An absorption refrigerator replaces the compressor with a heat-driven pair of vessels: water absorbs ammonia vapour from the cold side, and heating the ammonia-rich solution drives the vapour out again at high pressure. It therefore runs on a wood fire, a gas flame or a solar collector with no electricity, and in its simplest form with no moving parts at all.
Part of Refrigeration
A refrigeration cycle that takes heat instead of shaft work as its energy input, built around ammonia dissolving in water, and why it suits a village without electricity.
The Absorber And Generator Do The Compressor’s Job With Heat
A compressor takes low-pressure vapour from the evaporator and delivers it to the condenser at high pressure. In an absorption machine a chemical trick does the same job. The corpus carrier sets out the four parts, which this page keeps and sharpens:
- Generator. An ammonia-water solution is heated. Ammonia boils at a much lower temperature than water, so it leaves as vapour while most of the water stays behind as liquid.
- Condenser. The ammonia vapour is cooled and liquefied, exactly as in Condensation.
- Expansion and evaporator. The liquid ammonia passes through a restriction and boils in the cold box, as in Expansion and Evaporation.
- Absorber. The ammonia vapour leaving the evaporator meets the water-rich “weak” solution left in the generator, and dissolves in it. The resulting “strong” solution returns to the generator, and the cycle repeats.
Everything from the condenser to the evaporator is identical to the compressor machine. Only the step that creates the pressure difference changes. Dissolving a gas in a liquid is much easier to arrange than squeezing a gas, and the corpus carrier says plainly that this is why absorption needs no compressor, no electric motor and no precision machining.
Why A Village Should Build This One First
- Its energy is heat. A wood fire, a gas flame or a focused solar collector will do. The carrier’s table puts a small unit at 1 to 2 kg of wood an hour for a cold box of about 0.1 m³, and a larger one at 3 to 5 kg an hour for a walk-in room.
- Its parts are ordinary. Steel vessels, steel tube and a restriction. The carrier says an old pressure cooker or steam boiler can serve as the generator; whether any particular vessel is fit for the pressure is a question for Pressure Vessels, not for the carrier.
- It has few or no moving parts. A pumped continuous machine needs one pump. A batch machine needs none, which is the next section.
- The compressor machine needs shaft power. At 50 to 100 W of continuous mechanical power for a small household-size cooler (the carrier’s figure), a village without a running engine, wheel or turbine cannot keep one going. A fire it can.
Three Ways To Arrange The Cycle
| Arrangement | How the solution moves | Moving parts | Practical note |
|---|---|---|---|
| Batch (intermittent) | None: the same vessel is heated, then cooled | None | Heat for a spell, then cool for a spell; cold arrives in the cooling phase |
| Continuous with pump | A pump lifts strong solution to the high-pressure generator | One pump | Steady cooling; needs a pump that can handle the pressure difference |
| Pressure-equalised (diffusion) | A heated riser lifts liquid by bubbles; an extra gas keeps the total pressure even | None | Steady and silent, but needs a third gas and finicky balance |
The carrier says “pump or gravity-feed” for the continuous machine. Gravity alone cannot return solution from the low-pressure absorber to the high-pressure generator unless the machine is the pressure-equalised kind; a pumped machine needs a pump, and a batch machine avoids the problem. The batch and pressure-equalised designs are standard in the history of the machine (recalled, flagged); the carrier does not describe them.
The batch machine is the easiest to build and the best first project. One steel vessel holds strong solution and is connected by a pipe to a second vessel that serves as condenser and evaporator in turn. Heat the first vessel, and ammonia distils across to the second, which is kept cool in water, where it condenses. Remove the fire and let the first vessel cool: its solution starts absorbing ammonia again, the pressure falls, the liquid in the second vessel boils, and that second vessel gets cold. Put it in the cold box or a bucket of water and it makes cold, and with enough cooling effect, ice. It has no valves and no pump; it makes cold in bursts, not continuously.
Steady Cooling Needs Rectification Or Water Gets Into The Evaporator
The generator’s vapour is not pure ammonia. Water boils off with it in small quantities. If that water reaches the evaporator it does not boil at the low temperature the ammonia does; it accumulates there and the evaporator warms (recalled behaviour, flagged). A continuous machine therefore puts a rectifying section between the generator and the condenser: a short column or coil in which water vapour condenses and runs back down. The carrier’s description leaves this out; the plain four-part sketch above works as a batch machine only if the vessel is not driven too hot.
Worked Example: Cold From One Batch
Every figure here is an assumption chosen for the arithmetic, except the 25 to 30% charge strength and the 334 J/g for ice, which are the carrier’s and the corpus’s. The ammonia evaporation heat of about 1,370 J/g is a recalled value, flagged.
- Charge: 5 kg of solution at 30% ammonia by mass holds 5 × 0.30 = 1.5 kg of ammonia.
- Assume 60% of that is distilled over and condensed: 1.5 × 0.60 = 0.90 kg of liquid ammonia.
- Best-case cooling when it all boils: 0.90 kg × 1,370 kJ/kg = 1,233 kJ.
- Ice equivalent: 1,233 ÷ 334 = 3.7 kg of ice at its freezing point.
This is a best case. It ignores the part of the liquid that flashes at the restriction or on first contact with the low pressure, the heat the vessel and the water in it must lose first, and heat leaking in from outside. The real figure is lower, and the only way to know it is to measure. It does show that a kilogram or so of ammonia in a small sealed machine is enough to make a few kilograms of ice a day.
Common Mistakes
- Treating the fire as the controller. Too much heat drives water over with the ammonia, and a blocked or closed line lets pressure build. The heat must be something you can remove instantly.
- Skipping the absorber cooling. Dissolving ammonia releases heat, and a warm solution holds less ammonia; the carrier says to cool the absorber with air or water.
- Mixing up strong and weak. The strong solution is the ammonia-rich one leaving the absorber; the weak solution is what is left in the generator. Reversing them reverses the cycle.
- Not testing the pressure vessel first. Do not fire a vessel that has not been shown to be fit for the pressure; see Pressure Vessels.
Safety
A fire under a sealed vessel of ammonia and water is a pressure-vessel hazard first
The generator is a closed vessel, heated, holding a toxic, flammable gas. It must have a relief device that the operator cannot defeat or isolate, discharging where nobody stands, and the heat source must be one that can be cut at once. Operate the machine outdoors or in a separate ventilated building, away from living quarters and from the cold room people walk into. Never open any joint while the machine is warm or charged.
What’s Next
- Learn next: Ammonia-Water System — the continuous machine, its balances and its failure modes
- Combine with: Building an Absorber — materials, assembly and testing