Why vacuum is required in VAHP?

Why vacuum is required in VAHP (Vapour absorber heat pump) ???
In machine Which part vacuum is generated???

vacuum is generate in evaporator so therefor water vapor makes in just 3.5 to 5 Degree centigrade, it help to DM water cooling. as per rule of nature when atmospheric pressure decries then with boiling point of water is decries.

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