TRACI, A MULTIPURPOSE CO 2 COOLING SYSTEM FOR R&D

Transcription

1 Paper No: GL-208 RACI, A MULIPURPOSE CO 2 COOLING SYSEM FOR R&D B. Verlaat(a,b), L. Zwalinski(b), R. Dumps(b), M. Ostrega(b,c), P. Petagna(b) and. Szwarc(b,c) (a) National Institute for Subatomic Physics (NIKHEF) Amsterdam, NL 1098 XG 105, Netherlands, bverlaat@nikhef.nl (b)european Organization for Nuclear Research (CERN) CH 1211 Geneva 23, Switzerland, Lukasz.Zwalinski@cern.ch (c) adeusz Kościuszko, Cracow University of echnology Warszawska 24 St Cracow, Poland, omasz.szwarc@cern.ch ABSRAC CO 2 cooling is gaining interest as a cooling method for scientific equipment such as particle detectors. he merit of using CO 2 cooling is a high heat transfer capability resulting in small and efficient cooling channels. o support the many CO 2 research projects at CERN and its cooperating institutes, a multipurpose CO 2 cooling system named RACI (ransportable Refrigeration Apparatus for CO 2 Investigation) is under development. RACI is optimized for reduced costs and easy operation while keeping maximal flexibility. A new concept named I-2PACL (Integrated 2 Phase Accumulator Controlled Loop) is developed to minimize the piping and control, while maximizing the operational range for use. Several prototypes have been developed. he first results show that the I-2PACL method works well. An operational range of -35 C to +22 C was measured and a cooling capacity up to 250 watt was reached depending on the cooling temperature. A stable operation was observed with minimum control. wo prototypes have been handed over to clients and the operation of the system turned out to be easy and understandable by the non-expert users. his paper describes the design and test results of the current prototypes and will discuss the plans for the future. 1. INRODUCION High energy physics particle detectors like the ones constructed for the Large Hadron Collider (LHC) at CERN in Geneva (CERN 2009) need lightweight cooling systems to cool the wide spread silicon sensors and the attached read-out electronics. he cooling hardware inside the detector must be radiation hard and low mass as it may not obstruct the particle tracks. he cooling temperature is in general between -20ºC and - 40ºC as the silicon must be kept cold in general below -10ºC to avoid the outcome of radiation damage of the silicon sensors. Applications of room temperature cooling are also present in specific detector technologies. he current LHC detectors use mainly Fluorocarbon cooling systems both in single and 2-phase (Postema et. al. 2011); one detector was designed for evaporative CO 2 cooling. Both CO 2 and fluorocarbons are radiation hard, conventional refrigerants are not suitable in a radiation environment. CO 2 cooling has very good physical properties which allow small diameter tubing. With respect to the traditional Fluorocarbon systems the cooling system mass can be reduced by about 80% (Verlaat et. al. 2011). he successful demonstration of CO 2 cooling in the LHCb-Velo detector and the ability of saving a considerable amount of applied mass has convinced the particle physics community seriously to investigate CO 2 cooling as candidate for the future detector cooling. Almost all the new silicon detectors consider now CO 2 cooling as the baseline for their detector structures. New detectors considering CO 2 cooling are (Verlaat et al. 2011): the CMS upgrade pixel, the CMS racker upgrade, the Atlas IBL, the Atlas inner detector upgrade, and the KEKb Belle-2. 1

2 Paper No: GL CO 2 COOLING RESEARCH New silicon detector development is focused on light weight thermal mechanical support structures to support the silicon sensors and the attached read out electronics. In general these are carbon based structures optimized for a good mechanical stability. hese structures need to accurately place the sensors in the 3D space of the detector. Mechanical stiffness, low mass and low thermal expansion are the important design parameters. hese support structures also need to conduct the heat from the electronics into embedded cooling structures. So the materials must have a high thermal conduction relative to their density. Popular materials are carbon foams and high conductive carbon fibers. Cooling pipes are often made from titanium as this metal has a low CE and density compared to other metals. A typical silicon detector thermo mechanical structure is shown in figure 1. Developments of silicon detector support structures are ongoing in many high energy physics laboratories all over the world. he heat transfer optimization to the evaporative CO 2 cooling in the cooling channels is an important research topic as the cooling channel must be designed as small as possible. A detector is composed in general of many similar substructures. ypical powers of these substructures are maximum a few hundred watts. o support the CO 2 cooling developments, a multipurpose easy-touse CO 2 system is needed to provide the high energy physics laboratories CO 2 cooling for their research. A system called RACI (ransportable Refrigeration Apparatus for CO 2 Investigation) is under development at CERN together with Nikhef. he design goal of RACI is a mobile CO 2 cooling station which is easy to operate and which can be produced in series by outside labs or companies for an affordable prize. RACI must give the user an evaporative CO 2 flow from room temperature down to -40 C for a cooling power up to a few hundred watts. hese requirements are in general enough to support the development of the detector substructures. Larger cooling plants are needed in the future to test assembled detector structures. 3. RACI I-2PACL CONCEP Figure 1: A typical silicon detector support structure, carbon fibers with carbon foam and a titanium cooling pipe (photo SLAC). he CO 2 cooling systems developed for high energy physics experiments so-far use a concept called 2PACL (2-Phase Accumulator Controlled Loop). his method is compliant with the requirements of a typical silicon detector and is used successfully in the LHCb experiment at CERN (Verlaat, 2008). A 2PACL is a pumped 2-phase loop where the saturation temperature is controlled in an accumulator filled with liquid and vapor. he pressure in the accumulator is controlled by cooling or heating the liquid vapor mixture. he circulating CO 2 in the loop is condensed and sub cooled in a heat exchanger cooled by an external cold source which is normally a commercial HFC chiller. In the RACI I-2PACL the cooling of the accumulator is achieved internally by the CO 2 liquid. All the liquid is pumped through a heat exchanger in the accumulator and is heated to the saturation temperature of the CO 2 in the accumulator. his approach is different than a standard 2PACL where the liquid is heated by an internal heat exchanger between the inlet and outlet of the connected evaporator. he integration of the heat exchanger into the accumulator makes the system easier to regulate over a wide range of temperature from the cold limit of the chiller all the way to room temperature. he I-2PACL principle is patent pending. 2

3 Accumulator Paper No: GL-208 Experiment connection 5 4 Flow regulation RACI layout P, 3 Experiment venting Concentric hose P 10 Local experiment box 6 condenser P 5 Heater compressor EV R-404A Chiller R404a Chiller R404a evaporator/ CO 2 condenser 1 CO 2 pump CO 2 loop Figure 2: RACI I-2PACL concept 3 4. RACI DESIGN At CERN two RACI prototypes are made to support the LHCb- Velo and Atlas-IBL experiment. Figure 3 shows a picture of the two units. RACI is constructed mainly with commercial off the shelve components. he design is made modular such that fabrication and integration are relatively easy. he chiller is a Danfoss air cooled R-404A condensing unit OP-LCHC015 with a custom assembled high pressure heat exchanger AXP10-10 from Alfa Laval (see figure 5). he refrigerant injection is done via a thermostatic expansion valve. he CO 2 piping is constructed in a two dimensional layout squeezed in-between compressed foam layers. One foam layer can easily be removed for easy access to the CO 2 tubing and accumulator. he CO 2 connection to the experiment is via a flexible concentric tube like the transfer tube used in a 2PACL (Figure 2). A small local box with two shut-off valves and a flow regulating valve is present on the other end of the concentric hose. his box will be placed near the experiment; the RACI unit itself can be placed up to 3m away from the attached experiment. 4.1 CO 2 system design he CO 2 piping is located in the foam box as shown in figure 4. Figure 3: wo RACI prototypes he prototypes are made of standard Swagelok connectors; the next version will be orbital welded with VCR fittings. he accumulator is made of a large pipe with 2 Swagelok connector as flanges. he accumulator heater is a Watlow firerod heater in a thermo siphon concept. he pump is a Gather 1M-J/6/X-SS/S/Q/K200/DLC gear pump suitable for a system pressure of 200 bar. he pump can provide a pressure head up to 5 bar and deliver a mass flow of about 3 g/s. he flow to the experiment is regulated by a Swagelok needle valve with

4 Paper No: GL-208 a Cv value up to he condenser for the CO 2 is an Alfa- Laval high pressure heat exchanger AXP10-10 which is located outside the foam box in the chiller assembly. he tubes used are imperial sizes ⅛ for the liquid lines and ¼ for the vapor lines. Imperial sizes are used as Swagelok offers more variety of components for imperial sizes. 4.2 Electrical control design he control system is made of standard electrical components. No PLC is included; all the digital control logic is relay based and achieved by the digital output of PR4116 signal conditioners and PR5115 signal calculators from PRelectronics. One Watlow EZ-Zone S PID controller is included for the 250 watt accumulator heater. A second heater is mounted on the chiller evaporator outlet as dummy load in case of a too low load for the compressor is present. he dummy load heater switches on at a lower limit of the suction pressure controlled by one of the PR conditioners. Both heaters are protected in two faults for overheating via a serial thermal switch chain and internal thermocouples. he pump differential pressure is interlocked by two thresholds. An upper threshold (ca. 6 bar) and a lower threshold (1 bar) are protecting the pump for undesired flow conditions. At start-up the lower boundary is neglected via a set-reset mechanism made by relays. he pressure difference is calculated by the PR calculator using the suction and discharge pressure measurement. Figure 4: CO 2 piping covered in ice during the 1 st cooling test. (he foam layer was removed) All the sensors used for controls can be readout by a National Instrument integrated DAQ system via the 0-10 volt output of the PR conditioners. he DAQ system is only for monitoring, no computer is needed for operating the system. In total two pressure sensors and four temperature sensors are needed for the control. An additional 4 temperature sensors and a flow meter are optional and can be read-out via the free slots in the NI-DAQ. Figure 5: he Danfoss chiller and the Gather CO 2 pump Figure 6: he power distribution, the relays, and the heater controller. Figure 7: he PR conditioners and NI-DAQ 4

5 emperature ( C) Paper No: GL-208 he user interface is kept simple as inexperienced users must be able to operate the system. All the control buttons are integrated in the door panel and accessible from the outside. Figure 3 shows the units with the user interface clearly visible. he start-up and switch-off is done via a simple on/off buttons. he start-up procedure is controlled by the same PR conditioner/relay logic which also takes care of the interlocking. In total 4 interlocks are present (pump, compressor and the two heaters). he interlocks are indicated with lights integrated in the door panel and can be reset manually using reset-buttons next to the indication lights. he accumulator controller interface is also on the same door panel and is the small black square on figure 3. Emergency stop buttons and a general power switch are also present. he two RACI prototypes have a frequency control of the pump, but commissioning showed that a fixed flow for the pump is the best option. he flow regulation works by branching off manually the amount of needed CO 2. he remaining CO 2 flows via the by-pass back to the condenser. 5. COMMISSIONING he two prototypes were built for urgent use in experiments. Limited time was available for system commissioning. At the first run the system functioned well, only small parameter changes had to be made. Some attention points were discovered such as a reduced cooling capacity at lower temperatures. he next prototypes will therefore have improved insulation to increase the efficiency. he I-2PACL concept proved to be indeed a useful principle as it is self stabilizing over a large range of temperature. Limited user knowledge was needed to operate the system. 5.1 emperature regulation he temperature regulation was tested by adjusting several pressure set-points and switching the power of the attached experiment on or off. A measurement of several steps is shown in figure 8. Around 9:15 the system was started with a heating of the accumulator. he system pressure increased and the loop was filled with liquid consequently. he chiller starts directly and sub cools the liquid in the condenser. he pump starts as soon as the liquid arrives at the pump which is detected by measuring several degrees of sub cooling at the pump inlet. he cold liquid flow starts cooling down the accumulator and the system pressure decreases. Once the accumulator pressure reaches the set-point, the heater takes over and maintains the pressure at the given set-point. A stable temperature CO2 Chart liquid temperature itle (at pump) CO2 temperature Accumulator saturation temperature 150 W on Power off -50 9:00 10:30 12:00 13:30 15:00 16:30 ime (hh:mm) Figure 8: emperature measurement of RACI to demonstrate the full temperature range. 150 W on around 0 C was achieved. Around 10: watt was applied to the attached experimental heater. he system was able to maintain the evaporator temperature of the system constant. he liquid temperature (blue line) increases due to the power input, but has no effect on the evaporator temperature in the experiment. Around 11:30 the set-point was lowered to about -20 C, around 12:20 the experiment power was switched off again. Under all conditions the temperature remains stable. Around 13:30 a set-point was requested which is too low for the system. It was unable to reach the set point as the accumulator approaches the liquid temperature. Per design the accumulator temperature cannot be similar to the liquid temperature so a guaranteed sub cooling is present for the pump. Around 15:00 the set point was increased and the power was switched on again. hree set points were chosen to demonstrate the temperature range up to room temperature. At all the set points and power cycles the system remained stable. 5

6 Heat load (watt) Paper No: GL System capacity As shown in figure 8, there is a lowest temperature limit for the accumulator. his lower limit depends on the liquid temperature which is depending on the suction pressure of the chiller s compressor. In figure 9 a graph is shown of the minimum measured evaporator temperature as a function of the external applied power. RACI was able to cool the experiment down to -35 C. A maximum of 250 watt was cooled at -22 C. 300W 250W 200W 150W 100W 50W Heat Load without control Heat Load 6. SUMMARY AND CONCLUSIONS 0W emperature ( C) o support the particle physics community for their detector cooling development a Figure 9: Capacity of the RACI prototype multipurpose CO 2 cooler is under development called RACI (Multipurpose Refrigeration Apparatus for CO 2 Investigation). A new concept called I-2PACL was developed to simplify the design and increase the temperature range from cold (chiller depended) to room temperature. he new I- 2PACL concept works as designed; a very stable temperature control was achieved with a standard heater controller. he operation of the system turned out to be relatively easy and understandable by non-experts. wo prototypes have been built and three new prototypes are under construction (see figure 10). Lessons learned from the two first prototypes have changed the design of the new prototypes. he initial cooling capacity was not achieved ( C). he next prototypes will have therefore a larger chiller capacity and improved insulation to increase the cooling capacity at lower temperatures. he liquid by-pass will be done with a check-valve instead of a manual needle valve to simplify the flow control. A flow meter and an experiment heater interlock will also be added. For future development the cooling power will be increased, however the goal remains to have a small and affordable mobile unit. At the moment the chiller is a standard R-404A chiller, in the future it will be investigated how to make it full CO 2. Figure 10: he 3 new RACI prototypes under construction 7. REFERENCES CERN, 2009, he CERN Large Hadron Collider: Accelerator and Experiments Vol 1&2. ISBN : Postema H, Verlaat B, 2011, Cooling in HEP Vertex and racking Detectors, PoS(VEREX 2011)003, 20th International Workshop on Vertex detectors, Rust, Austria Verlaat B, Colijn A.P, Postema H, 2011, he Future of CO 2 Cooling in Particle Physics Detectors, ICR11- B2-309, International Conference of Refrigeration, Prague, Czech Republic Verlaat B. et al. 2008, CO 2 cooling for the LHCb- VELO experiment A CERN, 8th IIF/IIR Gustav Lorentzen Conference on Natural Working Fluids, Copenhagen, Denmark, CDP