/s C bar kg/s 3 Rows 15 28~ ~ ~120 75~ ~ Rows 19 28~ ~ ~120 75~ ~0.042 HX Type Number of Cases Condenser mode Test ranges Tair_on Maflow Tref_in Pref_in Mref_in C l/s C bar kg/s 3 Rows 16 19~ ~ ~90 60~ ~ Rows 17 19~ ~ ~90 60~ ~0.040

13 MODEL VALIDATION-DETAILED MODEL Row 3 Row 2 Row 1 Row 2 Row 1 Comparison of simulation and test results for refrigerant temperature and heat release variation along pipe from refrigerant inlet to outlet (3-Row) Comparison of simulation and test results for refrigerant temperature and heat release variation along pipe from refrigerant inlet to outlet (2-Row)

14 MODEL VALIDATION-SIMPLE MODEL Comparison of prediction and measurement for CO2 gas cooler heat release Comparison of prediction and measurement for CO2 condenser heat release

15 SIZING OF CO2 GAS COOLER/CONDENSER IN A CO2 BOOSTER REFRIGERATION SYSTEM Control Specification of CO2 Booster Refrigeration System Compmonent Parameter Control Evaporating temperature 5⁰C Evaporator Superheating 10K Compressor Capacity Variable speed Gas cooler Supercritical pressure Constant 80~120 bar Subcritical pressure Floating t=10k /Condenser Gas cooler approach temperature 2K with Fan Speed Condenser subcooling 3K with Fan Speed Vessel Pressure bar ( 3⁰C Sat) Transcritical- Subcritical Cycle Transition ambient temperature 25±1⁰C CO 2 Vessel EC 326 Valve opening 25 ICMT Vessel pressure 32 T Gas cooler pressure Valve opening AKV-MT Temperature outlet of gas cooler Valve opening ICM AK-CC 550A Air off 100% -3 EKC 347 MT Cabinet VO2 Fan speed Gas cooler 40 T MT pressure MT pressure AK2-XM-205B VLT-2800 Comp. speed 1 HT Compressor 27 To LT Cabinet T = temperature sensor = pressure transducer From LT Cabinet &LT Compressor Tested CO2 booster refrigeration system and controls (HT cabinet and compressor only)

16 AIR FLOW RATES REQUIRED BY TWO SIZES OF COILS AT FIXED COOLING CAPACITY AND VARIOUS OPERATING STATES Variation of condenser air volumetric flow rate with ambient temperature at fixed system cooling capacity (10kW) Variation of condenser air volumetric flow rate with ambient temperature and refrigerant pressure at fixed system cooling capacity (10kW)

17 REFRIGERANT SIDE PRESSURE DROPS FOR TWO SIZES OF COIL AT FIXED COOLING CAPACITY AND VARIOUS OPERATING STATES Variation of condenser refrigerant drop with ambient temperature at fixed system cooling capacity (10kW) Variation of gas cooler refrigerant pressure drop with ambient temperature and refrigerant pressure at fixed system cooling capacity (10kW)

18 NUMBER OF COMPRESSOR AND COP FOR TWO SIZES OF COILS AT FIXED COOLING CAPACITY AND VARIOUS OPERATING STATES Variation of number of operational compressor and cooling COP with ambient temperature at fixed system cooling capacity (10kW) condenser mode Variation of number of operational compressors and coolong COP with ambient temperature and refrigerant pressure at fixed system cooling capacity (10kW)-GC mode

19 CONCLUSIONS The performance of gas cooler/condenser and its integration with the rest of the components were investigated experimentally in a CO2 booster refrigeration test rig. The CO2 gas cooler/condenser models were developed using both the distributed and lumped methods and were validates against test results The CO2 gas cooler/condenser model with simple mode is applied to evaluate the effect of coil sizes and pipe arrangements on system performance. It is found that to maintain a fixed system cooling capacity at various ambient conditions, a larger size is required for operation in the condenser mode. Refrigerant side pressure drop is another issue to be considered in coil optimal design and sizing.

Design optimisation of CO 2 gas cooler/condenser in a refrigeration system

Design optimisation of CO 2 gas cooler/condenser in a refrigeration system Y.T. Ge *, S.A. Tassou, I Dewa Santosa K. Tsamos RCUK National Centre for Sustainable Energy Use in Food Chains (CSEF) School