PoS(Vertex 2007)009. CO 2 cooling experience (LHCb) Martin van Beuzekom, Ann Van Lysebetten 1, Bart Verlaat

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1 CO 2 cooling experience (LHCb) Martin van Beuzekom,, Bart Verlaat Nikhef Kruislaan 409, 09 SJ Amsterdam, The Nerlands Ann.Van.Lysebetten@cern.ch The rmal control system of LHCb Vertex LOcator (VELO) is a two-phase CO 2 cooling system based on 2-Phase Accumulator Controlled Loop (2PACL) method. Liquid carbon dioxide is mechanically pumped in a closed loop, chilled by a water-cooled freon chiller and evaporated in VELO detector. The main goal of system is permanent cooling of VELO silicon sensors and of heat producing front-end electronics inside a vacuum environment. This paper describes design and performance of system. First results obtained during commissioning are also presented. The 6th International Workshop on Vertex detectors Lake Placid, NY, USA 2-2 September, 2007 Speaker Copyright owned by author(s) under terms of Creative Commons Attribution-NonCommercial-ShareAlike Licence.

2 . LHCb and Vertex Locator The LHCb experiment [] is dedicated to precision measurements of CP violation in B sector and to search of rare B decays. All angles and some sides of unitarity triangle are addressed by a multitude of B decay channels for which an efficient trigger system is needed. Powerful particle identification is anor requirement set by physics goals of experiment. Two crucial components of LHCb experiment are high track reconstruction efficiency and efficient vertex identification. The ability to distinguish primary and secondary vertices on an event by event basis is essential for LHCb physics program. The Vertex Locator (VELO) [2] has to provide precise measurements of track coordinates close to interaction region. The VELO detector consists of two halves each equipped with 2 silicon modules along beam direction. The active part of silicon starts at a distance of mm from beam. The detector is mounted in a setup similar to Roman pots. A design impression, with only a few detector modules installed is given in Figure. Fig. : One of two VELO detector halves with some modules installed. The detector will be operated in an extremely harsh radiation environment which is strongly nonuniform. The damage to silicon at most irradiated area during one year of operation is equivalent to that of MeV neutrons with a fluence of. 0 4 /cm 2. The modules need to be cooled in order to hold annealing and prevent rmal runaway of silicon. The temperature of silicon sensors is required to be below 0 C at all times. As VELO is operated in vacuum (~0-4 mbar) cooling of sensors must be achieved by conductance to refrigerant. A conductance path causes rmal gradients and due to VELO module layout a cooling temperature of -2 C is required. The refrigerant must be radiation hard, cope with se low temperatures and have a cooling capacity up to 00 W per detector half. The cooling system, inside LHCb acceptance, must be a low mass system. To avoid risk of leaks into vacuum, no connectors can be used. We refore use failsafe orbital welds and vacuum brazing connections. 2. The VELO Thermal Control System (VTCS) The VELO Thermal Control System (VTCS) [] meets all se constraints. The detector waste heat is transported through a cascade of three hydraulic systems. The detector is connected to tertiary system, which is a mechanically pumped CO 2 loop, cooled by a secondary freon chiller. The secondary chiller on its turn is cooled by primary system based on chilled water. Figure 2 shows a block scheme of this cascade. 2

3 Detector waste heat Qt Environmental heat leak Qt Tertiary system (CO 2 ) Work (Pump) Qt2 Qt4 Control heat Qs=Qt+Qt2+Qt+Qt4 Environmental heat leak Qs Qs2 Secondary system (R404a) Qs4 Work & Control heat heat (Compressor) Primary system (Water) -40 C -20 C 0 C 20 C Fig 2: Block scheme and heat balance representation of VTCS 2. The tertiary CO 2 system. The tertiary system uses a 2-Phase Accumulator Controlled Loop (2PACL) [4] method with CO 2 as working fluid. A schematic overview and rmodynamic principle can be found in Figure and Figure 4 respectively. The numbers in parenses in following refer to nodes in se figures. The VTCS systems pressure is regulated by accumulator, a vessel always containing a saturated mixture of vapour and liquid CO 2. The accumulator is parallel connected between condenser and pump at node in Figure. The low hydraulic resistance between evaporator outlet and accumulator (node 0 to ) assures that pressure in evaporator is close to pressure in accumulator. When in saturation this pressure refore directly determines temperature of evaporator. In this way it is possible to control evaporator temperature over a long distance without complex local actuators, making distant evaporator completely passive. This approach greatly reduces risk of failures in a hostile radiation area which is difficult to access. Heat out 2-Phase Accumulator Heat in Heat out Condenser 0 Qp=Qs+Qs2+Qs+Qs4 9 Flooded evaporator Heat in Pump Heat exchanger Restrictor Fig. : Schematic overview of CO 2 system.

4 2x0 4 Tertiary VTCS in P-H diagram Pumping Expansion Internal heat exchanger 0 4 brings evaporator pre-expansion per definition right above saturation (-)=-(0-) Saturation line 0 C P [kpa] Accumulator pressure = detector temperature Pump is sub cooled Heating/Evaporating 0 Capillary expansion brings evaporator in saturation C -20 C -0 C C x h [kj/kg] Cooling/Condensing Fig 4: VTCS operation in Pressure-Enthalpy diagram for CO Detector load (9-0) In VTCS-2PACL liquid carbon dioxide is mechanically pumped (node to ) to evaporator by a membrane pump. The liquid goes to detector through a 0 m long internal heat exchanger (node -) where liquid is heated up through contact with CO 2 returning from detector (node 0 to). The liquid is n adiabatically expanded over restrictions (node to 9). The expansion brings CO 2 from a sub-cooled liquid state at node into saturation at node 9 before it enters evaporator in detector. The detector waste heat is removed by latent heat of evaporation of saturated CO 2 (node 9-0). The liquid-vapour mixture having a typical vapour quality of 0. to 0. is returned through return line of heat exchanger (node 0-). In this heat exchanger mixture is already partly condensed by exchanging heat with liquid of inlet line. The mixture is n furr condensed and sub-cooled by freon chiller in CO 2 condenser (node to ). The sub-cooled liquid is fed back to pump and cycle is started again. 0 C A useful feature of 2PACL is ability of turning two-phase loop into a singlephase loop. Increasing accumulator s pressure high enough will suppress any formation of vapour in loop. This feature is a useful at start-up (see Section 2..). Sub-cooled liquid is defined as a liquid which has a temperature a few degrees below its boiling point. 4

5 2.2 The VTCS design The cooling plant, containing active and control components, is situated about 0 m from passive CO 2 evaporator. The evaporator and cooling plant are linked through transfer line. This gives advantage that control part is always accessible as it is situated behind shielding wall and well protected from hostile radiation environment. The 0 m transfer line is internal heat exchanger between liquid feed and vapour return line. The liquid feed line lies inside vapour return line. The cooling plant consists of freon chiller filled with refrigerant R07a and active parts of CO 2 system. The chiller unit is a commercially engineered Controls PLC item, while CO 2 system Accumulators is designed and built at Nikhef, Amsterdam. The CO 2 unit contains membrane pumps, accumulators, all necessary Primary valves and plate water condensers. The latter are supply Secondary freon unit commercially available brazed plate heat exchangers Tertiary CO 2 unit reinforced with steel end plates to withstand a CO 2 transfer line CO 2 pumps maximum system pressure of 0 bar. A photograph of Fig.: The Freon chiller rack, accumulators and CO 2 rack. cooling plant is shown in Figure. It was installed in LHCb experimental site during summer 2007 and entered a commissioning phase in months after. Redundancy is built in to assure permanent cooling capacity. With three pumps CO 2 can always be pumped through circuit. The freon chiller contains a water (main) and air (backup) cooled condenser. Each detector half is cooled by its own CO 2 loop and is filled with about. kg of CO 2. The entire system is controlled by a Siemens 400 series PLC The accumulator The accumulator is a vessel always containing a saturated mixture of vapour and liquid CO 2. With accumulator system pressure and hence saturation temperature can be increased or decreased. The accumulator contains a rmo siphon heater of kw which allows for pressure increase by evaporation of liquid in mixture. Conversely, a cooling spiral fed by freon chiller permits mixture to condense and hence a pressure decrease. The cooling capacity is as well in order of kw. The maximum operating pressure of accumulator is 0 bar while it has volume of 4.2 liter. This vessel falls under European directive for high pressure vessels and was thus CE certified.

6 2.2.2 The evaporator The passive evaporator is situated inside vacuum of detector. The liquid flow is distributed through restrictions over parallel cooling connections for each detector half. The connections are obtained by vacuum brazing and orbital welding. A photograph and Fig. 6: The evaporator (left side) being installed in VELO detector half (right side) a design impression of evaporator body are shown in Figure 6. The distribution capillaries have an inner diameter of 0.mm and bring CO 2 to each of module stations. All stations are equipped with aluminium cast cooling blocks, also called cooling cookies. In this process stainless steel evaporator capillaries are imbedded in aluminium, by melting aluminium around capillaries. The flat side of cooling cookies are attached to VELO modules by Thermflow T70 [] for a good rmal connection. The return capillaries are connected through a manifold to return line. The temperature of cooling cookies and furr system temperatures are monitored by 4 wire PT00 sensors. 2. The VTCS performance 2.. Operating VTCS A simulation model was made to study rmal equilibria of different operational states. Figure 7 shows simulation results of various situations from warm startup to cold operation. During commissioning start-up phase was verified. This corresponding data are shown in Figure. The different operational states are indicated in se figures by letters A to E. Prior to start-up temperature in loop is equal to environmental temperature. This is situation in pressure- P [bar] 2 x D 9,0, C E 9 0 9,0 VTCS start-up and operating cycles x h [kj/kg] -40 C -20 C Fig. 7 Simulation results for start-up cycle in pressure-enthalpy diagram, B 0, A 0 C 20 C 40 C 6

7 CO2 Cooling experience enthalpy diagram along 20 C isorm in two-phase region (A). It is unknown where liquid or vapour is situated. To prime system accumulator is heated. The increasing pressure will cause loop to be filled with liquid. The vapour in loop condenses; liquid is supplied by accumulator. Once loop is flooded Pump pressure (Bar) pump can be switched on and Accumulator pressure (Bar) liquid CO2 of room temperature is now pumped Accu level (%) around in loop (B). Next cooling by freon chiller is switched on and CO2 liquid is cooled down Accu heating/cooling (W) (C). The pumped liquid will time be at about -40 C, while evaporator stays relatively Evaporator (ºC) warm when in liquid mode. Transfer return (ºC) The rmal difference Pumped liquid (ºC) between pumped liquid (node E A B C D in Figure ) and returned liquid (node ) is Fig. Data taken to illustrate VTCS start-up mainly due to heat leak of transfer line. Once cold liquid is circulating through evaporator, accumulator pressure and hence temperature can be lowered to required set-point; this is a saturation temperature of -0 C in Figure 7 and -2 C in Figure. During accumulator cool-down evaporator is cooled simultaneously. In Figure accumulator cooling is expressed as a negative heating power. During this cool down, accumulator liquid level is increasing due to increased vapour volume in system Temp ( C), Pressure (bar), Level (%) : 6 :0 0 A M 0 :4 :0 0 A M 2 :0 0 :0 0 P M : 2 :0 0 P M 2 :2 4 :0 0 P M T im e (h h :m m :s s ) In situation (D) accumulator has reached its desired set-point temperature. The evaporator is saturated and its temperature is dictated by accumulator, even though re is no heat to absorb. The VTCS is now ready for a detector power-on. The complete start-up procedure is implemented in PLC and just requires a one push-button action. It takes about 2 hours to go from room temperature to a set-point temperature of -2 C VTCS evaporator performance Once situation D is achieved detector can be switched on. The powered-on situation is indicated in E of figures 7 and. Figure 9 zooms in on situation E of Figure. The shown test results were not obtained by detector evaporator but by a parallel mounted evaporator containing an electrical heater. Situation E in figures and 9 shows behaviour when a load of 600 W is applied to one half. This represents. times expected detector load per half. 7 T e m p e r a tu r e ('C ), P r e s s u r e (B a

8 CO2 Cooling experience Power (Watt), Level ( ) Temperature ('C) Evaporator performance in lab temperature [oc] Heat load =0 Watt (.4x nominal) -20 Mass flow Mod2 Mod6 evaporator dry out Transition from bubbly to annular flow :4:2 2:7:6 :2:00 :26:24 :40:4 ::2 4:09:6 Time Fig.0: Performance of evaporator at reducing flow, heat load on module was 0 W. total mass flow [g/s] 2.. Both evaporators were fully characterised in lab after being assembled. One of phenomena that have been investigated is dryout. This happens when mass flow is so low (and hence vapour quality so high) that no liquid film on capillary walls is formed any longer. The heat transfer is strongly reduced resulting in strongly increasing cooling block temperatures. The characterisation tests of evaporators demonstrated that Before load is applied, we observe that temperatures at Evaporator Temp (ºC) evaporator and at accumulator are stable at Accu Temp Set-point (ºC) -2.4 C and -24. C respectively. The difference Accu level ( ) of.4 C between evaporator and 600 Watt accumulator is Detector Power (Watt) Accu Cooling Power (Watt) evaporator offset which is caused by flow resistance in return line After load is applied, we observe a temporary -.00 Pumped Liquid Temp (ºC) increase in evaporator and accumulator pressure and :40:4 PM :4:00 PM ::2 PM 2:02:24 PM 2:09:6 PM 2:6:4 PM 2:24:00 PM hence a temperature Time (hh:mm:ss) increase of about. C. Fig.9 Data taken to illustrate evaporator performance This increase is caused by increasing amount of vapour in return line. Cooling down accumulator will cause pressure to go down with an increased accumulator level as result. Within 7 minutes system is stabilised again and evaporator temperature has increased from -2.4 C to -2.0 C. The increased evaporator offset of 0.4 C is due to increase of vapour in return line, because vapour has more flow resistance compared to liquid. All se results are within specifications of system.

9 typical mass flow at which this phenomenon occurs is -6 g/s, as can be seen in Figure 0. The nominal flow for our evaporator is 2 g/s. Hence we are well above this limit.. Module cooling performance Once modules were installed on detector bases ir electrical [6] and rmal performance [7] was checked in lab. A small scale cooling system blowing liquid CO 2 through evaporator tubes at -2 C was used during se tests. All modules are equipped with NTCs to monitor rmal performance. Per module side 2 NTCs are installed, NTC0 close to cooling cookie region and NTC close to silicon. Left detector (A-side) ΔT (setpoint NTC) Cooling system to silicon Δ T(Cooling Cookie NTC0) Transfer cooling to module Fig. : Thermal performance of VELO modules for both halves. Right detector (C-side) Δ T (NTC0-NTC) Module performance Δ T(setpoint-Cooling Cookie) CO 2 heat transfer The results of measurements for 42 modules are shown in Figure. Several differences in temperature are plotted against module number. The difference in temperature between silicon and set-point temperature is about 20 C (diamonds in Figure ). This total difference is result of several contributions, temperature difference (i) from CO2 system to cooling cookie (blue squares), (ii) due to coupling of cooling interface to TPG 2 (downside triangles), (iii) along TPG of module (upside triangles) up to silicon. The largest contribution is 2 TPG stands for Thermo Pyrolythic Graphite and is used as a rmal highway to improve module heat conductivity. 9

10 coupling of cooling cookie interface to TPG. An overview of temperatures reached on silicon with a set-point temperature of -2 C is given in Table. T on silicon ( C) Left/A Half Right/C half Mean -4.2±.4 -.2±. Min Max Table : Summary of rmal performance of VELO modules. The measurement conditions in lab are not exactly same as in final system. The vacuum quality is a bit lower (0 - mbar instead of expected 0-4 mbar) and all modules are not cooled and operated (heat load put on modules) simultaneously. 4. Conclusions Mid July 2007 VELO Thermal Control System was installed in experimental site. During subsequent commissioning performance of system was verified. The results found are matching design parameters of system. Set-point temperatures were tested down to -2 C until now and system proved stable operation. Detector loads up to 00 W/half were tested. The redundancy to keep silicon at all times below 0 C is built in and testing has started. The rmal performance of modules was verified and at a cooling temperature of -2 C all modules are below required 0 C. The implementation of PLC control proved successful and main operation of system just requires a one push-button action. We are looking forward to enter final commissioning phase with VELO detector installed. References [] LHCb Coll., LHCb Reoptimized Detector design and performance, CERN/LHCC [2] LHCb Coll., The LHCb VELO technical design report, CERN/LHCC [] B. Verlaat, Thermal performance Testing of VTCS evaporator and VELO module, NIKHEF EN0-0, 200. [4] B. Verlaat, Controlling a 2-phase CO 2 loop using a 2-phase accumulator, International Conference of Refrigeration 2007, Beijing, ICR07-B2-6 [] [6] F. Da Cunha Marinho, Quality assurance tests of LHCb VELO modules, These proceedings. [7] T. Theunissen and A. Van Lysebetten, Thermal performance of VELO modules, LHCb note, to be published 0