Reciprocating Piston Air Compressors

Piston compressors are old-timers in the compressor family. Working principle is just piston reciprocating to change cylinder volume and compress gas. Structure you can see and touch, what's broken is obvious, so still used in plenty of applications today.

Overall Layout

Looking at a two-stage piston compressor outside in, bottom is the base, cast iron or welded steel plate, mainly load bearing and doubling as oil sump.

Crankcase sits on the base, crankshaft and connecting rods installed inside. Inspection ports on the side for replacing bearings and checking rods.

Cylinder arrangement varies, vertical or V-type, depends on manufacturer design and power. Two-stage machines always have two cylinders: first stage cylinder bigger, second stage smaller. Easy to understand: first stage handles larger gas volume, second stage handles gas already compressed once, smaller volume so cylinder made smaller.

Cylinder head removable because valve assemblies mount on it, and valves are what fail most often. More on that in the failures section.

Coolers at two spots: one between stages called intercooler, one after second stage called aftercooler. Air-cooled and water-cooled both exist; water-cooled works better but needs cooling water source; air-cooled simpler but bigger and less effective in summer.

Electric motor drives crankshaft, belt drive mostly, some direct-coupled. Direct-coupled more compact but locks motor and compressor speeds together; belt drive can adjust speed by changing pulleys.

Core Components

Component

Crankshaft Assembly

Component

Piston & Rings

Component

Valve Assembly

Crankshaft is alloy steel forging, converts rotary to reciprocating motion, basic physics. Crank radius determines piston stroke; bigger radius, longer stroke.

Connecting rod links crankshaft on one end, piston on other. Crankshaft end called big end, bearings inside, high forces and temps there, worn bearings cause abnormal sounds from whole machine. Piston end called small end, linked through wrist pin.

Piston itself is cast iron or aluminum, key is those piston rings on it. Rings have two functions: top ones called compression rings handle sealing, bottom ones called oil rings scrape oil. Compression rings worn, gas leaks into crankcase during compression, discharge volume drops, crankcase pressure rises. Oil rings bad, lube oil gets into cylinder and gets compressed and discharged, downstream equipment suffers.

Cylinder liner inner wall is honed for surface finish. Some models have replaceable liners; worn liner just swap a new one, way cheaper than whole cylinder block. Small machines mostly have integral cylinders though, no liner swap option.

Compression Cycle

Take a single-stage cylinder through one complete cycle.

Piston at highest position is top dead center (TDC), lowest is bottom dead center (BDC). Piston moves down from TDC, cylinder volume increases, internal pressure drops. Drops below intake manifold pressure, intake valve pushed open by outside air pressure, air rushes in. During this discharge valve stays closed, held shut by low internal pressure.

Mechanical Process

Piston Compression Cycle Visualization

Piston reaches BDC, starts back up. Intake valve closes (spring holds it, plus rising cylinder pressure also pushes), cylinder becomes sealed space. Piston keeps going up, volume shrinks, pressure rises.

Exceeds discharge side backpressure, discharge valve pushed open, compressed air squeezed out. Piston goes all the way to TDC, discharge basically done.

Single-Stage vs Two-Stage

Compressing straight from atmospheric to 8 bar (ratio about 8), by adiabatic compression discharge temp reaches 260-270°C. At that temp lube oil carbonizes, valves can't take it, piston rings wear faster, machine life tanks.

So high compression ratio jobs split into two stages. First stage compresses to 2-3 bar, discharge temp kept within 130-140°C, through intercooler back to near ambient, then into second stage to target pressure. Each stage has small ratio, temps stay manageable.

Two-stage also uses less energy than forcing single-stage. How much depends on pressure, cooling effectiveness, machine efficiency; roughly same discharge pressure two-stage saves about 10%, some claim up to 15%, varies by machine.

Even higher pressures like 20-30 bar and up, there are three-stage or four-stage, same principle.

Piston Machine Positioning

After screw machines came out, regular 7-8 bar industrial air supply basically got taken over by screw machines. Screw machines have strong continuous operation, low vibration, no discharge pulsation, less worry for most factories.

Application

High Pressure Systems

Application

Special Gas Compression

Piston machines retreated to niches where screw machines don't quite fit:

One is high pressure. Screw machines at 15 bar already considered high, above that rare. Blow molding needs 30-40 bar, some leak testing needs tens to hundreds of bar, piston machines still the mainstay.

Two is intermittent use. Auto repair shop inflating tires, small workshop occasional use, short run times, frequent starts/stops. Piston machines are adequate and cheaper.

Three is special gases. Compressing hydrogen, natural gas, needs diaphragm piston machines, membrane isolates process gas from lubricated parts. Screw machines can do it but sealing demands are tougher.

Four is backup. Some factories have screw machine main systems but keep one or two piston machines for emergencies.

Failures

Valves mentioned earlier, break most often. Plate fatigue fracture, seat eroded by gas flow cutting grooves, spring fracture, all common. Valve goes bad, symptoms are decreased discharge volume, elevated discharge temp, higher energy use. Experienced mechanics can tell which cylinder's valve is bad just by listening.

Piston ring wear is gradual. Compression ring sealing declining after several thousand hours on new machine is normal; concern is how fast. Noticeably bad after just few hundred hours, check whether liner is scored or lubrication has problems. Oil rings bad, symptoms are oil in discharge, crankcase oil level dropping fast, oil contamination in downstream piping and equipment.

Bearing problems relatively rare but serious when they happen. Usually abnormal sound comes first, then bearing temp rises. Catch early and just replace bearings; ignore it and journal gets damaged, major overhaul.

Cylinder liner wear is long-term. Unless lubrication problems or contamination got in, many years of normal use before it noticeably affects efficiency. By then other parts also near end of life, whether to just swap liner needs calculation.