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Discover the fascinating world of cryogenics, from the effects of temperature on matter to how refrigeration systems work. Learn about superconductors, electronics, and detectors in this informative workshop.
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Superconducting Electronics and Detectors Workshop “Jefferson Lab Cryogenic Operation” Mathew C. Wright December 1, 2015 https://www.jlab.org/conferences/super2015/
What is Cryogenics Temperature is a measure of the average kinetic energy of the particles in matter • Lower energy = cooler • Higher energy = warmer • Is there an absolute minimum and maximum temperature ? • Absolute maximum is theoretically 250,000,000,000,000,000,000,000,000,000,000 Degrees Fahrenheit • Absolute minimum is accepted to be -459.7 Degrees Fahrenheit Effects of temperature on the state of matter • Solid: matter that has a fixed volume and shape • Liquid : matter that has a fixed volume but not a fixed shape • Gas: matter that does not have a fixed volume or a fixed shape • Condensation point: the temperature at which matter becomes a liquid • Freezing point: the temperature at which matter becomes a solid
What is Cryogenics Interesting Facts: • Cryogenics is the study or production of very low temperatures between -238 and -459.7 Degrees Fahrenheit. • Your home refrigerator and automobile air-conditioning systems use a refrigerant called Freon • JLAB uses helium as a refrigerant because it is the only known substance that is not a solid at -456 F and normal atmospheric pressure • Helium will only freeze at -458 F and pressures greater than 25 times normal atmospheric pressure • JLAB maintains more than 27,000 gallons of liquid helium on site for cooling the cryomodules and magnets
A Common Idea of What a Refrigerator is: A dictionary definition of what a refrigerator is: A space where food is kept cool by means of ice or mechanical refrigeration. An engineering definition of a refrigerator: A device that moves energy from a low temperature source to a high temperature sink.
How Does a Household Refrigerator Work? Background: • Heat naturally flows from high temperature to low temperature. • All fluids have a temperature and pressure at which they boil when heated and condense when cooled. A Simplified Refrigerator Process: • Refrigerant is compressed, causing higher pressure and temperature where heat can be removed. • Coils on the back or below the refrigerator let the hot refrigerant gas dissipate its heat to the surrounding environment and condense the fluid. • High-pressure fluid flows through an expansion valve. You can think of the expansion valve as a small hole. On one side of the hole is high-pressure liquid. On the other side of the hole is low-pressure and low temperature. • Liquid refrigerant in coils, inside of the refrigerator vaporizes, absorbs heat from inside of the refrigerator, and turns back into gas. The cold gas returns to the suction side of the compressor to repeat the cycle.
How Does Jefferson Lab’s Refrigerator Work? • A simplified process of Jefferson Lab’s refrigerator: • Helium is compressed by oil flooded helium compressors. The oil reduces the temperature and keeps the compressor from overheating. Because helium molecules are small, the oil helps to create a seal between moving parts. • The majority of the oil is separated from the helium, cooled in separate water heat exchangers, and returned back to the compressor. The water that provides the cooling comes from a cooling tower, which uses the same principles of vaporization explained in step four of “How Does a Household Refrigerator Work?” • Helium is sent to a device that removes the remaining oil so it does not freeze at cryogenic temperatures and cause damage. • After the helium is at high pressure, ambient temperatures, and has no oil or other contaminations, the helium gas goes to the 4 Kelvin Cold Box (4KCB) where it goes through several stages of turbines and heat exchangers. The turbines and heat exchangers remove energy from the helium and provide helium at approximately 4 Kelvin and 3 atmospheres of pressure. • The cold gas is transferred from the 4KCB to the cryomodules through a heat exchanger in the Sub-Atmospheric Cold Box (2KCB). There are approximately 47 cryomodules and each one has its own expansion valve. With the pressure drop across the expansion valve and the correct conditions, the helium is liquefied in the cryomodule. • The cryomodule is similar to the inside of household refrigerator in that the contents are kept cold. Like the household refrigerator, the energy from the cryomodules causes the helium to vaporize. • The 2KCB reduces the pressure in the cryomodule to change the properties of the liquid helium. At 0.048 atmospheres, liquid helium becomes a “super fluid,” which has no resistance to heat, i.e., helium does not boil below 0.048 atmospheres. Instead the heat is conducted to the surface and is vaporized from the surface of the liquid helium bath. • At the exit of the 2KCB, helium returns to the 4KCB and goes through the same heat exchangers that cooled the gas in a previous stage. The helium is then returned to the suction side of the compressors slightly above 1 atmosphere and ambient temperature.
Energy Potential of Different Temperature Refrigerators A 2 Kelvin helium refrigerator requires approximately 1000 times more energy input than that of a household refrigerator to get the same amount of cooling. Between 50 and 263 Kelvin, the difference in required energy per amount of cooling changes little. But, the difference in required energy per amount of cooling does change significantly as colder refrigeration is required.
CHL1 vs CHL2 • CHL 1 • CHL 2 • Limited compressor discharge range (17 to 21atm) • Limited turndown capacity • Turbine flow controlled with inlet valve control • Turbine critical speeds result in capacity discontinuity • Manual adjustment of for mode transition • No local controls • Compressor Power input at 2.1K:4.2 MW (3700W @ 2.1K) • Wide compressor discharge range (6.5 to 20atm) • Large turndown capacity • Turbine flow controlled by discharge pressure • Continuous performance through capacity range • Automatic adjustment for mode transition • Compressors and the cold box have local controls • Compressor Power input at 2K:2.9 MW (3700W @ 2.1K)
JLab Floating Pressure Technology CHL 2 : Operates on Ganni Cycle Floating Pressure Technology • System capacity is varied from 20-100% at approximately constant system exergetic efficiency by varying the high pressure supply to cold box inlet between 6.5 and 18 atm with all 7 turbines operating with fully open inlet valves • The JLab compressor skids and the gas management are designed to support this variable pressure operation with out affecting the oil removal efficiency NASA James Webb Telescope: 20K Refrigeration System Performance
Keeping the Machine Reliability as high as possible • Down Time • Customer Down Time • Equipment Down Time • Performance • Fill Rates • LN2 usage/Helium Losses • Power Consumption • System Parameters • Maintenance Schedules / Coordination with other groups - Relay any transients to changes to management / Senior engineers • Engineer/Design new equipment • Maintain All the operational Documentation • Process Diagrams • Procedures • Etc…
Jefferson Lab Cryogenic Operation • Down-time • Any time loss to scheduled beam operations due to the cryogenics system and back to data collecting • Accounts for the restoration of the entire machine • The amount of time to recover from an outage is exponential to the amount of time that the cryogenic plant is down • Cryo down-time resulting in stopped physics • 1999 through 2008 ~ 1.57% average down time • November 2008 major down time of ~60 hours • 2008 down time ~ 2.6% • Main compressor failure without redundancy
Major Contributions to Down Time • Utilities Failures • Electrical Power • Power spikes • Phase imbalance • Cooling Water • Cooling tower accumulates debris • Pumping system failures • Instrument (control) air • Faulty / failure of pneumatic control valves due to moisture contamination
Major Contributions to Down Time • Control systems (CAMAC) • Old technology, uses lots of power, generates lots of heat (more heat, higher failure rates). • Laboratory grade hardware, not designed for industrial environment. • Highest failure rates in control system; electric valve cards, crate controllers, power supplies. • Replacement components are getting harder to find. • Aging system • Control cards • Carbon purification systems • Compressors • Carbon Steel components • Vacuum Jackets • Water Piping
Contamination • Dirty Sources (mostly Air & H2O) • Helium introduced into the system is always scrubbed • Clean gas from vendors is not always clean • Contamination can be found in Liquid dewars • Liquid dewars have been used as a source of clean helium • Loads from different users • Magnets • Cryogenic targets (sometimes sub-atmospheric) • Sub-atmospheric cryomodules
Contamination • Oil carryover • Improperly sized oil coalescers • 10 cm/s max velocity • Saturated carbon resulting in oil breakthrough to the cold box • Oil coats heat exchangers and reduces effectiveness • Can result in months to years of down time
History • Original Conditions • 4 GeV (235 g/s at 2K – design with margin) • Forced cryo plant to run at max design point • No redundant equipment (compressors or turbines) • Improvements made • Implemented Ganni cycle • Replaced the old 2K Cold Box with Jefferson Lab design (1998) • Added the Stand-by Refrigerator (SBR) • Present conditions • 6 GeV max load (235 g/s) • 4 GeV turn down (190 g/s)
System Cycle • Ganni cycle reduces how hard the system has to run • Invalidates the traditional philosophy that the design (‘TS’) condition is the optimal operating condition for as-built hardware and load • The traditional philosophy • Controls to the design T-s condition by adding heat, throttling valves and/or bypass of flow to maintain a fixed (design) load. • Utilizes a floating pressure system • A constant pressure ratio system that maintains a constant efficiency for a variable load
Redundancy • Up to 5.5 GeV using the Ganni cycle • SBR 2nd stage compressor (1/2 size of main compressor) can be used instead of one of the three main 2nd stage compressors • The SBR compressor can be run while maintenance on one of the main compressors is performed • Only two 1st stage compressors are required compared to the three needed to reach the operating condition • The SBR can be used to keep 4K liquid helium in the LINACs during scheduled physics down-time • Opportunity to replace charcoal in oil removal system of main machine and any other maintenance requiring the main machine to be down
Preventative Maintenance (PM) • Do not over maintain • The longer the process piping is open, the higher the probability of contaminating the system • Oil has a high affinity to absorb moisture from surrounding atmosphere • Mistakes can be made when performing PM • Jefferson Lab runs compressors for 74,000 hours between maintenance • Vendor recommends 25,000 hours on helium compressors
Conclusion • To minimize down-time and keep reliability up • Keep system free of contamination • Do not over drive the system • Have redundant systems for backup • Do not over maintain the system • Pay your cryogenics operators more • Attention • Money • Compliments
