The evaporator is such a major component of a direct-die system that I felt it warranted its own section in this guide. As mentioned before, the purpose of the evaporator is to allow the transfer of thermal energy from the hot CPU into the cold refrigerant vapor. As I see it, these are the main performance factors which you should hope to achieve in your evaporator:
* Low resistance conduction path(s) from your heat source to the areas in contact with refrigerant vapor. The amount of power which can be transferred per unit temperature difference in a conductor depends on the thermal resistance of the material, the width of the conducting channel and the length of the conducting channel. Whilst there are complex mathematical packages available to simulate this I feel it is best just to think about it and through trial and error evolve your designs to perform more effectively.
* Large surface area which is in contact with the refrigerant vapor. This will allow more power to be transferred for a given temperature difference between the evaporator and the refrigerant.
* Turbulence in your design. Whilst refrigerant will normally boil very violently, I believe that the vapor in the centre of the flow path may remain reasonably unaffected, especially if your flow channel is large. Creating harsh angles or rough surfaces in you design will cause disruption to the normal flow pattern and mix the inner and outer refrigerant allowing more of the inner layers contact to the evaporator surface.
* A small difference between the pressure of the refrigerant entering the evaporator from the metering device (typically capillary tube) and the suction line. Higher pressures in the evaporator with respect to the suction line will lead to higher evaporator temperatures and the possibility of more liquid refrigerant passing to the suction line.
Nearly every change you will make to an evaporator design will trade off one (or more) of these factors against another; for example, higher surface area will almost always mean a more resistive conduction path and greater turbulence will cause increased pressure difference. You should aim to produce a good balance and target your design to your application.
Occasionally someone will mention evaporator "mass". In my opinion this is completely irrelevant to performance and merely a by-product of the other factors and usually comes from low resistance conduction path(s).
Bookmarks