The chiller is used to transfer heat away from the pump head. Cooling the pump head ensures that only liquid carbon dioxide reaches the pump. This is important because the unit cannot pump gaseous carbon dioxide. The chiller essentially does two things. It counteracts the heat of compression which occurs inside the pump head, and it removes heat caused by friction of the piston moving back and forth. Both of these heat sources need to be controlled. If the pump head is not cooled, liquid carbon dioxide will enter and immediately flash to gas. The pump will cavitate and will operate inefficiently or not at all.