EP0373975B1 - Pumping unit for obstaining a high vacuum - Google Patents

Abstract

PCT No. PCT/FR89/00494 Sec. 371 Date Aug. 14, 1990 Sec. 102(e) Date Aug. 14, 1990 PCT Filed Sep. 27, 1989 PCT Pub. No. WO90/07061 PCT Pub. Date Jun. 28, 1990.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 P1, 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 P2<P1, 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 P1.

Description

The present invention relates to a pumping assembly for obtaining high voids.

It is well known that to obtain pressures below 10⁻³ mbar, a primary pump and a secondary pump are combined in series. When the assembly starts, only the primary pump is started until the pressure upstream of the primary pump drops to a value P₁ such that the secondary pump can be started. The secondary pump is then started and the two pumps, primary and secondary, operate simultaneously in series, permanently. The desired pressure in the enclosure is thus reached after a more or less long time.

Such a pumping assembly requires electrical energy to power the pump drive motors. This energy can come either from a mains supply or from a storage battery integrated in the pumping unit.

The invention aims to save the electrical energy consumed during pumping operations. The invention is particularly advantageous in the case of portable assemblies powered, precisely, by an accumulator battery, allowing the increase, for a battery of given weight and size, of the duration of the autonomy of the pumping assembly.

The subject of the invention is therefore a pumping assembly for obtaining high voids, comprising a primary pump and a secondary pump associated in series, the secondary pump sucking in an enclosure to be emptied, and comprising means for starting the pump. secondary when the pressure upstream of the primary pump drops below a value P₁, characterized in that a passive tank, followed by an isolation valve are interposed between the discharge of the secondary pump and the suction of the primary pump, and in that it comprises means for controlling the closing of the isolation valve and stopping the primary pump when the pressure in said passive tank reaches a value P₂ <P₁ and opening of the isolation and restart valve of the primary pump when the pressure in said passive tank again reaches the pressure P₁.

We will now give the description of an example of implementation of the invention with reference to the appended drawing in which:

FIG. 1 schematically represents a pumping assembly according to the invention.

Figure 2 is a curve representative of the operation of the pumping assembly.

FIG. 1 therefore schematically represents a pumping assembly which comprises a secondary pump 1 with its drive motor 2, connected on the side of its suction to an enclosure 3 in which it is desired to carry out a high vacuum, and on the side of its discharge, to a primary pump 4 with its drive motor 5, this primary pump 4 delivering to the atmosphere.

The pumping assembly shown is, for example, portable and autonomous and thus comprises a storage battery 6 for supplying energy to the assembly. The storage battery supplies an electrical control circuit 7 which comprises, among other things, a three-phase DC-AC converter for supplying motors 2 and 5. Lines 8 and 9 show these supplies.

As is known, the secondary pump 1 can only operate below a certain pressure P₁ called priming pressure. Also, when the assembly starts, only the primary pump 4 is started, and when the pressure upstream of the primary pump drops below this pressure P₁, the secondary pump 1 is started automatically. It is known that the intensity absorbed by the drive motor 5 is an increasing function of the suction pressure. Also, the secondary pump starts up when the current absorbed by the drive motor 5 drops below a value which corresponds to this priming pressure P₁. To this end, the control circuit 7 comprises for example a current relay operating for a predetermined value of the current in the line 9.

According to the invention, a passive tank 10 followed by an isolation valve 11 are interposed between the discharge 12 of the secondary pump 1 and the suction 13 of the primary pump 4. The passive tank 10 is only one simple cavity having a certain volume, it is for this reason that it is called passive.

The control circuit 7 comprises a relay operating between two maximum I₁ and minimum I₂ values of the motor 2 driving the secondary pump 1, values I₁ and I valeurs of the current which correspond to two values of the pressure P in the isolation tank 10: the first P₁ priming value and a second P₂ <P₁ value. This pressure P₂ corresponds to a value P _l of the pressure in the vacuum enclosure 3. This pressure P _l is the suction limit pressure of the secondary pump 1.

Thus, when the pressure in the reservoir 10 reaches the value P₂, the control circuit 7 controls, through line 14, the closing of the valve 11 and the stopping of the drive motor 5 of the primary pump 4. Conversely, when the pressure in the isolation tank 10 rises to the value P₁ owing to the fact that the secondary pump 1 is still operating and to the degassing of the walls of the enclosure 3, the control circuit 7 controls the re-opening of the valve isolation 11 and restarting of the primary pump 4. The pressure in the reservoir 10 drops again to the value P₂, which again causes the primary pump 4 to stop and the the isolation valve 11. The pressure in the isolation tank 10 thus oscillates between these two values P₁ and P₂, with an operation for the first time of the two pumps followed by a second time where only the secondary pump operates.

Figure 2 shows this operation.

From time 0 to time t₁, the pumping unit starts and only the primary pump 4 operates. At time t₁, the pressure in the reservoir 10 reaches the value P₁ and the secondary pump 1 is started. At this time, the intensity absorbed by its drive motor 2 is maximum and equal to I₁. The pressure decreases until P₂ at time t₂, the current absorbed by the motor 2 has dropped to a minimum value I₂, the relay trips and the primary pump 4 is stopped and the valve 11 closed. From t₂ to t₃ only the secondary pump works. In t₃, the primary pump starts up again and the valve 11 opens again, etc ... From t₃ to t₄ the two pumps operate, from t₄ to t₅, only the secondary pump 1 operates .....

If we define the pumping flow Q as the product of the flow S by the pressure P of the pumped flow, we have Q = PS.

It was said above that the pressure P₂ is the pressure in the reservoir 10 at the time when the suction of the secondary pump 1 reaches its limit pressure P _l . At this moment, the regime is permanent, and the flow Q pumped by the primary pump 4 is equal to the degassing flow Q₁ of the enclosure 3.

.Now, at this moment, the flow pumped by the primary pump is Q = P₂ S = Q₁, if S is the pumping flow rate of the primary pump 4. We thus have, $$\text{P₂ =}\frac{\text{Q1}}{\text{S}}$$

.

ta étant le temps d'arrêt de la pompe primaire 4 : t₃ - t₂ ou t₅ - t₄ sur la figure 2.
On a ainsi $$\text{ta =}\frac{\text{V}}{\text{Q₁}}\text{(P₁ - P₂)}$$

The on-off time ratio of the primary pump 4 is directly linked to the importance of the degassing flow Q₁ of the enclosure 3 and to the magnitude of the volume V of the tank 10. The following relationship links these different quantities: $$\text{P₁ - P₂ =}\frac{\text{Q1 your}}{\text{V}}$$

ta being the stopping time of the primary pump 4: t₃ - t₂ or t₅ - t₄ in figure 2.
We have thus $$\text{ta =}\frac{\text{V}}{\text{Q₁}}\text{(P₁ - P₂)}$$

Thus, the stop times ta are all the greater the greater the volume V of the reservoir 10, the greater the priming pressure P₁ of the secondary pump 1 and the degassing flow Q₁ of the enclosure 3 is smaller.

Furthermore, the running time t _m of the primary pump 4 (corresponding to the times t₂ - t₁ or t₄ - t₃ or t₆-t₅ in FIG. 2) depends on the volume V of the reservoir 10 and the pumping rate S of the pump primary 4.

The following relationship links these elements: $${\text{t}}_{\text{m}}\text{=}\frac{\text{2.3 V}}{\text{S}}\text{log}\frac{\text{P1}}{\text{P₂}}$$

est plus petit et que le débit S de la pompe primaire 4 est plus grand.Thus, this running time of the primary pump 4 is all the smaller the smaller the volume V of the reservoir 10, the more the pressure ratio $$\frac{\text{P1}}{\text{P2}}$$

is smaller and that the flow rate S of the primary pump 4 is greater.

We have : $$\frac{\text{tm}}{\text{your}}\text{=}\frac{\text{2.3}\frac{\text{V}}{\text{S}}\text{log}\frac{\text{P1}}{\text{P2}}}{\frac{\text{V}}{\text{Q₁}}\text{(P₁ - P₂)}}\text{=}\frac{\text{2.3}\frac{\text{Q1}}{\text{S}}\text{log}\frac{\text{P1}}{\text{P2}}}{\text{P₁ - P₂}}$$

est faible, que le débit de pompage S de la pompe primaire est grand et que la différence des pressions P₁ - P₂ est grande.Thus, this ratio is lower the lower the degassing flow Q₁ of the enclosure, the lower the ratio of $$\frac{\text{P1}}{\text{P₂}}$$

is low, that the pumping rate S of the primary pump is large and that the difference in pressures P₁ - P₂ is large.

For example, if the flow rate S of the primary pump 4 is: S = 3.6 m³ / h = 1 liter / second, the degassing flow Q₁ = 10⁻² mbxliter / second, the maximum pressure of priming P₁ = 40 mb, minimum pressure P₂ = 4.10⁻³ mb.
We then have t _m = 9.2 seconds and t _a = 4000 seconds $$\frac{\text{tm}}{\text{your}}\text{=}\frac{\text{2.3}}{\text{1000}}\frac{\text{tm}}{\text{tm + ta}}\text{=}\frac{\text{2.3}}{\text{1002.3}}\text{≃ 2.310⁻³}$$

Thus, the energy consumed by the primary pump 4 with such a pumping assembly during a time t of use of the assembly represents 2.3 10⁻³ of the energy consumed by the primary pump if this primary pump 4 had been running continuously for all this time t, instead of only working intermittently. The secondary pump operating continuously from time t₁.

This shows the advantage of the invention and particularly in the case of an autonomous assembly powered by battery.

The invention also applies in the case where the primary pump 4 is a fixing pump, for example a static pump of the "molecular sieve" or zeolite type. Pumping by capture of molecules is only effective at very low temperatures and this type of pump requires a powerful cooling system, for example by circulation of liquid nitrogen.

In this case, the drive motor 5 does not exist and is replaced by the cooling system. The control circuit 7 therefore acts on this cooling circuit 5 to stop it, under the same conditions in which the drive motor was stopped in the case of the use of a rotary primary pump delivering to the atmosphere.

Claims (2)

1. A pumping assembly for obtaining a high vacuum, the assembly comprising a primary pump (4) and a secondary pump (1) associated in series, the secondary pump (1) sucking from an enclosure (3) to be evacuated, and comprising means (7) for starting the secondary pump (1) when the pressure upstream from the primary pump (4) drops below a value P₁, the assembly being characterized in that a passive tank (10) followed by an isolating valve (11) is interposed between the outlet (12) from the secondary pump (1) and the inlet (13) of the primary pump (4), and 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 P₂ < P₁, 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 P₁.
2. A pumping assembly according to claim 1, characterized in that said primary pump is an absorber pump (4) provided with a cooling device (5), said means for stopping the primary pump (4) acting on the cooling device (5).

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