Adaptation of refrigeration compressors for Joule-Thomson cryocoolers fed with gas mixtures Maciej Chorowski Pawel Dorosz Agnieszka Piotrowska Wroclaw University of Technology, Poland I CEC 26 – ICMC 2016, March 8th, 2016

Outlines 1.Joule-Thomson coolers – open system 2.Joule-Thomson coolers supplied with gas mixture – closed system 3.Hermetic refrigeration compressors 4.Additional cooling of the compressor – technical solutions 5.Test results 6.Conclusions

Joule-Thomson coolers – open system 1.High working pressure, min. 70 bar for N 2, up to inversion pressure 2.Low thermodynamic efficiency 3.Limited working time (need of pressure tank replacement) 1.Simple construction 2.No moving parts working at low temperatures 3.Short start-up time

Joule-Thomson coolers – motivation of the system design J-T coolers Micro capacity below 1 W Low capacity up to 10 W Medium capacity up to 50 W Cooling power: CLOSED SYSTEM REQUIRED (reduction of working pressure) University of Twente, Netherlands Wroclaw University of Technology, Poland

Joule-Thomson cooler – closed system Working fluidNitrogenGas mixture Working pressure100 – 200 bar15 – 25 bar Temperature at 1 bar77.8 K80 – 120 K Phase change inside the heat exchanger NOYES Temperature difference at the cold end of HE 70 – 90 K5 – 15 K AZOT HEAT EXCHANGER AFTERCOOLER COMPRESSOR HEAT EXCH. JT VALVE NITROGEN MIXTURE

J-T cooler with hermetic piston compressor Refrigeration technology Cryogenics Working medium Refrigerants (both one component or mixtures) R 134a, R404a Pure hydrocarbons R290, R600a 4 -5-component mixtures of nitrogen and hydrocarbons (methane, ethane, propane and i-buthane) Pressure ratio limit 825 Temperature limit of outlet medium (discharge side) 150°C (to avoid thermal decomposition of oil) Inlet medium (cooling of the electric motor winding) Low temperature vapor of the refrigerant Gas mixture vapor at ambient temperature COMPRESSOR ELEMENTS ELECTRIC MOTOR

Compression process Poisson equation Nitrogen  Methane  Ethane  Propane  I-butane  Mixture 

Compressor test stand compressor aftercooler throttling valve

Temperature measurements of the compressor outer surface – nitrogen supplied nitrogen

5-components mixture Temperature measurements of the compressor outer surface – mixture supplied

Solutions of the compressor cooling methods 1.Compressor located in the stream of the after-cooler air - classic refrigeration technology v

Hermetic compressors Calculation results of the required compressor surface Compressors: Danfoss VTZ serie nitrogenmixture

2.Process modification (to decrease temperature of gas entering compressor ) – not effective enough 3.Enhancement of the compressor outer surface – from 15% up to 300% required – technically non feasible Solutions of the compressor cooling

4.Modification of the construction – assitional cooling of oil Temperature of the compressor shell – no additional cooling Temperature of the compressor shell – additional cooling Outlet temperature of gas – no additional cooling Outlet temperature of gas – additional cooling

J-T coolers designed, manufactured and tested at WUT REFRIGERATION SUB-SYSTEM CRYOGENIC SUB-SYSTEM CRYOGENIC SUB-SYSTEM REFRIGERATION SUB-SYSTEM

Cooling effect vs. temperature

COP of Joule-Thomson cooler designed at WUT J-T cooler Cooling effect, W Carnot efficiency Trend line

Test results of J-T cooler 1 – compressor inlet temperature, 1p – compressor inlet pressure, 2 – compressor outlet temperature, 2p – compressor outlet pressure, 4 – temp. before throtlting valve, 5 – temp. after throtlting valve, 6 – temp. after evaporator, 2N – temperature of nitrogen before throtlting valve, 3N – temperature of nitrogen after throtlting valve

Conclusions 1.Laboratory tests confirm the possibility of using hermetic refrigeration compressors for Joule-Thomson coolers supplied with gas mixtures. 2.Depending on the volumetric efficiency, hermetic refrigeration compressors can be used without any modifications (for low efficiencies), for high efficiencies the modifications are the must. 3.We have proposed and investigated additional compressor cooling based on external oil circulation – this solution is applicable to majority of the comercial hermatic compressors.