Roots pumps and screw pumps are rarely compared together, but they have many similarities and also differences.
Roots pumps usually refer to Roots vacuum pumps, but there are also LC Roots pumps that transport liquids, which can transport high-viscosity media, and screw pumps can replace Roots pumps in this respect.
There is also a type of Vacuum screw pump that can be used for gas transportation, so what are the similarities and differences between it and the Roots vacuum pump? This article will compare Roots pumps and screw pumps in the following aspects:
- What is a Roots pump?
- What is the difference between a screw vacuum pump and a twin screw pump?
- What is the difference between a screw vacuum pump and a Roots vacuum pump?
- Screw pump and LC Roots pump
- Why are screw pumps gradually replacing LC Roots pumps for conveying high-viscosity media?
What is a Roots pump?
There are two types of Roots pumps, one is the Roots pump that transports liquids, called LC Roots pumps; the other is Roots vacuum pumps, which are mainly used to transport gases.
1.1 Roots pump for liquid delivery
The oil delivery Roots pump is essentially the same as the rotary lobe pump, but the rotor is Roots type. Its rotor is the same as that of the cam pump, there are both double-lobe, three-lobe, and multi-lobe. There are many kinds of shapes. It can transport liquids with a viscosity of tens of thousands of centipoise.
The Roots pump mainly has two rotors with opposite rotation directions located in the pump body, which are driven by a pair of synchronous gears. The rotors of the Roots pump mesh with each other in the pump body, but there is a gap. The size of the gap depends mainly on the viscosity of the liquid. If the viscosity exceeds a certain range, the gap must be adjusted to increase.
1) LC roots pump working principle
LC Roots pump is a liquid power machine, which can compress liquid from a low-pressure state to a high-pressure state, and make the liquid move under a fixed pressure difference. Its working principle is: the liquid is injected into the working chamber of the pump from the container through the inlet pipe.
Due to the movement of the rotating shaft, the liquid in the working chamber will be driven by the rotating central shaft, and the liquid will be squeezed into the outlet pipe through the volume change, and the liquid will be injected into the pressure container through the outlet pipe to achieve the purpose of pressure rise.
In the operating principle of the LC Roots pump, the liquid flows from the inlet pipe of the pump into the pump working chamber, is squeezed to the outlet pipe under pressure due to the volume change, and finally flows into the pressure container from the outlet pipe, so as to achieve the purpose of liquid pressure lift.
There is usually an impeller inside the LC Roots pump. When the shaft rotates, the inflowing liquid will be stirred by the blades of the impeller to form a vortex flow. After passing through the blades distributed around the shaft, the blades at the rear end of the impeller will exert friction on the liquid. force. As the liquid moves between the vanes, friction, and volumetric changes exert pressure on the liquid, causing it to flow toward the outlet of the impeller.
In addition, LC Roots pumps are generally equipped with two safety valves, one inlet safety valve, and one outlet safety valve. When the pressure of the pump exceeds the set upper limit, the outlet safety valve will automatically open to let the liquid flow generated by the pump go out, in case the pressure of the pressurized container exceeds its tolerance; and when the pressure of the pump is lower than the set When the lower limit is reached, the inlet safety valve will open to prevent the pressure of the inlet liquid from being too low and protect the pump components from damage.
When using the LC Roots pump, first of all, make sure that the pump shaft and drive shaft are in a balanced state, so as to ensure the normal operation of the pump; secondly, check the seal roll of the pump regularly to ensure that there will be no leakage in the pump system; finally, also Regularly check whether the shaft of the pump is deformed to prevent excessive friction and affect the normal operation of the pump.
LC Roots pump is one of the most commonly used liquid pumps at present. Because of its simple structure, low noise, high-temperature resistance, corrosion resistance, energy saving, and other advantages, it is widely used and favored by the majority of users. In addition, when using the Roots pump, attention should be paid to maintenance and regular inspection to ensure the normal operation of the pump.
2) What are the characteristics of the LC Roots pump
This pump has the characteristics of low rotation speed, high efficiency, small volume, large flow rate, low power consumption, strong self-priming ability, no need to draw oil, and convenient use.
a) The Roots pump has a compact structure, few parts, easy maintenance and operation, and reliable operation.
b) The relative sliding speed of the Roots pump rotor is very low, the operation is stable, the wear is small, the noise is small, and the service life is long.
c) The area of the suction side of the Roots pump is large, the volume between the teeth changes slowly, the pressure pulsation is small, and the suction capacity is high, up to 5m.
d) The flow rate of the Roots pump is directly proportional to the speed, and the flow rate can be automatically adjusted with the help of the governor.
e) The Roots pump is equipped with a heat preservation jacket, which can heat the pump through heat preservation steam or heat transfer oil, and the heat preservation temperature can reach 350°C.
f) The machining precision of the Roots pump is high, and the volumetric efficiency is greater than 85%.
3) Application of LC Roots pump
The pump can be widely used to transport petroleum and petroleum products (gasoline can be replaced with copper pump blades) and various oils and liquids in factories. Such as crude oil, paint, lubricating oil, glass glue, fatty acid, toothpaste, and light alkaline (soap processing) liquids, etc.; food processing can transport various liquids, such as soybean milk, yeast, syrup, etc. The loading and unloading of various oil tankers and tankers is fast, efficient, easy to use, and can work in forward and reverse directions.
1.2 Roots vacuum pump
A roots vacuum pump refers to a variable displacement vacuum pump that is equipped with two leaf-shaped rotors that rotate synchronously in opposite directions.
Roots vacuum pumps have been tested for long-term operation in petroleum, chemical, plastic, pesticide, steam turbine rotor dynamic balance, aerospace space simulation, and other devices. At the same time, it is also widely used in the petroleum, chemical industry, metallurgy, textile, and other industries. Vacuum pump accessories are used as vacuum pump silencers for noise control of vacuum pumps.
1) Working principle
The Roots pump is a vacuum pump without internal compression, usually with a very low compression ratio, so high and medium vacuum pumps need a backing pump.
A vacuum pump relies on the synchronous and counter-rotating pushing action of a pair of leaf-shaped rotors in the pump chamber to move the gas to achieve air extraction.
Roots vacuum pump refers to a mechanical vacuum pump with a pair of synchronous high-speed rotating shoe-sole-shaped rotors. This pump cannot pump air alone. The front stage needs to be equipped with oil seals, water rings, etc. to directly discharge the atmosphere.
Its structure and working principle are similar to the Roots blower, and its suction port is connected to the vacuumed container or the main pump of the vacuum system when it works. There is no contact between the rotor and the rotor, between the rotor and the pump casing of this vacuum pump, and the gap is generally 0.1 to 0.8 mm; no oil lubrication is required.
Rotor profiles include arc lines, involutes, and cycloids. The volume utilization rate of the involute rotor pump is high, and the machining accuracy is easy to guarantee, so the involute type is mostly used for the rotor profile. The speed of the Roots vacuum pump can be as high as 3450-4100 rpm; the pumping rate is 30-10000 liters/second (1 liter=10-3 m3); the ultimate vacuum: single-stage is 6.5×10-2 Pa, double-stage It is 1×10-3 Pa.
The ultimate vacuum of the Roots vacuum pump depends not only on the structure and manufacturing accuracy of the pump itself but also on the ultimate vacuum of the backing pump. In order to improve the ultimate vacuum of the pump, Roots pumps can be used in series.
Schematic Diagram
2) Performance characteristics
The characteristics of the Roots vacuum pump are a fast start, low power consumption, low operation and maintenance costs, high pumping speed, high efficiency, insensitivity to a small amount of water vapor and dust contained in the pumped gas, within the pressure range of 100 to 1 Pa With a large pumping rate, it can quickly remove the gas released suddenly.
This pressure range is exactly between the oil-sealed mechanical vacuum pump and the diffusion pump. Therefore, it is often connected in series between the diffusion pump and the oil-sealed mechanical vacuum pump to increase the pumping capacity in the intermediate pressure range. At this time it is also called a mechanical booster pump.
Roots vacuum pumps are widely used in smelting, degassing, and rolling in vacuum metallurgy, as well as in vacuum distillation, vacuum concentration, and vacuum drying in chemical, food, and pharmaceutical industries. Vacuum pump accessories are vacuum pump silencers for vacuum pump noise control.
What is the difference between a screw vacuum pump and a twin screw pump?
In the previous article, we mentioned in detail what is a twin-screw pump
Then, what is a screw vacuum pump? and what is the difference between it and a twin-screw pump?
2.1 Similarities
1) Both twin-screw pumps and screw vacuum pumps are positive displacement pumps;
2) Both the synchronous gears drive the two screws to run;
3) Can contain a small number of solid impurities;
4) The installation types are vertical, horizontal, and skid-mounted.
2.2 Differences between the screw vacuum pump and twin screw pump
1) The conveying medium is different
The screw pump mainly transports liquid, while the screw vacuum pump mainly transports gas, and the transport medium is obviously different.
2) The components are different
The pump casing of the screw vacuum pump is a double-layer casing structure. The front and rear outer layers are metal pump casings, and the inner casing includes a volute, a front fender, and a rear fender. The front volute and the rear volute can be made of high-chromium alloy cast iron or rubber. The impeller is composed of blades, back blades, front cover, and rear cover.
The twin-screw pump is mainly composed of a pump body, a bushing, a spiral sleeve, and a driving and driven shaft.
3) Different application conditions
Twin screw pumps are mainly used in
Oil field: used as a multi-phase mixed pump for oil, gas, water, trace fine solid particles, etc., and a crude oil pump;
Shipbuilding industry: used as marine loading pump, ship bottom stripping and sewage treatment, main engine lubrication pump, fuel oil pump;
Petrochemical industry: used as loading and transport pumps for various resins, pigments, paraffin, paints, inks, latex, various oil products, crude oil, heavy oil, etc.;
Thermal power plants: used as heavy oil, crude oil delivery pumps, main engine lubricating oil pumps;
Food industry: used as a delivery pump for alcohol, honey, syrup, fruit juice, animal and vegetable oil, milk, soy sauce
Screw vacuum pumps are mainly used for dry vacuum pumps, mainly used in the semiconductor industry, chemical industry, thin film industry, petrochemical, food, and other processes such as evaporation, drying, polymerization, sublimation, distillation, rectification, negative pressure concentration, etc.
What is the difference between a screw vacuum pump and a Roots vacuum pump?
1.1 Different performance characteristics
The screw vacuum pump has a simple and compact structure, does not require any working medium, has no oil pollution, no sewage discharge, and has remarkable energy saving and emission reduction effects.
The Roots vacuum pump starts quickly, consumes less power, has low operation and maintenance costs, high pumping speed and high efficiency, and is insensitive to a small amount of water vapor and dust contained in the pumped gas;
1.2 The scope of the application is different:
Screw vacuum pumps are widely used in pharmaceutical, chemical, semiconductor, and other fields that require highly clean vacuum.
Roots vacuum pumps are widely used in smelting, degassing, and rolling in vacuum metallurgy, as well as in vacuum distillation, vacuum concentration, and vacuum drying in chemical, food, and pharmaceutical industries.
1.3 Different working principles
The screw vacuum pump uses a pair of screws to perform synchronous high-speed reverse rotation in the pump casing to generate suction and exhaust.
The Roots vacuum pump is equipped with two leaf-shaped rotors that rotate synchronously in opposite directions. There is a small gap between the rotors and between the rotor and the inner wall of the pump casing without contacting each other.
Screw pump and LC Roots pump
In terms of liquid transportation, both the screw pump and the LC Roots pump have strong adaptability to the viscosity of the medium and are especially suitable for conveying high-viscosity medium.
They all have a good self-priming ability and can transport media with particulate impurities;
Why are screw pumps gradually replacing LC Roots pumps for conveying high-viscosity media?
1) Medium viscosity
The viscosity range of the medium conveyed by the screw pump is higher
2) Maintenance cost
The LC Roots pump has a large volume, and the maintenance cost is much higher than that of the screw pump
3) Speed
The rotation speed of the LC Roots pump is 50r/min–500r/min, and an additional reducer is required to reduce the speed when conveying high-viscosity media.
The screw pump has a lower speed, does not need a reducer, and is more energy-efficient when transporting high-viscosity media.
Therefore, companies that use LC Roots pumps as viscous liquid delivery pumps can give priority to replacing them with screw pumps to save energy, reduce emissions, and reduce costs.
