One common question we hear from customers is, what vacuum pump do I need for my laboratory application? The answer depends on a variety of factors such as the type of application, working pressure required, flow rate needed, chemicals the pump will be exposed to, and what type of control is needed. While other factors may influence pump selection, we will address how these considerations are key to finding the right pump for your application.
Vacuum level
One of the major criteria in selecting a pump is to define how much vacuum your process requires. This is driven primarily by the type of application you are running. “Vacuum” in applied work simply refers to the pressure below atmospheric pressure. Knowing the depth of vacuum needed will determine your pump technology options. Certain pump technologies, such as diaphragm pumps, can support processes down to about 0.5 mbar. This is sufficient for common processes like filtration, rotary evaporation, and drying, as well as many distillation and fore vacuum pump applications. Other applications like freeze drying need deeper vacuum that other pump technologies such as screw pumps and rotary vane can provide.
Pumping speed
The rated pumping speed for vacuum pumps, also referred to as flow rate, means the maximum amount of gas a pump can move when not producing vacuum. This last phrase is critical, because in most applications, you need to move air, gases or vapors under vacuum conditions. The rated pumping speed is the maximum flow that the pump produces, which occurs when the pump generates the minimum pressure differential between its inlet and outlet. The pumping speed that needs to be considered for your application is not the maximum rated pumping speed but the pumping speed at the working pressure of your application. The way to check this is to review the pump’s performance curve or ask your manufacturer’s representative. If you get this wrong, your process may take a lot longer than anticipated. The graph illustrates the point where two pumps with the same rated maximum pumping speed can have different performance at the same working vacuum level.
With this in mind, we have created two different calculators to help size pumps for applications like vacuum drying or solvent removal. Generally speaking, these applications all center on the idea of inducing a solvent or mixture of solvents to evaporate at reduced temperature by pulling a vacuum. The Vacuum Drying Process Time Calculator can be used to estimate how long a process will take for a given pumping speed. The Vacuum Drying Process Flow Rate Calculator estimates what flow rate is needed to complete a process in a given period of time. Both calculators provide first-order estimates based on the ideal gas law and a few process-specific parameters.
Another common application for vacuum pumps is to pump down a chamber where no vapors are generated. This process sounds like a relatively simple calculation would be sufficient – where the flow rate would equal the volume that needs to be pumped out divided by time – but it is a little more involved. To help, we have developed the Chamber Evacuation Flow Rate Calculator intended to help estimate the pumping speed needed to evacuate a vacuum chamber.
As helpful as these calculators may be, it’s always best to check with the pump manufacturer to make sure the pump selected has the appropriate pumping speed at your working vacuum.
Reliability
When evaluating the reliability of a vacuum pump, pump maintenance and durability are critical to consider. For pump maintenance, both parts and labor can impact the lifetime ownership costs of the vacuum pump, especially if frequent maintenance is needed. By virtue of their design, some pumps require regular maintenance while others have relatively low requirements.
Consider and check the compatibility of the chemicals that will run through the wetted parts of the pump. If your process involves corrosive vapors, a pump with a fluoropolymer flow path is more durable and will provide much better corrosion resistance than an oil-sealed or water-sealed pump. Importantly, a durable pump suited for your work will also require much less maintenance.
Control
Selecting the right pump for your operation also includes consideration of whether you just need the full design vacuum level of the pump or whether precise vacuum control is needed. Unless you are using vacuum for uncontrolled evaporation, the best vacuum for your applications are not always the deepest, but the right vacuum level. Vacuum that is insufficiently deep may significantly extend process times. Vacuum that is too deep may cause foaming or evaporate too much of your process liquids. To get the right vacuum, you may need a pump with vacuum control for precision.
Vacuum control like that can be found with VACUUBRAND’s VARIO® -control technology, combines the VACUU·SELECT® touch screen controller with variable motor speed. Pumping speeds automatically adjust based on process demands to minimize process time and maximize process efficiency. Set a precise vacuum level or run a preconfigured application for automatic evaporation.
Because it is often difficult for customers to gather all of this information, we strongly recommend working with the manufacturer to size a pump for their particular needs. BrandTech® Scientific offers a variety of VACUUBRAND® vacuum pump technologies that address these considerations. Contact us to learn more about our vacuum technology and for help selecting a pump for your process.
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