Screw Vacuum Pump
Screw Vacuum Pump
What is a screw vacuum pump?
A screw vacuum pump is a dry rotary positive displacement pump utilizing two screw-shaped rotors to move air, powder, and fluid by creating a vacuum. One screw has a right-hand thread, and the other has a left-hand thread. A screw vacuum pump is considered dry because the pumping gas doesn’t require water or oil for sealing or lubrication in the vacuum stages. Instead, dry pumps rely on the precise clearances between the pump’s rotary and static components. However, a screw vacuum pump is not entirely oil-free, as gears and bearings lubrication is usually done using oil or grease. The screw vacuum pump is also rotary because the screws rotate in opposite directions at high rotational speeds and intermesh with tight clearances. The rotation is synchronized through a precision gear drive arrangement, with each screw rotor mounted to a separate shaft. Positive displacement means the pump works by mechanically trapping a volume of gas and moving it through the pump, creating low pressure on the inlet side.
Figure: showing the configuration of the screw vacuum pump
Due to the opposite rotation of both screws, the chamber connected with the suction port enlarges, and the gas is transported into the compression chamber. The transfer gas moves axially along the screws straight from the inlet to the outlet. The screw vacuum pump’s dry and non-contacting operation eliminates the need for lubrication in the pumping chambers. Consequently, the dry vacuum design ensures that the pumping medium is not contaminated or polluted during operations. Screw vacuum pump manufacturers design them to safely and reliably transfer a variety of mediums such as corrosives, organics, inorganics, and solvents because of their oil-free, non-contacting screw configuration. Screw a vacuum pump apply in distillation, transformer drying, house vacuum, filtration, handling problem gasses (such as flammable and corrosive gases), sterilization, evaporation, etc.
Components of a screw vacuum pump
The electric motor is the source of the mechanical power that runs a screw vacuum pump. The electric motor can run using an alternating current (AC) or Direct current (DC).
The pump casing contains the screws, bearing housing, and any other pump’s internal component. The pump casing is usually sealed to prevent fluid leakage under pressure. The pump casing has two types: axially split or radially. In an axially split casing, the two halves open parallel to the screw axis, while in a radially split case, the two halves open perpendicularly to the screw axis. A screw vacuum pump uses an axially split pump casing since it’s easy to maintain and provides excellent hydraulic performance. Screw vacuum pump manufacturers create pump casings from a wide range of materials, including grey cast iron, carbon steel, alloy steel, and stainless steel.
Helical screw-shaped rotors
A screw vacuum pump comprises two parallel non-contacting screw-shaped rotors (one driver and one driven screw) that rotate synchronously through a precision gear. The driver screw (male rotor) rotates the driven screw through a timing gear, helping to move the fluid at a constant volumetric level at any location. The driven screw (female rotor) turns about its axis due to the torque transmitted by the timing gear. The driven and driver screws rotate in opposite directions while maintaining a fixed clearance area, which helps to move the fluid along the rotating angle and provides a non-pulsating discharge. Screw vacuum pumps use variable pitch rotors, which help to compress the fluid at each pitch change while transporting it to the outlet. The screw vacuum pump manufacturers use high-tensile steel and alloy steel to manufacture the screws. The rotors are usually coated with spray-on polytetrafluoroethylene to prevent corrosion.
The driving shaft connects the pump assembly with the electric motor via flexible couplings. Screw vacuum pump manufacturers make driving shafts from mild steel, stainless steel, or carbon steel to withstand heavy loads, high speeds, and vibration, which can cause extreme stress during operation.
The primary function of a timing gear is to transmit rotary motion from the drive screw to the driven screw. It also helps to maintain a tight clearance between the drive and driven screw so that the screws do not come into direct contact during operation.
The bearings support the screws and ensure they align perfectly in the pump assembly. A screw vacuum pump comprises two kinds of bearing: upper and lower bearings. The upper bearings mainly carry heavy loads during pumping, including axial and radial forces. On the other hand, the lower bearings bear minor loads and help maintain a perfect alignment of the pump’s components.
Suction and discharge ports
The suction port connects to the suction pipe, permitting fluid entry into the pump, while the discharge port connects to the discharge pipe directing the pumping fluid out of the pump and to the required destination. Suction and discharge pipes are attached to the pump casing via flanges or pipe unions.
Pressure relief valve
A crew vacuum pump accumulates a lot of pressure during operation, which may destroy the pump’s components or put the pump operator at risk of injuries. A pressure relief valve is thus used to return the extra pressure to the intake, thereby protecting the pump and the operator.
It removes contaminants from the pumped gas stream before reaching the pump. Screw vacuum pump manufacturers design a dust separator to filter dust and other pollutants with a size larger than the rotor/stator clearance.
It is a stainless-steel muffler located at the pump outlet to reduce noise generated to an acceptable level.
Thermostatic control valve
It is to control the temperature by automatically varying the cooling water flow rate. It works together with a thermostatic switch which stops the pump to prevent damage if the pump temperature reaches a preset limit.
Figure: showing the components of the screw vacuum pump
How does a screw vacuum pump work?
When the pump is started, the screws turn synchronously in opposite directions at high rotational speeds inside the pump casing and without touching. As the screws counter-rotate, voids of increasing volume are created in front of the rotor chamber, drawing in the transfer medium. Compartments of the transfer medium are trapped within the spaces between the rotors and the housing and are transported to the discharge, with each turn of the thread serving as a pump stage. Since the screw threads are tapered at the discharge end, they compress the trapped medium and transport it to the discharge port, where it discharges against atmospheric pressure. Pump cooling can be achieved by circulating water or air in the cooling jacket.
Types of screw vacuum pump
Single-ended screw vacuum pumps
In a single-ended screw vacuum pump, the pumping medium (gas) enters through one end and exits from the other.
Figure: showing the configuration of a single-ended screw vacuum pump.
Double-ended screw vacuum pump
In the double-ended screw vacuum pump, the transfer fluid enters in the middle and exits from both ends of the pump
Figure: showing a double-ended screw vacuum pump
The screw vacuum pump can be classified according to the vacuum generated as follows:
High vacuum: 7.5-3 to 7.5-7 Torr
Utara high vacuum: 7.5-7 to 7.5-11 Torr
Extreme high vacuum: < 7.5-11 Torr
Applications of screw vacuum pump
Screw vacuum pumps are designed to cater to a wide range of industries, such as medicals, chemicals, water treatment, packaging, nuclear power, pharmaceuticals, etc. Typical uses include the following:
- Screw vacuum pumps are used in distillation (normal, short path & molecular).
- They are used for house vacuum (central or general/laboratory vacuum service, pilot plants).
- They are also used in reactor service.
- They are used for transformer drying.
- They are used for evaporation and filtration purposes.
- Screw vacuum pumps are also used in solvent recovery (fuel vapor).
- They are also used in sterilization (ethylene oxide).
- They are also used for pumping flammable, low auto-ignition temperatures and corrosive gasses.
- They are used to convey corrosives, organics, inorganics, and solvents.
Advantage of screw vacuum pump
- Screw vacuum pumps are perfect for applications requiring contamination-free operation.
- Its operation is quiet and free of vibration and pulsation, even at high speed.
- It is highly energy efficient and reliable.
- The pump runs dry without any damage. It eliminates the need for lubrication in the pump stages.
- Its robust bearing design compensates for the axial loads, while all radial loads are self-compensated.
- It can handle gases containing a high level of moisture or other vapors.
- It can manage extremely high fluid flow rates.
- The screw vacuum pump has compact constructions and sizes, facilitating easy installation and maintenance.
- It has good vapor pumping characteristics because the high operating temperature reduces the condensation of the pumped vapors.
- A screw pump can auto-ignition when flammable gases are processed because of the high operating temperature.
Disadvantages of screw vacuum pump
- Screw vacuum may become extremely hot with discharge temperatures reaching 350° C (662° F) because it doesn’t use water or oil in the pumped gas stream.
- The high operating temperature of screw vapor may pose the risk of vapor polymerization on screw surfaces which can cause pump failure.
- The screw vacuum pump requires continuous purging of the pump bearings to prevent premature failure due to extreme heating.
- Thermal growth on the rotor may affect the tight clearance leading to backflow (gas leakage).
- A screw vacuum pump is expensive relative to other pumps.
Troubleshooting screw vacuum pump
High absorbed power
- Wrong pump speed. Check that the pump speed matches the design.
- The suction filter is blocked. Inspect, clean, or replace the filter.
- Pump parts are contacting during operation. Inspect, repair, or replace contacting parts.
Low capacity or loss of flow
- Low pump speed. Check that the motor speed is correct according to the screw vacuum pump manufacturer’s guide.
- The motor rotation direction is wrong. Ensure motor rotation direction matches the direction indicated by the pump manufacturer on the nameplate
- Worn-out screws. Inspect and replace screws if it is necessary
The pump is vibrating and noisy
- Some internal components are worn out or in contact during operation. Inspect the pump for worn-out or in-contact parts and repair or replace them if necessary.
- There is solid ingress. Inspect whether the pump contains particles of size exceeding the screw vacuum pump manufacturer’s specification.
- The relief valve is opening and closing rapidly. Increase the relief valve set pressure as directed in the pump manufacturer’s guide.
Loss of suction
- The suction filter is clogged. Check the inlet filter and clean or replace it.
- There is a system blockage. Inspect suction pipes for blockage
Low discharge pressure
- Worn-out screw. Inspect the screw for wear and replace it if it is necessary
- Low NPSH. Ensure the pump’s NPSH meets the screw vacuum pump manufacturer’s specifications.
- The bypass valve is in operation. Check the bypass valve operating pressure and adjust it.
A screw vacuum pump is a dry rotary positive displacement pump that uses two screw-shaped rotors to move air, powder, and fluid by creating a vacuum. The rotation of the drive screw and driven screw is synchronized through the use of precision gear, and they intermesh with tight clearances. As the screw rotates in the opposite direction, their threads intermesh, creating voids that trap the pumping medium and move it axially from the suction to the discharge port. Screw vacuum pump manufacturers design them to safely and reliably transfer a variety of mediums such as corrosives, organics, inorganics, and solvents because of their oil-free, non-contacting screw configuration. Screw a vacuum pump apply in distillation, transformer drying, house vacuum, filtration, handling problem gasses, etc. Screw vacuum pumps are perfect for contamination-free applications and provide quiet, energy-efficient, reliable operation. However, they may overheat during the process, and the high operating temperatures may cause vapor polymerization or thermal growth on the rotor, leading to pump failure.