When pumping atmospheric air (or gas) in a vacuum system, however “pure” it may appear to be, it will invariably contain some vapour.
During the compression process in the pump, this vapour will condense. Failure to remove it will form a contaminant which will prevent the pump from achieving its optimum vacuum pressure. Also, the condensate can enter the pump’s mechanism, for example the oil in oil-sealed rotary pumps, where, as a contaminant, it can have a detrimental effect.
In simple terms, a gas ballast valve incorporated into the system will allow a flow of air into the final part of the compression cycle and allow the vapour to be expelled without condensation or affecting the pump’s overall performance.
While the gas ballast theory is simple, the rationale behind its use in vacuum pumps is still sometimes shrouded in confusion and misinformation. As a result, the subject is frequently considered esoteric.
So, to give you a better understanding of gas ballast, let’s talk a bit about its history and how it works.
Originally developed by Wolfgang Gaede in 1935, gas ballast was a significant addition to mechanical, oil-sealed vacuum pumps.
When used in mechanical, oil-sealed vacuum pumps, the gas ballast allows the pumping of vapours without them condensing and thus contaminating the pump’s sealing-oil. By reducing any potential condensation in the sealing oil, the pump can achieve its vapour duty at near full specification, though the ballast flow does impact the ultimate pressure of the pump.
In vacuum practice, the gas stream evacuated from the vacuum chamber will often contain water vapour, solvent vapour and/or other unwelcome contaminants.
These contaminants occur because they have been “converted” (under vacuum pressure) from liquid to gas phase then (i) flowing back to the pump, where they are “converted” back from a gas to a liquid contaminant within the pump oil, or (ii) when using ballast exiting the pump itself, without contamination.
In simple terms, gas ballasting is when the introduction of the ballast gas flow allows the condensable elements of air or gas that is being pumped to be expelled from the pump before it condenses, contaminates the system and/or pollutes the sealing oil.
For instance: if the pump is “working” a gas that would naturally condense in the pump, the gas ballast mechanism opens the exhaust valve before the vapour has had a chance to condense. The result is that this condensable vapour is discharged (to the atmosphere).
A pump that has been subject to condensed vapour can be “cleaned-up” by using gas ballast.
To do so, the pump inlet port should be closed, and the pump allowed to run with the gas ballast valve in the open position. Continue purging for several hours or overnight (depending on the size of the system and the degree of contamination).
However, if the pump is “smoking” or emitting a visible mist in large quantities, you could return to a room full of “oil-fog”. Venting into an extraction hood or using exhaust mist filters will help to eliminate this problem. The use of a gas-ballast oil return kit resolves this issue.
It should be remembered that as pumped gases and vapours exit the system, they can enter the working environment. This means that depending on their content and/or concentration, they may pose a hazard.
Gas ballasting, which is associated with oil-sealed rotary vane and dry scroll pumps, is frequently employed in freeze-drying, rotary evaporation, distillation, and gel drying.
The food industry faces around 1.3 billion tonnes of food wasted or lost annually worldwide. Vacuum technology helps combat food waste in numerous ways. This includes vacuum packaging that extends the longevity of food by reducing the growth of bacteria and fungi. Additionally, vacuum cooling helps keep fruit and vegetables fresh directly after harvesting by enabling food containers to be evacuated to pressures of > 6 mbar. Vacuum drying helps preserve fresh fruit and meat through Vacuum Microwave Drying. Here the products are heated by microwave to 35-60C while the vacuum pump keeps the pressure around 10 mbar. The water content then evaporates. Through Freeze Drying the products are cooled to -20-40C and the water sublimates at pressures below 0,1 and -1 mbar from the solid phase.
Dry Claw/screw pumps in food industry application are more often replacing wet pumps. Oil-sealed rotary vane pumps were the norm for the food industry. However, these pumps bear risks as the oil could end up polluting your workspace or, even worse, your food. Dry vacuum pumps avoid these risks and ensure product freshness.
Overall, dry vacuum pumps provide a low cost and efficient pump alternative for vacuum packaging and processing in the food industry. They also ensure that no dangerous gas molecules can get into contact with the food during processing.
For a vacuum pump system, a vital consideration in its design is the conductance. Conductance in vacuum systems is the characteristic of a vacuum component or system to readily allow the flow of gas and can be thought of as the inverse of resistance to flow. Its units are that of the volumetric capacity of gas flow in a passive component (or aggregate component of a vacuum system), such as an opening or a pipe, divided by time.
The conductance between two points is defined as the gas flow rate flowing through a device divided by the pressure drop that is driving it. Therefore, the dimensions that describe conductance are the same as pumping speed (volume per unit time) but conductance is a phenomenon usually used to describe piping and openings, not the vacuum pump.
Conductance in a vacuum system is equivalent to conductance (inverse of resistance) in an electric circuit. Parallel conductance is the sum of each conductance, and series conductance is the inverse of the sum of each inverse conductance. If a system suffers from low conductance, it can significantly reduce the effective pumping speed. This phenomenon becomes increasingly important as the pressure reduces and the flow transitions from a viscous regime to a molecular regime.
The conductance of a vacuum system needs to be calculated and incorporated into system design and pump selection. Failure to do so can seriously impact how long the vacuum chamber will take to reach the desired pressure and may cause an ‘over-specification’ of a pump. A well-designed system will increase vacuum speed and efficiency, as well as minimise energy costs; the costs of failing to take conductance into account can be severe, these include delayed equipment start-up, plant downtime and process inefficiency.
Pumping speed has, of course, a direct effect on the amount of time it takes to achieve a given vacuum level. The resistance from the system’s components means that the net (or effective) pumping of a pump is less than the rated speed of the pump and less than the conductance of the most restrictive component. The lesson here is that it is not good enough to select a pump on its rated speed because too little conductance can mean that it can never achieve its rated speed (and lead to over-specification).
There are three modes of flow within a system. In higher pressure systems, viscous flow (or continuum flow) is experienced where molecule-to-molecule collisions dominate the flow pattern; here there is a linear relationship between conductance and pressure. As the pressure drops so does the density of the medium, which increases the space between molecules, whereby the mean free path before molecule-to-molecule collisions occur is greater.
As the pressure drops further it enters molecular flow, where the interaction between the molecules no longer has an effect and molecule-surface collisions dominate flow behaviour. Here the conductance is independent of pressure. In between these two states there is a transition flow which is non-linear – also called Knudsen flow – and can be seen as an ‘admixture’ of viscous and molecular flow.
The graph below (for air at 293K) shows the conductance of a one metre length pipe for a range of diameters and pressure. Conductance changes as 1/length:
Calculating a system’s total conductance is vital to selecting the correct pump and ensuring it all performs as expected. So how do you calculate this? The conductance of the vacuum piping can be calculated (or charts used) for various pressures, pipe diameters, pipe lengths and orifice sizes using a vacuum simulation software. However, when it comes to the individual system components, the conductance needs to be based on the specifications and information supplied by the manufacturers, as well as nomograms and literature.
When making calculations it is important to bear two basic concepts in mind. Firstly, the mass flow rate throughout the system remains constant. This means that as the local pressure decreases, the speed increases in a predictable manner and allows the conductance to be calculated.
Secondly, is the impact of the three types of flow that we have already discussed exist in vacuum systems. Conductance will vary depending on the mode of flow through the system. For any component, conductance will change depending on the flow type.
Whether you are working in the Research and Development field, with analytical instruments, or otherwise – you need a vacuum system that ensures safe operation, is highly reliable and built-for-purpose to meet your operating requirements.
And that’s where we can help.
Our team of experts at Vacuum Science World have many years of experience in the design, modelling and specification of vacuum systems and will provide you with tailored recommendations for your system and equipment specification.
For all applications, we can support you to develop a highly reliable, robust, and practical vacuum solution and method of operation designed for your unique system and project needs.
Just tell us your system details and requirements – chamber dimensions, fore-lines, desired pressure, construction material, application, processes, and other relevant details – and we’ll do the rest.
Vacuum pumps are used in a wide range of industries, from the food and beverage industry to the medical industry. Considering that different industries have different needs, even the smallest difference in function and application can make an impact. As such, different types of vacuum pumps were invented to serve specific needs. Two of the more popular types are the rotary vane vacuum pump and the dry claw pump.
In today’s blog, let’s review the differences between these two pumps:
A rotary vane vacuum pump is a positive-displacement pump, wherein its suction and discharge capabilities are facilitated by rotating vanes on a motor. At the suction side, pressure is created from the rotation to pull in the pump medium, while at the discharge side, the vacuum pump simply loosens its chamber to allow the pump medium to exit. The rotary vane vacuum pump can work with oils, gas, and other fluids, and is prized for its versatility – these pumps can virtually be employed universally throughout low and medium vacuum ranges.
The most common places to see a rotary vane vacuum pump in action are in the aerospace or automotive industries. In addition, they can also be used in laboratory freeze dryers or in HVAC applications.
Dry claw vacuum pumps also operate based on the principle of rotation, albeit with two closely placed rotors, or claws. These claws never touch, but as they rotate the motions cycle between drawing in and compressing air. This pump type is known for its relatively low level of noise and is thus deemed beneficial for facilities where pneumatic machines and conveyors are used.
In addition to this, dry claw vacuum pumps are also used in packing, wood processing, and wastewater management applications. This pump type is prized for its mechanical strength and efficiency and does not require any oil to operate.
As you can see, these pump types can be quite similar on some fronts, but by virtue of their different methodologies, each pump type will be more suitable for certain applications compared to the other. The best way to be sure of whether you have acquired the correct pump for your needs is to consult with an expert. So, give GlobalVac a call any time you have questions about vacuum pumps – you can be assured that we will have the answers!
Anyone without a deep understanding or knowledge of pumps might think that vacuum generation is simply a question of “plugging in a pump”, starting it up and waiting for the vacuum to drop to the required level.
But the reality is that there is far more to the process.
Vacuum pumps are used to remove air or gas molecules from a sealed volume thus creating a vacuum. The vacuum level can be controlled, for example, with a process gas at a specific pressure.
Finding the right vacuum pump not only requires a good understanding of the necessary vacuum level and application – it also requires an understanding of process conditions, the operating range and the benefits and limitations of each specific vacuum pump type.
In this blog, we’ll briefly outline the four factors to consider when choosing between different types of vacuum pumps.
The pump selection heavily depends on the level of vacuum that needs to be obtained. Typically, the different pressure ranges in vacuum technology are defined as follows:
In rough- and medium vacuum most gas molecules are within the volume of the vacuum chamber, whereas in ultra-high vacuum (UHV) and extreme high vacuum (XHV) most of the remaining molecules will be on or in the chamber walls respectively. Thus, different pump technologies will be required for the different vacuum pressure ranges.
It is also important to consider whether it is mainly about pumping down to the required pressure level or – for example – holding a specific pressure level while certain gas loads are introduced to the vacuum system (e.g., for process reasons). While some vacuum pumps are optimized for pump-down processes (but might struggle with high process gas loads), others are more capable of handling high gas loads.
Depending on the target vacuum level, a mix of different vacuum pump technologies might be necessary. Primary vacuum pumps, i.e., those operating in the rough and medium vacuum ranges, exhaust to the atmosphere and can operate in isolation. High and ultra-high vacuum pumps, such as turbopumps and diffusion pumps, need to exhaust to or operate with a primary pump to create a level of vacuum they can work from. Ion, non-evaporable getter (NEG) and cryopumps need initial evacuation and then occasional primary pump support (e.g., during reactivation or regeneration process steps).
Dependent upon the application area and medium to be pumped, the choice of pump(s) will vary. For example, rotary vane (RV) pumps are suited for a wide range of low and medium vacuum applications, including research and development, analytical instruments, industrial and coating activities, freeze drying, process engineering and many more.
The use of oil as a sealant and coolant allows for very good pumping performance and suitability for many applications, including where dirt, dust or condensates might be present.
Scroll pumps, on the other hand, provide hydrocarbon-free vacuum by compressing gases using two tip-sealed spirals rotating eccentrically against each other. This results in low operation and maintenance costs. Compared to RV pumps, scroll pumps applications are mainly limited to processes, without dust or dirt which could damage the tip seals in a short period of time.
Taking this into account, the impact of the application on the selected pump technology needs to be evaluated carefully, as well as the potential impact of:
Evaluating the impact of the vacuum pump on the application or process is just as important. There are several variables that can influence the choice between different types of vacuum pumps, including – but not limited to:
Returning to the example products above, RV pumps are at a disadvantage because they cannot generate hydrocarbon-free vacuum due to oil emissions. Scroll technology, on the other hand, while capable of producing hydrocarbon-free vacuum, carries the risk of particle emission due to wearing down of the tip seal.
As well as considering what needs to be achieved, initial capital costs, operating costs and maintenance needs should be assessed.
Taking two high vacuum pump technologies, turbomolecular pumps (TMP) and oil diffusion pumps, as examples, it is fair to state that the initial costs for TMP usually will be significantly higher compared to an oil diffusion pump. However, considering the cost of ownership over a five-year period, oil diffusion pumps might cost more due to higher energy and maintenance costs. For some products, economic advantages might apply from a certain pump size/performance class.
There are two classifications for vacuum pumps. Primary pumps exhaust directly to atmospheric pressure (such as rotary vane, scroll, diaphragm, screw and multi-stage roots pumps) and secondary pumps which require the use of a primary pump to continuously support their operation (turbomolecular pumps and diffusion pumps) or to evacuate to a pressure at which they can begin to be operated ion getter, titanium sublimation, non-evaporable getter and cryogenic pumps). Roots booster pumps are often combined with primary pumps to be a ‘primary’ pump pair but are strictly to be classed as secondary pumps.
Effective vacuum generation requires an understanding of needs and the different types of vacuum pumps available. Choosing the wrong pump can be a costly mistake and potentially damaging to your operation should it not perform as required.