Theory of Motorized Barometric Vacuum Pump

Abstract

Here we present different features of a new invented piston vacuum pump. These features are chiefly the new mechanical and electrical mechanisms used for desired control of on/off input and output valves of vacuum chambers of the piston vacuum pump and using the method of complete immersion in oil as a gas-tight medium. Also, contrary to the current piston vacuum pumps, the more vacuum is increased the less energy is consumed.

Basic Ideas

At the advent of technology, for creating high vacuum (when it was necessary eg for manufacturing electrical lamp), the vacuum created above the mercury column in a barometer was used. Such a vacuum is reckoned as a high vacuum because during the process of its creation no even smallest aperture toward the outer air exists. So, no air molecule exists there, and only there we have the vapor pressure of the liquid (mostly mercury) used in the barometer. (Under vacuum, vapor pressure of mercury in room temperature is such small as permitting creation of high vacuum.)

Clearly, the handy methods for using the vacuum created above the mercury column were hard, because it necessitated that a powerful man to move the system regularly upwards and downwards in order that oscillating motion of the mercury surface could pump the air out of the vacuum vessel. By progress of the vacuum techniques, the duty of upward/downward motion of the system was rendered semi-automatic. But this technique was not so developed and continued because other vacuum techniques were developed which seemed more practical. In the design of Motorized Barometric Vacuum Pump, there is a kind of return to the development of the same above-mentioned technique by delivering the same oscillating movement to an electric motor.

The vacuum chamber above the mercury column in the barometer is created because of the weight of the mercury column (overcoming the outer air pressure). This is a forced mechanism. But, is weight the only forced mechanism being able to make a vacuum cavity in a liquid? Certainly this is not the case. Such a nearly perfect vacuum cavity or chamber in a liquid can also be created by pulling a syringe inside a liquid. Our new vacuum pump creates such a vacuum cavity in the oil regularly by transferring the motion of an electric motor to relevant pistons oscillating in related cylinders merged in oil.

Using the mentioned mechanism of syringe and its development to make this kind of pump is in fact using the mechanism of piston vacuum pumps in quite specific conditions. Among several kinds of vacuum pumps, piston vacuum pumps are cheapest because of their relatively simple and routine mechanism although their pumping speed is relatively high. But anyhow, usually the ultimate pressure gained by them is also relatively high (about 10 to 20 torr). That the ultimate (or remaining) pressure in them is relatively high is chiefly because of the mechanisms used for their outlets and inlets (in vacuum chambers) which we describe them and that how they’ve been obviated in our pump, here.

One of the sealing mechanisms of vacuum chambers in these current pumps is chiefly the contact surfaces between the ball or valve and the opening of gas exit in these chambers in the vicinity of the outer air pressure. Such a sealing mechanism necessitates that in the (short) time interval necessary for the outlet being closed due to the air pressure, (occurred in the suction stage of the pump piston,) a little air enters the chamber. This shortcoming has been obviated in our pump because the valves in it are submerged in oil, and the above-mentioned suction causes the entrance of some oil not gas into the chamber. It is probable that for obviating this shortcoming we use some ball or valve in the outlet that due to its weight or the force of a spring keeps closing of the outlet and so prevents from air coming inside (in suction stage). But, such a remedy has itself this deficiency that the air pressure in the vacuum chamber cannot decrease below the least necessary pressure for overcoming the ball weight or the spring pressing force. Such a deficiency does not exist in our pump, because use of a suitable spring holds the outlet (a little) open in a normal condition when we have no pressure difference at the opening.

For the inlet too, in the current piston pumps, a ball or valve is used that in a normal state closes the inlet due to the pressure exerted by a pressing spring (at the back of the ball or valve). This causes that the valve becomes open only when the gas pressure difference at the inlet is able to overcome the pressure due to the spring. This means that the gas pressure inside the vessel (where we want to evacuate it) cannot become zero because its excess over the gas pressure inside the vacuum chamber is to overcome the pressure of the valve spring. On the other hand since the ball or valve must fit in closing the inlet, existence of spring pressure is necessary. To obviate this shortcoming in obtaining zero pressure, we used a solenoid for electrical controlling of the process of opening of the inlet. Electric current of the solenoid is directed through a microswitch controlled by a circular mechanical guide being conformed by the position of the piston.

Another shortcoming of the current piston vacuum pumps is in the interval existent between the piston and the cylinder through which (even though fitted by o-ring or by similar material) some air molecules penetrate into the vacuum chamber. Such a shortcoming does not exist in our pump because the cylinder is submerged in oil and instead of air molecules some oil molecules penetrate into the chamber.

Also in the current piston vacuum pumps usually the electric motor should consume so much energy to overcome one atmosphere pressure in each strike. But in our pump this is not the case. As you can see schematically in the following figure, two pistons in a cylinder are either coming close to each other or getting away from each other.

In the stage 1 the pump is full of oil and the two pistons start to get away from each other. As a result, the additional volume V2 is created between the two pistons which sucks air from V1. In the stage 2, in which the two pistons start to come close to each other, this air of the volume V2 will be pumped out and the same additional volume V2 will be created at the two ends of the cylinder which will suck air from V1 again. In the stage 3 which the two pistons are again start to get away from each other, the air existent in the volume V2 at the two ends of the pump will be pumped out and again the volume V2 created between the two pistons will suck air from V1. This process will continue in this manner and air is gradually is decreased in V1. As we can see, contrary to the current piston vacuum pumps, pressure difference between the two ends of each piston is not pressure difference between the outside air pressure (one bar) and the vacuum created by piston suction which this matter would exert much force on the electric motor especially when the pressure inside the vacuum vessel decreases. Rather, pressures on the two ends of each piston have always only small difference with each other, and this difference will even become smaller when the pressure inside the vessel approaches zero. This causes no extra pressure will be exerted on electric motor.

More about this pump

This pump is formed by a cylinder that at the two ends and at the middle of itself is attached to three vacuum chambers (a, b, and c in the following figure). These three chambers are separated by two pistons (d and e). These pistons gain their oscillating movements (which for each piston is opposite to the other one) from a crank (being rotated by an electric motor).

In each vacuum chamber, there exist an inlet (f in the following figure) and an outlet (g). The inlet is opened and closed by a solenoid (i), and the outlet is kept open in normal states by a weak spring.

Alternation of the current-taking of the solenoid is secured by a microswitch (k in the following figure). This microswitch is switched properly by a properly designed guide mounted on the crank shaft.

The least pressure we could obtain using ordinary hydraulic oil is about 1 torr. The most important parameter that prevents this pump from creating very high vacuum is degassing of oil under vacuum condition especially considering that the oscillating oil is regularly mixing with air. To obviate this barrier in order to gain high(er) vacuum(s), I’ve designed (as R&D of this project) to place the inlet and outlet of this vacuum pump in a chamber which is being evacuated of air (to a moderate or even low vacuum) by another (ordinary) vacuum pump. In such a condition, oil is already degassed totally and there will be no air molecules to be mixed with the oscillating oil.

Click Animation of Motorized Barometric Vacuum Pump to see generally how this pump works.