It ain't necessarily so
It ain't necessarily so

("It Ain't Necessarily So" from the opera Porgy and Bess, by George and Ira Gershwin, 1935)

 

IT AIN'T NECESSARILY SO

How many times have PTOA Readers and Students read that the main purpose of the Rotating Equipment identified as a  "Pump" is to increase the PV Pressure in liquids?

Likewise...

PTOA Readers and Students have read many times that the main purpose of the Rotating Equipment known as a "Compressor" is to increase the PV Pressure in gases or vapors.

Well, those statements ain't necessarily so when the subject is a Vacuum Pump!

Vacuum Pumps move air or other gases.

So why aren't these "pumps" called "Vacuum Compressors?"

 

WHOA!  STOP RIGHT HERE!

The PTOA Department of Redundancy Department highly suggests that PTOA Readers and Students stop right here and take a moment to review PTOA Segment #149 which features Atmospheric Pressure (P_atm) and PTOA Segment #150 which features the relationship between Absolute Pressure (P_abs), Atmospheric Pressure (P_atm), and Gauge Pressure (P_gauge). 

• A Vacuum Pressure is any Pressure below Atmospheric Pressure, meaning below 1 atm or 14.7 psi or 101.3 kPa. Vacuum Pressures must be created by Vacuum Pumps or Special Dynamic Pumps. A Vacuum Gauge indicates how much lower than Atmospheric Pressure the measured and indicated Vacuum Pressure is. 
• Atmospheric Pressure (1 atm, 14.7 psi, 101.3 kPA) is the pressure that every PTOA Reader and Student feels on their skin daily whether they know it or not!
• Gauge Pressure is the net PV Pressure that is sensed and measured in psi above Atmospheric Pressure. Gauge Pressure must also be created by the help of Rotating Equipment like Pumps and Compressors..
• Absolute Pressure (P_abs)= Atmospheric Pressure (P_atm) + Gauge Pressure  (P_gauge) and is measured in psia.

 

The language used to discuss Vacuum Pressures is confusing!

A  "strong Vacuum" or "high Vacuum" or "greater Vacuum Pressure" factually means that less PV Pressure is sensed and measured.

Alternatively, a "weak vacuum" or "slight vacuum" infers that a higher PV Pressure … yet still below Atmospheric Pressure … is sensed and measured.

The PTOA uses the lingo "greater magnitude of Vacuum" to infer that less PV Pressure is sensed and measured relative to another sensed and measured Vacuum Pressure.

 

DIFFERENCES BETWEEN COMPRESSORS AND VACUUM PUMPS

The major differences between Compressors and Vacuum Pumps are defined by the environment that surrounds the equipment and what the expected service of the Rotating Equipment is.

 

Both the Suction Pressure and Discharge Pressure of a Compressor are Gauge Pressures measured in PSI

The job of the Compressor truly is to add the PV Pressure to a flowing gas.

Unlike liquids, gases are truly "compressible."

The act of forcing the gas into a smaller volume or volumetric flowrate increases the PV Pressure of the gas. "Compressibility" was featured in PTOA Segment #153.

Compressors have an inlet Suction PV Pressure measured as a gauge pressure in psi and an even greater Discharge PV Pressured measured as a gauge Pressure in psi.

 

 

The Suction Pressure of a Vacuum Pump is BELOW Atmospheric Pressure and
The Discharge Pressure of a Vacuum Pump is AT  Atmospheric Pressure

The main job of the Vacuum Pump in the process industries is to remove air or other gases from a container or other fixed volume (aka, "degasification").

The ability to suck gases at Atmospheric Pressure out of a fixed volume is used in packaging, filtration, cooking, pasteurization, distillation, deodorizing, sterilization, chilling, freeze drying … and evisceration of meat and poultry products (yuk!).

Perhaps the most common use of a Vacuum Pump is the common household food vacuum packer like the one shown in the nearby gif.

The Suction PV Pressure of a Vacuum Pump is always less than 1 Atm … hence, the Vacuum Pump  "pulls a Vacuum!"

How strong a vacuum does a Vacuum Pump pull?

The nearby graphic shows an increase in vacuum strength from the left hand side to the right hand side on the X axis. The red Vacuum Forming double head arrow indicates a measurement in  units of Torr while a Vacuum Pressure is being created.  The right hand side of the Vacuum Forming arrow indicates the slightest of Vacuum Pressures.

The below bullet points define the Vacuum Ranges in order of producing an increasingly strong Vacuum Pressure:

• Rough Vacuum Range:  760 to 1 Torr.
• Medium Vacuum Range: 1 Torr to 1X10^-1 Torr.
• High Vacuum Range: 1 X 10^-1 Torr to 1 X 10^-7 Torr.
• Ultra High Vacuum Range: 1X10^-7 Torr and stronger Vacuum Pressures.

 

PTOA Readers and Students learned all about the Torr unit of measurement in PTOA Segment #149.

Process Industries use vacuum generating technology to generate Vacuum Pressures in the "Rough Vacuum" and Medium Vacuum" ranges. Otherwise stated, industrial Vacuum Pressures range from 760 Torr to 1 X 10^-1 Torr.

In the nearby chart this range spans just from the third to the fourth hash mark on the X axis.

The "High Vacuum Range" stretches from the right side of the fourth hashmark on the X axis to the tenth hashmark on the X axis. The "Ultra High Vacuum Range" extends from the right side of the tenth hashmark on the X axis to the end of the X axis.

For process industries there is a more relevant way to gauge the strength of a Vacuum Pressure as shown on the nearby chart. PTOA Readers and Students need to remember that "Hg" is the symbol for the element "mercury."

The top horizontal line on the chart is Atmospheric Pressure measured as 0 inches Hg and 760 Torr (aka, 760 millimeters of Hg).

The bottom horizontal line on the chart represents a "Complete Vacuum" measured as 29.92 inches Hg and 0 Torr (aka, 0 mm Hg).

The scale on the left side of the vacuum chart illustrates an increasingly strong Vacuum Pressure in terms of inches Hg. The scale on the right side of the vacuum chart indicates an increasingly strong Vacuum Pressure in terms of Torr (even though the numbers descend in magnitude).

In the process industries, a Vacuum System may typically be designed to pull a 24 to 28 in Hg Vacuum Pressure.

Now … what about the Discharge PV Pressure of a Vacuum Pump?

The Vacuum Pump Discharge PV Pressure is  Atmospheric Pressure (1 atm).

Otherwise stated, every Vacuum Pump discharges the air or gas at the the top horizontal line in the chart … Atmospheric Pressure.

Unlike the Compressor, the Vacuum Pump does not compress the gas it moves at all!

The Vacuum Pump simply sucks in air or any gas... or a liquid … from a container and discharges the air or gas at 1 atm (Atmospheric Pressure, 14.7 psi).

The vacuum cleaner shown in the nearby picture is sucking up air and a solid … dirt... with a vacuum created by a Vacuum Pump. Then the dirt is discharged into the blue container which is at Atmospheric Pressure.

 

More Differences between Vacuum Pumping and Air Compression

"Vacuum pumping of air" and "air compression" have three distinct differences:

• In a Vacuum Pump the maximum change in Pressure (Discharge Pressure - Suction Pressure) cannot exceed 29.92 in of Hg which is 14.7 psi.
• The volume of air that is sucked into the Vacuum Pump on successive strokes decreases as the magnitude of the vacuum increases.
• Because the volume of air being sucked into the Vacuum Pump decreases as the magnitude of the vacuum increases, heat is dissipated through the Vacuum Pump structure and must be removed by heat exchange within the Vacuum System.

 

The distinct differences between vacuum pumping and air compression explained above reveal why Vacuum Pumps cannot be called "Vacuum Compressors."

However, there actually are some designs of mild Air Compressors which can be tweaked to operate as a Vacuum Pumps.

For Vacuum Systems that can be tweaked to operate either as a Compressor or Vacuum Pump, it has been determined that Compressor Mode requires four times the energy than Vacuum Pump Mode requires.

Why?

Because the Vacuum Pump is merely sucking gas from a closed volume and expelling the gas at 1 atm.

The Compressor is infusing energy in the form of elevated PV Pressure into the gas. Otherwise stated, the act of "compressing a gas" requires four times more energy than evacuating a gas.

 

 

SELECTING A VACUUM PUMP

The selection of a Vacuum Pump is strongly influenced by the operating environment.

The selection of a Vacuum Pump is also determined by the total volume of the Vacuum System from which the air or gas needs to be vacated because the target Vacuum Pressure must be established throughout the entire system.

The "Pump Down Time" is the time it takes to establish the target Vacuum Pressure throughout the entire Vacuum System.

"Pump Down Time" determines the Vacuum Pump speed.

Perhaps a single stage Vacuum Pump will suffice to establish the target Vacuum Pressure throughout the Vacuum System in a reasonable "Pump Down Time."

If not, then a Two-Stage or Multistage Vacuum Pump with an Ejector booster could be installed to establish the target Vacuum Pressure throughout the Vacuum System in a reasonable "Pump Down Time."

 

TYPES OF VACUUM PUMPS

Brilliant PTOA Readers and Students … meaning those who read the PTOA Segments in the intended, sequential order … already know about the Pump Family Tree.

Vacuum Pumps are included in the Reciprocating-Action PD Pump Family, the Rotary-Motion PD Pump family, and the Special Dynamic Pump Family.

 

Special Dynamic Vacuum Pumps

PTOA Readers and Students learned all about the Special Dynamic Pump family members called Ejectors and Eductors in PTOA Segment #208. Both of these Dynamic Special Pump designs use a "motive fluid" and the  "Venturi Effect" to create a Vacuum Pressure.

The descriptor "Ejector" infers that the motive fluid is a gas or vapor and that the Vacuum Pressure  is continuously generated 24/7 to evacuate a vessel or system.

The Steam Jet Ejectors on the top of a Vacuum Distillation Tower establish the Vacuum Pressure which allows the heavy crude fractions to boil at much lower temperatures than they would boil at Atmospheric Pressure. Three vertically-mounted Steam Jet Ejectors are visible in the nearby picture of a Vacuum Distillation Tower.

Vacuum Eductors have a wider range of service in the process industries spanning the food and beverage industries, chemical and petrochemical industries, pharmaceuticals, waste reduction, and the power generating and energy industry.

 

 

Reciprocating-Action Vacuum PD Pumps

The nearby Single Stage Bellows-Diaphragm Vacuum Pump gif is an example of a Reciprocating-Action PD Pump.

This Vacuum Pump sucks gas out of a closed system and delivers it to a confined space where it is trapped. The bellows-diaphragm extends into the chamber and displaces the trapped gas, exhausting it into the surrounding atmosphere.

Guess what?  Leakage out of a vacuum system is not a thing! There is no need for the seals which would be required in other types of piston pumps!

The Single Stage Bellows/Diaphragm Vacuum Pump can attain a 22 to 24 in Hg Vacuum Pressure.  Adding a second stage increases the magnitude of the vacuum to 28 in Hg.

 

Rotary-Motion Vacuum PD Pumps

Brilliant PTOA Readers and Students are all ready knowed-up on the following Rotary-Motion PD Pumps which can all be manufactured to perform as Vacuum Pumps:

• Straight Lobe and Helical Lobe Pumps are a version of Gear Pumps which were featured in PTOA Segment #212. Lobe Vacuum Pumps can pull a 22 in Hg vacuum and are commonly used in combination with Rotary Piston Vacuum Pumps, Liquid Ring Vacuum Pumps or Eductors to establish and sustain a stronger Vacuum Pressure throughout a Vacuum System. Such a combination of vacuum technology would have a high pumping capacity with good vapor handling capability.
• Helical Screw Pumps were recently featured in PTOA Segment #214 and can generate a 27 to 28.5 in Hg vacuum. The Helical Screw Vacuum Pump exhausts with pulseless action while vacating gas from a system.
• 

  Becker Rotary Vane Vacuum Pump

  Sliding Vane Pumps were featured in PTOA Segment #213. A Single-Stage Sliding Vane Vacuum Pump has one Rotor and Vane mounted on the drive shaft. The Single-Stage Sliding Vane Vacuum Pump can attain a 0.01 Torr vacuum.  When two Sliding Vane Vacuum Pumps are in series (aka a "Compound Sliding Vane Vacuum Pump") the generated vacuum increases to 0.0001 Torr. The maximum air displacement is 75 cubic feet per minute (cfm) at this Vacuum Pressure.

•  A  Rotary Piston Vacuum Pump can have a one, two, or three sets of cams and pistons. The more cams and pistons, the greater the magnitude of vacuum that can be pulled, typically 27 to 28.5 in Hg. The graphic below illustrates the Intake and Exhaust strokes of a Rotary Piston Vacuum Pump.

 

There is one more Rotary-Motion Vacuum Pump that has an interesting, unique feature which has not yet been described.

 

The Form and Function of a Liquid Ring Vacuum Pump

Truth be known, Liquid Ring Vacuum Pumps are difficult to visualize because they have a moving ring of liquid, typically water.

The movement of the rotating liquid ring is analogous to the reciprocating action of a piston in a cylinder.

The long, rotating Impeller blades that are attached to the eccentrically-placed Rotor move into and out of the liquid ring with each rotation.

Uh-oh. Fred is getting  understandably confused!

Just read a bit more and then it will be time to You Tube and Chill with a Nash Liquid Ring Vacuum Pump You Tube.

One design of a Liquid Ring Vacuum Pump has the Rotor mounted eccentrically within a circular Housing/Casing as shown in the nearby graphic.

A different design will have an elliptically-shaped Housing/Casing and a centered Rotor.

Whichever design is employed, the eccentricity of the Rotor with its bladed Impeller relative to the Housing/Casing is crucially important to the function of the Liquid Ring Vacuum Pump.

The eccentricity of the Rotor to the Housing/Casing is crucial to forming the rotating liquid ring and the rotating liquid ring is required to create the vacuum.

How does the Liquid Ring Vacuum Pump work?

The Rotor must first turn so that the rotating liquid ring will form. Then the air or gas that is sucked into the pump will be trapped between successive blades, the liquid ring and the Housing/Casing.

The eccentricity of the Rotor to the Housing/Casing makes it possible for the liquid ring to move away from the bladed Impeller, which creates the Vacuum Pressure.

As the Impeller continues to rotate, the wedge-shaped volume between two successive Impeller blades once again enters the liquid ring and the decreasing volume displaces the gas through the Discharge Port.

Single Stage Liquid Ring Vacuum Pumps can produce a 24 in Hg vacuum; the Two Stage Liquid Ring Vacuum Pump model can achieve a 28 in Hg vacuum.

The air or gas that is discharged from a Liquid Ring Vacuum Pump is discharged a a constant, non-pulsing delivery.

Okay, Fred …

It is time to You Tube and Chill with  the Nash Liquid Ring Vacuum Pump You Tube which can be accessed HERE or directly below.

At the one minute mark, this nifty Nash Liquid  Ring Vacuum Pump You Tube actually shows a liquid ring being formed in an experimental Liquid Ring Vacuum Pump.  Unfortunately, the crucial eccentricity of the Rotor and the liquid ring is not clearly discernable until the animation.

The animation more clearly shows the eccentricity of the Rotor relative to the liquid ring.

The gas that is sucked into the Vacuum Pump is captured between two successive Impeller blades and hence squeezed into a smaller and smaller volume until the gas is displaced and discharged at around the 5  o'clock position on the Rotor

Be certain to give this great Nash You Tube a THUMBS UP!

 

AND THAT'S ALL THE PTOA HAS TO SAY ABOUT PUMPS!

Next the PTOA Process Variable Pressure Focus Study Area will feature Dynamic and Positive Displacement Compressors which will be easy to learn because Compressor technology is similar to Pump technology!

 

TAKE HOME MESSAGES: Vacuum Pumps are called "Pumps" even though they are often used to move … or vacate... gases.

A Vacuum Pump differs from mild Compressor because:

• Their Suction Pressure is BELOW Atmospheric Pressure and their Discharge Pressure is AT Atmospheric Pressure. Otherwise stated, the Vacuum Pump "pulls a vacuum at the Suction."

• The above statement infers that the maximum change in Pressure in a Vacuum Pump (Discharge PV Pressure - Suction PV Pressure) is 29.92 in Hg or 14.7 psi.
• The volume of air that is sucked into the Vacuum Pump on successive strokes decreases as the magnitude of the vacuum increases.
• The Vacuum Pump does not add energy in the form of PV Pressure into the gas that flows through it. Otherwise stated, a Vacuum Pump cannot compress a gas but rather just vacates air or gas and discharges the air or gas at Atmospheric Pressure.

 

The Vacuum Pump must be sized to establish a target Vacuum Pressure in a Vacuum System.  The "Pump Down Time" is the time it takes to establish the target Vacuum Pressure throughout the Vacuum System.  The "Pump Down Time" will determine the Vacuum Pump speed.

The typical range of Vacuum Pressures used in the Process Industries range from 22 in Hg to 28.5 in Hg.

There are models of Vacuum Pumps in the Reciprocating-Action PD Pump Family, the Rotary-Motion PD Pump Family, and the Dynamic Special Pump Family (Ejectors and Eductors).

Liquid Ring Vacuum Pumps employ a rotating liquid ring and an eccentrically situated Rotor with Impellers to generate a 24 in Hg vacuum; a 28 in Hg vacuum can be established with a Two Stage Liquid Ring Vacuum Pump.

Many thanks to the Nash Pump Company for their You Tube which explained the form and function of the Liquid Ring Vacuum Pump.

PTOA Readers and Students have finished learning about Pumps and are almost done with the PTOA PV Pressure Focus Study Area.

 

©2020 PTOA Segment 0215

PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT PUMPS

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