Lesson System components · Compressor
Compressor: types and function
The compressor is the pump that moves refrigerant vapor from low pressure (evaporator) to high pressure (condenser). It does not “make cold”—it enables the pressure difference the rest of the cycle needs. Knowing hermetic vs semihermetic, and reciprocating, scroll, rotary, screw, centrifugal, tells you how the machine fails, how it sounds, and what you can service in the field.
Job description
- Draw low-pressure vapor from the suction line.
- Reduce volume / raise pressure (and temperature).
- Discharge high-pressure vapor into the condenser.
Compression ratio ≈ absolute discharge pressure ÷ absolute suction pressure. High ratio means hot discharge, more stress, less capacity.
Hermetic, semihermetic, open
| Style | Shell | Field service |
|---|---|---|
| Hermetic | Welded; motor inside | Replace assembly; no valve plate service |
| Semihermetic | Bolted; motor inside | Valves, oil, some internals serviceable |
| Open | External drive | Shaft seal; belt/coupling issues |
Residential scrolls are usually hermetic. Commercial racks often use semihermetic recip or screws.
Reciprocating
Pistons and valves. Sensitive to liquid slugging. Valve plates and gaskets matter on semihermetics. Clearance volume and valve condition affect volumetric efficiency.
Scroll
Orbiting scroll traps and compresses pockets of vapor. Common on comfort A/C. Generally quieter; still hates liquid and debris. Many are one-way; wrong rotation (three-phase) means no pumping.
Rotary, screw, centrifugal
- Rotary: small appliances and some ductless.
- Screw: larger commercial / industrial; oil injection often critical.
- Centrifugal: large chillers; surge and vane control are specialty topics.
What the compressor needs to live
- Dry vapor at the inlet (adequate superheat for the design)
- Correct oil type and level
- Reasonable compression ratio (clean condenser, proper evaporating T)
- Electrical integrity (capacitors, contactors, voltage)
Sounds and symptoms (qualitative)
- Slugging: knock, foam, broken internals
- High ratio: very hot discharge, amp climb, protector trips
- No pumping: equalized pressures, wrong rotation, valves/scroll failure
- Short cycling: controls, low charge, oversized, dirty evaporator icing
Technician mindset
Do not condemn a compressor until:
- Contactor, capacitor, and voltage are proven.
- Pressures show it truly cannot pump with a clear system.
- Superheat shows it was not fed liquid for months.
- Condenser rejection is restored.
A “bad compressor” call is expensive; many are system failures that killed a good pump.
Compression ratio in daily service
As suction falls or head rises, ratio climbs. High ratio means hot discharge and less mass flow—dirty condensers and iced evaporators look like “weak compressors.” Fix ratio drivers before pricing a compressor.
Nameplate literacy
RLA, LRA, refrigerant, oil, and rotation notes live on the shell. Photograph plates before semihermetic teardown. Heed three-phase rotation stickers after contactor work.
Capacity control you may meet
Unloaders, hot-gas bypass, VFDs, and digital scrolls change “normal” amps. A lightly loaded rack compressor may sit far from full-load RLA and still be healthy.
Failure signatures
Broken recip valves: pressures drift toward equalizing. Scroll with debris: metallic noise and dirty driers. Open drive: shaft seal oil film. Match signature to type.
Field case
Situation. Three-phase rooftop: “compressor not cooling.” Amp draw low, suction and discharge nearly equal after running. Contactor pulls in.
How to think it.
- Equalized pressures while “running” → not pumping.
- Check rotation (phase); reverse two legs—pressures separate and cooling returns.
- Document: scroll/recip on three-phase must have correct rotation after any power work.
Takeaway: type + electrical basics beat premature compressor changeout.
In the field
Symptom
No cooling, noise, trips, equalized pressures
Where to look
Type, rotation, valves/pumping proof, SH history, condenser
Likely causes
- Electrical, liquid abuse, high ratio, mechanical failure
What to measure
- Amps, P suction/discharge, SH, discharge T
What not to do
- Replace compressor without proving cause of death
Checklist
- I state: compressor raises vapor pressure; it does not create cold alone
- I identify hermetic / semihermetic / open on site
- I recognize recip vs scroll vs larger types in context
- I check electrical and rotation before condemning
- I link high ratio to dirty condenser / low suction