Two families: positive-displacement (reciprocating, screw) trap and squeeze a fixed volume — high pressure ratio, flow nearly independent of pressure; and dynamic (centrifugal, axial) add velocity then convert it to pressure — huge flow, flow that falls as pressure rises.
Compression heats the gas (T₂/T₁ = rp^((n−1)/n)). Staging with intercooling caps the discharge temperature and cuts the power toward the isothermal ideal — the second model shows how much.
A dynamic compressor runs on a map of speed lines bounded by surge on the left (flow reversal — destructive) and choke on the right. An anti-surge recycle valve keeps the operating point safely right of the surge line. The first model lets you drive it into — and out of — surge.
1 · A pump for gas — and why that’s different
Mechanically a compressor and a pump are cousins: both add energy to a fluid to raise its pressure. The difference is that liquids are essentially incompressible and gases are not. Squeeze a gas and its volume shrinks, its density rises, and — crucially — its temperature climbs. That single fact drives most of what makes compressors their own discipline:
- The work shows up as heat. Unlike pumping, a big fraction of compression work raises the gas temperature, which sets material limits, demands intercooling, and makes how you compress (in one step or several) an economic decision.
- Volume changes through the machine. The gas leaving is far denser than the gas entering, so inlet and discharge ends are sized very differently — and “flow” has to be stated carefully (actual inlet volume, mass, or standard volume).
- It can become unstable. A dynamic compressor pushing against more pressure than it can make will let the flow reverse — surge — something no centrifugal pump does in the same violent, cyclic way.
If you have read the pump fundamentals guide, much will rhyme — pump curves, the affinity laws, BEP — but each idea gains a thermodynamic twist here.
2 · The two families
Every compressor falls into one of two camps, distinguished by how it raises pressure:
| Positive displacement | Dynamic (turbo) |
|---|---|
| Traps a fixed volume and mechanically reduces it. | Accelerates the gas with an impeller/blades, then a diffuser converts velocity to pressure. |
| Reciprocating (pistons), screw, scroll, rotary-vane, diaphragm. | Centrifugal (radial), axial. |
| High pressure ratio per stage; modest flow. | Very high flow; modest pressure ratio per impeller (so multiple stages). |
| Flow is nearly constant with discharge pressure (a near-vertical curve). | Flow falls as discharge pressure rises (a drooping curve) — and can surge. |
| API 618 (recip), API 619 (screw). | API 617. The workhorse for large gas service. |
The rule of thumb: need a lot of pressure from a modest flow (a small high-pressure gas stream, hydrogen make-up, a reciprocating duty) → positive displacement. Need to move a great deal of gas at a moderate ratio (pipeline boosting, refrigeration, air separation, process gas) → dynamic. Screw compressors sit usefully in the middle for air and refrigeration.
3 · The thermodynamics: why it gets hot, and why staging helps
Compressing a gas from P₁ to P₂ always raises its temperature. The path depends on how much heat escapes during compression:
- Isothermal — compress infinitely slowly with perfect cooling so temperature never rises. This needs the least work, but it is an unreachable ideal.
- Adiabatic (isentropic) — compress with no heat removed at all. The hottest case, and more work than isothermal.
- Polytropic — the real machine, somewhere in between, captured by a polytropic exponent
nand a polytropic efficiencyηp.
Two consequences are practical and constant: a high single-stage ratio can drive the discharge temperature past what the metal, the gas, or the lube oil will tolerate (a hard ceiling, often ~150–200 °C); and more work is spent than necessary because you are compressing hot, low-density gas. Staging with intercooling solves both — split the ratio across stages and cool the gas back down between them. Each stage stays cooler, and because each later stage compresses cold (denser) gas, the total power falls toward the isothermal ideal. The model makes the trade-off concrete:
Interactive — Staging & intercooling
Live modelShaft power vs number of stages
rp,st = rp^(1/N), (n−1)/n = (k−1)/(k·ηp), polytropic head H = (1/m)·R·T₁·(rp,st^m − 1) with R = cp(k−1)/k; shaft power = ṁ·N·H/ηp, each stage intercooled back to T₁. Ideal-gas, constant properties, perfect intercooling assumed — illustrative, not for design (real plants add intercooler ΔT, pressure drop and an EOS).4 · The compressor map & surge
A centrifugal compressor doesn’t have a single curve — it has a map. For each running speed there is a curve of pressure ratio versus inlet flow, and because of the affinity laws (flow ∝ speed, head ∝ speed²) faster speeds sit higher and to the right. Overlay them and you get the family of speed lines, often with efficiency “islands” drawn on top. The machine’s usable world is bounded on two sides:
- Surge — the left limit. As flow drops at a given speed, the curve climbs to a peak. Push past that peak and the flow can no longer hold against the discharge pressure: it breaks down and reverses. Pressure collapses, the check valve slams, flow re-establishes, pressure builds, and it reverses again — a violent, audible cycle several times a second that hammers thrust bearings, seals and blades. Surge can destroy a machine in seconds.
- Choke (stonewall) — the right limit. At very high flow the gas reaches sonic velocity somewhere in the stage and the curve falls off a cliff; you simply can’t push more through.
The whole art of operating a dynamic compressor is staying in the band between them — and well clear of the surge line. Drive the model below into surge, then open the recycle valve and watch it recover:
Interactive — Compressor map & surge
Live modelThe compressor map
Anti-surge control: the recycle valve
Because surge is so destructive, every dynamic compressor has an anti-surge (recycle) system. A controller watches the operating point relative to the surge line; as flow falls toward the anti-surge control line (a margin to the right of true surge), it opens a recycle valve that routes discharge gas back to the suction. That extra flow keeps the machine safely loaded even when the process demand collapses — during start-up, trips, or turndown. It works, but every kilogram recycled is compressed for nothing, so the recycle gas is hot and the power is wasted; good control sits just clear of surge, not far from it. You saw both effects in the model: open the valve and the operating point marches right, away from surge, while the “recycle power” climbs.
5 · Reciprocating compressors & the PV diagram
Positive-displacement machines tell their story through the PV (indicator) diagram — pressure versus cylinder volume over one revolution. It is to a reciprocating compressor what the map is to a centrifugal one:
Two ideas fall straight out of that loop:
- Clearance volume & volumetric efficiency. The gas trapped in the clearance space at top-dead-centre doesn’t get discharged — it re-expands on the return stroke and delays the intake of fresh gas. The bigger the clearance (and the higher the ratio), the less fresh gas drawn per stroke: lower volumetric efficiency. It is also the basis of a neat capacity-control trick — adding adjustable clearance pockets turns capacity down without throttling.
- Valves are the weak point. Each cylinder breathes through spring-loaded suction and discharge valves cycling thousands of times a minute. They are the number-one reciprocating-compressor maintenance item — and a classic target for ultrasound and PV-analysis monitoring.
6 · What goes wrong
| Failure | Where & why |
|---|---|
| Surge damage | Dynamic machines run too close to (or into) the surge line — thrust-bearing and seal damage, blade fatigue. Prevent with anti-surge control and monitoring. |
| Valve failure | Reciprocating cylinders — fatigue, fouling, sticking. The dominant recip maintenance cost. |
| Fouling | Deposits on impellers/internals shift the map left and rob efficiency — wash or clean on condition. |
| Seal & bearing wear | Dry-gas seals, oil seals, journal & thrust bearings — the same rotating-equipment failure modes as pumps, at higher stakes. |
| Lube-oil & cooling | High discharge temperatures stress oil and intercoolers; degraded cooling pushes temperatures past limits. |
The same reliability toolkit applies. Compressors are prime candidates for the methods in the rest of the Academy: vibration analysis for rotordynamic and surge symptoms, alignment & balancing for the train, thermography on coolers and bearings, and online predictive monitoring tying it all into the CMMS. The criticality of large compressors usually puts them at the top of any RCM study.
Key takeaways
- A compressor is a pump for compressible gas — the work heats the gas, volume shrinks through the machine, and dynamic types can surge.
- Two families: positive displacement (high ratio, steady flow) and dynamic (high flow, drooping curve, runs on a map).
- Staging with intercooling caps discharge temperature and pushes power toward the isothermal ideal — with diminishing returns per stage.
- The map is bounded by surge (left) and choke (right); an anti-surge recycle valve keeps the operating point safely clear of surge.