Why CO₂ as coolant?

• Radiation hard (used as coolant in nucleair power plants)
• Non toxic
• Non flammable
• Low pressure drop in micro channel tubes
• Good thermo dynamic properties
• Widely available at low cost
• No need to recover or recycle

Principal of operation

The CO₂ is used in a two phase cooling system. The coolant is supplied as a liquid, the heat is taken away by evaporation.

In total there are 54 heat inputs each will generate 40 Watts of heat. The heat of every input is cooled by a 1.1 mm outer diameter pipe with an inner diameter of 0.9 mm. The capacity of the cooling pipe was tested and found to be > 50 Watts. The heat transfer coefficient between pipe and coolant was > 20 kW/m²K.

Description

A schematic drawing of the cooling system is shown in this eps file.

At room temperature the system is filled with CO₂ gas at a pressure of 40 bars. After switching on the R507 compressor (9) the heat exchanger (6) will be cooled and the CO₂ condenses. The volume of the gas container (11) is so chosen that at 20 bars there is sufficient liquid in the system to start the pump (5). The pump pressure is limited by the maximum pressure valve (12) to 70 bars. The needle valve (4) is set to have a throughput of 15 grams CO₂ with a pressure drop of 45 bars. Pressure behind the valve is then 25 bars and the temperature 0 ^°C. In the heat exchanger (3) the incoming liquid is cooled by the return gas and liquid. To make sure that the coolant is equally divided between the 57 cooling pipes the restrictions (2) generate 10 bars of pressure drop for 0.25 grams/sec of CO₂ liquid flow. The right part of the diagram indicates a "normal" cooling system with compressor and condenser. The evaporating temperature is set with the pressure regulator (10). With this regulator the temperature in the detector can be changed (from -30 to 0^°C (?)).