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The Humm™ free-piston Stirling engine contains two moving parts that float on helium gas bearings to eliminate contact
wear. Freezing is performed by continuous evaporation and condensation of the cooling medium permanently sealed
within the thermosiphon where ultra low temperature is
constant from top to bottom

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• The thermosiphon is completely sealed and contains no moving parts. It is embedded within a composite non-CFC urethane foam and vacuum insulated panel wall.
• All cooling is performed by a simple condensation and evaporation process dependent on the thermodynamic properties of the cooling medium as it changes phase from a liquid to a gas at different temperatures.
• HUMM™ technology is not refrigerant specific; it can use any of variety of off-the-shelf cooling fluids such as DuPont SUVA 95®, hydrocarbons such as ethane (widely used in Europe) or other refrigerant fluids commonly used in global markets for household refrigerators and consumer goods.
• The Shuttle™ is permanently charged at the factory with a micro-charge of cooling fluid sealed within the tube.
• While the free-piston Stirling engine can operate in any orientation, the thermosiphon is gravity fed. The system uses no compressors, pumps or suction or discharge lines and does not require on/off cycling. By eliminating cycling, component wear and energy spikes from start/stop functions is eliminated and a steady-state temperature condition is maintained at the desired setpoint without a sawtooth curve common to cycling refrigeration systems.
• As the liquid cooling fluid absorbs heat from the interior, it changes phase from a liquid to a gas, moves up through the thermosiphon and condenses on the cold head of the free-piston Stirling engine, here the gas condenses to a liquid and runs back down the thermosiphon. This process is essentially isothermal and maintains the same temperature from top-to-bottom and creating a uniform interior temperature of better than ±-0.75°C.
  • Note: An optional multi-point temperature mapping record is available; see Accessories or contact Stirling Ultracold for details.
• Because the free-piston Stirling engine is so efficient much less heat is generated and discharged to the room than for conventional compressor cooled devices. An ultra-quiet axial-flow micro fan similar to a personal computer fan cools the Stirling.