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Deciding What Type of Vacuum Pump to Use

By Aimee O'Driscoll, 23 September 2019

A crucial part of most rotovap setups is the pump. We discussed pumps in a previous post where we covered general considerations such as vacuum pressure, pump speed, and vacuum control.

Here, we’ll look at which type of pump you should use. We’ll discuss the most common types of pumps and closely examine the advantages and disadvantages of oiled versus oil-free pumps.

Types of Vacuum Pumps

The two main types of pumps used in rotary evaporator setups are diaphragm (membrane) pumps and rotary vane pumps.

Diaphragm Pumps

Also known as membrane pumps, diaphragm pumps are positive displacement pumps that utilize a series of chambers, diaphragms, and valves. The valves open and close to move air from one chamber to another.

Animation of a diapragm pump.

Diaphragm pumps are dry pumps, meaning they are oil-free. We’ll discuss this further below, but oilless pumps are generally better for rotovap applications. One major benefit of oil-free pumps is they require very little maintenance. They tend to cost more upfront, but you can save time and money in the long run.

Within the category of diaphragm pumps, you have a range of options. You can choose the speed and pressure required, and in some cases, what type of regulation you want, for example, RPM or valve-regulated pumps.

Many diaphragm pumps come with parts that have special coatings, making them resistant to corrosion by common chemicals. For example, PTFE might be used to coat some pump parts.

 

Diaphragm pumps.

Three different diaphragm pumps: A chemical resistant vacuum pump, a valve-regulated vacuum pump, and an RPM-regulated vacuum pump.

Rotary Vane Pumps

Rotary vane pumps are another type of positive displacement pump. They comprise vanes of variable length mounted to a rotor on an off-center drive shaft. The vanes form chambers which vary in size which the rotor rotates. When larger, the chambers draw in fluid, and when smaller, they expel it. These pumps pumps use oil to make a tight seal and lubricate parts.

Rotary vane pumps usually perform better than diaphragm pumps in that you can achieve a deeper vacuum. They are also relatively cheaper than diaphragm pumps. However, they could end up costing you in the long run. They require regular maintenance, which is costly in itself. And if they’re not maintained properly or are inappropriate for a given application, they could need to be replaced very quickly.

Oil-sealed pumps should be operating as close to ultimate vacuum pressure (the lowest pressure that can be achieved by the pump) as possible. Operating at higher pressures can lead to more frequent maintenance and damage to the pump. It may also void the pump warranty. Very low pressures typically aren’t required for rotovap processes, making oil pumps generally unsuitable for these applications.

What’s more, it’s very difficult to size an oiled pump correctly for a rotovap application. Having an over-powered pump could result in vapor not having enough time to condense in the condenser. That being said, if a process occurs faster than expected, the pump could be exposed to higher pressures, resulting in premature wear and damage. This means that even if a pump seems like it should be appropriate, it may not be in practice.

Summary of Oil-Sealed Versus Oil-Free Pumps

The following table compares oiled against oil-free pumps:


Oil-Sealed Pumps

Oil-Free Pumps

Medium to high vacuum levels

Low to medium vacuum levels

Lower upfront cost

Higher upfront cost

Costly to operate and maintain

Lower long-term operating costs

Difficult to size correctly for a given application

A given pump can work for a range of applications

Regular maintenance on parts such as hydrocarbon exhaust and oil mist filter

Little maintenance required

Need to change and dispose of oil

No hazardous waste disposal

Subject to issues such as blocked passages resulting in downtime and costly repairs

Less susceptible to problems

Negative environmental impact due to release of hydrocarbons and oil disposal

Better for the environment

Vacuum Control

As mentioned, one decision you need to make when choosing a pump is what type of control you need. Some pumps have built-in vacuum control while others don’t. Those that do may control vacuum via a number of different methods.

For example, “Plus” models within the LabTech VP series of vacuum pumps include a manual control regulator. This operates by simple venting, allowing outside air in. Another example of built-in vacuum control can be found in the Heidolph RPM regulated pumps. These have a digital control and operate by adjusting their speed. They come to a stop when the desired vacuum level is reached and then run intermittently at low speed to maintain the vacuum.

 

Pumps with built-in vacuum control.

A VP18RPLUS Vacuum Pump (left) and a Heidolph RPM Regulated Pump (right).

If there is no built-in vacuum control, you can use a digital vacuum controller or a standalone manual vacuum regulator. The latter is the most inexpensive option, although it is possible to juryrig a vacuum controller if you’re on a very tight budget.

Vacuum controllers.

A LabTech VC1000 Vacuum Controller (left) and a Heidolph Manual Vacuum Controller (right).

Certain rotovap models come with built-in vacuum control, so you may not need this feature in your pump. For example, some Hei-VAP rotovaps come with a smart “Control-Box” which allows you to control connected vacuum sources and chillers. It includes a customizable solvent library, the option to quickly find boiling points, and the ability to save specific processes.

Two Hei-VAP models.

The Hei-VAP Ultimate Control and Hei-VAP Expert Control both come with a smart Control-Box.