Published on 1/26/2017
Categories: Insider tips

Roughing pump in high-vacuum furnaces for beginners [1/2]

Roughing pump in high-vacuum furnaces for beginners [1/2]

In this article you will find important tips to help you operate your pumping system in an energy and cost-saving manner. More specifically, I’m going to explore the topic of roughing pump operation in high-vacuum furnaces. In this first part of the article, we are going to see more in detail the pressures which can be reached in the high-vacuum system and the various types of pumps you may choose for you furnace with all their pros and cons. Then, this introductory chapter will lead us through the roughing pump operation which will be analyzed in the second part of the article.

What does “high vacuum” mean?

As is known, the maximum vacuum required in vacuum furnaces depends on the process and materials used in the thermal treatment of the load. Maximum vacuum is the value of pressure that can theoretically be reached in the furnace when it is cold, load-free and clean.

In conventional thermal treatments, the maximum vacuum is deemed sufficient when the pressure that can be reached is in the 1E-2 Pa range.

In the most severe industrial applications, where there is the risk of the load being contaminated by traces of oxygen or other residual gases, the pumping system must be able to reach working vacuums in the 1E-4 Pa range.

Finally, the characteristics of the pumping system and the entire vacuum system used in laboratory furnaces or in the research or nuclear sectors must be such as to allow even lower pressures to be reached. This is why we speak about ultra-high vacuum (UHV).

How does the roughing pump work?

The roughing pump is the component that primes the other pumps in series on the vacuum line, evacuating the system until a pressure level is reached at which a booster pump (known as a lobe-type roots pump) can be activated, followed by other types of diffusion, turbo molecular, cryogenic, ionic and other pumps that can be used when the pressure is reached.

Depending on the flow rate, the pumping system is proportioned in relation to system volume, pumping cycle speed, load degassing and contamination produced by the load (vapors, dust, binders, etc.) in the pump itself. It is for this reason that mechanical strength and wear influence the choice of pump type.

The first choice concerns the decision to adopt a vane pump or a rotary piston pump.

Vane vacuum pumps: the working principles

Two-stage or single-stage vane pumps are lubricated, cooled (air or water) pumps.

Two-stage vane pumps are generally of limited capacity; these enable very high vacuums to be reached by measuring the final pressure on the suction flange. Oil has the function of reducing lubrication (minimum lubricant flow capacity) and avoids the re-transfer of gases dissolved in the oil from the oil tank to the suction.

In these pumps, in addition to the ability to reach high vacuums, it is important to assess a second characteristic, namely ballast resistance. This measurement is carried out with the ballast fully open and must have values close to the maximum vacuum (pressure no greater than a decade).

Ballast resistance measures the pump’s ability to remove contaminants such as water vapor, solvent vapors and other contaminants pumped from the vacuum chamber that could saturate it. The presence of condensed vapors in the oil causes a rapid reduction in the vacuum obtainable. The condensation of vapors in the pump oil is a consequence of the pump’s high compression ratio, required to discharge against atmospheric pressure with a compression ratio in the 1E5 - 1E7 range. There are laboratory tests where vacuum values in the 1E-2 Pa range have been reached for these types of pumps.

The decontamination operation for a pump saturated with vapors occurs by opening of the gas ballast valve, normally incorporated into the pump itself. This allows the introduction of air or nitrogen, in the case of the removal of combustible vapors, into the compartment between the rotating vanes and the stator, raising the internal pressure. Through the injection of gas (air or nitrogen), the compression ratio is reduced by around 1 tenth, and, as a result of this operation, the pump is capable of removing the vapors from the system, even starting from a maximum partial pressure value of 30 mbar.

But let us consider single-stage vane vacuum pumps now. What are the peculiar and specific features of the sigle-stage vane pump? And what about oil-sealed rotary piston pumps? When and under which conditions should be dry vacuum pumps used?

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Single-stage vane pump and two-stage pump: the pros and cons

Then there is a type of high-capacity lubricated single-stage vane vacuum pump, in which the lubrication oil has a non-secondary function consisting of transferring the heat, caused by the compression, to an oil-water or oil-air heat exchanger. This type of pump is a valid solution for high-volume environments to be evacuated, even if the maximum vacuum does not reach the values of two-stage pumps.

The maximum final vacuum is due to the large quantity of oil circulated, which tends to retain gases and vapors by dissolving them; these vaporize and return to the pump suction. There is, however, a positive effect due to the high temperature of the pump and oil, whereby it is difficult for the pump to become contaminated due to the effect of condensable vapors, which are removed.

No vane vacuum pumps have a long lifespan if dust or small solid particulates are aspirated. In fact, today’s vanes are no longer metallic, but made from a cotton and phenol resin composite.

Rotary piston pumps and dry vacuum pumps: the pros and cons

Rotary piston pumps, whether single-stage or two-stage, have greater similarity to small-capacity vane pumps, in terms of the vacuum level that can be reached. Rotary piston pumps display a high capacity to withstand wear, and are still widely used, despite their higher price.

Finally, there are dry rotary pumps. These are non-lubricated pumps, with evident advantages, since maintenance is drastically reduced and there is no disposal of waste oil. For the user, another very interesting factor is the energy consumption, which is far lower if compared to that of similar lubricated pumps. These aspects are defining in directing choice towards a product with a much higher price, but which is fully compensated by the reduction in operating costs.

However, the dry vacuum pump appears to be somewhat delicate, given the reduced play between the stator and rotor. It is also hot and noisy. The installation of a silencer on the exhaust is always required. In addition, the furnace connected to it must be “clean”. Its use is generally to be avoided in dirty or particularly polluting systems or processes. However, these must be assessed on a case by case basis: in certain processes, the fact that the pump is dry can even allow it to be washed periodically in order to remove any contaminants.

Before going into the second part of the article about the most critical aspects of the roughing pump operation, do you want to share with us your experience in the different vacuum pumps? Do you have something to add?

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