A refrigerator moves heat from a cold place to a warmer one by passing a sealed fluid through four steps: compress it, cool it until it condenses, drop its pressure, and let it boil in the cold space. Heat will not climb that hill by itself, so the machine must spend work (a compressor) or heat (an absorption cycle), and it is exactly a heat engine run backwards.
Part of Refrigeration
The four-step cycle that every mechanical or heat-driven cooler uses, and why it is the mirror image of an engine.
An Engine And A Refrigerator Are The Same Cycle In Opposite Directions
An engine takes heat from something hot, turns part of it into work, and rejects the rest to something cooler. A refrigerator takes heat from something cold, is fed work, and rejects the sum of the two to something warmer. The components are the same kind: a fluid, a cylinder and piston or equivalent, a hot heat exchanger and a cold one. Only the direction of travel round the cycle is reversed.
That is why a refrigeration compressor and an engine cylinder look alike. A reciprocating compressor is a piston in a cylinder with valves, driven by a crank, which is what the corpus describes in Crank Mechanism and the engine tree describes in Piston Assembly. A reader who understands one machine can reason about the other, because the same temperatures govern both. Engine Theory is the other half of this idea.
The energy bookkeeping is the same for both. For the refrigerator, heat rejected at the warm end equals heat absorbed in the cold space plus the work (or heat) fed in. Nothing is made or lost; it is only moved. For an engine, heat taken in equals work out plus heat rejected.
The Four Steps Each Do One Job
The corpus carrier names them; each has its own page.
| Step | What happens | Where heat goes | Page |
|---|---|---|---|
| Compression | Vapour is squeezed to a high pressure and gets hot | Work is added | Compression |
| Condensation | Hot vapour gives up heat to the air or water and turns to liquid | Heat out | Condensation |
| Expansion | The liquid passes through a restriction and its pressure falls | None added or removed | Expansion |
| Evaporation | The cold liquid boils in the cold space, taking heat from it | Heat in | Evaporation |
The point of the fluid is that its boiling temperature depends on pressure. At the low pressure of the evaporator it boils colder than the cold space, so heat flows into it. At the high pressure of the condenser it boils (and condenses) hotter than the outside air, so heat flows out of it. The compressor is what creates the two pressures; the restriction is what holds them apart. The carrier’s warning about the expansion device is worth repeating: without a real restriction between the two sides there is no pressure difference, and no cold.
When The Heat Goes To The Room, It Is A Heat Pump
The warm end of a refrigerator is not waste if you want warmth. The same machine, pointed the other way, is a heat pump: it takes heat from a cold source and delivers more heat to the warm side than the work it consumed, because the delivered heat includes what it carried across. No efficiency figure is quoted here; the reasoning is the bookkeeping above.
Worked Example: Following One Unit Of Energy Round The Loop
Assume, for illustration only, that a machine removes 3 units of heat from a cold box using 1 unit of work.
- Heat absorbed in the evaporator: 3.
- Work added at the compressor: 1.
- Heat rejected at the condenser: 3 + 1 = 4.
The condenser is warmer to the touch than the box is cold, and it always rejects more than the box gave up. A cold room with a hot, stuffy machine behind it is behaving correctly. The numbers are placeholders chosen so the arithmetic is easy to follow; real figures depend on the temperatures the machine works between, which Compression treats.
Common Mistakes
- Thinking cold is made. Cold is only the absence of heat; the machine moves heat, and that is why the rear of the machine is warm.
- Forgetting the pressure difference. A pipe that is open from one side of the loop to the other cannot hold two pressures, and cools nothing.
- Putting the cold side where the heat cannot get out. The warm end must have air or water to give its heat to.
Safety
The loop is a sealed pressure vessel full of a hazardous fluid
Every refrigerant in this tree is toxic, flammable, or both, and the high side runs at a pressure that can burst a weak pipe. Never open, cut, weld or heat a charged circuit. Read Safety before building anything and Pressure Vessels before pressurising anything.
What’s Next
- Learn next: Working Fluids — which fluids can be used and why the choice matters
- Combine with: Absorption Refrigeration — the same cycle driven by heat instead of shaft work