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The dry ice chiller consists of two nested 7 gallon plastic containers. 

 

          

 

The structures in the diagram in the lower container serve no purpose other than to prop up the second container.  The upper container is about half full of coolant.  Keep this coolant at a low temperature by the addition of chunks of dry ice.  The current coolant is 70% isopropyl alcohol which has a freezing point of -29 C.

 

The lower container also contains some coolant.  The temperature of this coolant is kept at a set point temperature, for example -5.  The coolant from the lower container is constantly circulating through the patient circuit hex with a centrifugal pump.

 

Open the valve slightly to allow coolant to be pumped from the lower container into the upper container at a controlled slow rate.  This causes the liquid level in the upper container to rise above the perforations, and some of the colder liquid to overflow through the pipe into the lower container.  The black mesh on the pipe is necessary to prevent small bits of dry ice from getting into the pumps.  In this manner, you may easily control the temperature of the circulating coolant.  No computer control is used.