Lab Manager Magazine's Independent Guide to Purchasing a Vacuum Pump

Laboratory vacuum pumps are one of the most ubiquitous pieces of laboratory equipment, and also one of the most difficult to choose. Vacuum pumps come in many different designs with an array of features, and it is important to carefully consider the applications for which the pump will be used before selecting a particular model.

The first decision to be made when purchasing a laboratory vacuum pump is whether a rotary vane (oil-sealed) pump or diaphragm (oil-free) pump is needed:

Rotary vane pumps are the traditional vacuum pump and are still widely used. However, they are only needed in applications where a greater vacuum is required.
Diaphragm pumps are appropriate for most laboratory applications. They are easier to maintain and, if pumps with chemical-resistant diaphragms and valves are selected, may be used for corrosive solvents without the use of a cold trap.

For most laboratory applications, the oil and excess vacuum of a rotary vane pump creates greater service demands and a greater need for cold trap coolant to protect the oil. Moderating the excess vacuum may be accomplished by introducing air in order to reduce the vacuum to required levels. However, this practice increases noise, releases a malodorous, hazardous oil mist into the lab, and reduces the cold trap's ability to protect pumps, leading to an increased risk of pump corrosion. After reading this guide, check out the latest vacuum pump models at LabWrench.com.

Diaphragm (oil-free) vacuum pumps provide a vacuum in a range needed for most laboratory applications. Even high boiling-point solvents, such as DMF, can be evaporated at near room temperature (<100°F), or DMSO at about 125°F, with a good diaphragm vacuum pump.

Diaphragm Pumps

Having decided on a diaphragm pump, the purchaser should decide whether a chemical-resistant pump is needed, or whether a pump suited only for non-corrosive applications is sufficient (remembering to consider possible future uses). Corrosion-resistant components significantly increase the cost of a vacuum pump.

Next, the buyer should consider the depth of vacuum required to meet his or her application requirements.

A. Ultimate vacuum: 200–70 mbar vacuum

This level of vacuum is appropriate for:

Vacuum filtration
Degassing solutions
Liquid aspiration
Extraction

The pressure range 200–100 mbar is common for central vacuum system pressures.

Flow rate
The necessary flow rate required is typically determined by the number of vacuum applications that will be operated simultaneously. More applications will require a bigger pump.

Vacuum monitoring and control
Gauges are recommended when monitoring and recording of process parameters is required, and when manual operations depend on knowledge of application conditions. Manual and electronic controllers may be helpful in special cases for these normally low-control, intensive applications.

Manual control: Pinch or bleeder valves can add sensitivity to aspirations and reduce risk of filtration breakthrough.
Two-point control: may be useful to prevent evaporation of volatile solvents during filtration.
No control: For some applications, control of the vacuum is considered unnecessary and the pump is simply run at its full capacity.

B. Ultimate vacuum: 70–10 mbar vacuum

This level of vacuum is appropriate for the following applications when working with low to moderate boiling-point solvents:

Rotary evaporation
Vacuum concentration
Gel drying
Vacuum ovens

C. Ultimate vacuum: 10–1 mbar vacuum

This level of vacuum is appropriate for the following applications when working with high-boiling-point solvents:

Rotary evaporation
Vacuum concentration
Gel drying
Vacuum ovens

Flow rate
The necessary flow rate required is typically determined by the volume of liquid being evaporated and the rate at which the vapors are produced. These factors combine to determine the total quantity of vapor that must be moved per unit of time.

Vacuum monitoring and control
Gauges are recommended when trying to operate these more control-intensive applications manually, and to record process parameters if needed. Manual control may be sufficient for simple applications, but electronic controllers offer significant performance and productivity advantages. Whether you need a 70-10 mbar vacuum or 10-1 mbar vacuum, you’ll need to choose from the following control options:

Manual control: may be adequate for the simplest evaporative applications. Otherwise, equipment cost savings may be more than offset by reduced staff productivity. This type of control is best when pressure conditions are monitored with a gauge.
Two-point control: Two-point control is typically a sound approach for routine evaporative operations. It requires test runs and set-point programming, and performs well with an in-line cold trap.
Adaptive control: Self-regulating vacuum control reduces manual oversight and improves laboratory productivity by avoiding test runs and programming. This type of control is particularly useful when the pump is not protected by a cold trap or condenser, as it can prevent sample bumping in critical applications. It may permit the recording of vacuum conditions for ISO/GLP needs.

Rotary Vane Pumps

Rotary vane pumps should only be used for demanding evaporation, filtration and drying processes that cannot be supported by diaphragm pumps

A. Ultimate vacuum: 1–10-3 mbar vacuum

This level of vacuum is appropriate for:

Freeze drying
Schlenk lines

Rotary vane pumps may also be needed for evaporating high boiling-point solvents at or below room temperature. Using rotary vane pumps for rotary evaporation and concentrators will often cause bumping. For high-flow applications like aspiration and filtration, rotary vane pumps will generate high loads of oil mist.

Flow rate

Rotary vane pumps are typically used for applications that are more dependent on ultimate vacuum than flow rate. However, larger pumps may be needed for large freeze dryers or large Schlenk-line manifolds.

Vacuum monitoring and control

Gauges are helpful to ensure that the system is leak-tight, and for providing the fine vacuum levels needed for these applications.

Manual control: Manual control is typical, but not adequate, control for many rotary vane applications. The use of bleeder valves (a controlled leak) to moderate vacuum pressures will often lead to high loads of oil mist that must be controlled.
Two-point control: Control of vacuum with a solenoid valve may be helpful when using rotary vane pumps for evaporating high boiling-point solvents.
No control: For some applications, control of the vacuum is considered unnecessary and the pump is simply run at its full capacity.
Dry scroll pumps: are an excellent alternative to rotary vane pumps where oil-free pumping is desirable. This technology is suitable for noncorrosive liquids and delivers low ultimate pressure at high speed. Dry scroll pumps are oil-free, durable, reliable and easy to maintain.

Definitions

1. Ultimate vacuum

The ultimate vacuum specification of a pump describes the lowest pressure that the pump can achieve—that is, the pressure at which it can no longer move any vapor. Select a pump with an ultimate vacuum specification that is below the pressure at which you plan to operate your application. For example, don’t select a 10 Torr pump if you need to operate your application at 10 Torr.

2. Flow rate

The flow rate is the volume of gas a pump can move per unit of time. No matter the ultimate vacuum needed, larger applications will need higher flow-rate pumps. The flow rate in vacuum pump specifications (also called pumping speed or free air displacement) refers to the maximum speed the pump can move vapors at atmospheric pressure—that is, when there is no vacuum. When operating at the vacuum levels needed for your application, pumps with the same flow rate specification can differ greatly in their actual pumping speed (if a pump can’t generate sufficient flow at the desired vacuum level, the application will proceed much more slowly or, in extreme cases, not at all). Flow rate curves show what proportion of the specified flow rates are available over the vacuum range provided by the pump. The purchaser should inquire about a pump's flow curve to compare flow rates at operating vacuum.

Control options

Manual control: Either simple on/off control, or manually adjustable control of flow with pinch valves or bleeder valves. This provides a very approximate control of vacuum by manually matching pumping speed to vapor flow rates, which vary with vacuum pressure. A gauge should be used to monitor vacuum pressure conditions to guide manual operation.
Electronic two-point control: Controls vacuum as a thermostat controls temperature. A target pressure is set and an electronic controller keeps the application pressure within a narrow range near the target by turning the pump on and off, or by opening and closing a solenoid valve to permit or restrict pumping of the application. Often capable of programmable setpoints and ramps.
Electronic adaptive control: This is the highest level of control. Relying on instantaneous detection of application conditions, motor speed is continuously varied to provide the pumping speed needed to maintain target pressures and reduce risk of over-pumping (bumping). It is useful for protecting critical samples and for fully automating evaporative applications.

3. Unit conversions

Many different measurement units are used to describe vacuum pressures. Most common are millibar (mbar) and Torr (mm of mercury). These are absolute measures of pressure that start at 0 and range to 1000 mbar or 760 Torr at standard atmospheric pressure. These are better for measuring vacuum pressure than gauge pressures that count up from atmospheric pressures for two reasons. First, atmospheric pressures differ from place to place, so using atmospheric pressure as a base is a less reliable measure. Second, mbar and Torr scales result in simple numbers at working vacuum pressures in the laboratory (such as 20 mbar or 15 Torr), whereas a gauge pressure measured in inches of mercury reaches 29 inches at about 30 mbar, and then must measure to three or four decimal places to describe vacuum conditions common in the lab.

Take away messages:

More vacuum is not always better! Match your vacuum pump to the vacuum needed for your applications, and you’ll get better productivity, less risk to samples of over-pumping, reduced service demands and less noise.
For performance, convenience and service, never use an oil pump when an oil-free diaphragm pump can do the job.
If you plan to put corrosive vapors through the pump, buy a pump designed for corrosive vapors. Your pump will then serve you for a long time.