Industrial & Mass Production (1910–1940)Thermodynamics & Consumer Technology

Einstein & Szilárd's Absorption Refrigerator

US 1,781,541

Zero-Moving-Parts Hermetic Closed Loop & Dalton's Partial Pressure Evaporation

Inventor(s)Albert Einstein, Leo Szilard

Grant Date1930-11-11

Filing Date1927-12-16

LocationBerlin, Germany

Einstein and Szilard's 1930 absorption fridge: one total pressure, no shaft seal, heat instead of a compressor. A third gas drops the refrigerant's partial pressure so it boils in the box. Written after newspaper reports of families killed by leaking SO₂.

USPTO PDF

Engineering Analysis & Physical Principles

How It Works: Step-by-Step Mechanical & Physical Breakdown

In the 1920s, refrigerators frequently killed people when rotating compressor shaft seals wore out and leaked lethal toxic gases (methyl chloride and sulfur dioxide). Albert Einstein and his former student Leo Szilard spent five years designing a completely hermetic refrigerator with no moving parts, no motor, and no seals.

The Core Breakthrough Mechanism

The entire closed system is kept at a uniform constant pressure (10 atmospheres). Heat applied to a boiler drives off ammonia gas into an evaporator containing liquid butane. By Dalton's Law of Partial Pressures, the presence of the ammonia gas drastically drops the butane's partial pressure, causing the butane to flash-evaporate at sub-zero temperatures (-10°C) and produce refrigeration without any mechanical compressor.

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Detailed Component Architecture

1Uniform-Pressure Hermetic Chamber

Welded steel tubes holding three fluids at equalized 10 atm pressure.

Because the pressure is identical everywhere in the system, no high-pressure/low-pressure throttle valves or rotary compressor seals are required, eliminating the primary source of mechanical failure and chemical leakage.

19th-C. Term: Closed hermetic circuit maintaining uniform total internal pressureModern: Hermetically sealed absorption refrigeration loop

2Thermosiphon Bubble Pump Boiler

A thermal heat source (gas flame or electric heater) driving fluid circulation.

Heat boils the ammonia-water solution in the generator. Rising vapor bubbles act as miniature pistons (airlift pump), lifting the liquid solution upward against gravity without any electric motor.

19th-C. Term: Thermosiphon lifting generatorModern: Bubble pump / Thermosiphon heat pipe

3Dalton Partial Pressure Evaporator

Mixing volatile butane with auxiliary ammonia gas to force evaporation.

Total pressure is constant: $P_{total} = P_{butane} + P_{ammonia} = 10\text{ atm}$ . Injecting ammonia gas lowers $P_{butane}$ below its saturation point at -10°C, absorbing latent heat $\Delta H_{vap}$ from the refrigerator cabinet.

19th-C. Term: Evaporation under reduced partial pressureModern: Dalton absorption evaporator

Governing Physical Equations & Principles

Dalton's Law of Partial Pressures in Evaporation

P_{total} = P_{butane} + P_{ammonia} = \text{const.} \implies P_{butane} = P_{total} - P_{ammonia} \ll P_{sat}(T_{cold})

The partial pressure of a gas is proportional to its mole fraction. Lowering the partial pressure of butane induces rapid boiling at sub-freezing temperatures without pulling a mechanical vacuum.

Thermosiphon Buoyancy Convection

\Delta P_{buoyancy} = (\rho_{liquid} - \rho_{vapor}) g \cdot h

Density differences between boiling vapor bubbles and liquid columns create a continuous upward driving pressure that circulates refrigerant passively.

Why It Still Matters

Einstein and Szilard's zero-moving-parts absorption cycle is used today in recreational vehicle (RV) refrigerators, remote solar-thermal cooling stations, and hermetic molten-metal cooling systems in advanced nuclear reactors.

Legal Claims Decoder (2 Numbered Claims)

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Claim #1Independent Master Claim

Verbatim Historical Legal Text

“1. An apparatus for refrigeration comprising an evaporator, an absorber, a generator, and a condenser connected in a closed hermetic circuit maintaining a substantially uniform total internal pressure, said circuit containing a volatile liquid refrigerant, an auxiliary evaporating gas, and an absorption liquid, said auxiliary gas being adapted to reduce the partial pressure of the refrigerant in the evaporator causing evaporation thereof at low temperature, and said absorption liquid being adapted to dissolve said auxiliary gas in the absorber to permit separation and recycling.”

Plain English Engineering Translation

Covers the constant-pressure, three-fluid absorption refrigeration cycle with no moving parts using refrigerant evaporation into an auxiliary gas and selective absorption in a solvent liquid.

Key Protected Innovations:

Uniform constant-pressure closed refrigeration loopDalton partial pressure evaporative cooling without compressor sealsZero-moving-parts thermosiphon circulation

Historical Legal Impact:

The master patent claim for hermetic non-mechanical absorption cooling systems.

The Historical Bottleneck

1920s kitchen refrigerators used SO₂ or methyl chloride behind a shaft seal. When the seal went, families died in their sleep. Einstein and Szilard read the Berlin newspaper accounts and decided the compressor was the wrong machine to put in a home.

Why Prior Art Failed

•Rotating shaft seals leak. Always, eventually.
•The motor hummed and needed oil.
•Service calls were part of the product.

The Breakthrough Insight

“Hold the whole loop at one total pressure (about 10 atm) so you need no shaft and no throttle. A third gas (ammonia) in the evaporator drops the partial pressure of the butane; Dalton's law does the work a vacuum pump would have done. Heat, not a crank, drives the circulation.”

Patent Wars & Legal Litigations

Vs. Electrolux / Platen–MuntersInfringement Challenge

Rival Claim & Defense:

von Platen and Munters already had a continuous absorption cycle (water, ammonia, hydrogen) that Electrolux owned.

Litigation Conflict:

Einstein–Szilard's butane/ammonia/water variant, with a later electromagnetic liquid-metal pump, was close enough that Electrolux bought the portfolio (about $750 in the usual telling) rather than litigate.

Final Resolution & Judicial Outcome:

Electrolux kept the silent-fridge market. Einstein and Szilard took the fee. The cycle became a thermodynamics homework problem.

After the Grant

Szilard moved on to the chain reaction (see Fermi). Einstein's name on a fridge is a historical curiosity that still gets undergraduates to look up partial pressures.

Civilizational Impact

RV and off-grid absorption fridges still run a cousin of this cycle on a propane flame. The 1920s death-by-leak problem was actually solved by Midgley's CFCs plus a welded compressor; those CFCs then became their own crisis.

Historical Fact

Einstein used the Electrolux money to help students and refugees leaving Germany. The sum was not a fortune. The gesture is why the story survives.

Further Context

The newspaper deaths were real. So was the later realization that a hermetic compressor plus a safer refrigerant was the mass-market path.
No moving parts is a slogan. The Einstein–Szilard pump, when they added one, moved liquid metal with a traveling magnetic field. Still no shaft seal.