CO₂

  • Carbon Dioxide (CO2)
  • 一般属性

    在热动力属性方面,二氧化碳不是很适合作为制冷剂。但是,二氧化碳具有多种独特的热力学物理属性。
    • 较高的传热效率
    • 压力损失相对敏感
    • 极低粘度

    在实际应用中,二氧化碳系统具有极高性能,主要原因是当二氧化碳作为二次侧液体时具有较好的热交换能力和极低的泵功率,同时在冬季寒冷气候中运行时能够达到极低的冷凝压力。

    对比其他制冷剂,二氧化碳系统的效率更多取决于应用和气候。所有制冷剂在冷您温度提高时系统效率均会下降,相对而言二氧化碳制冷剂的幅度最低。二氧化碳优秀的热力学物理属性能够起到一定补偿,但有限制。

    二氧化碳在较高温度时具有较高的含能量,当热量回收用于加热卫生热水或类似应用时,整个系统的效率将非常高。

     

  • 环境影响

    在环境影响方面,二氧化碳是一种非常理想的制冷剂,其 ODP 为 0,GWP 为 1。它是大气中自然存在的物质,而且数量巨大。

     

  • 压力与温度

    二氧化碳是一种高压制冷剂,高工作压力需要高效率运行。静态时,环境温度可能达到或超过零界温度,压力可能超过零界压力。因此,系统通常设计承受压力最高 90 bar,或者有时候配备一个小型静态冷凝机组来保持低压。

    同时,二氧化碳具有较低的压缩压力比(比 HFC 和氨低 20-50%),这能够提高容积效率。在蒸发温度为 -55 - 0 ºC 时,二氧化碳的容积性能是氨的 4-12 倍,因此压缩机所需排量更小。

    二氧化碳的三相点和临界点与工作范围非常接近。正常系统运行过程中,可能达到临界点。系统运行期间,三相点可能达到,当系统中含液体零件暴露在大气压力下时出现干冰。通风运行期间需采用必要措施防止出现干冰。

     

  • 材料相互作用

    二氧化碳不会与常见金属、Teflon®、PEEK 或氯丁橡胶组件发生反应。但是,它会在弹性体中扩散,以及导致丁基橡胶(IIR)、丁腈橡胶(NBR)和乙丙橡胶(EPDM)膨胀。

    二氧化碳液体密度约是氨的 1.5 倍,因此蒸发器需要更高的混合型冲注,例如大型工业系统中的大型板片式冷水机。高密度意味着高油循环,因此工业系统需要高效油分离器。

     

  • 成本效益

    CO2 是许多行业的副产品,因此它的价格较低。但是,由于压力高(跨临界系统)且复杂性高(跨临界和亚临界系统),因此 CO2 系统比传统系统昂贵。由于采用了增压系统,系统复杂性似乎有所降低;同时,随着 CO2 装置数量增加,统计显示其成本逐渐接近采用 HFC 的系统。

    其次,大型 CO2 系统(尤其是工业制冷)比乙二醇替代方案更节省开支,因此初投资成本和整个生命周期花费更低。

     

  • 应用

    不同于大多数制冷剂,二氧化碳在实际中用于三种不同制冷循环:
    • 亚临界(复叠系统)
    • 跨临界(二氧化碳系统)
    • 二次侧液体(二氧化碳作为挥发性盐水)

    技术采用取决于应用和系统所在地。目前在多种应用中,二氧化碳广泛采用。

    • 工业制冷二氧化碳通常与氨组合使用,在复叠系统中或作为挥发性盐水
    • 食品/零售制冷
    • 热泵
    • 冷藏运输

    丹佛斯相信二氧化碳将在多种商业制冷应用系统中作为主要制冷剂。新的F-gas法规明显推动此方向发展。

     

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