The present invention relates to a vacuum pump used for an exhaust pump, for example, in a semiconductor manufacturing apparatus, and more particularly, to a vacuum pump which is operated under the condition that pressure of gas passing through an exhaust port of the pump is substantially equal to or close to the atmospheric pressure; with the vacuum pump being a dry-type pump employed in a process having such a tendency that reaction products are liable to stick to the inside of the pump.
A dry-type vacuum pump is advantageous in that a clean vacuum can be obtained because there is no oil or water in a conduit where gas fed from a suction port passes, meanwhile an effect to remove heat generated when the gas is compressed is restricted so that the temperature inside the pump becomes high. For the reason, conventionally, a cooling jacket is provided on the outside of a heat generating portion in order to cool the same by water. Referring to FIG. 7, a conventional dry vacuum pump includes a rotor 4 rotatably supported by bearings 6 in a casing 3, with the vacuum pump including a suction port 1 and an exhaust port 2, and a stator 5 securely fixed in the casing 3. Gas suctioned from the suction port 1 is successively compressed in multi-stage due to the compression function of a pump mechanism unit comprised of the rotor 4 and the stator 5, and is then discharged via the exhaust port to the atmosphere. In the compressing process of the gas, heat is generated by compressing the gas and the amount of the compression heat of the gas increases as the gas arrives nearer the exhaust port 2. For the purpose of removing this compression heat, in the conventional example shown in FIG. 7, a cooling jacket 7 is provided on an exterior of the stator 5 for cooling the stator 5 by water supplied from a water supply port 8.
An example of this type of conventional technique is disclosed in, for example, Japanese Patent Unexamined Publication No. 62-29796 or Japanese Utility Model Unexamined Publication No. 64-46495.
In the above-described conventional technique, water is mainly used as a cooling medium to have so large specific heat and so large thermal conductivity that its cooling effect is very preferable. However, the conventional technique is disadvantageous in that, when gas suctioned by a vacuum pump is one whose sublimation temperature is high, i.e., which is liable to be solidified even at a low temperature, the gas is transferred into the solid phase if the interior of the pump is cooled excessively, and the gas is solidified to adhere to or accumulate on the interior of the pump as a reaction product so that a conduit in the pump is clogged and a rotor is unfavorably locked. In order to resolve these problems, as disclosed in Japanese Utility Model Unexamined Publication No. 64-46945, the temperature of a stator is maintained at a predetermined value by controlling an amount of circulating cooling water. However, if an amount of the cooling water is decreased to be less than a predetermined amount, the overall cannot uniformly be cooled, which results in a problem that an efficiency of the vacuum pump is degraded. Further, a flow meter is required for controlling the amount of the cooling water. Since bleaching powder precipitates at a narrow portion of the flow meter, there also occurs a problem that the temperature of the pump cannot be reliably controlled.
Incidentally, though it is suggested to provide a heater only at an exhaust port of the vacuum pump so as to prevent the sublimate gas from solidification, the method of heating the gas by provision of the heater is disadvantageous in that the heater is sometimes not operationally reliable. A sublimate gas is a gas which separates out of a solid which is at or below a sublimation temperature.