使气体在可变容积中膨胀，输出外功制冷的膨胀机(通常由电动机制动吸收外功)。这种膨胀机分立式和卧式两种。采用较多的是立式结构,曲轴、连杆、字头、活塞、进气阀和排气阀等是运动件，分别装在机身、气缸和中间座中,其作用近似于往复活塞压缩机，但其进、排气阀系借进、排气凸轮定时启闭。活塞膨胀机由于存在进、排气阀流动阻力、不完全膨胀、摩擦热、外热与内部热交换等引起的冷量损失，一般绝热效率为: 高压膨胀机65 ~ 85%，中压膨胀机60 ~ 70%。20世纪50年代相继出现的不用凸轮传动机构的无阀和单阀膨胀机，减少了膨胀机的运动件,提高了机器运转可靠性，已在小型深低温设备上得到广泛的应用。60年代，采用加填充剂的聚四氟Z烯密封元件代替用油润滑的金属制密封元件,避免润滑油带入深低温精馏区或液化区，保证了安全。 An expander that expands gas in a variable volume and outputs external cooling (usually absorbed by electric motor braking). There are two types of expanders: discrete and horizontal. The most commonly used structure is a vertical structure, with the crankshaft, connecting rod, letterhead, piston, intake valve, and exhaust valve as moving parts, installed in the body, cylinder, and intermediate seat respectively. Their function is similar to that of a reciprocating piston compressor, but their intake and exhaust valve systems are timed to open and close by the intake and exhaust cams. Due to the flow resistance of the inlet and outlet valves, incomplete expansion, frictional heat, and external and internal heat exchange, the piston expander suffers from cooling loss. The general insulation efficiency is 65-85% for high-pressure expanders and 60-70% for medium pressure expanders. In the 1950s, valveless and single valve expansion machines without cam transmission mechanisms emerged one after another, reducing the number of moving parts and improving the reliability of machine operation. They have been widely used in small deep and low-temperature equipment. In the 1960s, PTFE sealing elements with fillers were used instead of metal sealing elements lubricated with oil to prevent lubricating oil from entering deep low-temperature distillation or liquefaction zones, ensuring safety.
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