Absorption Refrigerator

Fill a syringe with carbonated water, seal the tip, and pull the plunger. Gas comes out of solution. Push, and it goes back in.

Now do it with cloudy ammonia solution and a little headspace, outdoors or with the window wide open, and note how eagerly the vapour is taken up.

That eagerness is the whole machine. A liquid that will drink a gas can lower the pressure above that gas — which is the only thing a suction stroke actually does.

Household ammonia is an irritant. Work in moving air, never mix it with bleach.

Put a thermometer in water and stir in ammonia solution, logging the temperature.

Expect it to rise.

Absorption is exothermic — dissolving the vapour gives heat back out. That is why a real machine has a separate absorber that must be cooled, and why an absorption fridge is warm in two places rather than one.

Note the consequence now: heat comes IN at the generator and goes OUT at both the condenser and the absorber. Three heat exchanges, not two.

Warm the ammonia solution gently and watch vapour come off; smell the difference in the headspace above it as it heats.

Expect the solution to give up its dissolved gas as temperature rises — the reverse of step 1.

This is the generator, and it is where the energy actually enters the machine. Heat separates the pair; the affinity that made absorption easy is exactly what the flame must overcome.

Stop as soon as you have seen it. This step is about the direction of the process, not about collecting anything.

Compare two efforts: compress a syringe full of air, then try to compress one full of water with the tip sealed.

The air squashes. The water refuses.

Now note which one the two machines have to move. Perkins' compressor raises the pressure of a vapour, which takes real work. Carré's solution pump raises the pressure of a liquid, which takes very little, because there is nothing to squash — you are only pushing it along.

The energy did not vanish; it moved to the flame. Carré traded expensive mechanical work for cheap heat.

Look up or measure the efficiency of a gas-powered absorption fridge — a caravan or hotel minibar unit — against a compressor fridge of similar size.

Expect the absorption unit to move well under half as much heat per unit of energy in.

It is genuinely less efficient, and no arrangement of the components fixes that.

It wins anyway wherever it is chosen, because it is silent, has nothing to wear out, and will run on energy that is free or already wasted. Efficiency is only decisive when the energy has to be bought.

The patents. Ferdinand Carré patented the ammonia-water absorption machine in France in 1859 and in the United States in 1860. ⚠ The US number is not pinned here — the record for 1860s French inventors is thin and we would rather cite nothing than cite wrongly. His brother Edmond Carré had built an earlier absorption machine in 1850 using water and sulphuric acid; Ferdinand's contribution was the pairing that actually worked at scale.

It went to sea. In 1876 a Carré machine was installed in the ship Paraguay, which carried frozen meat across the Atlantic. That is the beginning of something enormous: the moment food production stopped having to happen near the people eating it.

The American Civil War made it. The Union blockade cut the Confederacy off from Northern natural ice, and Carré machines were imported to make ice locally. A refrigeration technology found its first large market because a war removed the cheaper alternative — the same economics that had kept Cullen's discovery idle for eighty years, running in reverse.

Where it is still the right answer. The Einstein-Szilárd refrigerator of 1926 is an absorption machine, patented by exactly the Einstein you are thinking of, and designed with no moving parts after a Berlin family died from a leaking seal on a compressor fridge. Today: caravan and boat fridges running on bottled gas, hotel minibars that must be silent, off-grid vaccine refrigerators on paraffin or solar heat, and enormous industrial chillers driven by waste heat from power stations, where the fuel has already been paid for.

Honest limits. Low efficiency, as step 5 shows. It is slow to start and dislikes being tilted, because the circulation depends on gravity and on liquids staying where they were designed to sit — which is why a caravan fridge must be level. And ammonia is toxic and corrodes copper, so these machines are built in steel and treated with respect.