Latest / Mechanical Engineering Made Simple / (#26) Reciprocating vs. Centrifugal: The Compressed Air Compressor Wars and the Hidden Cost of Leaks

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(#26) Reciprocating vs. Centrifugal: The Compressed Air Compressor Wars and the Hidden Cost of Leaks · Mechanical Engineering Made Simple

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  Welcome back, design junkies, to mechanical engineering made
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  simple. Great to be here.
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  We are diving deep today into, well, something fundamental
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  compressed air. It's pretty much everywhere in
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  industry. Absolutely.
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  So our our mission today is digging into the engineering
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  trade-offs. We're looking at selecting
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  sizing air compressors using our source material as the guide.
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  Yeah. And we're focusing on the, let's
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  call in the big three, reciprocating Rotary and
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  centrifugal compressors, understanding, you know why
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  you'd pick one versus another. OK, let's start with the basics
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  then. What makes compressed air so
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  useful in the first place? It's actually quite unique.
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  The energy stored and compressed air can be directly converted to
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  work. You don't need that intermediate
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  step like converting from heat energy first.
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  Direct Energy transfer. That's a neat advantage.
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  It really is, and looking at the machines that make this happen,
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  one thing that jumped out from the research was the comeback of
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  an old favorite. You mean the reciprocating
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  compressor? Exactly.
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  Yeah, for a long time, maybe the late 50s through the mid 70's,
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  the centrifugal type really took over, you know, higher capacity,
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  less maintenance headache, right.
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  The reciprocator sort of fell out of favor.
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  It did, but then two big things shifted the landscape.
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  Energy costs went way up and a new process plant started
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  getting smaller, more downsized. And suddenly efficiency became
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  king again. Precisely.
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  The reciprocator, being a positive displacement machine,
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  just has inherently higher efficiency, so it's clawed its
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  way back into a really prominent role.
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  It's an efficiency play, pure and simple.
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  OK, let's start there then. What defines the reciprocating
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  compressor? Well, fundamentally it's
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  positive displacement. That means it traps a fixed
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  volume of air and then forces it into a smaller space.
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  It's also intermittent flow, working stroke by stroke.
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  And it's superpower. Pressure, definitely pressure.
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  It boasts the widest pressure range of any compressor type.
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  We're talking from vacuum service all the way up to get
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  this 40,000 PSIG. 40,000 PSIG. Wow, just to ground that for
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  everyone listening, how does that compare to say, typical
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  factory air? Oh, it's orders of magnitude
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  higher. Your standard industrial plant
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  air usually runs somewhere between maybe 100 and 150 PSIG.
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  So 40,000 is for really specialized stuff.
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  Absolutely. Things like polyethylene
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  production, maybe high pressure gas injection, it just
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  demonstrates the incredible range and frankly the robustness
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  of that reciprocating design. OK.
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  And that high pressure leads us to staging, right?
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  Why move to two stages instead of just doing it all in one go?
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  It's really about managing the heat of compression and
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  efficiency too. But heat is critical.
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  A single stage unit is fine for lower pressure, say 25 to maybe
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  100 PSIG, but. Beyond that?
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  Beyond that, the temperature gets really high, so a two stage
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  machine compresses part way. Then crucially, it cools the air
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  down in an intercooler, removes a lot of that heat, then it does
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  the final compression stage, usually taking it up to the 100
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  to 250 PSIG range. So the intercooler is key for
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  protecting the machine and for efficiency.
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  Both. Yes.
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  Heat is wasted energy, so removing it improves the overall
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  thermodynamic efficiency significantly, plus it prevents
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  overheating components. And there's a standard that
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  addresses this heat, specifically API 6/18.
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  Yes, API 618. That's the big one.
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  For heavy duty recipes. It recommends limiting the
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  discharge temperature to 300°F. 300F and that's an average.
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  That's the important part. Yes, it's an average.
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  You'll definitely have hot spots inside the cylinder that get
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  hotter than that, so managing that thermal load is vital for
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  the machine's longevity. OK.
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  Another critical point for reciprocators is oil free air.
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  Some industries just can't have oil contamination right?
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  Like food or electronics. Correct 0 tolerance for oil and
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  engineers have basically 2 main ways to achieve this with a
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  reciprocator. What's the first method?
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  Material science. You swap out the traditional
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  lubricated piston rings and packing rings for materials that
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  are inherently low friction or self lubricating.
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  Think carbon or Teflon based materials.
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  And the second path. Physical separation.
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  This is where you see the cross head.
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  Design the cross head. How does that work to keep the
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  oil? Out it uses something called a
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  distance piece. It's essentially A spacer,
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  sometimes A11, that creates a physical gap and isolated
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  chamber between the cylinder, where the air is and the
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  crankcase where the lubricating oil lives.
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  An engineered barrier. Exactly prevents that Lube oil
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  from migrating into the compression cylinder.
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  Got it. And regardless of oil or oil
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  free, one thing seemed non negotiable, inlet filters.
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  Absolutely mandatory. Think about it, Pipe scale dust
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  from the ambient air. That stuff will just destroy the
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  precision surfaces inside a cylinder bore very quickly.
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  Filters are cheap insurance against catastrophic wear.
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  OK. So the Reciprocator, high
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  efficiency, amazing pressure range, but maybe more complex
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  with valves and maintenance. What happens when we move to the
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  Rotary family? What's the big advantage there?
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  Simplicity mainly. The single most important common
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  feature for Rotary compressors is they don't have those suction
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  and discharge valves like a reset.
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  Fewer moving parts to breakdown. Exactly, and they also tend to
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  run smoother, less shaking force.
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  That often means simpler, less massive foundations are needed,
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  which is a cost saving right there.
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  Let's talk about the most common Rotary type, the Rotary screw,
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  the twin rotor version. Right.
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  So here you have two precisely machine rotors that mesh
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  together. As they turn, they trap pockets
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  of air and the volume of those pockets decreases along the
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  length of the rotors, which compresses the air.
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  It's a constant volume variable pressure machine.
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  And many of them are oil flooded.
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  You mentioned that. Yes, that's very common.
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  Oil is injected directly into the compression chamber as it's
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  happening. Why deliberately mix oil with
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  the air you're compressing? Seems counterintuitive.
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  It does, but it serves 2 brilliant purposes.
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  One, it seals the tiny gaps, the clearances between the rotors
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  and the housing that boosts sufficiency.
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  See Two and this is huge. It directly cools the air during
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  compression. Direct cooling.
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  Yeah, which means you can achieve really high pressure
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  ratios like up to 21 to 1 in just a single casing because
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  you're constantly removing that heat.
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  It's a very effective design. But there's a dry version to the
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  dry screw. What are the trade-offs there?
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  Well, without the oil for sealing and cooling, they need
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  really precise timing gears to keep the rotor synchronized so
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  they don't crash into each other.
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  And because of those gears and the dry operation, they create a
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  lot of high frequency noise. Noise that needs special
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  handling. It does.
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  You can't just use a simple volume bottle or muffler like
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  you might on a recipe. Dry screws usually need a
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  specific, often proprietary silencer from the manufacturer
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  designed to tackle that particular high frequency
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  pulsation. OK.
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  Moving through the Rotary family, what about the sliding
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  vein compressor? OK, picture this.
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  You have a rotor, but it's mounted eccentrically off center
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  inside a cylindrical housing. The rotor has slots, and in
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  these slots are veins that can slide in and out.
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  As the rotor turns, centrifugal force throws the veins outward
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  so they seal against the housing wall.
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  Because the rotor is eccentric, this creates these Crescent
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  shaped pockets of air that get smaller as the rotor rotates.
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  Compressing the air. Where are these typically used?
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  They're good general purpose machines, often use single stage
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  up to about 50 PSIG, or maybe staged up to 125 PSIG.
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  And they're actually excellent for vacuum service, too, down to
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  about 28 inches of mercury. But the sources mentioned a
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  pretty scary failure mode vein breakage.
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  Yes, that's the Achilles heel. Those veins wear down over time,
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  constantly sliding against the cylinder wall.
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  You have to monitor that wear very closely.
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  Because if one breaks. If a vein breaks or wears too
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  short, it can get jammed or worse, wedge itself between the
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  rotor and the cylinder. That can completely destroy the
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  cylinder. It's a catastrophic, often non
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  repairable failure. Very expensive.
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  Yikes. OK, that makes the next one, the
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  low compressor sound appealing just for its simplicity, even
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  though it's compression method is.
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  Odd. It is definitely
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  counterintuitive. Lobe compressors don't really
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  compress the air inside the machine at all.
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  Their clearance type machines, meaning the rotors which look
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  like figure eights or lobes, just trap air and push it
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  towards the discharge port. The actual compression only
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  happens when that air hits the back pressure already existing
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  in the discharge pipe system. So the pipe does the
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  compressing, not the machine itself.
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  Essentially, yes. The rotors are just movers of
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  air volume. Because of this, they are
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  inherently low pressure devices. What's the limit typically?
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  For larger units, say above 3000 CFM, the practical upper limit
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  is only about A10PSI pressure differential.
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  They move bulk air, but not to high pressures.
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  Interesting. OK, last one in the Rotary
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  group, the liquid ring compressor sounds wet.
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  It is. It uses a rotor to drive a
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  captive ring of liquid, usually water, inside the casing.
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  Again, the rotor is eccentric, so it forms chambers between its
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  blades and the liquid ring. As it rotates, these chambers
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  shrink, compressing the trapped air.
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  You mentioned the efficiency isn't great.
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  No, it's relatively low, maybe around 50% because you're
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  spending energy just slinging that liquid around.
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  So why use it? Two key benefits.
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  One, it's inherently oil free because the liquid acts as the
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  sealant and coolant, 2 and this is often the main reason it can
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  handle entrained liquids in the incoming Airstream, and it
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  actually scrubs particulates out of the air down to Micron cell
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  uses. So if you have really dirty wet
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  air, this is your go to machine. Exactly.
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  It cleans as it compresses. OK, that covers the rotaries.
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  Now let's shift to the powerhouse, the machine that
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  really dominated for decades in terms of sheer volume, the
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  centrifugal compressor. Yeah, if you measure by a
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  capacity CFM or just total installed horsepower,
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  centrifugals are king. These are dynamic machines,
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  totally different from positive displacement.
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  Dynamic, meaning they use high speed rotating impellers to
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  fling the air outwards, adding velocity, and then a diffuser
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  section converts that velocity into pressure.
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  They're designed for huge flows. Think 1000 up to maybe 30,000
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  CFM or even more. And they seem built for
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  reliability. Very much so.
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  They're inherently oil free in the air path.
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  The lubrication system for the bearings and gears is sealed
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  off. The design is relatively simple,
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  dynamically balanced, so they don't need the massive heavy
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  foundations that a big reciprocator would.
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  And runtime. Typically very long.
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  You're often looking at three years or more between major
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  overhauls and seeing six year run times isn't unusual at all.
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  They're built for continuous heavy duty service.
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  Given those huge capacities, getting the sizing right must be
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  absolutely critical. The sources highlighted a common
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  pitfall when comparing bids. This is a big one.
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  Yes, you must compare apples to apples.
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  That means specifying the required capacity based on the
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  actual conditions, the compressor inlet, the real inlet
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  pressure, temperature and relative humidity.
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  Not just asking for standard cubic feet per minute.
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  SCFM. Exactly because different
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  manufacturers might use slightly different definitions of
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  standard conditions, additions, temperature, and pressure.
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  If you just ask for SCFM without specifying the inlet conditions,
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  you could end up with a machine that's significantly undersized
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  or oversized for your actual needs.
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  Always anchor it to actual inlet cubic feet per minute ACFM.
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  OK. For quick back in the envelope
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  calculations, there's a rule of thumb for volumetric efficiency.
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  Yeah, a useful one for plant air systems around 100 PSIG.
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  It's typically in the range of 4 to 4.2 SCFM per brake
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  horsepower. For practical estimating, using
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  4 SCFM per horsepower is usually close enough. 4 SCFM per
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  horsepower. Good to remember.
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  And when designing a new system you can't just aim for the
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  expected load right? You need margins.
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  Absolutely. You need some headroom.
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  The general guideline is for water cooled compressors add
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  about a 30% capacity margin over your calculated system demand
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  and. For air cooled.
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  Bump that up to 40%. Air cooled machines are more
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  sensitive to ambient temperature swing, so they need a bit more
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  buffer. Now you can trim these margins
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  back, but only if you have really solid historical data
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  from your specific plant showing exactly what your air usage
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  patterns are. Otherwise, stick to the 30 or
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  40%. Makes sense, and selecting the
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  right compressor isn't just about flow and pressure.
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  Our sources listed a whole bunch of factors.
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  Oh yeah, it's a whole system analysis you've got to consider,
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  Let's see the air requirements, obviously the type of driver,
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  electric motor turbine, the location, any regulations like
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  noise limits, maintenance, access and get ability, even
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  things like the subsoil conditions for the foundation.
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  And costs too, not just the purchase price.
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  Right, power costs, which are huge over the life of the
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  machine, and maybe water availability and cost if you're
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  looking at water cooled options. It's a complex decision matrix.
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  You might choose a quieter, more expensive machine if you're
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  right next to a residential area, for example.
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  OK, Speaking of costs, let's wrap up with something that
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  really hit home in the research. The immediate, very real cost of
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  just not maintaining the system properly.
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  Specifically air leaks. Yes, the silent killer of
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  efficiency. It's genuinely shocking how much
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  energy is just wasted, bled off into the atmosphere through
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  leaks and pipes, fittings, hoses, tools.
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  The numbers in the source material were pretty stark.
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  Using their example cost about $0.10 per 1000 cubic feet of
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  air. OK, let's take their example.
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  A single leak from just a 3/8 inch diameter opening that's not
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  huge. You might barely hear it in a
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  noisy plant, right? At 100 PSIG.
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  That single small leak wastes 6,671,890 cubic feet of
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  compressed air every single month.
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  Wow. Almost 7,000,000 cubic feet just
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  gone. And based on that $0.10 per 1000
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  cubic feet cost, that translates directly to $667.19 wasted every
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  month from one small leak. That's incredible.
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  It's more than $20 a day just vanishing.
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  Exactly, and most plants have dozens if not hundreds of leaks
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  like this or smaller. It adds up incredibly fast.
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  It's just pouring money, or rather energy down the drain.
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  So the big take away there is pretty clear there isn't.
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  It absolutely. Before you even think about
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  buying a bigger compressor because you think you don't have
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  enough air, do a thorough leak audit and repair program.
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  Fixing those leaks often pays for itself very quickly and
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  might completely eliminate the need for a larger machine.
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  Efficiency starts with stopping the waste.
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  You got it. Seal the system first.
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  OK. So as we wrap up, it really
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  brings us back to that central tension we started with.
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  Especially now with high energy costs, it's often a choice
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  between say the big centrifugal machine, high capacity, maybe
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  lower maintenance, simpler foundation versus the
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  reciprocator, maybe lower initial capacity per dollar, but
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  that higher inherent efficiency. It really crystallizes the
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  decision, doesn't it? It's that balance, initial
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  capital cost versus long term operating cost, mainly driven by
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  energy consumption. And in today's world, energy
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  cost is often the dominant factor over the machine's
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  lifetime. A fascinating engineering trade
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  off. A choice between output now and
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  efficiency forever. Lots to think about until the
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  next equation. Design junkies.
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  We are mechanical engineering made simle.