A mechanical refrigerator is the four-step cycle driven by a compressor, and its true cost is the continuous shaft power the compressor needs: about 50 to 100 W for a household-size box and 200 to 500 W for a walk-in room. It is more efficient and easier to control than an absorption machine, but only a settlement with a steady engine, wheel or turbine should depend on it.
Part of Refrigeration
A refrigeration machine built around a motor- or engine-driven compressor: its parts, its power needs and where that power can come from.
The Compressor Replaces Heat With Shaft Power And Needs A Reliable Source Of It
The compressor machine has the same condenser, restriction and evaporator as the absorption machine; only the pressure-raising step differs. The corpus carrier lists five components: a gas-tight compressor, a condenser, an expansion valve or capillary, an evaporator and a motor or engine. It gives the power: a 0.2 m³ refrigerator needs roughly 50 to 100 W of continuous mechanical power; a walk-in room needs 200 to 500 W from an engine or turbine.
The word continuous matters. A refrigerator does not store its cold for long, and a stopped compressor is a box slowly warming. The carrier notes that a hand crank can only sustain the required power for very short periods. The corpus gives 50 to 75 W for one person cranking a chain pump (Chain Pump), so one person at a crank is at the bottom of the carrier’s range for a small fridge and cannot do it for a working day.
Where The Shaft Power Can Come From
| Source | What it offers | The catch |
|---|---|---|
| Engine | Steady power, any hour | Needs fuel and attention; see Engine Theory |
| Water wheel or turbine | Free power while the water runs | Must be near water; speed varies with the flow |
| Electric motor | Easy to couple and control | Needs a generator and a supply behind it |
| Hand or treadle | Needs no equipment | Only for short periods |
An engine and a compressor are cousins: both are pistons in cylinders with valves, and the engine tree’s Piston Assembly and Valve Train describe the same parts. The corpus’s overshot-wheel example in Gas Laws in Practice gives 785 W from a 20 L/s flow falling 5 m through an 80%-efficient wheel, which is comfortably above the carrier’s range for a small box. Joining a prime mover to a driven machine without gears is covered in Direct Coupling (written for a turbine and generator) and with a belt in Belt Drive.
What You Need
- A gas-tight compressor (see Compressor Types).
- A condenser, restriction and evaporator, built as the earlier pages in this tree describe.
- A drive with a guard, and a speed that the compressor was designed for.
- A relief device on the high side that cannot be defeated.
- Oil suitable for the fluid. The compressor needs lubrication, and oil carried round the circuit must return or be separated; how it mixes with the refrigerant depends on the fluid (recalled behaviour, flagged).
Method: Deciding Between Compressor And Absorption
Step 1: Count the shaft power available at the hours the box must run. If the answer is “most hours, reliably”, a compressor is viable.
Step 2: Count the fuel or sunshine available for heat. If shaft power is scarce and a fire or collector is not, choose Absorption Refrigeration.
Step 3: Count the skill on hand. A compressor needs a gas-tight shaft seal and valves; an absorption machine needs sound pressure vessels and careful welding.
Step 4: Build the one you can maintain. The carrier’s advice is to start with absorption and add a compressor machine when reliable power exists.
Worked Example: Can The Wheel Run A Small Box?
Inputs: the corpus gives 785 W for the overshot wheel in the example above. The carrier gives 50 to 100 W for a 0.2 m³ box.
- Fraction of the wheel’s output a small box takes: 100 ÷ 785 = 0.127, which is about 13%, at the top of the range.
- Fraction at the bottom of the range: 50 ÷ 785 = 0.064, about 6%.
- A walk-in room at 200 to 500 W: 200 ÷ 785 = 0.25 (25%) to 500 ÷ 785 = 0.64 (64%).
So a wheel of that size can run a small box with room to spare and a walk-in room with little left over. These are the carrier’s figures; they take no account of losses in the drive or of the wheel’s output falling in a dry season.
Common Mistakes
- Building a compressor machine with no steady power. It will cool only while someone cranks.
- Using a compressor that is not gas-tight. A leak lets refrigerant out and air in.
- Forgetting the drive guard. Belts and flywheels injure.
- Assuming any air compressor will do. Its materials and seals may not suit the fluid.
Safety
A compressor is a pressure vessel with a moving drive attached
The discharge side is hot and under pressure, a belt or flywheel can catch clothing and hair, and the charged circuit holds a hazardous fluid. Guard every moving part, fit and test the relief device, and never service a charged circuit. See Pressure Vessels.
What’s Next
- Learn next: Compressor Types — which compressors a settlement can build or salvage
- Combine with: System Construction — piping, seals and assembly