TUBE COOLING

TUBE COOLING

In a X-ray tube, only less than 1% of the electrical power supplied

is converted to X-rays. The remaining electrical power (over 99%)

is converted into heat. This large amount of heat may melt the

target and therefore heat should be removed very quickly from

the target. Hence, efficient cooling systems are necessary for the

X-ray tube. In general, targets are made by inserting a layer of tungsten in copper block, and X-ray tubes are usually enclosed in metal cases, which are filled with oil for insulation purpose. The heat produced on the focal area is conducted quickly into the anode disk, stored temporarily, later transferred to the insulating oil by radiation. This oil surrounds the glass envelope as well as copper block (static oil cooling). The heat taken up by the oil is transferred to the housing (metal case) by convection process. In some designs, fan is used to assist the convection process and removes the heat from the housing. In the case of rotating anode tube, the molybdenum neck is so long and prevents the heat conduction to the rotor. If the anode assembly is coated black, it will promote the heat radiation process. The rate of heat radiation is proportional to fourth power of anode temperature. Hotter the anode, greater the rate of heat dissipation. For prolonged operation, the oil around the tube is connected by two pipes to a oil reservoir with radiator and pump, where the oil is cooled additionally by an air current and water (circulating oil cooling). In X-ray tubes used in CT scan and angiography work, oil is pumped through an external heat exchanger. In some modern tubes, the anode is earthed and water is allowed to circulate through the anode. Sometimes the water is additionally cooled by Freon gas.