Choosing the correct vacuum pump for your lab's vacuum oven is a significant decision. The right pump means efficient and accurate results, while the wrong one could cause headaches and failures. This guide is here to help you figure out the key things to consider and what features to look for when picking a vacuum pump for your lab oven.
Vacuum ovens provide a highly effective way to remove excess moisture from a substance by combining elevated temperature and reduced pressure. When the pressure around the substance is reduced, increased evaporation will occur at lower temperatures. When drying is performed at higher temperatures, lowering the pressure will enable faster evaporation. The vacuum oven regulates the desired temperature, while the vacuum pump lowers the pressure within the oven chamber. This vacuum technology is commonly used across various industries including medical, pharmaceutical, food and beverage, medical technology manufacturing, aerospace, and electronics. To make sure every lab gets just what it needs, companies provide a range of vacuum ovens and pumps. Let's dive into the top recommended vacuum options for ovens and the important things to keep in mind when buying one.
Type of vacuum pumps:
There are several types of vacuum pumps that can be used for vacuum ovens, each with its own advantages and limitations. The most common types include:
1. Diaphragm pumps: These are dry pumps that use a series of valves with pulsating motions to move air. This design allows them to operate without oil, effectively reducing hydrocarbon production and making them more eco-friendly. The valves are often made from polytetrafluoroethylene (PTFE), also known as Teflon, which provides high resistance to chemicals, vapor damage, and corrosion. These properties make diaphragm pumps ideal for handling acidic, corrosive, or viscous substances. Their resistance to most materials and elemental damage also makes them suitable for concentration and evaporation processes.
These pumps can be used for low-to-medium vacuum applications in the first place. Used with a vacuum controller (or, on some vacuum pumps, a built-in vacuum controller) you can run the pump at lower vacuum / higher pressure. Many other types of pumps cannot do that. If you were to hook these other types of pumps to a vacuum controller to regulate the vacuum, they could overheat and experience a mechanical failure. Diaphragm pumps might cost more upfront, but they save you time and money on maintenance in the long run.
2. Rotary Vane Pump: The rotary vane pump (RV pump) is a compact vacuum pump that offers high performance and lower upfront costs. RV vacuum oven pumps can achieve medium vacuum levels, making them practical for high-boiling solvents like methanol and aqueous solutions. RV vacuum pumps operate using oil, which ensures an airtight seal and consistent lubrication of the working parts, crucial for cooling the pump’s rotors. These pumps should always be used with a cold trap which is a condensing device to prevent contamination in a vacuum line or pump. If you are looking for a space and money saving option without compromising on functionality or form, the rotary vane vacuum pump is an ideal choice.
The lifespan of rotary vane pumps is largely dependent on proper maintenance. These pumps can achieve deep ultimate vacuum levels and offer variable displacement capacities from low to high. They are effective for samples containing solvents, aqueous vapors, or substances with high boiling points. Regular oil changes are crucial for maintaining vacuum pumps. It is recommended for certain models to perform maintenance on these pumps every 3,000 hours or around 4-6 months of use. Follow the manual's maintenance schedule, but consider more frequent changes based on usage. The Oil´s appearance is a good indicator when the pump needs maintenance. Don't wait for the oil to look dark or cloudy; stick to recommended intervals in the pump´s manual to ensure optimal performance and prevent unexpected failures.
3. Dry Scroll Pump
Dry scroll vacuum pumps use two spiral scrolls to compress air and vapor towards the vacuum oven’s exhaust. These pumps may be more expensive initially, but they are a worthwhile investment due to lower operational costs, minimal maintenance, and no need for oil as a lubricant. They are especially effective for freeze-drying processes, handling water vapors better than other pumps.
The metal scrolls inside may be able to handle samples with up to 20% acids, especially in chemical-resistant models. Scroll pumps achieve deeper vacuums and higher displacement than diaphragm pumps and are used in degassing, distillation, and concentration. It is advised to replace scrolls every 40,000 hours of use or annually for some models. Always refer to the manual's maintenance schedule and guidelines for best practices.
Key factors to consider when choosing a vacuum pump.
When selecting a vacuum pump for your vacuum oven, consider the following factors:
- What solvents are you working with?
It is essential to choose a pump that is compatible with your specific application's chemicals. Some pumps may not be suitable for certain chemicals, leading to corrosion and potential damage.
Solvents with low boiling points, such as acetone or methylene chloride, require less vacuum to evaporate than benzene or acetonitrile. For high boiling point solvents like water and toluene, a deeper vacuum is necessary. If you are working with corrosive solvents, ensure that your pump has a corrosion-resistant flow path to protect it from corrosive vapors and to avoid the need for inconvenient cold traps.
When it comes to vacuum pumps, there are two main types: oil-lubricated and oil-free. Each has its advantages and disadvantages, but for most laboratory and industrial applications, an oil-free pump is preferred.
Oil-lubricated pumps require regular maintenance and can contaminate the environment with oil mist. On the other hand, oil-free pumps operate without any lubrication, reducing maintenance costs and minimizing the risk of contamination.
Therefore, selecting an oil-free pump with excellent chemical compatibility would be a better decision for optimal performance and longevity.
- Depth of vacuum and flow rate.
Another important thing to consider when picking a vacuum pump is the flow rate. It is crucial because it tells you how efficiently the pump can drain contents from a machine. Usually, higher flow rates mean the job gets done quicker. Assessing the pump’s ability to generate the desired volume and mass flow rate will help determine if it meets your process requirements.
Choosing the right vacuum pump depends on your oven's size, how deep you need the vacuum to be, and whether the pump can handle the vapors it will encounter. For ovens under 2 cubic feet, a pump with a 20 – 40 L/min flow should do the trick. If your oven is bigger than 2 cubic feet, go for a pump with an approximately 60 L/min flow rate. It is recommended the vacuum depth to be below 10 torr for oven applications.1
If you have a small system, you do not need a big pump. Since the system is small, you will not need as much flow to clear it of air. But it is different for larger systems. The bigger the system, the more helpful a higher flow rate is. Otherwise, you might be waiting a long time for the system to reach the desired vacuum pressure.
For instance, when using a vacuum oven for purging low-boiling point volatiles from a product, achieving high-vacuum depths along with heat is necessary to completely remove some solvents. If the oven is compact, a small direct-drive pump can quickly evacuate the system. Conversely, for a larger oven, a pump with higher free-air displacement may be required.
- Vacuum control options
There are a few options available for vacuum control.
Manual control You can only make an approximate and indirect adjustment using a manual control valve. Depending on the valve type, you alter the effective flow rate to the pump by either introducing air or narrowing the cross-section. This lets you regulate an approximate vacuum level. However, setting an exact and consistent pressure directly is not possible. For certain applications, like filtration, this may be adequate. But for others, such as evaporation, manual control can be challenging. Mixtures with varied boiling pressures require constant manual readjustment of the control valve. This demands a certain level of expertise and continuous process monitoring. Even with this effort, manually controlling an individual vacuum process remains difficult.
Two point control: A vacuum controller lets you electronically set an initial vacuum point and manage valve control with "Open" and "Close" switching points. When the set pressure is exceeded, the electromagnetically actuated valve opens fully, evacuating the system until the pressure returns to the set value. The vacuum fluctuates between the two valve switching points within the defined hysteresis range. In two-point control evaporations, the first boiling point can be detected automatically, unlike with manual control. However, the pump runs continuously and doesn't respond to boiling pressure changes in the sample vessel during the process. Only the initial vapor pressure is detected, adapted as a constant setpoint, and maintained, potentially causing the process to halt. The operator may need to readjust the setpoint during the process.
Adaptive vacuum control: This control method is especially beneficial as it adjusts the pump’s motor speed as needed, allowing pressure values to be set continuously and with pinpoint accuracy. The pump aligns with the desired vacuum at all times, operating only as fast as necessary. Additionally, for evaporations, the fully automatic vacuum controller not only identifies different boiling points but also adapts to changing process conditions using the unique vapor pressure tracking system. This ensures optimal results in the shortest time possible, all at the touch of a button, without requiring monitoring or intervention.
- Cost.
Before spending a lot on a vacuum pump, take a moment to assess your resources and needs. When figuring out the value of these products in your lab, it is important to consider both the upfront costs and ongoing maintenance expenses. As a bit of research will show, the initial price is just one piece of the puzzle.
The most significant expense in the lifespan cost of a vacuum pump is its energy consumption. Energy costs can be five times greater than the initial purchase price, with maintenance costs accounting for about 30% over the pump’s lifespan.
Choosing an energy-efficient and reliable system can significantly reduce total ownership costs. Key expenses include initial purchase (10%), maintenance (30%), energy consumption to power the vacuum pump (50%), and additional expenses (10%).2
Remember, investing in a high-quality, oil-free vacuum pump is a smart, long-term move for your vacuum system. Keeping an eye on it, maintaining it regularly, and making sure everyone knows how to use it properly will help it last longer and run efficiently.
References:
1. https://knf.com/fileadmin/Local_files/USA/Downloads/Lab_downloads/literature/Brochure_Lab_Select-the-Best-Pump-for-Your-Vacuum-Oven_KNF_USA.pdf
2. https://www.elmorietschle.com/en-us/blog/all-you-need-to-know-about-vacuum-pumps.
