CO 2 COOLING FOR THE LHCB-VELO EXPERIMENT AT CERN.

Size: px

Start display at page:

Download "CO 2 COOLING FOR THE LHCB-VELO EXPERIMENT AT CERN."

Cuthbert Douglas
6 years ago
Views:

1 GL th IIF/IIR Gustav Lorentzen Conference on Natural Working Fluids 7-10 September 2008 Copenhagen, Denmark Paper number: xxx CO 2 COOLING FOR THE LHCB-VELO EXPERIMENT AT CERN. B. Verlaat*, A. Van Lysebetten, and M. Van Beuzekom. *Author for correspondence National institute for subatomic physics (Nikhef), Kruislaan SJ Amsterdam, The Netherlands, bverlaat@nikhef.nl ABSTRACT CO 2 as evaporative coolant has gained interest for the use in high energy particle physics experiments. It is applied as coolant in the Vertex Locator (Velo) of the LHCb detector, which is an experiment at the Large Hadron Collider at CERN in Geneva. Silicon particle detectors like the Velo have special requirements on cooling. The sensors which are spread over a large volume must be kept at a stable cold temperature (-7 ºC) at all times while the attached electronics generate a substantial amount of waste heat which has to be taken by the cooling system. The cooling infrastructure in the detector needs to be of low mass and the construction materials including the coolant need to be radiation resistant. CO 2 as coolant is a good option for this application as it can withstand a large amount of radiation and has excellent thermal behavior in small diameter tubes. The CO 2 cooling system for the Velo uses the 2-Phase Accumulator Controlled Loop (2PACL) method. This method supplies low quality CO 2 into the evaporator at a constant pressure and requires no active components inside the detector. This paper describes the design of the Velo detector and the Velo Thermal Control System VTCS). It explains the 2PACL method and test results of the VTCS are presented and experience gained during VTCS commissioning are described. 1. LHCB AND THE VERTEX LOCATOR. A novel CO 2 cooling system is developed for cooling the Vertex Locator (VELO) of the LHCb experiment. LHCb is a particle detector studying Cp violation which must give the answer to the question why the universe exist only out of matter, and why anti matter has disappeared. The LHCb detector (LHCb Coll., 2003) is one of the 4 new build particle detectors constructed around the collision points of the new Large Hadron Collider (LHC) at CERN in Geneva.

The silicon wafers are mounted on a module containing the read out electronics and mechanical support. The silicon modules are situated in a vacuum which is separated only by a 0.

2 1.1 The Vertex Locator The VELO (LHCb Coll., 2001) is the sub detector closest to the collision point in LHCb. It exists of 21 double and 2 single silicon wafer layers which are situated approximately 1 cm away from the LHC proton beam. The silicon wafers are mounted on a module containing the read out electronics and mechanical support. The silicon modules are situated in a vacuum which is separated only by a 0.3mm foil from the LHC beam vacuum. The maximum pressure difference between the 2 vacuums is 5 mbar, which results in a very complicated system for pumping down and inflating the vacuum systems simultaneously. The reason for a secondary vacuum for the silicon modules is the out gassing of the silicon modules, and the ability of installing detector hardware without exposing the LHC beam vacuum volume to the outside air. The 0.3 mm aluminum foil around the silicon stations acts also as a faraday cage protecting the silicon and electronics from the electromagnetic interference of the proton beam. Secondary Vacuum LHC Beam Vacuum Silicon Figure 1: Artists impression of the VELO experiment the Fig 2: Schematic side cut of the VELO experiment (beam vacuum white / detector vacuum yellow) The silicon stations suffer from a high dose of ionizing radiation induced by the LHC proton beam. The radiation causes damage to the silicon crystal structure resulting in an increase of leakage current (The ROSE coll. 1999). Permanent cooling of the sensors is needed to avoid the outcome of this damage. A silicon temperature less than -7ºC is sufficient to minimize the effects of radiation damage. 1.2 Velo module design. The silicon modules consist of a carbon fiber TPG 1 laminate at which on both sides an electronics hybrid with a silicon sensor are glued. At the bottom of this laminate the CO 2 cooling evaporator is mounted on one side and the carbon fiber support paddle on the other side. The paddles are mounted on a stiff aluminum base frame. Figure 3 show a picture of a constructed Velo half with the discussed items clearly visible. The beetle read-out chips are located on the hybrid at the edge of the silicon and generate a substantial amount of waste heat which needs to be taken away by the CO 2 cooling system. The aluminum base is the positional reference of the modules and must therefore be maintained on room temperature. This is achieved by heaters since the thermal connection via the paddles will otherwise cause the base to cool down by the CO 2 evaporators. Table 1 and 2 show the temperature requirements and the heat dissipation of the Velo components. 1. Thermal Pyrolytic Graphite, High conductive material λx,y=1800 W / mk, λz=10 W / mk

Table 1: Operational and survival temperature limits of VELO key components Operational Survival temperatures: temperatures: Before irradiation: After irradiation: Silicon Wafers -12 C / -5 C

3 Table 1: Operational and survival temperature limits of VELO key components Operational Survival temperatures: temperatures: Before irradiation: After irradiation: Silicon Wafers -12 C / -5 C (Nominal tip = -7 C ) -30 C / 100 Long-term: -30 C / 0 C Short term: 0 C / 24 C (100 hours) Hybrid -20 C / 80 C -40 C / 50 C RF-Foil <20 C NA Module base frame 18 C -22 C (Stable) NA Table 2: Power dissipation of VELO hardware inside the detector vacuum Per VELO Per PU RF-Foil Module Total for module module Base VELO Nominal power 21.8 W 13.3 W 8 W 70W 1048 W Maximum power 27.5 W 16.2 W 8 W 139W 1394 W 1.2 CO 2 evaporator design. Each silicon module is connected to a dedicated parallel evaporator branch. One branch consists of a 1 meter long stainless steel capillary of 1.5x0.25mm, which is embedded in an aluminum plate connected to the carbon fiber/ TPG laminate. The aluminum plate is casted around the tube in a special developed procedure by melting the aluminum around the tube in a vacuum oven. Melting the aluminum under vacuum will cause the aluminum to join to the stainless steel forming inter metallic phases. The melting procedure will give a lot of freedom in pipe geometry inside the aluminum and a perfect thermal Figure 3: Assembled Velo half with silicon modules, cooling, and module base contact between the pipe and the aluminum plate. The cooling block shape is achieved by the shape of the casting mould. The way the casting is done looks similar to baking of cookies, this is the reason why the aluminum blocks are called cooling cookies. Each evaporator branch has a 1.3 meter restriction capillary of 1x0.2 mm at the inlet for a good flow distribution over all the evaporator branches. The presence of the evaporator in a vacuum system gives high constrains to the leak tightness therefore the complete evaporator assembly (figure 4) is made of stainless steel tubes all joined together with vacuum brazing or orbital welding. No connectors are present inside the vacuum system. The inlet manifold connected to the inlet capillaries is outside the vacuum vessel and is accessible. This way it is possible to connect cooling to individual channels, a feature which is used by module commissioning. During laboratory commissioning the cooling was achieved by a CO 2 bottle blow system.

Figure 4: VTCS CO 2 evaporator assembly Fig 5: Velo Module with aluminum cooling cookies. The Velo Thermal Control System (VTCS) (Van Beuzekom, 2007) is a cascade system of three hydraulic systems.

This chiller is a commercial type chiller with a gas compressor, water condenser, evaporators and expansion valves.

4 Figure 4: VTCS CO 2 evaporator assembly Fig 5: Velo Module with aluminum cooling cookies. The Velo Thermal Control System (VTCS) (Van Beuzekom, 2007) is a cascade system of three hydraulic systems. A chiller condenses the evaporated CO 2 generated in the Velo module evaporators back to liquid and reject the waste heat to the cold water system of CERN. This chiller is a commercial type chiller with a gas compressor, water condenser, evaporators and expansion valves. The water system is called the primary cooling system, the chiller the secondary and the CO 2 loop the tertiary cooling system. Figure 6 shows a schematic block scheme 2. VELO THERMAL CONTROL SYSTEM Detector waste heat Qt1 Environmental heat leak Qt3 Tertiary system (CO 2 ) Work (Pump) Environmental heat leak Secondary system (R404a) Work & Control heat heat (Compressor) Primary system (Water) -40 C -20 C 0 C 20 C representation of the cascade systems, with the main heat flows and the system temperature distribution. The tertiary cooling system is a hybrid 2-phase / single phase mechanically pumped loop using CO 2 as working fluid. It can operate in a single phase or in a two-phase cooling mode. During start-up it operates in single phase mode, once cooled down it can be set to a 2-phase loop for a more efficient heat transfer and accurate temperature control. Qt2 Qt4 Control heat Qs1=Qt1+Qt2+Qt3+Qt4 Qs2 Qs3 Qs4 Qp1=Qs1+Qs2+Qs3+Qs4 Figure 6: Block scheme and heat balance representation of the VTCS

5 Accessible and a friendly environment Accumulator 8 gas R404a R507a Chiller chiller 2-phase Condenser 1 Cooling plant area 7 liquid Cooling plant: Sub cooled liquid CO 2 pumping CO 2 condensing to a R507a chiller CO 2 loop pressure control using a 2-phase accumulator Pump 2 2-phase liquid Transfer lines (~50m) Concentric tube Inaccessible and a hostile environment VELO area 2-phase 6 Evaporators liquid Figure 7: Block scheme and heat balance representation of the VTCS 3 Restriction 4 2-phase 5 liquid Evaporator : VTCS temperature -25ºC Evaporator load Watt Complete passive The main unit of the VTCS is placed 50m away from the Velo detector behind a thick concrete shielding wall. This wall is shielding the system from the radiation of the LHC. Behind this shielding wall it is also possible for people to work. The VTCS is designed such that all active hardware is located in this safe zone. The cooling hardware in the experimental area consists only of tubes and passive devices such as restrictors and check valves. Local sensors (pressure and temperature) are only for monitoring and are not important for the cooling system operation. Malfunctioning of inaccessible vital hardware is so reduced to an absolute minimum. The CO 2 evaporators in the detector are connected to the VTCS via 50 meter long concentric transfer lines. The liquid feed transfer tube (1/4 x0.035 ) is situated inside the vapor return tube (16mx1mm). The concentric construction acts as a long counter flow heat exchanger needed for conditioning the evaporator inlet flow to a low vapor quality. The flow in the evaporator must be of low vapor quality to achieve a stable liquid expansion in the flow distribution capillaries. The cooling system must maintain the silicon wafers cold, powered or unpowered. The evaporator outlet vapor quality is therefore variable. The evaporator temperature to achieve the silicon temperature requirement is between -25ºC and 30 C (Verlaat, 2005). This temperature can be set in the system by the accumulators and is called the VTCS setpoint temperature. 2.1 The 2PACL principle. To achieve liquid expansion over the capillaries and a low vapor quality together with a remote controlled pressure in the evaporators the 2-Phase Accumulator Controlled Loop (2PACL) principle is developed at NIKHEF (Verlaat, 2007). This principle is controlling the system Heat out 2-Phase Accumulator Heat in Heat out 1 Condenser 13 Pump 3 5 Heat exchanger 10 Restrictor Flooded evaporator Figure 8: Simplified representation of the 2PACL method pressure and hence the evaporator pressure using a 2-phase accumulator. This vessel is parallel mounted to the system and contains per design always a content of liquid and vapor. The guaranteed presence of a saturated mixture in the accumulator makes the system pressure to be a function of the accumulator temperature. In this way the system pressure can be tuned independent from the freon chiller temperature cooling the CO 2 condenser. 9 Heat in

6 The 2PACL system works as long as the freon chiller is colder than the accumulator temperature. The freon chiller condenses and sub-cools the CO 2 so the pumps can operate free from cavitation. The remaining sub cooling after the pump is heated up by the concentric transfer line such that the evaporator inlet remains in saturation. The operating condition of the evaporator is independent of the amount of pump sub cooling. This sub cooling may vary in a wide range so a controlled condenser temperature is not needed. The accumulator temperature is the only accurate controlled item in the CO 2 loop. 2x10 4 Tertiary VTCS in P-H diagram Internal heat exchanger 10 4 brings evaporator pre-expansion per definition right above saturation (3-5)=-(10-13) Saturation line 10 C P [kpa] C -10 C Accumulator pressure = detector temperature Pump is sub cooled C -30 C 10 3 Capillary expansion brings evaporator in saturation -40 C Detector load (9-10) x h [kj/kg] Figure 9: VTCS operation in the Pressure-Enthalpy diagram for CO VTCS construction The VTCS chiller and CO 2 loop are installed in racks on one of the LHCb service platforms. The CO 2 loop is constructed from orbital weld components from Swagelok and Cajon VCR connectors. The CO 2 pump used is a Lewa CO 2 membrane pump. The accumulators are self engineered stainless steel vessels containing cooling spirals and a thermo siphon liquid heater. There are Controls PLC 2 identical CO 2 loops, each cooling 1 Accumulators Velo detector half. Both CO 2 loops are cooled by 1 water cooled chiller for normal operation and an aircooled back-up chiller for redundancy. The chillers are self Primary water engineered R507a chillers using supply standard refrigeration hardware such Secondary freon unit as Bitzer compressors, Danfoss line components and SWEP plate heat Tertiary CO 2 unit exchangers. The CO 2 condensers are also SWEP plate heat exchangers CO 2 transfer line CO 2 pumps reinforced for the high CO 2 system pressure. The design pressure for the Fig. 10: The VTCS cooling plant installed at LHCb CO 2 loop is 135 bar, the test pressure 170bar.

7 2.2 VTCS installation commissioning and testing The VTCS installation started at the LHCb 10 experiment in July Commissioning work is ongoing but the system was successfully operated for the detector 0 commissioning in March and May The VTCS is operated during the vacuum -10 period at the end of March 2008 at several set point temperatures varying from 0ºC to -30ºC. In this period it was not yet possible -20 to switch on the detector to induce load on the system. Figure 11 and 12 show the temperature results of this vacuum -30 commissioning period. The first half of the detector was commissioned in May 2008 with a cooling temperature of +10ºC. -40 During this commissioning the vacuum system was under atmospheric pressure. To avoid too much pressure difference between the 2 vacuum systems the cooling temperature was limited to +10ºC. Detector commissioning with cooling under vacuum is foreseen in July Measured Temperature (ºC) Condenser inlet TR Condenser Inlet (TRTT101pvss) TR Pump Pump subcooling sub (VRTT112pvss) cooling TR Pump Outlet (TRTT115pvss) Pump outlet Evaporator TR Evaporator Inlet (TRTT047pvss) TR Evaporator inlet outlet (TRTT048pvss) TR Accu Saturation (TRPT102tsat) = Setpoint control Evaporator outlet Accumulator saturation Setpoint Temperature (ºC) Figure 11: Steady state temperature results of the commissioning under vacuum Start-up A Accumulator saturation (Set-point) T Evaporator temperature C Condenser inlet Pump Outlet Temperature (TRTT120pvss) C C-Side RF-Foil temperature (TRTT043pvss) P C-Side Module Base Temperature (TRTT038pvss) C Condenser outlet / Pump sub cooling Pump outlet RF-foil Sp=0ºC Sp=0ºC 5 Measured Temperature (ºC) Sp=-10ºC Sp=-20ºC Sp=-25ºC Sp=-30ºC Sp=-30ºC Time from 29 March clock time (Hours) 2 Days + 6 hours cooling under vacuum at several set point temperatures Figure 12: Transient temperature results of the commissioning under vacuum. Due to the absence of the detector the VTCS is tested using dummy by pass heaters. These heaters are mounted on top of the Velo parallel to the evaporator and are a good representation of the detector heat load. Figure 13 shows the response of the VTCS to a nominal heat load change from 0 to 600 Watt. The system pressure is raised and compensated by cooling the accumulator.

8 The accumulator level has increased and the evaporator pressure is back to normal. The pump sub cooling is increased due to increase of the freon chillers compressor suction pressure. The VTCS was tested under several heat loads and set point temperatures. At the nominal set point of -25ºC the system was able to remove 1600W. The lowest achieved set point was -40ºC. These numbers are far beyond the systems requirement, so dummy load tests have shown that the VTCS is able to meet the requirements of table 1 and Evaporator Temp (ºC) Temperature ('C) Accu Temp Set-point (ºC) Accu level ( ) 600 Watt Detector Power (Watt) Accu Cooling Power (Watt) Pumped Liquid Temp (ºC) :40:48 PM 1:48:00 PM 1:55:12 PM 2:02:24 PM 2:09:36 PM 2:16:48 PM 2:24:00 PM Time (hh:mm:ss) Figure 13: VTCS response to a load step Power (Watt), Level ( ) 3. CONCLUSION AND DISCUSSION The VTCS has demonstrated that the 2PACL method with CO 2 is a good principle of controlling the temperature of the Velo silicon modules. The benefits from this system such as small cooling channels and no active components inside the detector has been notified by other particle detectors as a possible future cooling system. Both the 2 large CERN experiments Atlas and CMS are considering a similar system for their upgrade inner detectors. The challenge will be to upgrade the discussed principle from a 2.5 kw system towards a 100 kw system. 4. REFERENCES [1] LHCb Coll. 2003, LHCb Reoptimized Detector design and performance, CERN/LHCC [2] LHCb Coll. 2001, The LHCb VELO technical design report, CERN/LHCC [3] The ROSE Coll 1999, R&D On Silicon for future Experiments, CERN/LHCC note, [4] B. Verlaat 2005, Thermal performance testing of the VTCS evaporator and VELO module, NIKHEF EN [5] B. Verlaat 2007, Controlling a 2-phase CO 2 loop using a 2-phase accumulator, International Conference of Refrigeration 2007, Beijing, ICR07-B [6] M. Van Beuzekom, A. Van Lysebetten, B. Verlaat 2007, CO 2 cooling experience (LHCb), The 16th International Workshop on Vertex detectors, Lake Placid, NY, USA, PoS 009

DESIGN, MANUFACTURE AND TEST RESULTS OF THE VTCS CO 2 EVAPORATOR FOR THE LHCB EXPERIMENT AT CERN.

HEFAT2010 7 th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics 19-21 July 2010 Antalya, Turkey DESIGN, MANUFACTURE AND TEST RESULTS OF THE VTCS CO 2 EVAPORATOR FOR THE LHCB