Vacuum Pumps - Dry Vs Liquid Ring

Vacuum is any system of reduced
pressure, relative to local (typically atmospheric) pressure. Achieved with a pump, vacuum systems are commonly used to

• Remove excess air and its constituents.
• Remove excess reactants or unwanted byproducts.
• Reduce the boiling point.
• Dry solute material.
• Create a pressure differential for initiating transport of material

Liquid-ring and dry pumps offer the most advantages for the chemical process industries (CPI). Both of these pump types have bearings sealed off from the pumping chamber and do not require any internal lubrication because the rotors do not contact the housing. Both, when employing a coolant system, prevent the coolant from contacting the process fluid and causing contamination, and both use mechanical shaft
seals for containment.

Liquid Ring Pumps
In the cylindrical body of the pump, a sealant fluid under centrifugal force forms a ring against the inside of the casing (See Figure below).



The source of that force is a multi-bladed impeller whose shaft is mounted so as to be eccentric to the ring of liquid. Because of this eccentricity, the pockets bounded
by adjacent impeller blades (also called buckets) and the ring increase in size on the inlet side of the pump, and the resulting suction continually draws gas out of the vessel being evacuated. As the blades rotate toward the discharge side of the
pump, the pockets decrease in size, and the evacuated gas is compressed, enabling its discharge.

The ring of liquid not only acts as a seal; it also absorbs the heat of compression, friction and condensation. Popular liquid choices include water, ethylene glycol, mineral oil and organic solvents.

Advantages

1. Simpler design; employs only one rotating assembly.


2. Can be fabricated from any castable metal.


3. Little increase in the temperature of the discharged gas.


4. No damage from liquid or small particulates in the process fluid.


5. Maintenance and rebuilding are simple.


6. Slow rotational speed (1,800 rpm or less), maximizing operating life.


7. Can use any type of liquid for the sealant fluid in situations where mingling
   with the process vapor is permissible.


8. No lubricating liquid in the vacuum chamber to be contaminated.


9. Accommodation of both condensable vapors and noncondensables, while operating as both a vacuum pump and condenser.

Disadvantages

1. Mixing of the evacuated gas with the sealing fluid.


2. Risk of cavitation requires a portion of process load to be noncondensable under operating conditions.


3. High power requirement to form and maintain the liquid ring, resulting in large motors. Therefore consume more energy.


4. Achievable vacuum is limited by the vapor pressure of sealant fluid at the operating temperature. Hence require Chilled water & hence operating cost is higher than dry pumps.

Screw Pump
Rotary-screw pumps dominate as dry pumps in the CPI, particularly in larger-size pump applications. Other type of dry pumps are Rotary-claw and rotary-lobe.

Two long helical rotors in parallel rotate in opposite directions without touching, synchronized by helical timing gears. Gas flow moves axially along the screw without any internal compression from suction to discharge. Pockets of gas are trapped with in the convolutions of the rotors and the casing, and transported to the discharge.




Compression occurs at the discharge port, where the trapped gas must be discharged
against atmospheric pressure. Each convolution of the rotor acts similarly to a stage in series with the one behind it; at least three convoluted gas pockets in the rotor are required to achieve acceptable vacuum levels.

See the figure & saving nos in my previous article - Guaranteed Energy Saving in Vacuum Pump.

Advantages

1. Rugged rotor design, constructed of sturdy cast or ductile iron without any flimsy rotating components.


2. Noncontact design facilitated by timing gears.


3. High rotational speed reduces the ratio of gas slip to displacement, increases net pumping capacity and reduces ultimate pressure.


4. Multiple staging provides inlet pressures below 1-mm Hg absolute while discharging to atmosphere.


5. No contamination of evacuated gas & so process vapors can be recovered thru downstream condensation. Very cost effective in such situations.


6. Due to lack of condensation, pump can be fabricated of standard, inexpensive cast iron.

Disadvantages

1. Cannot handle particulate matter, nor large slugs of liquid.


2. May discharge gases at high temperatures.


3. Most difficult to repair or rebuild.


4. May require a gas purge for cooling, or to protect the bearings and seals from the process gas.


5. Due to high operating temperatures,some process gases may polymerize.

Source: CHE Fact Sheet