Additionally, the cooling effect from boiling (latent heat of vaporization when water boils from liquid to gas) can lead to freezing, especially in systems without adequate heat input to combat this “loss” of heat energy to as the expanding gas absorbs more heat energy from its surroundings. The formation of ice as water boils will significantly slow the removal of water from a system.

Sublimation, the direct transition from solid to gas, occurs more slowly than boiling and at deeper vacuum levels (less absolute pressure), which can further complicate system dynamics as it will take longer to remove that water from the system and the water will be even further expanded at the greater depths of vacuum. 

Vacuum System Performance

Vacuum systems, particularly those using positive displacement pumps, have a fixed capacity. With each rotation of the compression element a fixed volume of gas is captured and compressed up to atmospheric pressure. When boiling occurs, the combined process flow with the steam and any gas in the system may exceed the pump’s capacity.

The effect is that it will take significantly longer to evacuate the combined vapor load, leading to “flat spots” in pump-down curves. These flat spots represent inefficiencies and delays in reaching the desired vacuum level where the system “stalls out.” During this time, the boiling of the liquid and the additional vapor load generate enough pressure on the surface of remaining liquid to slow or stop the boiling.

As the pump removes more gas from the system, the pressure will minutely drop, increasing the boiling rate. On and on this cycle goes until all water has either boiled or frozen, at which time one would see the vacuum level in the process swiftly drop again.  This second drop is indicative that no more liquid water is present in the system, and the remaining liquid has frozen.

RELATED: IMPLEMENTING VACUUM SYSTEM INNOVATIONS IN THE REAL WORLD: HOW INSTALLATION & WEATHER AFFECT PERFORMANCE AND SYSTEM DESIGN

Improving Pump-Down Time

To enhance evacuation speed and system efficiency, two key strategies are commonly employed:

Boosters

These increase the volumetric capacity of the system and are especially effective at pressures below approximately 150 torr. Boosting effectively increases the system pressure at the inlet of the backing pump. This increased pressure directly correlates to a greater mass flow rate that the pump can process which increases its ability to remove products (vapor, gas, etc.) from the system. Boosters can be staged (i.e., have multiple boosters in-line) to compound their effectiveness for systems that require very deep levels of vacuum.

Figure 2. Example of vacuum booster on system capacity.

Heaters

By raising the temperature of the liquid, heaters increase the boiling point and vapor pressure, which improves flow rate and reduces pump down time. Heating can also prevent freezing and/or speed up sublimation rates as well which are significantly slower than boiling rates.

Figure 3. Example of freeze-drying system using a heater to increase the rate of removal of water from the product while under vacuum.

Liquid Separation Techniques

Imagine this: you have a system where the depth of vacuum is not deep enough to boil water inside the system. There is still a significant amount of liquid water or other liquids present. What do you do for these types of systems?

Effective separation of liquids from gas streams is vital in vacuum systems to protect the vacuum pump. Remember that liquid water is considered an incompressible liquid meaning its volume cannot be reduced. If the liquid works its way into a vacuum pump’s compression chamber it can cause severe damage to the internals. Common techniques include:

• Centrifugal Separation: Utilizes tangential inlets and rotational motion to separate liquids from gases. The gas spins, and the liquids sling to the outside of the vessel, contact the vessel walls, and are pulled down by gravity to collect in the bottom of the vessel.
• Change of Direction/Elevation: Alters the flow path to reduce velocity, changes elevation and allows liquids to drop out to be collected at the bottom of the separation element.
• Baffling/Tortuous Path: Slows the flow and captures liquids using demister pads of similar media. As the liquid slows and gets stuck within the baffle or filter media, it will then fall to the bottom of the vessel.

Liquid separators often employ one or more of the above strategies for more efficient separation and removal of the liquid from the vacuum stream. Centrifugal separation is popular for high flow, low vacuum level systems as it still maintains the velocity of the gas stream and does not cause significant pressure drop. Filtration or baffling often follows a change of direction as the second stage of filtration to remove finer liquid vapors or droplets that are still entrained in the gas stream.

Figure 4 (left): Example of a centrifugal separator to remove condensables from vacuum flow.

Figure 5 (right): Combination separator that uses both change of direction/elevation and baffling/tortuous path methods to remove liquid from a vacuum gas stream.

Draining Liquids Under Vacuum

Now that the liquids are collected at the bottom of the separation vessel, they will need to be drained out. In pressurized systems (like compressed air systems) where the pressure gradient from high pressure to low pressure works away from the system and will push any condensate or collected liquid out, but in vacuum systems the pressure gradient works against the flow direction needed for the removal of the liquid. 

Draining liquids from a vacuum system requires careful handling to avoid system damage or contamination. Best practices include:

• Venting the system to atmosphere before draining
• Isolating and venting the drain location to ensure safe liquid removal

Venting the system to atmosphere will effectively remove the pressure gradient so all pressure is equalized, and gravity can drain out the liquid. However, it is not always possible to constantly stop a system, vent to atmospheric pressure, drain the collected liquid, then pull the system down again. In order to drain collected liquid in a vacuum system on the fly. One will require a series of valves and an additional collection volume that can be isolated from the system, vented to atmosphere, then opened to drain. T

his process can be manually or automatically done with a simple control system. The frequency at which this needs to be done is directly related to the amount of liquid the system will collect while operating and the collection volume needs to be sized appropriately to not cause any disruptions to the overall system operation.

Figure 6. Vacuum draining system required to drain liquid from a vacuum system by isolating and venting a small vessel from the rest of the system with automatic valves.

Filtration Methods

Filtration is essential for removing particulates and vapors from a gas stream. Common methods include: 

• Separation (change of direction, demister pads): For capturing entrained liquids in the gas stream
• Cold traps, coalescers, and condensers: For vapor removal
• Filter media: Such as paper or polyester for solid particulates

Figure 7 (left). Internal of liquid removal filter showing the change of direction and demister pad inside the filter to coalesce and remove vapor from a vacuum gas stream.

Figure 8 (right). Vacuum condenser with a heat exchanger inside that cools the gas stream to condense vapor inside the filter for collection.

Typically, when filtering out a gas stream the first process is to remove liquids, then vapors, then particulates last. Demister pads and separators can remove a significant amount of liquid from the gas stream. Cold traps and coalescers can remove condensable loads within gas streams. If there is water that has boiled off inside the system due to the depth of vacuum, it still may be able to condense back from a vapor to a liquid (or frozen into a solid) on a condenser.

The benefit of removing the vapor from the system is that the load to the vacuum pump will be significantly reduced and a much smaller vacuum pump can be used. If the liquid cannot be condensed, then coalescing filter media can be used to remove the vapor from the gas stream before it can condense inside the pump.

Figure 9. Liquid ring vacuum pump assembly, full recirculating with heat exchanger for seal liquid and stainless-steel separator.

Liquid-Tolerant Vacuum Systems

Some systems are specifically designed to handle high-moisture environments. These systems typically include: 

Liquid Ring Vacuum Pumps

Liquid ring vacuum pumps are great for high liquid load applications – especially with water. As that water is drawn into the pump it will join with the liquid ring and help keep it replenished. This reduces the constant seal water flow required to the pump depending on how much water is drawn into the pump through the process. Liquid ring pumps are also great for processes with a high condensable vapor load.

As the pressure rises inside the pump during compression, any condensable vapor will transition from gas to liquid and that liquid will mix with the seal liquid. The seal water can be treated to neutralize any corrosive or aggressive compounds as well to promote long life and make the liquid stream safer for operators.

RELATED: LIQUID RING VACUUM PUMP DOS AND DON’TS

Figure 10. Internal view of a liquid ring vacuum pump. A wet inlet gas mixture will simply mix with and join the liquid ring and may reduce the amount of seal liquid required at the seal water inlet to the pump (depending on how much is entering the pump).

Temperature Controlled Pumping Chambers

Some vacuum systems, namely dry screw vacuum pumps, have jacketing on the compression chamber that can be controlled to maintain the pumping chamber at a specific temperature. This can be especially handy for condensable vapor systems where the temperature in the compression chamber, if carefully selected and controlled, can stay warm enough to keep any vapors in the gas stage so they can pass right on through the pump without condensing which can be harmful to the pump internals.

Figure 11. Dry screw vacuum pump with liquid cooling jacket (shown in blue) where the pumping chamber temperature can be controlled to keep the pumping chamber hot enough to prevent vapors from condensing inside the pump.

Multistage Separation

Multistage separation and filtration will get the most efficient system in place to protect your vacuum pumps, which can be costly to repair or replace if damaged by ingesting liquids. Many filtration systems are only fractions of the cost of vacuum pumps, so the investment in proper filtration is a no-brainer. 

Separation systems are larger than filtration systems as they require sufficient space to change the direction of flow, incorporate baffling or demister pads, and have enough capacity to collect the liquid.

These systems are applied only in high liquid load systems. While some standard options exist, the system may need to be engineered based on the processes specific requirements, type of liquid, and volume of liquid collected per hour/shift/day.

Figure 12. A simple vacuum filter with a demister pad or baffling can significantly reduce the liquid load on a system, extending the life of your vacuum pump.

Clog & Corrosion-Resistant Materials and Designs

Choosing the right materials of construction for your separation, filtration, and vacuum system are just as critical as employing the systems themselves. Temperature, pressure/vacuum rating, filtration and separation efficiency, and general material compatibility are all critical criteria in selecting a system that will have as long of a life cycle as possible.

If the process is just liquid water, then carbon steel vessels are acceptable, but liners or coatings (i.e., epoxy) would be recommended to prevent rusting and corrosion. Stainless steel might be recommended or required if there are any aggressive components in the gas stream.

Advanced Filtration Components

Filtration materials should consider any trace chemicals as well; it might be very helpful to have an activated carbon filter media to remove hydrocarbons or other nasty parts of the process as well. If in doubt, please consult your local vacuum pump manufacturer for their recommendations in selecting the appropriate materials.

Figure 13. Multi-stage filtration using change of direction, mesh baffling pad and cyclonic tubes to spin the gas and further reduce the liquid content entrained in the gas stream.

Best Practices

To ensure optimal performance and longevity of vacuum systems, the following best practices are recommended:

System Design

Match separation and filtration components to the system's flow rate and pressure requirements.

Operational Practices

1. Use a dry gas sweep and purge to prevent condensation.

Dry sweep gases (can be as simple as atmospheric air that will naturally “dry out” as it expands into the vacuum system) are especially helpful to remove water or other condensables that may have found their way into the piping, separator, or pump prior to shutting down. Sweeping out those condensables before the pump cools can help ensure that the internals of the pump are dry and ready for the next operation.

Figure 14. Example of an atmospheric sweep/purge arrangement where the automatic valve (lower valve) will isolate the pump inlet from the process. The smaller valve (on top) opens to allow atmospheric air in, which expands under vacuum, decreasing its dewpoint and helping to remove water from the inside of the pump before shutting down.

2. Maintain temperature control to avoid freezing or overheating.

Temperature control can significantly affect pump down times, system capacity, and general process effectiveness. There might be an ideal range of temperature for your system and having the right equipment to heat or cool it to maintain within that specific range will be crucial to repeatable, reliable operation.

3. Monitor liquid levels continuously.

There is always the expected or designed for liquid content in a system but there will always be an opportunity for an unexpected amount of liquid to enter a system and cause havoc. By using continuous level monitoring and control (as opposed to timer based liquid level control) you can protect your system against unexpected high liquid loads.

Example: If a line breaks and extra water is sucked into your system, or if an operator leaves a line on that's being cleaned, level monitoring can protect your system.

Figure 15. Vacuum separator with liquid level monitoring (indicated with red arrows) with signals sent to PLC to control automatic drain sequence and overflow protection. This system will shut down the vacuum pump if the liquid level rises above the high-high (top) level switch.

4. Implement automatic draining and overflow protection.

Just like with continuous liquid level monitoring, overflow protection is also critical. Whether you’re able to have a system overflow that will dump to a catch vessel or monitoring that will trip out the system if high liquid levels are detected – it is crucial to have the right monitoring and safeties to protect your system, facilities, and most importantly your employees.Automatic draining systems help maintain liquid levels within design limits regardless of system operation and are highly recommended.

Design Considerations

When designing a vacuum system, several factors must be considered when discussing liquid loads:

1. System flow rate

This will dictate how big everything will need to be.

2. Operating pressure and temperature

The operating pressure will dictate if the liquids boil to gas, freeze to solid, or stay liquid. And what phase it will be in will dictate how you will deal with it as well as the impacts to the system flow.

3. Type and quantity of liquid

Questions to ask here are what kind(s) of liquid(s) will be in the system? How much is expected per hour/shift/day/week?

That will help dictate what kind of level monitoring, overflow protection, drain cycle timing, drain vessel size, and more.

Figure 16. Example of a duplex vacuum system designed for high liquid content with automatic drain sequencing to allow for the receiver tank to be drained without interrupting the process.

Each of these variables influences the overall performance and reliability of the vacuum system. If you are experiencing issues with liquids in your vacuum system and need support in modifying or design a new system to deal with that liquid – please contact your local Rogers branch. We have a host of solutions at our fingertips that we would be happy to discuss with you to find the right one for your application.

Meet the Author

Jackson Redline is a mechanical engineer and project manager working in the Engineered System Solutions team. Jackson started at Rogers Machinery fresh out of college and was recently recognized in Plant Engineering 2023 Leaders under 40.