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Piston Compressor

5 min read

Parent
🔧
Air Compression
Building compressors from simple to sophisticated
Current
⚙️
Piston Compressor
Single and multi-stage reciprocating compressors
Sub-Topics
1️⃣
Single-Stage Design
Up to 8 bar (120 psi) from one cylinder
2️⃣
Two-Stage with Intercooler
Higher pressure and better efficiency

A piston compressor is a cylinder, a close-fitting piston, an inlet valve and an outlet valve driven by a crank, and its sweep sets the ceiling on its output: a 100 mm bore and 100 mm stroke at 300 RPM sweeps 235.6 L/min of free air. Real delivery is lower, so time how long it takes to fill a tank of known volume, and remember that reaching 7 bar gauge takes at least 1.13 kW at the shaft.

Part of Pneumatic Systems

The reciprocating piston compressor sucks in air on one stroke and squeezes it into a tank on the other. It is the buildable route to tool-pressure air.

The Sweep Sets the Ceiling and the Valves Set the Rest

Each revolution the piston sweeps its bore area times its stroke. For the carrier’s cylinder of 100 mm bore and 100 mm stroke:

  1. Swept volume = π / 4 x 0.1² x 0.1 = 7.854 x 10⁻⁴ m³ = 0.785 L
  2. At 300 RPM = 5 revolutions per second, free air = 7.854 x 10⁻⁴ x 5 = 0.003927 m³/s = 235.6 L/min

That figure is a ceiling. Air left in the clearance space at the top of the stroke re-expands and fills part of the next stroke (see Single-Stage Design), the valves open late and close late, and the piston rings leak. Do not quote the swept figure as the delivery.

What You Need

Following the carrier article:

  • A cylinder of cast iron or steel, 75-150 mm bore and 100-150 mm stroke, bored and honed to better than 0.8 μm Ra. Use Cylinder Boring for the boring-bar technique only; its steam-engine roundness table (plus or minus 0.030 down to 0.003 inch) is a different standard from the finish given here.
  • A cast-iron piston with 2-3 piston rings, turned to fit with about 0.05 mm clearance; see Piston Rings.
  • Inlet and outlet valves, reed or plate type, from spring-steel strip 0.3-0.5 mm thick.
  • A crank, a connecting rod and a flywheel, driven by a belt from a water wheel, windmill or motor; see Crank Mechanism.
  • A receiver tank with a relief valve, gauge and drain; see Receiver Tank Construction.

Method: Commission the Compressor

Step 1: Turn it by hand with the outlet open It should turn smoothly with a faint suction and puff at each stroke. A hard spot means a tight ring or bore.

Step 2: Check the valves Blow into the outlet port: it should pass air out and not back. Suck on the inlet: it should pass air in and not out.

Step 3: Fit the relief valve before the first pressure run The relief valve goes on the compressor side of any shut-off valve, so the compressor can never pump against a closed line.

Step 4: Time a fill Fill the empty receiver and note how long it takes to reach 7 bar gauge.

Worked Example: Finding the Real Delivery

A 100 L (0.1 m³) receiver starts at atmospheric pressure. At 7 bar gauge it holds 8 bar absolute, so the free air that has gone into it is 0.1 x (8 - 1) = 0.7 m³ (700 L).

  1. At the ideal 235.6 L/min the fill takes 700 / 235.6 = 2.97 minutes.
  2. Suppose the fill actually takes 4.5 minutes. Delivery = 700 / 4.5 = 156 L/min, which is 156 / 235.6 = 66% of the swept volume.

The receiver warms as it fills, so measure the time to a given gauge reading twice and average. Keep the 66% as an example, not a promise; your machine will be different.

Power check. Delivering 235.6 L/min of free air at 7 bar gauge needs between 827 W (isothermal) and 1,130 W or 1.13 kW (adiabatic) at the shaft, before friction. See Gas Laws in Practice for the working. A water wheel of the corpus’s reference size, 20 L/s falling 5 m at 80% efficiency, gives 785 W (Water Wheel Types), which is not enough.

Common Mistakes

  • Valves that stick or flutter. Reed valves too stiff delay opening and waste work; too weak they slam and break.
  • No check valve or relief valve. The air can leak back through the compressor when it stops, and a closed outlet can pump to destruction.
  • Over-oiling. Excess oil is carried into the hot discharge line and the tank; see Lubrication for oiling bearings and use only a few drops for the cylinder.
  • No unloading at start. Starting a compressor against a full tank takes much more torque than starting it against an open outlet.

Safety

The stop, the flywheel, the belt and the discharge line

A compressor on a belt and flywheel has a drive you cannot stop from where you stand. Put the stopping device (a belt shifter or a clutch) where the operator can reach it without crossing the belt, guard the belt and the flywheel, and time how long the flywheel takes to coast to rest so everyone knows it keeps turning after the power is cut. The cylinder head and discharge line run hot enough to burn; see Gas Laws in Practice for the figures. Never block the outlet or fit a shut-off valve between the compressor and its relief valve.

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