HVAC compressor types: scroll, screw, reciprocating, centrifugal

The compressor, the pump of the cycle

A compressor is the pump of the refrigeration cycle. It pulls low-pressure vapor off the evaporator, squeezes it up to a high pressure and temperature, and pushes it to the condenser. Everything else in the system is plumbing and heat exchange. The compressor is the only part that adds work, and it is the reason heat moves from a cold space to a warm one instead of the other way around.

Two things happen at once when it runs: pressure goes up, and refrigerant moves. Raise the pressure of the discharge vapor and you raise its saturation temperature above the outdoor air, so it can reject heat in the condenser. Drop the pressure on the suction side and you lower the saturation temperature below the space, so the evaporator can absorb heat. The compressor is what holds that pressure difference open.

Because it is the one moving heart of the machine, it is also the most expensive single part and the one that fails most expensively. A blower motor is a Tuesday. A burned compressor on a packaged rooftop unit can total the equipment. Most of what follows is about knowing which type you have, why it was chosen, and how to keep it alive, because the charge that feeds it decides a lot of that, which is its own subject covered in the refrigerant charging guide.

Positive displacement or dynamic?

Compressors split into two families by how they raise pressure. Positive displacement traps a fixed volume of vapor and physically shrinks it. Dynamic accelerates the vapor to high velocity and then converts that velocity into pressure. That single difference drives almost everything about where each type lands, how it behaves at part load, and how it fails.

Positive displacement is the larger family by far. Reciprocating, scroll, screw, and rotary all trap and squeeze. They make their pressure ratio by geometry, so they hold head well even when the suction pressure sags, which matters in refrigeration and heat pumps where conditions swing. The trade-off is moving parts in contact, valves or rotors or scrolls, that wear and need oil.

Dynamic means centrifugal in this trade. An impeller flings the vapor outward, a diffuser slows it down, and the kinetic energy becomes a pressure rise. There is no trapped volume and no positive seal, so a centrifugal can move enormous flow smoothly, but it depends on flow to stay stable. Let the flow fall too far and it surges, which positive displacement machines never do. That is the core reason centrifugal owns the large-tonnage chillers and nothing small.

What does a reciprocating compressor do?

A reciprocating compressor uses pistons in cylinders to trap and squeeze vapor, the same way a piston engine moves air, just run backward as a pump. A crankshaft drives the pistons, suction and discharge reed or plate valves open and close with the pressure swing, and each down-and-up stroke pulls in a charge of vapor and compresses it. It is the original refrigeration compressor and still the workhorse of supermarket racks and industrial refrigeration.

Recips hold up because they are simple and they tolerate the high pressure ratios that low-temperature refrigeration demands. You see them parallel-piped on a rack, several semi-hermetic compressors sharing a suction header, staging on and off to follow the case load. For capacity they can also unload individual cylinders: a solenoid holds the suction valve open on a cylinder so it pumps nothing, dropping the machine to stepped capacities like 75, 50, or 25 percent without stopping it.

The valves are the weak point. Reed and plate valves flex millions of cycles and they break, and a broken discharge valve shows up as a compressor that pumps poorly with a discharge line that runs hot and a suction that will not pull down. Recips also hate liquid. They are stiff, the clearance over the piston is small, and a slug of liquid that cannot compress will snap a valve plate or bend a rod. That intolerance is why the scroll pushed them out of residential AC.

The scroll compressor

A scroll compressor compresses vapor between two interleaved spirals, one fixed and one orbiting. The orbiting scroll does not spin, it traces a small circle, and as it orbits it traps a pocket of vapor at the outer edge and walks that pocket inward, shrinking it continuously until it discharges at the center port. The whole compression takes a little over two turns of the shaft, so it is smooth and quiet, with far fewer moving parts than a recip and no suction or discharge valves to break.

That smoothness and the low parts count are why the scroll took over residential and light-commercial air conditioning and heat pumps. It is the compressor in most split-system condensers and packaged units built in the last twenty years. Many scrolls are compliant: the orbiting scroll can move slightly axially or radially so the tips and flanks ride against the mating scroll with a controlled force, sealing well at running conditions and backing off to pass a slug of liquid or a debris particle instead of grenading. That compliance is a big part of why a scroll survives the floodback that would kill a recip.

For more capacity than one scroll gives, manufacturers tandem or manifold two or three scrolls on a common base into one circuit, staging them to follow load. Above that you move to screws. A scroll also runs in one direction only, so a miswired three-phase scroll spun backward will not pump and will get loud and hot fast. Check rotation on a three-phase scroll at startup, every time.

The screw compressor

A screw compressor compresses vapor between helical rotors that mesh like gears without touching. The common form is twin-screw: a male rotor with a few lobes drives a female rotor with more interlobe grooves, and as they turn, the meshing point sweeps along the rotors, shrinking the trapped gas volume from the suction end to the discharge end. There are also single-screw designs with one main rotor and two gate rotors. Either way the flow is continuous and smooth, with no pulsing valves, which is why screws run quietly at high capacity.

Screws own the middle and upper-middle of the chiller and industrial refrigeration market, where a recip would need too many cylinders and a centrifugal would be oversized or surge-prone at part load. Their signature capacity control is the slide valve: a movable section of the rotor housing slides to delay the point where compression starts, bleeding off capacity continuously, commonly from full down to roughly a quarter, and pairing well with a variable-frequency drive for the rest.

Screws live on oil. They are oil-flooded: oil is injected into the compression space to seal the clearance between the rotors, to cool the gas, and to lubricate. That means a screw machine carries a real oil-management system, a separator that strips oil out of the discharge gas, an oil cooler, a filter, and a pump or pressure-driven feed to return it. Lose oil flow or oil cooling on a screw and the rotors gall fast. The separator and oil return are not accessories on a screw, they are part of the compressor.

The centrifugal compressor

A centrifugal compressor is dynamic, not positive displacement. A high-speed impeller accelerates the refrigerant vapor outward by centrifugal force, then a diffuser slows that fast-moving vapor and turns its velocity into a pressure rise. There is no trapped pocket and no positive seal, so a centrifugal moves huge volumes of vapor smoothly and efficiently, which is exactly what a large chiller needs. This is the compressor in the big water-cooled machines that cool office towers, campuses, and district plants.

The catch unique to centrifugal is surge. The machine needs forward flow to stay stable. Drop the load too far or push the lift too high and flow through the impeller collapses and momentarily reverses, then re-establishes, then collapses again. That cycling is surge, and it shows up as a heavy rhythmic whoosh or rumble with swinging amps and pressures. Surge hammers the thrust bearing and is the failure mode to design and control around on every centrifugal. Capacity is trimmed with inlet guide vanes that pre-swirl the gas, usually combined with a VFD to push the surge line down and the efficiency up.

The newer machines are oil-free. Magnetic-bearing centrifugals levitate the shaft on a magnetic field with no mechanical bearings and no oil system at all, driven by a high-speed permanent-magnet motor on a VFD. Removing the oil removes the oil-fouling that robs chiller efficiency over time and removes a whole maintenance system, which is why oil-free magnetic-bearing centrifugals have taken over a large share of new mid-size and data-center chiller work. Cross-link the chiller plant startup guide for how these get commissioned.

What is a rotary compressor?

A rotary compressor is the small positive-displacement type that lives in window units, PTACs, refrigerators, and the indoor or outdoor sections of mini-splits. The common form is the rolling-piston design: an eccentric roller turns inside a cylinder and sweeps vapor ahead of a spring-loaded vane that rides against the roller, compressing the gas in one turn. There is no crankshaft-and-rod assembly and very few parts, so it is compact, light, and quiet.

Rotaries dominate the bottom of the range, roughly the fractional-horsepower to a few tons, where a recip would be bulky and a scroll is overkill or too costly. Most of the high-efficiency mini-splits you install run a variable-speed rotary on an inverter, which is a big reason a good ductless head can modulate so far down and hold a room dead steady. The physics are the same as the bigger machines, just at low capacity and tight tolerances.

For service, the rotary is almost always a fully sealed hermetic, so there is nothing to repair inside it. It is also direction-sensitive on the inverter-driven units, and like any small hermetic it is unforgiving of acid and moisture because there is no filter-drier the size of a beer can protecting it. Keep the system clean and dry and it runs for years.

Matching the compressor to the tonnage

The type follows the size more than anything else. Small loads go to rotary and scroll, mid loads to scroll and screw, large loads to screw and centrifugal. The ranges below overlap and shift by manufacturer and refrigerant, so treat them as typical territory, not hard lines, and confirm any specific machine against the manufacturer's data and its AHRI rating.

Where two types overlap, part-load behavior usually breaks the tie. A screw and a small centrifugal might both cover 200 tons, but if the load spends most of the year at half capacity, the screw with a slide valve and VFD often holds efficiency better and never risks surge. The selection lives in the load profile, not the peak.

How does each compressor control capacity?

Every type has its own way of following a load that is almost never at design. Match the control method to the type and you understand most of what you will see on the gauges and the amp clamp during part-load operation.

Reciprocating machines stage cylinders on and off with suction-valve unloaders, giving stepped capacities, and a parallel rack stages whole compressors. Scrolls stage in tandem or manifold sets, or run digital, where the top scroll lifts to unload for part of a cycle, or run a true inverter for smooth modulation. Screws use a slide valve for continuous turndown, usually paired with a VFD. Centrifugals throttle inlet guide vanes and ride a VFD, working down toward the surge line and no further.

The thing to carry: positive-displacement types can unload to small fractions and keep running, while a centrifugal has a hard floor set by surge. That floor is why large all-centrifugal plants stage multiple machines and lean on a VFD, and why a single centrifugal serving a load that drops to a fraction of design is the wrong selection no matter how efficient it looks at full load.

Part-load efficiency and where it pays

Equipment almost never runs at design load. A chiller might hit full tonnage a few hours a year and spend the rest below half, so the efficiency that actually shows up on the power bill is part-load efficiency, not the full-load number on the nameplate. That is the whole reason the industry rates chillers on a weighted part-load figure, IPLV or NPLV under the AHRI chiller standard, instead of a single full-load point.

At part load, the variable-speed machines pull ahead. A compressor slowed to match the load draws far less power than one cycling on and off or throttling against itself, because the work drops with the speed. Inverter scrolls, VFD screws, and VFD centrifugals all gain at part load for the same reason, and a centrifugal in particular gets more efficient as it unloads, right up until it nears surge.

The practical read for selection: a system that lives at part load justifies the variable-speed machine even if it costs more up front, while a process load that runs flat out most of the time may not. Size and select to the load profile, then prove the part-load numbers at commissioning. The chiller plant startup guide covers how those numbers get verified on a real plant.

What is the difference between hermetic, semi-hermetic, and open?

The difference is how the motor is enclosed with the compressor, and it decides whether you can ever take the machine apart. There are three arrangements: hermetic, semi-hermetic, and open-drive. The choice trades serviceability against leak-tightness.

A hermetic compressor seals the motor and the compressor together in a welded steel shell. There is no shaft passing out of the housing, so there is nothing to leak, but there is also no way in. When a hermetic burns out, you replace it. These are the small machines: most residential scrolls and nearly every rotary in a mini-split or refrigerator. A semi-hermetic also puts the motor and compressor in one housing, but the housing is bolted with gasketed covers instead of welded. You can open it, change valve plates, pull a piston, even rewind or swap the motor. That is why supermarket racks and many commercial chillers run semi-hermetic recips and screws, they are meant to be rebuilt in place.

An open-drive compressor has the motor outside the refrigerant housing entirely, connected by a shaft through a seal or by a coupling. The motor is fully accessible and can be any type, including an engine, but the shaft seal is a wear point and the most likely leak path on the machine. Open-drive shows up on large ammonia plants and some big chillers. Quick tell in the field: a weld seam all the way around means hermetic, bolt-on heads and covers mean semi-hermetic, an external motor with a coupling guard means open-drive.

Oil management by compressor type

Oil is not optional and it is not the same job in every machine. The compressor needs oil to seal and lubricate, but the oil rides around with the refrigerant, and getting it back to the compressor is half of refrigeration system design. How that is handled depends on the type.

Recips and scrolls hold oil in a crankcase or sump and rely on the gas velocity in the suction lines to carry oil back from the system. That puts the burden on the piping: long or poorly pitched suction risers trap oil, starve the compressor, and seize it slowly. Parallel racks add an oil-management system with a reservoir and float regulators that keep each compressor's level right. Screws are oil-flooded, so they always carry a discharge oil separator, a cooler, and a return path, because the oil is injected into the compression itself. Centrifugals with bearings keep a lubricated bearing oil system separate from the gas path, while magnetic-bearing oil-free centrifugals have no oil to manage at all, which is one of their main selling points.

The field lesson across all of them: oil that leaves the compressor and does not come back is a death sentence on a clock. Find the oil before you find the failure, and watch the sight glass and the oil pressure safety on any machine that has them.

Inverter and variable-speed compressors

An inverter compressor runs on a variable-frequency drive that changes the motor speed to match the load instead of cycling the compressor on and off. Slow the compressor and it pumps less refrigerant and draws less power, with the work falling off faster than the capacity, which is where the efficiency comes from. The technology started on scrolls and rotaries and is now standard on high-efficiency residential and light-commercial gear and common on screws and centrifugals.

Two payoffs beyond the energy. First, comfort: a variable-speed machine settles at a low steady output that holds tight space temperature and pulls more humidity, because it runs long and slow instead of blasting and stopping. Second, soft starts: ramping up avoids the inrush and the contactor hammering of a fixed-speed compressor slamming on across the line. A mini-split holding a room within a degree all day is doing it with a variable-speed rotary or scroll.

The cost is the electronics. The drive is now a part that fails, it makes electrical noise that can upset other equipment, and it needs the right line conditions and sometimes filtering. Troubleshooting shifts toward the drive and its fault codes as much as the mechanical compressor. When an inverter system acts up, read the drive before you condemn the compressor.

What kills a compressor?

Compressors rarely die of old age. They get killed, and the killers are a short list that repeats across every type. Know them and most failures are preventable.

Liquid is first. Floodback is liquid refrigerant returning down the suction line during running, washing the oil off the bearings and cylinders. Slugging is liquid actually in the compression chamber, usually on a flooded start, and because liquid does not compress it snaps valves, breaks scrolls, and bends rods in one stroke. Both trace back to charge, metering, airflow, and oil migration. The charging guide covers setting superheat so the suction comes back as vapor, which is the front-line defense.

After liquid: loss of lubrication, from oil that migrated away, never returned, or got washed thin, which lets metal touch metal until it galls or seizes. Overheating, from low charge, high head, a failing condenser, or restricted airflow, which cooks the motor windings and breaks down the oil so it stops lubricating. Short-cycling, where the compressor starts and stops dozens or hundreds of times a day, never builds oil pressure, and beats up the windings and bearings on every inrush. And contamination, the moisture and acid that come in with a leak or a sloppy repair and eat the windings from the inside, the classic slow path to a burnout. A crankcase heater fights one of these directly: it keeps the oil warm during the off cycle so refrigerant does not migrate into it and condense, which is what causes the flooded start in the first place.

Reading a compressor in the field

Three readings tell you most of what a compressor is doing: the amps, the suction pressure, and the discharge pressure. Take all three with the system running at a known condition and you can tell a healthy machine from a sick one before you ever open anything.

Amps against the nameplate rated-load amps is the first check. High amps with low capacity says the motor is working hard for little output, which points to high head, a mechanical drag, or a failing winding. Low amps can mean the machine is barely pumping, a worn recip with bad valves or a scroll losing its seal. Then read the gauges. The suction and discharge pressures, converted to saturation temperatures and compared against the coil conditions, give you superheat and subcooling, which is how you separate a charge problem from a compressor problem. That conversion and the targets live in the refrigerant charging guide, and a set of accurate gauges or a digital manifold is the tool that makes the call.

Two quick field tells. A recip that will not pull the suction down and runs a hot discharge usually has broken valves. A compressor drawing locked-rotor amps and tripping instantly is electrically or mechanically seized, and that is a replacement, not a repair. Before condemning any compressor electrically, check the windings to ground and winding to winding with the power off and the terminals discharged, so you are reading the motor and not a contactor or a wire.

Replacing a compressor after a burnout

Replacing the compressor is the easy half. The system that killed the first one will kill the second one if you do not find out why it died and clean up what it left behind. A motor burnout in particular dumps acid and contamination into every foot of the system, and that acid does not leave with the old compressor.

After a burnout, the work is decontamination. Recover the refrigerant, pull the dead compressor, and clean the system: typically oversized suction and liquid filter-driers to scrub acid and moisture, with an acid test of the oil, and on a bad burnout a second cleanup pass after the system has run a while to catch what the first driers missed. Skip that and the new compressor's oil turns acidic in weeks and you are back. The other non-negotiables are a deep vacuum to pull moisture and non-condensables, verified with a micron gauge that holds, and a correct charge, both covered in the charging guide.

Match the replacement before you wire it. The compressor is sized and built for the system's refrigerant, voltage and phase, capacity, and application range, and a near-miss substitution runs hot, floods back, or short-cycles. Confirm oil type too, because the wrong oil with the system refrigerant will not return or lubricate right. A compressor change is a system service, not a part swap.

Selecting and matching to the system

The compressor is matched to the system it lives in, and the mismatches are quiet at first and expensive later. Refrigerant, voltage and phase, capacity, oil, and application envelope all have to line up, and the envelope is the one people forget. A compressor rated for medium-temperature AC will not survive low-temperature refrigeration duty, and a low-temp compressor run at high suction can overload and overheat.

Refrigerant choice is moving under everyone right now. The shift to lower-GWP refrigerants, including the mildly flammable A2L blends, means a compressor and the system around it are designed for a specific refrigerant family, and you do not freelance a substitute. The charging guide covers the refrigerant side of that change. For selection, take the verdict simply: install the refrigerant, oil, and compressor the equipment was built and listed for, and confirm the application range covers the actual operating conditions, not just the design point.

Where a real choice exists, it is usually scroll versus screw versus centrifugal at the boundaries of their ranges, and the deciders are the load profile, the part-load hours, and the lift. Pick for the conditions the machine will actually see across the year. Specsmanship at full load means little if the system spends its life at forty percent.

Cold-climate heat pumps and vapor injection

Heat pumps lose capacity as it gets colder, because the suction pressure falls with the outdoor coil temperature and the compressor pumps thinner vapor. The fix that made cold-climate heat pumps practical is vapor injection, often called enhanced vapor injection or EVI, built into a scroll compressor.

An EVI scroll has an extra port partway through the compression. A small stream of refrigerant is flashed through an economizer to subcool the main liquid feed, then that intermediate-pressure vapor is injected into the scroll mid-compression. The result is more heating capacity and better efficiency at low ambient, holding capacity down into temperatures where a plain compressor would fall off badly and lean on electric resistance heat. Manufacturers publish capacity holding well below freezing on these, but confirm the specific machine's rated performance against its own data rather than a rule of thumb.

Most cold-climate machines pair vapor injection with an inverter drive, so the compressor both injects and varies speed, ramping up hard for capacity in the cold and settling down low in mild weather. When you service one of these, the economizer circuit and the injection valve are extra parts in the diagnosis that a conventional heat pump does not have.

The data-center and large-chiller compressor

Large cooling loads that run around the clock, data centers most of all, push toward two compressor types: the screw and the centrifugal, and increasingly the oil-free magnetic-bearing centrifugal. The driver is the duty. A load that runs twenty-four hours a day at high but variable demand makes part-load efficiency and reliability worth more than first cost.

Oil-free magnetic-bearing centrifugals fit that duty well. With no bearing oil there is no oil fouling the heat exchangers to erode efficiency over the years, no oil system to maintain, and the VFD gives strong part-load numbers across the swinging load a data center presents. They are often staged several to a plant so machines can drop off as load falls without driving any single one into surge. Where the lift or the part-load floor argues against centrifugal, VFD screws cover the duty with a slide valve and no surge limit.

The compressor is only one part of the answer in these plants. The redundancy, the staging sequence, and the proof of flow before any compressor starts are plant-level concerns, and the chiller plant startup and commissioning guide covers how a critical plant gets stood up and tested.

What to document

When you service or replace a compressor, the record is what lets the next person understand the machine without tearing into it. Capture the type and family, the model and serial off the nameplate, the refrigerant and oil type and amount, the rated-load amps and the measured amps, the suction and discharge pressures at a known condition, and the capacity-control arrangement. On a replacement, write down why the first one failed and what cleanup was done, because a burnout history changes how the next failure gets diagnosed.

Common mistakes

• Letting liquid back to the compressor from an overcharge or a low superheat, so it floods or slugs.
• Running a machine low on oil because the suction piping or oil return was never set up to bring it back.
• Short-cycling a compressor with a bad control or an oversized machine, beating up the windings on every start.
• Skipping the acid test and the cleanup driers after a burnout, so the new compressor turns its oil acidic in weeks.
• Mismatching a replacement on refrigerant, oil, voltage, capacity, or application range and calling it close enough.
• Leaving no crankcase heater, or a dead one, so refrigerant migrates into the oil and the next start is a flooded start.
• Starting a three-phase scroll or inverter machine without checking rotation, so it runs backward, hot, and loud.
• Condemning a compressor electrically without discharging the terminals and reading the windings to confirm it is the motor.

Field checklist

Standards and references

Compressor and chiller performance ratings come from AHRI. Positive-displacement compressors are rated under AHRI Standard 540, which sets how a manufacturer publishes capacity and power across operating conditions, so two compressors can be compared on the same basis. Water-chilling and heat-pump packages are rated under AHRI 550/590 in I-P units and 551/591 in SI, which define the full-load and part-load ratings, including the weighted IPLV and NPLV part-load figures used to compare chillers. Confirm the current edition, because these are revised on a cycle.

Around those ratings, the manufacturer's data is the controlling authority for any specific machine: the application envelope, the oil type and charge, the capacity-control range, and the allowable operating conditions all come from the compressor manufacturer and govern over any general rule of thumb. For the system the compressor lives in, ASHRAE covers the refrigeration-cycle fundamentals and equipment in its handbook volumes, and ASHRAE 15 governs machine-room refrigerant safety, which matters as the trade moves to A2L refrigerants. Refrigerant recovery and handling fall under the EPA Section 608 rules in the United States. Cite the standard that controls the point and verify the edition and the manufacturer data before you rely on a number on a submittal.

Units, terms, and acronyms

Compressor work spans a few unit systems and a pile of shorthand, and the same machine reads differently across a nameplate, a chiller schedule, and a manufacturer envelope.

Capacity is given in tons of refrigeration in North American HVAC, in BTU per hour for smaller equipment, and in kilowatts in SI and most chiller data, where one ton is about 12,000 BTU per hour or roughly 3.5 kW. Pressures read in psig on field gauges and in kPa or bar on metric data. Efficiency shows up as COP, EER, or SEER on packaged equipment, and as kW per ton or the part-load IPLV and NPLV on chillers. Match the unit to the document and convert deliberately.

Positive displacement

Compression by trapping a fixed volume of vapor and shrinking it; recip, scroll, screw, rotary

Dynamic / centrifugal

Compression by accelerating vapor with an impeller and converting velocity to pressure

Surge

Flow reversal in a centrifugal at too little flow or too much lift; the centrifugal failure mode

Slide valve

The continuous capacity control on a screw compressor, delaying where compression begins

Hermetic / semi-hermetic / open

Motor sealed in a welded shell, bolted serviceable housing, or external with a shaft seal

Floodback / slugging

Liquid returning while running, or liquid in the compression chamber, usually on a flooded start

EVI

Enhanced vapor injection, a mid-compression vapor port that holds heat-pump capacity in the cold

IPLV / NPLV

Integrated and non-standard part-load value, the weighted part-load efficiency of a chiller

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