A reciprocating compressor squeezes refrigerant with a piston in a cylinder, needs tight piston rings and self-acting valves, and can be built from a modified pump or engine cylinder. A rotary compressor traps and squeezes gas with a turning element and runs more smoothly but is harder to build, and a salvaged compressor of either kind must never be recharged with a fluid it was not made for.
Part of Refrigeration
The two compressor families the carrier names, how big a piston compressor must be for a given duty, and why materials and seals decide whether a salvaged one is usable.
A Piston Compressor Is A Pump That Must Not Leak
The corpus carrier describes the reciprocating compressor as the most common: a piston in a cylinder compresses the gas, with piston rings sealing the cylinder wall and reed or plate valves letting vapour in and out. It can be built from a modified pump or engine cylinder. The same parts appear in the corpus articles Piston Compressor, Piston Rings and Crank Mechanism, and in the engine tree’s Piston Assembly. A refrigeration compressor differs from an air compressor in what it demands of the seals: nothing may leak in or out, because the fluid is hazardous and air in the circuit spoils the cycle.
The carrier calls the rotary type harder to build from scratch but smoother in operation, and suggests salvaging units from old appliances. That advice needs a caution, covered below.
What You Need
- A cylinder with a true, smooth bore and a piston fitted with rings; see Cylinder Boring for how the corpus gets a cylinder round.
- Valves of the reed or plate type, light enough to open and shut on the pressure difference, strong enough not to break.
- A crank, connecting rod and bearings to drive the piston.
- A shaft seal, because the shaft passes out through the crankcase wall and is a leak path (recalled, flagged).
- Materials that suit the fluid: steel and iron for ammonia, with no copper alloys in contact with it.
Method: Sizing And Checking A Piston Compressor
Step 1: Compute the swept volume. Volume per stroke is π ÷ 4 × bore² × stroke. Multiply by strokes per minute for the displacement.
Step 2: Treat the result as an upper bound. Valve losses and the clearance volume left at the top of the stroke reduce the real delivery. The corpus air example says it is “before valve and clearance losses” and no figure for these losses is given here.
Step 3: Match it to the duty. The vapour flow needed depends on the cooling load and the fluid’s properties at the suction pressure, which come from that fluid’s property table.
Step 4: Check the discharge side is protected. The relief device and the gauge belong on the high side before the first run.
Worked Example: Swept Volume Grows With The Square Of The Bore
Take a small compressor with a 40 mm bore and a 40 mm stroke running at 300 rpm, chosen only for the arithmetic.
- Swept volume per stroke: π ÷ 4 × 0.04² × 0.04 = 5.03 × 10⁻⁵ m³, which is 0.0503 litres.
- Per minute at 300 strokes: 0.0503 × 300 = 15.1 litres.
The corpus air compressor in Gas Laws in Practice has a 100 mm bore and 100 mm stroke at 300 rpm and sweeps 235.6 L/min. The ratio is 235.6 ÷ 15.1 = 15.6. Check it from the dimensions: (100 ÷ 40)³ = 2.5³ = 15.6. Swept volume scales with the cube of the linear size when bore and stroke grow together, and with the square of the bore at a fixed stroke.
Salvaged Compressors Are Made For One Fluid
A sealed compressor from a domestic refrigerator or air-conditioner carries its motor inside the refrigerant circuit, with copper windings and an oil charge chosen for a particular fluid (recalled, flagged). Such a unit cannot be converted to ammonia, and opening one releases a refrigerant of unknown composition. The sensible uses for a salvaged sealed unit are to run it as it is, as a complete working refrigerator while power lasts, or to leave it alone. An open compressor of unknown history is a pressure vessel of unknown history, and the corpus rule in Vessel Construction applies: do not trust one whose service you cannot establish.
Common Mistakes
- Using an air compressor unchanged. Its aluminium, brass and rubber parts may be attacked by the fluid, and its seals leak at a rate air does not mind.
- Leaving out the shaft seal check. It is where the circuit loses gas.
- Running with worn rings. Gas leaks past the piston, delivery falls and the discharge temperature rises.
- Opening a sealed salvaged unit. It releases a refrigerant you cannot identify.
Safety
Refrigerant in a salvaged unit is of unknown type and pressure
Do not cut, braze or vent a sealed circuit. Many refrigerants are asphyxiants or flammable, and some produce toxic gas when heated. If a unit must be opened, do it outdoors, and treat the contents as hazardous. See Safety.
What’s Next
- Learn next: System Construction — piping and seals for the whole circuit
- Combine with: Cylinder Block — how a cylinder is cast and bored