Latest / Mechanical Engineering Made Simple / (#30) From Fridge to Fusion: The Essential Engineering of Cold, Cryogenics, and the Vapor-Compression Cycle

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(#30) From Fridge to Fusion: The Essential Engineering of Cold, Cryogenics, and the Vapor-Compression Cycle · Mechanical Engineering Made Simple

• 0:00
  Welcome back, design junkies, to Mechanical Engineering Made
• 0:03
  Simple. Today we are undertaking a deep
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  dive into the engineering of Cold War.
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  I mean, this is a field that touches everything from the beer
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  in your refrigerator to the magnets in a fusion reactor.
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  It really is a spectacular range of temperatures.
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  Yeah. And our mission today, I guess,
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  is to sort of chart that territory.
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  We're moving from standard refrigeration, you know, the
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  machinery that keeps us comfortable.
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  Air conditioning, basically. Right, air conditioning all the
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  way into the specialized, almost esoteric world of cryogenics.
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  That means producing temperatures below -150°C.
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  OK, before we dive into the deep thermodynamics, let's maybe
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  ground ourselves in some terms and a bit of history.
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  Good idea. So refrigeration is the broad
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  term right? Using mechanical or heat
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  activated machinery for cooling. And if it's for people comfort,
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  we call it air conditioning. Simple enough.
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  Yep, and this all sits on foundations laid well almost 2
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  centuries ago now. It wasn't just an overnight
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  thing, was it? Not at all.
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  The fundamental idea of using a machine to force cold.
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  Well, we look back to Jacob Perkins in London 1834.
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  He filed the first patent for a mechanically driven
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  refrigeration system and guess what he used as the working
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  fluid? Something weird.
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  Ether. Ether.
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  Wow. Yeah, but that patent proved you
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  could consistently and artificially create cold.
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  It really set the stage for, well, every cold chain we rely
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  on today. OK, let's unpack the core engine
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  then. The thing that powers like 99%
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  of our our modern cooling needs the vapor compression cycle.
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  Right. This is the heart of your home.
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  AC, commercial freezers, most big industrial chillers, it's
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  everywhere. And the beauty is its
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  simplicity, sort of. It's all about phase change.
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  Exactly. We take a working fluid, the
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  refrigerant, and we cycle it through states where it's forced
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  to evaporate and condense its specific controlled pressures.
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  It's just a continuous loop designed to grab heat from one
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  spot and dump it somewhere else. And this loop needs four key
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  parts. If you get these four, you
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  basically get refrigeration. Let's start with the one that
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  needs the energy input, the compressor.
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  OK, the compressor, it's the muscle of the system.
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  Its main job is to raise the pressure of the refrigerant
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  vapor. But just raising pressure isn't
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  the whole story, is it? No, because raising the pressure
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  does something crucial. It raises the vapor saturation
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  temperature. Really.
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  That's very deliberately so that the refrigerant is now,
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  thermodynamically speaking, hot enough its saturation
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  temperature is above the temperature of whatever we're
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  using to cool it down, like the air outside or maybe cooling
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  water. Making it hotter than the
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  surroundings, that's the key, because that dictates what the
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  next component does, the condenser.
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  Precisely. The condenser is just a heat
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  exchanger because the compressor made the vapor hot and
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  pressurized. This hot fluid can now actually
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  reject its heat to the cooler ambient air or water.
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  As it dumps that heat, the vapor condenses back into a liquid.
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  Typically it leaves the condenser as a sub cooled
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  liquid, meaning it's cooled just a tiny bit below its saturation
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  temperature at that pressure. OK, so we went from cold vapor,
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  squeezed it to hot vapor, cooled it to hot liquid.
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  Now we hit the restriction point, the expansion device.
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  Right, this could be a fancy valve, or in really simple
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  systems, just a long skinny bit of pipe called a capillary tube.
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  Why is this sudden pressure drop so critical?
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  This is where the magic happens. The cooling effect starts right
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  here. The expansion device takes that
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  high pressure liquid and lets it expand rapidly.
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  Big pressure drop. Imagine suddenly letting go of a
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  pressurized hose. The fluid's temperature plummets
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  immediately because it's boiling point.
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  It's saturation point just dropped way down.
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  The temperature coming out of this device is now way below the
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  temperature of whatever we're trying to cool.
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  Which brings us finally to the payoff, the evaporator.
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  This is where the useful cooling actually happens.
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  Exactly. The evaporator is the cold side.
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  That low temperature, low pressure fluid, which is usually
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  a mix of liquid and vapor, we call it Two phase fluid, flows
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  into the evaporator coils. It then absorbs thermal energy
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  from the products, like the warm air in your room or whatever
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  needs cooling. That absorbed energy makes the
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  rest of the liquid flash into vapor.
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  By the time it leaves the evaporator, it's typically a
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  super heated vapor all ready to go back to the compressor and
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  start the whole cycle over again.
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  OK, that makes the cycle really clear.
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  But let's tackle that unit of measurement.
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  We still hear the old term tons of refrigeration.
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  What's that about? Yeah, the ton.
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  It's a historical term. It's equivalent to 3.51 kilo R
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  kilowatts refrigeration. KWR kilowatts refrigeration that
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  R is important. It's critical for engineers,
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  Yeah, it specifies the actual useful cooling capacity, how
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  much heat you managed to remove. It's completely distinct from
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  the electrical power needed to run the system.
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  Which we might label keep WJ kilowatts electrical.
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  Exactly, Text carriage is the input power, so text carriage is
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  the result the cooling effect and text carriage is the
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  electrical cost to get that result.
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  Got it. Like a truck.
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  Text carriage is how much cargo it moves.
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  Text carriage is the fuel it burns.
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  You need both for efficiency. Precisely.
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  You always want the highest possible ratio of text carriage
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  A to tech. Scary JJJ.
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  That's an efficient system. OK, moving on from the hardware,
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  let's talk about the fluid itself, the refrigerant you
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  mentioned, there's no single perfect fluid.
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  Definitely not. No single refrigerant is cost
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  effective or even works across the huge range of temperatures
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  and alications we need globally. Selection is really complex.
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  You're balancing efficiency, the temperature range it works in,
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  material compatibility and crucially nowadays, safety and
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  environmental impact. And there's a standard for
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  classifying them, right? Arsh, HRE Standard 34.
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  Yup, NC X-ray 341992 gives us a system.
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  It classifieds them based on just two things, toxicity and
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  flammability. OK, break those down for us.
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  Toxicity is either Class A which is low toxicity, it's threshold
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  limit value, time, weighted average, or text TLV TA is over
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  400 parts per million or it's Class B toxic meaning the text
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  TLV TOV is below 400 PM. Then flammability is rated 1-2
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  or three. One is non flammable, 2 is
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  moderate, 3 is highly flammable. So for comfort cooling like our
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  home AC units we want the safest combo which is group A1, low
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  toxicity, non flammable. That's the gold standard for
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  places where people are directly exposed, yes, but this is
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  exactly where environmental policy just completely turned
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  the industry upside down, starting with the Montreal
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  Protocol back in 87. Yes, the ozone layer issue, that
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  was a huge deal. It was a global game changer, an
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  international agreement to restrict and eventually ban
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  substances that deplete the ozone layer, primarily only
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  corafluorocarbons, CFCS. The science was clear, CSCS were
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  destroying the ozone, so deadlines were set.
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  CFC production basically stopped by January 1st, 1996.
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  And the next group, HDFCS, hydrochlorofluorocarbons,
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  they're being phased out too by 2030.
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  Which meant engineers needed whole new families of chemicals
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  and new ways to measure their impact.
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  Absolutely. We suddenly had these two
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  critical metrics, usually compared back to the old villain
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  Refrigerant 11 or text R11. First, ozone depletion
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  potential, text ODP. How much damage does it do to
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  the ozone layer? 2nd, and maybe even more
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  worrying long term, the Halo carbon global warming potential
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  Text HTTP. How much does it contribute to
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  the greenhouse effect compared to CO2?
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  And some of those numbers were pretty shocking.
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  Weren't. They staggering take text R12,
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  an old common CFC. One single molecule of text or
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  traps almost as much heat in the atmosphere as 5000 molecules of
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  carbon dioxide 5000. Wow.
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  OK. So that really puts the climate
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  impact into perspective. It does, and that led to this
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  really stark trade off. Which brought us to the next
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  generation, the HFCS hydrofluorocarbons like text
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  R134 that's in many car ACS. Exactly.
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  We saw the first problem. HFCS have 0 text ODP because the
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  fluorine atom and attack ozone. Big win there, but they still
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  have significant text to GWP values.
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  We basically traded an ozone problem for a climate change
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  problem. Not ideal.
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  So the search continues. It does.
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  That's why there's such a push now towards natural
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  hydrocarbons, things like propane or butane.
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  Tex are 600. They offer zero Tex ODP and
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  practically 0 Tex GWPPS. Environmentally, they're great.
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  Not the catch. They are highly flammable.
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  Group A, three or sometimes B3. So using them means dealing with
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  significant safety risks and much more complex system
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  designs. Leak detection, ventilation,
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  charge limits. It adds a lot of engineering
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  overhead. OK, but before we leave
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  refrigerants, we have to mention the industrial workhorse ammonia
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  TXT 717. Ammonia, yeah, you can't ignore
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  it in the industrial world. It's rated B2, moderately toxic,
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  moderately flammable, so definitely not for your house.
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  Yet it dominates things like cold storage, food processing.
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  Why take the risk? Pure efficiency.
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  Ammonias month will be a vaporization.
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  That's the energy it absorbs when it boils is typically 6 to
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  8 times higher than the common Halo carbons. 6 to 8 times,
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  yeah. It means for every kilogram of
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  ammonia you pump around the system, you get massively more
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  cooling effect. It's just incredibly efficient
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  thermodynamically. So in big industrial plants with
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  strict safety protocols, leakage monitoring and trained
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  personnel, the efficiency gains often outweigh the risks.
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  OK, that makes sense. Now let's take that conceptual
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  leap. We're leaving the realm of
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  keeping food cold or rooms comfortable and entering
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  cryogenics temperatures below 150 Kelvin so -123 Celsius.
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  Even colder typically, yeah. Below -150 Celsius or 123
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  Kelvin, so below 150K. Right, 150K at these
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  temperatures even that super efficient ammonia system is
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  useless. The physics has to change.
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  It absolutely does in cryogenics.
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  The rule book changes that simple Joel Thompson cycle.
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  We touched on just expanding a gas through a valve to cool it.
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  It works great for many gases. But not for the really important
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  cryogenic ones. Fails completely for two vital
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  ones, hydrogen textes 2 and A and helium.
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  And why? What's the weird reason?
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  It's because at room temperature they have negative Joule,
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  Thompson and coefficients. Negative meaning when you expand
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  them through a valve from high pressure to low pressure.
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  Yeah, instead of cooling down like most gases, they actually
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  heat up. They heat up.
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  That's completely counterintuitive.
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  It is. Think of the JT effect is like
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  an internal friction or braking effect during expansion.
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  For most gases, it cools them. For hydrogen and helium at room
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  temp, it's like hitting the accelerator instead of the
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  brake. So you can't just expand them
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  from room temperature to get liquid text T2 or liquid BEX.
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  You have to pre cool them significantly first.
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  For hydrogen you need to get it down to liquid nitrogen
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  temperature around 77 Kelvin or even lower before it crosses
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  what's called the inversion temperature.
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  Below that temperature, the Jewel Thompson effect flips and
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  then expansion finally starts to cool.
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  It wow. OK, so that complexity leads us
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  to the real workhorse of deep cold, then the clod cycle or
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  expander cycle. Exactly.
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  The clod cycle is much smarter engineering, especially for
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  these gases. It tackles the inefficiency of
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  just dumping pressure across a valve.
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  How does it do that? Well, if you just expand gas
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  across a valve, that's an irreversible process
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  thermodynamically. You're essentially wasting the
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  potential energy stored in that pressure, generating entropy,
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  creating lost work W sub L. The Claude cycle prevents a lot
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  of that waste. It incorporates an expansion
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  engine. Think of it like a tiny
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  specialized turbine. So it makes the gas do work as
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  it expands. Precisely, you force the gas to
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  push a piston or spin a turbine as its pressure drops.
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  By extracting mechanical work, you achieve 2 things.
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  First, you get a much larger cooling effect for the same
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  pressure drop, ideally following an almost constant entropy path
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  on a temperature entropy diagram.
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  Making more efficient. Much more efficient.
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  And second, you recover some of that energy, reducing the
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  thermodynamic lost work that W sub L associated with simple
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  valve expansion. It makes the whole liquefaction
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  process far more effective, especially for gases like helium
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  and hydrogen. And efficiency is absolutely
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  everything down there, isn't it? Because the cost of removing
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  heat gets astronomical the colder you go.
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  It really, really does. The theoretical minimum work
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  text constants secuanor required to liquefy a gas just climbs
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  dramatically as you approach absolute 0.
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  To liquefy helium, for instance, takes about 26,700 joules per
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  mole. That's a lot of energy.
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  So because the energy investment is so huge, stopping heat from
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  leaking in becomes the biggest challenge.
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  Insulation is paramount. Absolutely critical.
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  The first line of defense is usually vacuum insulation.
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  Just pump all the air out between 2:00.
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  Walls. Basically, yes.
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  Create a vacuum jacket that eliminates heat transfer by
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  conduction and convection because there's no medium, no
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  air molecules to carry the heat across.
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  Leaving only radiation. Leaving only thermal radiation,
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  Heat traveling is electromagnetic waves, and
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  that's governed by the Stefan Boltzmann law.
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  The key part of that equation Q equals Sigma AF one 2 * ( t one
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  to the power of 4 -, t two to the power of 4 ) isn't the
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  complexity, it's that T to the power of 4 dependence.
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  Right temperature to the power four.
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  That means even a tiny temperature difference between
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  the warm outer wall and the cold inner wall results in a
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  surprisingly large amount of heat leaking in via radiation,
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  especially at room temperature radiating towards cryogenic
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  temperatures. Exactly, which is why for really
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  high performance systems we need super insulation.
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  Super insulation sounds serious. It is.
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  It takes a vacuum concept and adds another layer of defense
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  against radiation. You fill the vacuum space with
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  many, many layers of thin, highly reflective material,
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  often aluminum coated mylar plastic film separated by low
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  conductivity spacers. Each reflective layer acts like
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  a radiation shield, bouncing most of the incoming thermal
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  radiation back out. With enough layers you can cut
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  radiative heat transfer down by orders of magnitude.
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  But it sounds incredibly delicate.
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  That vacuum has to be perfect. It has to be very, very good.
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  For super insulation to work effectively, you need to
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  maintain extremely high vacuum levels, typically below 10 to
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  the power of -3 Tor. If you lose that vacuum, if air
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  leaks in conduction and convection come roaring back and
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  the insulation basically fails. OK, insulation covered last
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  critical piece materials. You can't just use standard
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  steel pipes or tanks at, say, 4 Kelvin liquid helium
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  temperature. There's this risk of brittle
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  failure. Brittle failure?
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  Yes, it's a huge concern in cryogenics.
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  It's when a material suddenly fractures at a relatively low
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  stress level with little or no warning, no stretching or
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  deformation beforehand, it just snaps.
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  That sounds terrifying for a pressure vessel.
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  It is, and many common engineering materials,
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  especially steels that are strong and ductile at room
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  temperature can become extremely brittle, like glass when they
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  get very cold. So what works?
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  What metals stay tough when they're super cold?
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  We need materials that retain their toughness.
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  Generally that means looking at metals with a specific crystal
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  structure, face centered cubic or FCC.
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  FCC structure? Like what material?
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  Things like austinitic stainless steels, the 300 series, like
• 14:57
  three O 4 or 316, also aluminum alloys, copper, nickel alloys.
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  These FCC metals tend to stay ductile even down to very low
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  temperatures. We verify this using impact
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  tests like the Chirpy impact test, which measures how much
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  energy and material absorbs during fracture.
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  For really demanding applications like handling
• 15:16
  liquid hydrogen, engineers often use stainless steels with higher
• 15:19
  nickel content because adding nickel is known to lower the
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  ductile to brittle transition temperature, keeping the steel
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  tough even when extremely cold. Wow.
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  So we've journeyed today from like the compressor maybe
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  sitting on on your roof, keeping your house cool all the way down
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  to the specialized alloys inside a liquid helium tank operating
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  near Absolute 0. It's quite the range and that
• 15:39
  you know, if we connect this fundamental engineering, this
• 15:41
  mastery of creating and containing extreme cold, it
• 15:44
  leads directly to some truly amazing modern technologies.
• 15:47
  One of the most exciting is superconductivity.
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  Yes, this is where the physics gets really, really weird and
• 15:53
  wonderful. It really does.
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  Below a certain critical temperature, which we call text
• 15:58
  T visible T sub zero specific materials suddenly exhibit 0
• 16:03
  electrical resistance. Absolutely none.
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  This was first discovered way back in 1911 by high
• 16:08
  Kammerlingani's working with mercury cooled by liquid helium.
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  0 resistance means electricity can flow through these materials
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  forever, theoretically with no energy losses.
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  Heat. Which is the dream for power
• 16:20
  transmission, but also allows for incredibly powerful magnets.
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  Right, exactly. Because you can run enormous
• 16:25
  electrical currents through superconducting wires without
• 16:28
  the melting, you can generate incredibly intense magnetic
• 16:32
  fields. Superconducting magnets, often
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  made from materials like niobium, Tin X, Andy's NLA, are
• 16:37
  commercially available, creating fields up to 10 Tesla, sometimes
• 16:40
  even more. And that's the key technology
• 16:43
  behind things like MRI machines and maybe even high speed
• 16:46
  trains. Precisely.
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  It's fundamental to MRI particle accelerators and, yes, advanced
• 16:52
  magnetic levitation mag Levit promotion systems.
• 16:55
  You hear about maglev trains reaching maximum operating
• 16:57
  speeds of, say, 550 kilometers per hour.
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  That's enabled by powerful superconducting magnets, which
• 17:03
  of course rely on cryogenic cooling systems to function.
• 17:06
  It all ties back together. Amazing.
• 17:08
  From keeping cool to flying on magnets until the next equation
• 17:11
  designed junkies. We are mechanical engineering
• 17:14
  made simple.