Pump assembly for obtaining a high vacuum

A pumping assembly for obtaining a high vacuum, the assembly comprising a primary pump (4) and a secondary pump (1) associated in series, the inlet of the secondary pump (1) being taken from an enclosure (3) to be evacuated, the assembly further including means (7) for starting the secondary pump (1) when the pressure upstream from the primary pump (4) drops below a value P.sub.1, the assembly being characterized in that a passive tank (10) followed by an isolating valve (11) are interposed between the outlet (12) from the secondary pump (1) and the inlet (13) to the primary pump (4), and in that it includes control means (7) for closing the isolating valve (11) and stopping the primary pump (4) when the pressure in said passive tank (10) reaches a value P.sub.2 <P.sub.1, and for opening the isolating valve (11) and restarting the primary pump (4) when the pressure in said passive tank ( 10) returns to the pressure P.sub.1.

The present invention relates to a pump assembly for obtaining a high 
vacuum. 
It is well known that in order to obtain pressures of less than 10.sup.-3 
mbar, a primary pump is associated in series with a secondary pump. When 
the assembly is started up, only the primary pump is run until the 
pressure upstream from the primary pump has dropped to a value P.sub.1 
enabling the secondary pump to operate. The secondary pump is then started 
and both pumps, i.e. the primary pump and the secondary pump operate 
simultaneously, in series, and permanently. The desired pressure in the 
enclosure is thus achieved after some length of time has elapsed. 
Such a pumping assembly requires electricity to feed the motors driving the 
pumps. The electricity may be taken either from a mains supply or else 
from a storage battery integrated in the pumping assembly. 
The object of the invention is to economize the electrical energy consumed 
during pumping operations. The invention is particularly advantageous for 
portable assemblies which are powered, in particular, from storage 
batteries, the invention making is possible to increase the running time 
of the pumping assembly for a battery of given size and weight. 
The present invention thus provides a pumping assembly for obtaining a high 
vacuum, the assembly comprising a primary pump and a secondary pump 
associated in series, the inlet of the secondary pump being taken from an 
enclosure to be evacuated, the assembly further including means for 
starting the secondary pump when the pressure upstream from the primary 
pump drops below a value P.sub.1, the assembly being characterized in that 
a passive tank followed by an isolating valve are interposed between the 
outlet from the secondary pump and the inlet to the primary pump, and in 
that it includes control means for closing the isolating valve and 
stopping the primary pump when the pressure in said passive tank reaches a 
value P.sub.2 &lt;P.sub.1, and for opening the isolating valve and restarting 
the primary pump when the pressure in said passive tank returns to the 
pressure P.sub.1.

FIG. 1 is thus a block diagram of a pumping assembly comprising a secondary 
pump 1 having a drive motor 2 having its inlet side connected to an 
enclosure in which a high vacuum is desired, and having its outlet side 
connected to a primary pump 4 having a drive motor 5, said primary pump 4 
outputting to the atmosphere. 
The pumping assembly shown is, for example, portable and cordless, and 
therefore includes a storage battery 6 for powering the assembly. The 
battery feeds an electrical control circuit 7 which includes, inter alia, 
a DC-AC converter for providing a 3- phase AC to the motors 2 and 5. Lines 
8 and 9 represent these power supply connections. 
As is known, the secondary pump 1 cannot operate unless below a certain 
pressure P.sub.1 referred to as the priming pressure. Thus, when the 
assembly is initially started, only the primary pump 4 is switched on, and 
the secondary pump is started automatically when the pressure upstream 
from the primary pump falls below said pressure P.sub.1. It is known that 
the current taken by the drive motor 5 is an increasing function of inlet 
pressure. Thus, the secondary pump is switched on when the current taken 
by the drive motor 5 drops below a value which corresponds to said priming 
pressure P.sub.1. To this end, the control circuit 7 includes a 
current-sensitive relay, for example, switching at a predetermined value 
of the current taken by the line 9. 
According to the invention, a passive tank 10 followed by an isolating 
valve 11 are interposed between the outlet 12 from the secondary pump 1 
and the inlet 13 to the primary pump 4. The passive tank 10 is merely a 
cavity having a certain volume, that is why it is called "passive". 
The control circuit 7 includes a relay which operates between two values of 
the current taken by the drive motor 2 on the secondary pump 1: a maximum 
value I.sub.1 and a minimum value I.sub.2, which values correspond to two 
values of the pressure P in the isolating tank 10: the first value 
corresponding to the priming pressure P.sub.1, and the second value 
corresponding to a pressure P.sub.2 &lt;P.sub.1. The pressure P.sub.2 
corresponds to a value P.sub.l for the pressure in the vacuum enclosure 3. 
This pressure P.sub.l is the limiting inlet pressure for the secondary 
pump 1. 
Thus, once the pressure in the tank 10 reaches the value P.sub.2, the 
control circuit 7 closes the valve 11 via the line 14 and switches off the 
drive motor 5 of the primary pump 4. Conversely, when the pressure in the 
isolating tank 10 rises to the value P.sub.1 by virtue of the secondary 
pump 1 continuing to operate and the walls of the enclosure 3 degassing, 
the control circuit 7 reopens the isolating valve 11 and switches back on 
the primary pump 4. The pressure in the tank 10 drops again to the value 
P.sub.2, thereby switching off the primary pump 4 again and reclosing the 
isolating valve 11. The pressure in the isolating tank 10 thus oscillates 
between the two values P.sub.1 and P.sub.2, so that during a first period 
of time both pumps are in operation and during a second period of time 
only the secondary pump is in operation. 
FIG. 2 shows this operation. 
From time 0 to time t.sub.1, the pumping assembly is started up and only 
the primary pump 4 is in operation. At time t.sub.1, the pressure in the 
tank 10 reaches the value P.sub.1 and the secondary pump 1 is switched on. 
At this moment, the current taken by its drive motor 2 is at a maximum and 
is equal to I.sub.1. The pressure falls down to P.sub.2 at time t.sub.2, 
with the current taken by the motor 2 also falling down to its minimum 
value I.sub.2, thereby triggering the relay so that the primary pump 4 is 
stopped and the valve 11 is closed. From time t.sub.2 to t.sub.3, only the 
secondary pump is in operation. At t.sub.3, the primary pump is restarted 
and the valve 11 is reopened, etc. . . . From t.sub.3 to t.sub.4, both 
pumps are in operation, from t.sub.4 to t.sub.5, only the secondary pump 1 
is in operation . . . 
If the pumping flow Q is defined as the product of its volume rate S 
multiplied by the pressure P of the pumped flow, then Q=PS. 
It is specified above that the pressure P.sub.2 is the pressure in the tank 
10 when the inlet side of the secondary pump 1 reaches its limiting 
pressure P.sub.1. At this moment, conditions are steady, and the flow Q 
pumped through the primary pump 4 is equal to the outgassing flow Q.sub.1 
in the enclosure 3. 
At this moment, the flow pumped by the primary pump is Q=P.sub.2 
.multidot.S=Q.sub.1, where S is the volume rate of the primary pump 4. 
This gives P.sub.2 =Q.sub.1 /S. 
The ratio of on-time to off-time for the primary pump 4 is directly related 
to the degassing flow Q.sub.1 in the enclosure 3 and to the magnitude of 
the volume V of the tank 10. These two magnitudes are related by the 
following equation: 
EQU P.sub.1 -P.sub.2 =ta.Q.sub.1 /V 
where: 
ta is the stop time of the primary pump 4 (i.e. t.sub.3 -t.sub.2 or t.sub.5 
-t.sub.4 in FIG. 2). Thus: 
EQU ta=V(P.sub.1 -P.sub.2)/Q.sub.1. 
Thus, the stop times increase with increasing volume V in the tank 10, with 
increasing priming pressure P.sub.1 for the secondary pump 1, and with 
decreasing degassing flow Q.sub.1 from the enclosure 3. 
In addition, the on-time tm of the primary pump 4 (corresponding to times 
t.sub.2 -t.sub.1 or t.sub.4 -t.sub.3 or t.sub.6 -t.sub.5 in FIG. 2) 
depends on the volume V of the tank 10 and on the volume rate S of the 
primary pump 4. 
These quantities are related by the following equation: 
EQU tm=2.3(V/S)log(P.sub.1 /P.sub.2). 
Thus, the on-time of the primary pump 4 decreases with decreasing volume V 
of the tank 10, with decreasing pressure ratio P.sub.1 /P.sub.2, and with 
increasing volume rate S of the primary pump 4. 
This gives: 
##EQU1## 
Thus this ratio decreases with decreasing degassing flow Q.sub.1 from the 
enclosure, with decreasing ratio P.sub.1 /P.sub.2, with increasing volume 
rate S of the primary pump, and with increasing pressure difference 
P.sub.1 -P.sub.2. 
By way of example, if the volume rate S of the primary pump 4 is S=3.6 
m.sup.3 /h=1 liter/second, the degassing flow Q.sub.1 =10.sup.-2 
mb.liter/second, the maximum priming pressure P.sub.1 =40 mb, and the 
minimum pressure P.sub.2 =4.10.sup.-3 mb, then tm=9.2 seconds and ta=4000 
seconds, giving: 
EQU tm/ta=2.3/1000 tm/(tm+ta)=2.3/1002.3.apprxeq.2.3.times.10.sup.-3 
Thus, the energy consumed by the primary pump 4 in such a pumping assembly 
during a period of time t during which the assembly is in use corresponds 
to 2.3.times.10.sup.-3 times the amount of energy that would have been 
consumed by the primary pump if the primary pump 4 had been operating 
throughout the period t, instead of operating intermittently. The primary 
pump operates permanently as from time t.sub.1. 
The advantage of the invention is thus clear, particularly when used with a 
cordless assembly powered by a battery. 
The invention is also applicable to cases where the primary pump 4 is a 
fixing pump, e.g. a static pump of the zeolite or "molecular sieve" type. 
Pumping by capturing molecules is effective only at very low temperature 
and this type of pump requires a powerful cooling system, e.g. based on 
liquid nitrogen circulation. 
In this case, there is no drive motor 5, since the motor is replaced by the 
cooling system. The control circuit 7 thus operates by switching on and 
off the cooling circuit 5 under the same conditions as it switches on and 
off the drive motor for a rotary pump that delivers to the atmosphere.