Getting more from your pump
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Looking for the best OEM vacuum pump for your application? Here, John Jackson looks at some of the areas to consider and the questions to ask to put you on the right path, focusing on issues such as environmental conditions, pump materials, motor technology and the required lifespan.
With so many different types and styles of vacuum pumps to choose from, selecting the best one for a particular application can seem daunting. But by bearing in mind a few key criteria and requirements, you can select the most appropriate product secure in the knowledge that it will give you optimum performance over the longest possible lifespan.
A first consideration is the power supply. Such a fundamental criteria might seem obvious, until you start to think about where in the world your pump will end up, the voltages used in that part of the world, and the tolerances you can expect in fluctuations on that voltage.
As an example of what can go wrong, people often see DC as being a universal constant. Where ever the pump will end up, you can guarantee there will be a DC supply already there controlling other products, but will you know whether the control voltage is 6V, 12V or 24V? Similarly, AC voltages and frequencies change from country to country. The question of whether to use an AC product or a DC product in a given application will be addressed later, but which ever you choose it is important to consider the implications of variations in supply voltage. And where there are significant variations in voltage, it is prudent to build in tolerances which will ensure the pump will start even at the lowest voltage encountered, in the lowest temperatures expected.
Air quality
The quality of the air being used can have significant implications for vacuum pumps, affecting both the materials that can be used within the products and the level of filtration required. Corrosive chemcials, water, dust and other contaminants all have to be taken into account in choosing the most appropriate pump or compressor for the application.
As an example of the problems that can occur, a key application for vacuum pumps is in autoclaves, where they come into contact with steam. One of the materials often used for the diaphragm in diaphragm pumps is viton, but this swells over a period of time when it comes into contact with steam, compromising the pump's operation. In such an application, a pump using an EPDM diapgragm would be more appropriate.
In other applications, the presence of corrosive chemicals such as sulphuric acid would steer you away from certain pump materials. But even seemingly safe materials are not immune from attack. Stainless steel, for example, is suscpetible to corrosion from the presence of fluorine, even at trace levels. Where this gas was present, you might want to opt for a cast iron construction, perhaps with a nickel coating.
Certain contaminants would steer you away not simply from a particular material but from a whole design concept. The presence of water, for example, all but eliminates the possibility of using dry rotary vane pumps. Diaphragm pumps are more appropriate here. Similarly, the presence of dust can cause serious problems in pumps, but some principles of operation are much more susceptible than others. Filters can provide an effective solution, but it's important to ensure that the filter employed is sized correctly so as not to be too restrictive.
By considering the contaminants that are present, you can begin to eliminate inappropriate designs of vacuum pump, to ensure the product you actually end up using will give you the optimum lifespan in your application.
Another key factor in selection of the right vacuum pump or compressor for an application is the flow rate. This should be considered from the point of view of the working vacuum or pressure at the duty point, as well as any maximum or minimum expected values. Pumps, for example, are often expected to purge systems every few cycles, requiring operation at a higher pressure than the duty pressure. It is also important to consider how likely it is that leaks or blockages will occur in the pipework over time, and to select a slightly higher specified product at the outset.
Lifespans
Perhaps the most important criteria in selecting a vacuum pump is the anticipated lifespan of the product in which it it to be used. With the pump being the highest cost component of most systems, it is rare to find any backup. At the same time, it is rare for a system to build in any manual operation option. All this makes the lifespan of the pump a vital consideration. Often lifespan versus cost is a compromise, which requires asking how long you actually want the pump to last, and what the true cost of replacement will be.
In general, brushed DC designs come at lower cost, while AC designs give longer life, but selection of the most appropriate is more complicated than that. The usage profile of the application is key, taking into account factors such as temperature of operation and whether the pump will be subjected to continuous or intermittent duty. The lifespans of AC motors, for example, degrade significantly when they are started more than six times per minute. DC motors, however, are not really affected, while their flat torque curves also eliminate the starting problems that you can get with AC designs.
When systems run at high temperatures, condensation can occur when the system is switched off. Condensation can be a particular problem with several pump designs. Those using PTFE cups, for example, suffer from condensation problems, because the water droplets wash off the PTFE coating that the cup deposits, meaning it has to redeposit a new PTFE layer, which leads to the cup wearing out more quickly than it should. AC designs emit more heat, and for that reason often need cooling fans fitted, which adds to the overall size of the package. Of course, heating from the motor is only really a problem in larger pumps where that mass is significant, but ambient temperature is also a factor.
Where ultimate operating lifespan is the most important criteria, AC designs will generally give the best performance. A third option, however, is the brushless DC design, which gives a much longer life, but at a much higher cost.
Giving these criteria for selection some thought, will go a long way to refining the selection of the most appropriate pump for the application, ultimately helping both the designer and the supplier.
15 February 2006
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