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If I understand correctly (history shows I probably don't), then the only reason why the absorption chiller evaporators are restricted (I hope that's the right word) to extract just enough heat so the chilled water exits at around 3 Celsius is so that the danger of the chilled water becoming frozen, and thus not being able to be pumped, is avoided by a safe margin.

Other than that I understand that most of the heat transfer at the evaporator takes place not because of the temperature gradient, but because the refrigerant water is kinda forced to evaporate due to the vacuum headspace (okay, your eyes bleed, but you're the physicist, not me). So I suppose if the flow rate in the chilled water pipes is lower, and every molecule of water passing through can lose more heat, the chilled water could essentially freeze?

This leads me to my question: If water in the chilled water pipe, is replaced by air, can't we use it to freeze things in a freezer? Is it possible?

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  • $\begingroup$ The specific heat of water (per kg) is more than 4000 times higher than air and the density of more than 800 times lower. To match the specific heat of water in a system with the same volume, you would need to use compressed air at about 3 million times atmospheric pressure. $\endgroup$
    – alephzero
    Jun 2, 2021 at 22:19
  • $\begingroup$ @alephzero I think you've got it the other way around pal. I'm not saying to replace the refrigerant water with refrigerant air. I'm saying to replace the building-to-cool 's water-to-chill by air. $\endgroup$
    – El Flea
    Jun 3, 2021 at 8:32

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Of course we could go below freezing using absorption cooling. The question is who would want to.

Lithium Bromide chillers is what I'm familiar with, and the temperatures they can produce is limited less by the chilled medium and more the properties of the LiBr solution. Using brine as the heat transfer medium is common when we need to use chillers below freezing. But I can't get "more vacuum" in the evaporator since the vacuum is really low (high? I'm unclear what the usage really is, lol) already. Theoretically there should be other absorption solutions that will go to lower temperatures to take your evaporator to below-freezing temps.

The issue with these units is that they are a maintenance nightmare. It is really hard in the field to maintain complex equipment that maintains a very low pressure vacuum. I'm guessing the reason we don't see these in use for ice making is that ammonia refrigerant plants are cheaper to build and maintain.

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