The evaporator has two important functions. It is supposed to cool the processor as much as possible, while making sure that the surrounding of the processor, including the motherboard, doesn't receive any major cooling at all, to avoid condense water buildup.
Here's a quote from a previous cooling article at Tom's Hardware:
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Cooling a CPU below the freezing point of water requires a very well done insulation from the surrounding. You can certainly imagine that otherwise all the liquid in the air around it would instantly freeze at the CPU and the surrounding components, further away condensed water would cover the less cold components of the motherboard and this would definitely jeopardize if not even destroy any computer system due to short circuits. Thus the CPU needs to be insulated very well and to avoid any condensing water, the cover of the CPU should even be heated.
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