Appendix! Glossary of Terms for Cryocoolers and List of Organizations Adiabatic Compression and Expansion: Thermodynamic process of volume, pressure, temperature change, and also adiabatic process change that occurs without heat transfer to or from the system. Aftercooler: Water- or air-cooled heat exchanger used to cool compressed fluid leaving a compressor. Axial Compressor: A type of fluid compressor with a rotor carrying blades arranged radially on a drum or disks; corresponding blades are arranged on the stator. The fluid flows through the compressor in the axial direction increasing in pressure and density during the compress­ ion process. Beale Free-Piston Stirling Engine: A type of Stirling engine in which the piston and displacer move entirely under the action of fluidic forces. There are no connecting mechanisms between the piston and displacer. The load is direct coupled to the piston. Brayton Cycle: See Joule-Brayton cycle. Bucket Brigade Loss: Finkelstein's term for shuttle heat transfer. Centrifugal Compressor: A type of fluid compressor with a high-speed rotating impeller which accelerates fluid to a high centrifugal velocity, the energy of which is subsequently transformed to pressure energy in a volute casing. Claude, Georges: French scientist who conceived the combination of expansion engine and Joule-Thomson valve for gas liquefaction. Claude Cycle: An idealized thermodynamic cycle in which a fraction of the high-pressure fluid is expanded in an expansion engine and the remainder in a Joule-Thomson valve. The cold, low-pressure fluid from the engine cools the remaining high-pressure fluid passing to the JT valve. All the fluid is compressed adiabatically at ambient temperature and is cooled (at high pressure) prior to expansion in 375 376 Appendix I contraflow recuperative heat exchangers by the low-pressure return stream. Clearance: The amount by which a cylinder is greater in diameter than a piston or a bearing than the shaft rotating in it. Clearance Space: (a) The minimum volume of the compression and expansion spaces of Stirling or Vuilleumier engines. (b) The small volume in the cylinder above the piston at the end of compression (in a compressor) or at the start of admission in an expander. Coefficient of Performance (COP): The ratio of heat transferred to input work. For refrigeration the COP = heat (refrigeration efiect)/work supplied. For a heat pump the COP = heat rejected/work supplied (i.e., the inverse of thermal efficiency). Coldfinger: The long, thin cylinder of a cryocooler containing a displacer, or regenerative displacer. Refrigeration is generated at the end of the cold finger. Also cold sting. Collins, Samuel: American cryogenic engineer of the 20th century best known for development of helium liquefiers working on the Claude cycle. Collins Cryostat: A helium liquefier working on the Claude cycle. Compound Working Fluid: The working fluid of a Stirling engine that consists of two or more components and which may exist as a liquid gas, vapor, or dissociated elements. Compression Space: The variable volume of the working space in a Stirling engine where the working fluid is principally concentrated when the total system volume is decreased, the pressure rises, and heat is rejected to the cooling medium. In a prime mover, the compression space is cooler than the expansion space. In a refrigerator or heat pump, the compression space is warmer than the expansion space. Compressor: A machine used to elevate the pressure of the fluid; may be a reciprocating, rotary, or screw compressor. Constant-Enthalpy Process: Thermodynamic compression or expansion at constant enthalpy--e.g., a Joule-Thomson expansion. Constant-Entropy Process: Thermodynamic compression or expansion process that occurs reversibly with no transfer of heat and hence no change in entropy. Constant-Pressure Process: Thermodynamic heating or cooling process that occurs at constant pressure. This mayor may not be regenerative. Constant- Temperature Process: Thermodynamic heating or cooling process that occurs at constant temperature. This mayor may not be regen­ erative. Constant- Volume Process: Thermodynamic heating or cooling process that occurs at constant volume. This mayor may not be regenerative. Cooler: The heat exchanger provided to facilitate the transfer of thermal Glossary of Terms/List of Organizations 377 energy from the working fluid to the cooling medium, water, air, or some other fluid. Crank Drive: One form of kinematic drive consisting of a crank and connecting rod used to convert reciprocating to rotary motion and to convey power between pistons and drive shaft. Cryocooler: Any device, system, or ensemble capable of generating refriger­ ation at cryogenic temperatures, i.e., less than 120 K. Cryogenerator: A cryocooler capable of achieving refrigeration at cryogenic temperatures (less than 120 K). Dead Volume Ratio: That part of the total working space not included in the variable volumes of the expansion and compression spaces, expressed in terms of the variable volume of the expansion space. Direct Heating: A system in which the hot products of combustion pass directly over the heater tubes in which the working fluid flows, so that heat is transferred directly from the combustion products to the heater tube walls and hence to the working fluid. Discontinuous Piston Motion: The nonsinusoidal motion of the piston and displacers required to achieve the necessary volume variations of the idealized thermodynamic cycles. Displacer: A lightweight structural reciprocating element in a Stirling engine characterized by a large temperature difference but a negligible pressure difference across the upper and lower transverse faces. Double-Acting Engines: A family of Stirling engines having a single reciprocating element per thermodynamic system. There is a minimum number of two cylinders but no maximum number. Dual-Pressure Cycle: A thermodynamic cycle with two or more stages of expansion in engines or JT valves. Many variations are possible involv­ ing several stages of expansion and intermediate pressure separation of saturated liquid and vapor. Duplex Stirling Engine: Two Stirling engines arranged so that one, operating as a prime mover, receives heat at a high temperature and produces work to drive the second Stirling engine, acting as a cooling engine, refrigerator, or heat pump. Ericsson Cycle: An idealized thermodynamic cycle consisting of isothermal compression and expansion processes at different temperatures bounded by constant-pressure regenerative processes. Exhaust Gas Heat Exchanger: See Regenerative Cycle. Expansion Space: The variable volume of the working space in a Stirling engine where the working fluid is principally concentrated when the total system volume is increased, the pressure falls, and heat is ab­ sorbed. In a prime mover, the expansion space is hotter than the compression space. In a refrigerator or heat pump the expansion space is cooler than the compression space. 378 Appendix I Finkelstein Adiabatic Cycle: An idealized thermodynamic cycle for Stirling engines with no heat transfer in the compression and expansion spaces and infinite rates of heat transfer in the heat exchangers. Free-Displacer Engines: A form of Ericsson regenerative engine (Bush type) where the displacer moves under the action of fluidic forces. Used principally as a pressure generator or pump. Freezer: The heat exchanger provided in a refrigerator or heat pump to facilitate the transfer of heat to the working fluid from an external low-temperature source. Gifford-McMahon Engine: A regenerative expansion engine to generate refrigeration at cryogenic temperatures. Valves regulate flow of com­ pressed gas to and expanded gas from the expansion cylinder. Harmonic Piston Motion: The near sinusoidal motion of the pistons and displacers used in practical Stirling engines. Heater: The heat exchanger provided in a prime mover to facilitate the transfer of thermal energy from an external source to the working fluid. Heat Pipe: A device used in an indirect heating system in which an intermediate fluid is used to transfer heat from an external energy source to the working fluid. Usually the intermediate fluid a liquid metal, i.e., soli urn) is evaporated at the thermal inlet and condenses at the thermal outlet. Large rates of heat transfer can be effected with minimal temperatures differences. Heat Pump: A machine driven from external power supply absorbing heat at ambient temperature and rejecting the heat at some higher tem­ perature. Heylandt Crown: The addition of an extension to a reciprocating piston to remove the hot or cold fluid from the region where the piston rings and seals operate. Hybrid Free-Displacer-Crank-Controlled Piston Engine: A form of Stirling engine where the reciprocating piston has kinematic coupling to a rotating shaft but the displacer is oscillated under the action of fluidic forces. Indirect Heating: A system in which thermal energy from an outside source heats an intermediate fluid (i.e., sodium) which conveys the energy to the heater tubes and hence to the working fluid (see Heat Pipe). Intercooler: Water- or air-cooled heat exchanger used to cool compressed fluid between stages in a multistage compressor. Intermediate (Capacity) Cryocooler: Cryocooler having a refrigerating capacity less than 25 W at 1 K, 100 W at 4 K, 1 kW at 20 K, or 15 kW at 80 K. Isentropic Process: Thermodynamic process of volume, pressure, and tem­ perature change that takes place at constant entropy. Glossary of Terms/List