Three minutes to let you know the truth about vacuum pumps

A vacuum pump refers to a device or equipment that uses mechanical, physical, chemical, or physicochemical methods to extract air from the container being pumped and obtain a vacuum. Simply put, a vacuum pump is a device that uses various methods to improve, generate, and maintain vacuum in a closed space.

 

According to the working principle of vacuum pumps, they can be basically divided into two types, namely gas capture pumps and gas transfer pumps. Widely used in industries such as metallurgy, chemical industry, food, and electronic coating.

 

Structural features:

(1) Overall pump result type

The arrangement structure of the vacuum pump body determines the overall structure of the pump.

 

The vertical structure of the intake and exhaust ports is horizontally arranged, making assembly and connection of pipelines more convenient. But the center of gravity of the pump is relatively high, and its stability is poor during high-speed operation, so this type is mostly used for small pumps.

 

The intake port of the horizontal pump is located above and the exhaust port is located below. Sometimes, for the convenience of installing and connecting vacuum system pipelines, the exhaust port can be connected horizontally, meaning that the inlet and outlet directions are perpendicular to each other.

 

At this point, the exhaust port can be opened from both left and right directions, except for one end of the exhaust pipeline, the other end can be blocked or connected to a bypass valve. This pump structure has a low center of gravity and good stability during high-speed operation. This structure is commonly used for large and medium-sized pumps.

 

The two rotor shafts of the pump are installed perpendicular to the horizontal plane. This structure has easy control of assembly clearance, convenient rotor assembly, small pump footprint, but high pump center of gravity and inconvenient gear disassembly and assembly, and relatively complex lubrication mechanism.

 

(2) Pump transmission mode

The two rotors of the vacuum pump operate relatively synchronously through a pair of high-precision gears. The driving shaft is connected to the motor through a coupling.

 

There are mainly two types of transmission structure layout: one is that the electric motor and gear are placed on the same side of the rotor. The driven rotor is directly driven by the motor end gear, so that the deformation of the active rotor shaft is small, and the gap between the two rotors will not change due to the large torsional deformation of the active shaft, so the gap between the rotors is uniform during operation.

 

The advantages and disadvantages of this transmission method are: a. There are three bearings on the driving shaft, which increases the difficulty of pump processing and assembly, and the disassembly and adjustment of gears are also inconvenient; b. The overall structure is uneven, and the center of gravity of the pump leans towards the side of the motor and gearbox.

 

Features:

(1) There is a large pumping speed within a wide pressure range;

 

(2) The rotor has good geometric symmetry, resulting in low vibration and smooth operation. There is a gap between the rotors and between the rotor and the housing, which does not require lubrication and has low friction loss, which can greatly reduce the driving power and achieve higher speeds;

 

(3) The pump chamber does not require oil sealing and lubrication, which can reduce the pollution of oil vapor on the vacuum system;

 

(4) There is no compression or exhaust valve in the pump chamber. Simple and compact structure, insensitive to dust and water vapor in the extracted body;

 

(5) Low compression ratio, poor hydrogen extraction effect;

 

(6) The surface of the rotor is a complex curved cylindrical surface, which is difficult to process and inspect;

 

Maintenance:

The quality of a vacuum pump depends on its structure and the quality of the oil, and it must be protected when using a vacuum pump.

 

If volatile organic solvents are distilled, they will be absorbed by the oil, resulting in an increase in vapor pressure and a decrease in vacuum efficiency. If it is an acidic gas, it will corrode the oil pump. If it is water vapor, it will cause the oil to become an emulsion and damage the vacuum pump.

 

Therefore, when using a vacuum pump, the following points must be noted:

An absorption device must be installed between the distillation system and the vacuum pump.

 

Before distillation, the organic solvent vapor in the system must be thoroughly removed using a water pump.

 

If using a water pump for air extraction, try to use a water pump as much as possible. If there are volatile substances in the distillate, you can first use a water pump to reduce pressure and then switch to a water pump.

 

The pressure reducing system must be kept airtight, and all rubber stoppers must be of appropriate size and holes. Rubber tubes should be vacuum sealed. Apply vacuum grease to the ground glass.

 

According to the scope of use and pumping efficiency, vacuum pumps can be divided into three categories:

(1) A general water pump with a pressure of 1.333 to 100kPa (10 to 760mmHg) is considered a "coarse" vacuum.

 

(2) The pressure of the oil pump can reach 0. Good, good. 3 Pa (0.001 good 1mmHg) is considered a "second high" vacuum.

 

(3) Diffusion pump, pressure can reach below 0. Good Pa, (10-3mmHg) is a "high" vacuum.

 

If you want a lower pressure, you need to use an oil pump. A good oil pump can pump below. 3Pa (1mmHg).

 

In organic chemistry laboratories, there are two commonly used vacuum pumps: water pumps and vacuum pumps. If low pressure is not required, a water pump can be used. If the water pump is well constructed and the water pressure is high, the suction efficiency can reach 1067 to 3333Pa (8 to 25mm Hg).

 

The good pressure that a water pump can extract is theoretically equivalent to the steam pressure at the current water temperature. For example, when the water temperature is 25 ℃, 20 ℃, and 10 ℃, the pressure of water vapor is 3192, 2394, and 1197Pa (8-25mmHg), respectively.

 

When using a water pump for air extraction, a safety bottle should be installed in front of the water pump to prevent water pressure from falling and allowing water to flow backwards; Before stopping the pumping, release the air first and then turn off the water pump.

 

Lubricating oil:

In a vacuum pump, vacuum pump oil not only serves as a medium to obtain vacuum, but also plays a role in lubricating, cooling, and sealing mechanical friction points.

 

This is the most important performance of vacuum pump oil. Due to the high vacuum degree required by vacuum pumps, paraffin based narrow fraction lubricating oil is generally used. For diffusion vacuum pumps, low vapor pressure silicone oil or other synthetic oils can also be used.

 

The volume inside the vacuum pump chamber constantly changes to form an exhaust effect, requiring the lubricating oil to have appropriate viscosity and viscosity temperature characteristics.

 

Vacuum pumps are constantly developing at a high speed. Due to the high-speed friction between the sliding blades and the pump body, the oil temperature increases, making it easy for the oil to oxidize and decompose. Especially, diffusion pumps often work in high temperature environments, causing an increase in steam pressure in the system and a decrease in vacuum degree. Therefore, it is required that the vacuum pump oil has good thermal oxidation stability.

 

If the gas sucked in by the vacuum pump is corrosive, it will react chemically with oil and corrode the parts inside the pump; Inhalation of air often contains condensed water vapor, which causes emulsification of vacuum pump oil and corrosion of metals. Therefore, good corrosion resistance and emulsification resistance are required.

 

The main requirement is that the vacuum pump does not carry any lightweight components to avoid affecting the saturated vapor pressure of the oil.

 

Extreme pressure is an important performance indicator for vacuum pump oil, in order to understand the maximum pressure of a vacuum pump under good vacuum extreme pressure.

 

High motor temperature:

A certain company's 300MW unit is equipped with two mechanical vacuum pumps per unit, one in operation and one as backup.

 

The vacuum pump is used for vacuuming during unit startup and for extracting uncondensed gas from the condenser during normal operation.

 

The pump is equipped with a motor of 160KW, a speed of 590r/min, a rated current of 330A, a voltage of 380V, B-level insulation, and a normal operating current of 220-230A.

 

Every summer, the motor temperature exceeds the limit, so it is good to install temporary cooling fans, but the harvest is not significant.

 

Long term high-temperature operation of motors can cause insulation aging and shorten their service life. The analysis of the reasons for the high temperature of the vacuum pump motor is as follows.

 

Cause analysis:

(1) The motor has high power, high working current, and generates a large amount of heat.

 

(2) The fan speed is low, the air pressure is low, and the air volume is small.

 

(3) The number of fan blades is small, resulting in a lower air volume.

 

(4) The electric motor is contaminated with dust and oil, which reduces its heat dissipation capacity.

 

(5) The voltage of the bus where the vacuum pump motor is located is 380V. Due to uneven cable voltage drop and load distribution, the actual voltage applied to the motor is only 365V, resulting in a low voltage and high operating current.

 

Countermeasures——

The power and speed of the motor are matched with the vacuum pump and cannot be changed.

 

The fan is installed on the motor spindle, and the motor speed determines the fan speed and cannot be replaced.

 

Although increasing the number of fan blades can have a certain effect, increasing the number of blades makes it difficult to find dynamic balance. If not properly aligned, it can cause an increase in motor vibration.

 

(1) Extend the original fan cover by 40cm and install an axial flow fan with the same diameter as the fan cover inside. The motor power of the axial flow fan is 850W, the speed is 1489r/min, and the voltage is 380V.

 

The original fan will continue to be retained, and an additional power supply control will be installed for the axial fan. The axial fan will not be interlocked with the main motor.

 

Start the axial flow fan in a timely manner after the vacuum pump is started, and stop the axial flow fan 30 minutes after the vacuum pump stops to ensure sufficient cooling of the main motor;

 

(2) Regularly remove dust from the motor, keep the motor heat sink clean, and increase its heat dissipation capacity;

 

(3) Adjust the voltage of the bus where the vacuum pump is located to 400V.

 

Effect:

(1) Due to the high speed and high air pressure of the axial fan, the cooling effect is greatly enhanced. Under the same ambient temperature and load current, the temperature of the main motor drops by 12 ℃. The temperature of the main motor in summer has not exceeded the limit again.

 

(2) The axial flow fan can be manually controlled, and after the main motor is stopped, the axial flow fan can still operate, allowing the main motor to receive sufficient cooling.

 

(3) Try to balance the load distribution of the two bus sections as much as possible to prevent excessive voltage drop caused by excessive load on a certain bus section.

 

(4) After voltage adjustment, the operating current of the vacuum pump decreases to 210A, and the heat generated is relatively reduced.

 

(5) Slowing down the insulation aging speed of the main motor and extending its service life.