US 2935242 A
Description (OCR text may contain errors)
y 3, 0 2 A. L ORENZ 2,935,242
PUMPING APPARATUS Filed July 16. 1956 FIG. 1 I I? TO VACUUM EmA/v/cAL SYSTEM 532:
FIG 2. t
MECHANICAL 7'0 VACUUM SYSTEM h FORE 4'- 45 PUMP 4 43 49 65 7: VAR/ABLE SPEED TRANSMISSION 56 55 INDUCTION MOTOR J54 CONSTANT SPEED 63 MOTOR INVENTOR. ALBERT LORENZ A T TORNEYS PUMPING APPARATUS Albert Lorenz, Hanan (Main), Germany, assignor to W. C. Heraeus, G.m.b.H., Hanan, Germany Application July 16, 1956, Serial No. 597,945
(ltaims priority, application Germany July 23, 1955 3 Claims. (Cl. 230-11) This invention relates to pumping apparatus, and more particularly, to mechanical pumps used for high vacuum operation.
it has been discovered that a Roots type pump (also sometimes referred to as a Connersville blower, see Industn'al Chemistry, 4th edition, pages 696 and 697, by Riegel, published 1942 by Reinhold Publishing Corporation), is useful as a high vacuum pump when used in combination with a fore pressure mechanical pump of the rotary piston oil sealed type. Such a pumping system is described in U.S. Patent No. 2,721,694.
The advantages of a Roots type pump as a high vacuum pump becomes apparent only at pressures below about 20 mm. Hg, i.e., in the hue and high vacuum region, and only when operated at high rotary speeds. For a Roots pump to be operable at the high rotary speeds necessary to produce high vacuum. it should be of light construction. High vacuum Roots type pumps used in combination with conventional fore pumps have produced pumping systems with speeds at low pressures not heretofore customary for systems using only mechanical pumps. Moreover, the vacuum produced is dry, that is, a pumping vapor is not necessary.
However, the time required for pumping a system down from an atmospheric pressure to about 20 mm. Hg is determined by the capacity of the fore pressure pump adapted to handle large volumes of air at relatively high pressures. In order to increase the pumping speed in the higher pressure region, it was necessary, prior to this invention, to use several fore pumps, or use a fore pump considerably larger than necessary for continuous operation in the high vacuum range.
The former method is frequently used for evacuating large volumes, say 5300 cubic feet or more, such as may be encountered in evacuating large vacuum furnaces for casting of steel.
The light construction of a Roots pump necessary for high vacuum operation presents the danger of thermal overloading of the pump when operated at its normal speed at or near atmospheric pressure. The reason for this is that the power supplied to the pump is converted almost entirely into heat of compression of the pumped gases, and much of this heat must be absorbed by the pump. The amount of heat varies depending on the pressure diiierence between the pump intake and discharge, and would be excessive if the pump were operated at its normal operating speed near atmospheric pressure.
For this reason it has been customary practice to use only the fore pump in the initial stages of evacuating a system until the pressure is suitably reduced to permit satisfactory operation of the Roots pump.
This invention provides apparatus which permits the operation of the Roots pump at atmospheric and subatmospheric pressure to aid in the evacuation of the system without the danger of thermally overloading the Roots pump during its operation at near atmospheric States Patent pressure. Thus, the system may be evacuated with the apparatus of this invention in a much shorter time than heretofore practical without the necessity of using a fore pump or a plurality of fore pumps having a capacity larger than necessary for continuous operation in the high vacuum range.
Briefly, the invention provides high vacuum pumping apparatus which includes a fore pressure pump having an inlet and an outlet. A high vacuum Roots type pump having an inlet adapted to be connected to a system to be evacuated, has an outlet connected to the fore pressure pump inlet. A motor is provided for driving the Roots type pump, and means are provided for automatically regulating the speed of the Roots type pump to consume a substantially constant amount of power from the motor as the pump is operated over a wide pressure range from atmospheric to sub-atmospheric pressure.
These and other aspects of the invention will be more fully understood from the following detailed description of some examples taken in conjunction with the accompanying drawings in which:
Fig. i is a schematic diagram of one form of the invention in which a hydraulic torque converter is used to couple the Roots type pump to the motor; and
Fig. 2 is a schematic diagram in which a variable speed transmission couples the Roots pump to the motor and is controlled automatically to insure a substantially constant power consumption by the pump.
Referring to Fig. l, a Roots type pump 1i? has an inlet 11 adapted to be connected to a vacuum system (not shown). The Roots pump has an outlet 12 connected by a coupling 13 to an inlet 14 of a mechanical fore pump 16 of the rotary piston oil sealed type, which has an outlet 17 adapted to exhaust to the atmosphere.
The Roots pump is driven by a drive shaft 18 connected by a coupling 20 to the output shaft 21 of a hydraulic torque converter 22. An input shaft 24 of the torque converter is connected by a coupling 26 to a shaft 28 of a motor 30 which is supplied power from a source 31 through a lead 32 and a variable resistor 34.
In the operation of the apparatus of Fig. 1, say to reduce the pressure in the vacuum system from atmospheric to below 20 mm. Hg, the fore pump is turned on, and the motor to drive the Roots pump is also turned on. The hydraulic torque converter keeps the power supplied to the Roots pump constant by providing a variable speed drive between the motor and the pump. The torque converter is responsive to the load on the Roots pump and controls the speed of the Roots pump in accordance with this load to maintain the power supplied to the Roots pump constant. At the beginning of the pumping process, the pressure diiference between the Roots pump inlet and outlet may range from 300 to 600 mm. Hg without exceeding the power permissible for the Roots pump. The motor is of sufiicient size to supply this power. As the pressure in the vacuum system is reduced the Roots pump works against a smaller pressure diiferential and its speed increases so that the power supplied to it is kept substantially constant over a wide range. Once the high vacuum range is reached, the Roots pump is operating at its rated high rotary speed to produce and maintain a high vacuum in the system.
With the apparatus of Fig. 2, a Roots pump 49 has an inlet 41 adapted to be connected to a vacuum system (not shown), and an outlet 42 connected by a coupling A3 to an inlet 44 of a fore pump 45 having an outlet 46 which exhausts to the atmosphere. A drive shaft 47 on the Roots pump is connected by a coupling 48 to an output shaft 49 of a conventional variable speed transmission 50 which has an input shaft 51 connected by a coupling 52 to an output shaft 53 of an induction motor the desired amount of power.
3 a 'M-supplied power from-a suitable source 55 through lines 56;
The shaft of the induction motor extends through the motor, and the end of the induction motor shaft remote from the variable transmission is connected by a coupling 58 to a'first input shaft 59 ofa differential gear system 60. A second input shaft 61 of the differential is connected by a coupling 62 to the shaft of a constant speed motor 63. The differential has an output shaft 64 which is connected to turn a control shaft 65 of the variable speed transmission.
The differential output shaftturns in response to the difference in speeds between the induction motor and the constant speed motor. If the induction motor is running faster than the constant speed-motor the differential output shaft is turned in one direction,-and if the induction motor is turning slower than the constant speed motor,
the differential output s'haftis turnedin the otherdirection. Thus, the variable speed transmission control shaft is adjusted to keep the speed of the induction motor constant and equal to the speed of the constant speed motor. This keeps the power input from the induction motor to the Roots pump constant because theslip of the induction motor changes with the load, and the rotor speed of the induction motor is a measure of the power output of the motor. a 7 a The operation of the aparatus of Fig. 2 is as follows: The fore pump; induction motor, and constant speed motor are turned on to begin the evacuation of the vacuum system from atmospheric pressure to a high vacuum. The speed of the constant speed motor is set to cause the induction motor to turn at a rate to supply The load on the Roots pump is relatively high during initial stages of the evacuation, tending to cause the induction motor to slip, i.e., turn at a speed slower than the desired speed set by the constant speed motor. The difierential senses this difference and the diiferential output shaft turns the variable speed transmission control shaft in a direction to increase the speed ratio between the induction motor and the Roots pump. Thus, the induction motor is permitted torturn at an increased rate, and the Roots pump turns at a rate to provide the safe amount of power to aid the fore pump in evacuating the system. If the induction motor begins to turn faster than the constant speed motor, the variable speed transmission is adjusted to increase the speed of the Roots pump to consume the desired amount of power from the induction motor.
' The following example demonstrates how the apparatus of this invention reduced the time required to evacuate a vacuum chamber having a volume of 1765 cubic feet. The vacuum chamber was connected to the inlet of a high vacuum Roots pump with a pumping speed of 3000 cubic meters per hour at 1700 r.p.m. The Roots pump outlet was connected to the inlet of a mechanical fore pump having a pumping speed of 300 cubic meters per hour. Without using the automatic regulation of power consumption by the Roots pump, and using the fore pump only, as is the customary practice, 56 minutes were required to reduce the pressure in the vacuum chamber from atmospheric to 3 mm. Hg. Using the automatic power regulation of this invention and using the Roots 7 pump in conjunction with the fore pressure pump, with the power consumption of the'Roots pump being set for I-I.P., the pressure in the vacuum chamber was reduced from atmospheric to 3 mm. Hgin 22 minutes, i.e., conassua e 7 4 siderably less than one half the time required for evacuation by the conventional procedure.
During the evacuation of the chamber in accordance with this invention there was no pressure region in which the Roots pump was thermally overloaded.
1. High'vacuum pumping apparatus which includes a fore pressure pump having an inletand an outlet, a high vacuum Roots type pump having inlet adapted to be connected to a. system to be evacuated and an outlet connected to the fore pressure pump inlet, a pump induction motor for driving the Roots type pump, a variable speed transmission connected between the Roots type pump and the pump motor, the transmission having a control shaft, a differential gear system having a pair of rotatable input shafts and a rotatable output shaft which turns in response to a difference inrotational speed of the two input shafts, a constant speed motor connected to one of the input shafts and the shaft of the pump motor shaft being connected to the other input shaft, the differential output shaft being connected to the'control shaft for the variable speed transmission so that the speed ratio between the Roots type pump and .its motoris automatically adjusted to keep the speed of the pump motor automatically adjusted to substantially the same speed as the constant speed motor. 7
2. High vacuum pumping apparatus comprising a fore pressure pump. having an inlet and an outlet, a high vacuum Roots pump having an inlet adapted to be connected to a system to be evacuated and an outlet connected tothe .fore pressure pump inlet, a motor and transmission means coupled between the motor and the Roots type pump and responsive to the load on the Roots type pump for translating a substantially constant amount of power from the motor to the Roots type pump whereby the Roots type pump and the fore pressure pump may be simultaneously operated over a wide pressure range from atmospheric pressure to the desired subatmospheric pressure.
3. In a high vacuum pumping apparatus the combination which comprises a fore pressure pump having an inlet and an outlet, a high vacuum Roots type pump having an inlet adapted to be connected to a system to be evacuated and an outlet connected to the fore pressure pump inlet, a motor for driving the Roots type pump and variable speed transmission means connected between the motor and the Roots type pump and responsive to the load on the Roots type pump for controlling-the speed of the Roots type pump to supply the Roots type pump with a substantially constant amount ofpower during the opera- 7 tion thereof over the range of pressures at the outlet of the Roots type pump from atmospheric pressure to less than 20 mm. Hg pressure whereby the Roots type pump and the fore pressure pump may be operated simultaneously from atmospheric to the desired subatmospheric pressure in the system.
References v Cited in the file of this patent UNITED STATES PATENTS Van Atta Oct. 25, 1955