When the inlet pressure of the roots pump is higher than 1 Torr, the pressure difference between the outlet and the inlet may reach 15-20 Torr or more. The power of the pump is proportional to the pressure difference. Therefore, the large pressure difference can make power of the pump increase rapidly. As a result, the temperature of the exhaust gas, parts in the pump, and in particular the rotor will increase accordingly. If no cooling measures are taken (especially for the rotor), the thermal expansion of the rotor may make the gap disappear after long time operation. The rotor will be wedged, stuck, and even damaged.
There are two commonly used cooling methods.
1. Install an aftercooler on the exhaust side.
The cooling principle is that one end of the heat sink is set around the position that rotor head can sweep over. The length of the loose blades is equal to the axial length of the rotor. If there is no cooler, the exhaust gas flows back through the gap to be heated and then it will heat the rotor. If a water cooler is provided, the rotor can be cooled with the returned cooled gas. With this after-cooler, the permissive pressure difference will reach 85 Torr and it can still operate normally. While the maximum allowable pressure difference is 20 Torr if there is no cooler. The cooler can take up 80% of the heat of the compression work.
2. Use the inside of rotor itself to cool rotor.
The principle is shown as below. The cooling oil is also used to lubricate the bearings and gears to seal the circulating oil in the system. The rotor temperature is often lower than the pump body temperature in the operation. With increasing of the gas load, the temperature difference between the rotor and the pump body also increases. As a result, the gap in the pump may enlarge and the pumping speed may decrease accordingly. In order to avoid it, the amount of circulating oil is regulated by the pump load. When the load is particularly large, the pump body should be cooled to maintain the original gap.
Diagram of the internal cooler of the roots pump rotor
The figure shows the effects of whether the roots pump unit (geometrical pumping speed is 200003m/h) is cooled or not on the actual pumping speed.