# Vacuum pump basic knowledge and selection guidance

Vacuum pump of basic knowledge

1. The concept of vacuum

The word “vacuum” comes from the Latin word “vacuum”, meaning “empty” or “empty”. Vacuum refers to the gas state lower than the ambient atmospheric pressure in a given space, that is, the density of gas molecules in the space is lower than the density of gas molecules under the atmospheric pressure in the region, not the space without matter. Water ring vacuum pumps are used in low vacuum (105-103 Pa) applications

2. Unit of measurement of vacuum

In vacuum technology, the amount that represents the rarefaction of a gas in a vacuum state is called vacuum, which can be represented by pressure, molecular density, average free path, and the time constant to form a single molecular layer, etc., but is usually expressed by the pressure (residual pressure) value of the gas. The lower the gas pressure, the higher the vacuum; Conversely, the higher the pressure, the lower the vacuum.
The legal pressure measurement unit is Pa [PASCAL] and the symbol is Pa
1Pa= 1n.m-2 in addition, a percentage of vacuum can be used as a unit of measurement.

— percentage of vacuum (%) P — absolute pressure (Pa) pb-p represents the reading of the vacuum pressure gauge, and the pressure gauge (Pe) is compared with the percentage of vacuum (%) and pressure P

3. Unit conversion

1 atmosphere =1013.25hPa (hPa)
1mmHg =1Torr =1.333 hPa
1bar =1000 hPa (hPa)
1mbar =1 hPa (hPa)
1inHg =25.4mmHg =33.8 hPa

4. Related terms

◇ air volume — the air volume of the water ring vacuum pump refers to the volume of the gas, m3/min or m3/h, that passes through the suction state of the vacuum pump population per unit time when the inlet is given vacuum degree and the outlet is atmospheric pressure 1013.25hPa.
Maximum gas capacity — the maximum gas capacity of the water ring vacuum pump refers to the maximum gas capacity on the gas capacity curve, m3/min or m3/h.
Vacuum (or pressure) – water ring vacuum pump vacuum pump refers to the inlet in the vacuum state of gas rarefied, expressed in absolute pressure, Pa, hPa, kPa.
Limit vacuum degree (or called limit pressure) — the limit vacuum degree of water ring vacuum pump refers to the vacuum degree when the inlet air volume is zero, Pa, hPa, kPa.
Compression ratio — ratio of gas volume under suction pressure to gas volume after compression
Saturated vapor pressure — the vapor pressure of a substance whose vapor is in phase equilibrium with its condensed phase at a given temperature.

Vacuum pump guidance of selection

The working pressure of the vacuum pump shall meet the ultimate vacuum and working pressure requirements of the vacuum equipment. Generally, the vacuum pump is selected to be half to an order of magnitude higher than the vacuum equipment. The suction pressure range of 2BVX and 2BEX series vacuum pumps is between 33hPa and 1013.25hpa, within which the air volume varies with the suction pressure. Select the appropriate vacuum pump according to the air volume and vacuum. Ensure the vacuum required by the process or remove the gas that needs to be removed. The operating point of the vacuum pump as far as possible in the efficient area, that is, in the critical vacuum or critical exhaust pressure of the area. Avoid operating near maximum vacuum or maximum exhaust pressure. Operating in this area, not only the efficiency is very low, and the work is very unstable, easy to produce vibration and noise. For the vacuum pump with high vacuum degree, the operation in this area, often cavitation phenomenon will occur, the obvious sign of this phenomenon is the noise and vibration in the vacuum pump. Cavitation will lead to the vacuum pump body, impeller and other parts of the damage, so that the vacuum pump can not work.

1. Vacuum water diversion

Our company’s full-automatic vacuum water diversion device 2BYS series is suitable for vacuum water diversion in large and medium sized water plants.
Vacuum water diversion is suitable for all kinds of pumps, especially for large and medium vacuum pumps and vacuum pumps with long suction pipes. The suction pipe is connected to the apex of the vacuum pump housing. Generally, the vacuum pump is selected according to the pumping volume and the maximum vacuum value, and a standby vacuum pump is equipped. The 2BVX and 2BEX series vacuum pumps can meet the requirements of the maximum vacuum value of water diversion by the pump, so only the pumping volume can be calculated. The calculation method of pumping volume recommended by water supply and drainage professionals is as follows:

W- extraction volume m3/min
K- leakage coefficient 1.05~1.1
W1- air volume m3 in the suction tube
W2- vacuum pump volume. Calculate the suction inlet area of the pump multiplied by the distance from the suction inlet to the outlet valve
T- water diversion time, not more than 5min
The water column height of Hg- atmosphere is 10.33m
Zs- the difference of m between the pump shaft and the lowest water level in the suction well

2. Selection of vacuum pump under general working conditions

When the user does not have high requirements for the use of the environment and has enough circulating water, the form of supporting for the user is mostly small sets (i.e., the supporting form in the 2BEX sample). According to its transmission mode is divided into direct drive, reduction gear drive and belt drive three kinds. The principle of selection is to select according to the performance curve measured in the water ring vacuum pump test under standard conditions (water temperature 15℃, air temperature 20℃ and exhaust pressure 1013mbar). In non-standard conditions, the inlet rate or gas volume must be corrected.
Under standard conditions, in order to suction pressure P1 and the requirements of the Qc, containment on the sample performance curve for the corresponding water ring vacuum pump speed (n) and shaft power (P), when the vacuum pump rotating speed n is selected standard of conventional motor speed, selection of transmission mode for straight league or belt transmission (user) has special requirements. When the original speed n of the selected vacuum pump is not the standard speed of the conventional motor, and the product of the original speed n and the standard transmission ratio I of the reduction gear is consistent with the standard speed of the conventional motor, the reduction gear drive is selected. Otherwise choose belt pulley drive.
Selection calculation is illustrated with the process design parameters of a chemical plant as an example:
Design parameters:
Inlet gas temperature 35℃
The absolute inlet gas pressure is 200hPa
The volume of 3000 m3 / h
Working fluid temperature 19℃
Since the sample curve is measured at the inlet temperature of 20℃, inlet temperature of 15℃, exhaust pressure of 1013hPa, and the inhalation gas is saturated air, the above parameters need to be corrected.
The amount of air pumped under specified conditions in the sample
Where, is the speed correction coefficient, and is the specified speed in the sample; Is the actual speed;
Is the working fluid temperature correction coefficient
Is the suction pressure;
Is the saturated vapor pressure at the temperature of the sample and the actual working water;
, are the temperature of the sample and the actual working water (thermodynamic temperature).
The formula is limited to the water temperature within the range of 15℃±5℃, otherwise it shall be corrected by the sealed water temperature correction diagram.
Is the inlet temperature correction coefficient,

Is the inlet air temperature on the sample and the experimental (actual) air inlet temperature, and it can be seen that:

Considering the 10% margin, the volume is.
According to,, a 2BEX 303-0 water ring vacuum pump can be used to check the sample curve with a speed of 590rpm and an axis power of 64kW.
Safety factor of matching power K=1.1 — 1.2 (for paper mill K=1.3)
The selection of reduction gear is based on the mechanical power meter of reduction gear. Finally check the thermal power.
The belt drive is selected according to the manual.

3. Selection of water-ring vacuum pump units in thermal power plants

3.1 the term
Rated power (nameplate power, this condition is called TRL condition)
Under the specified back pressure condition, 3% water replenishment rate and rated power of the unit, and its steam intake is called rated steam intake (also known as rated flow rate and TMCR flow rate).
Maximum continuous power (TMCR condition)
Under the condition of rated steam intake, the back pressure is the back pressure (designed back pressure) determined by taking into account factors such as annual weighted average temperature and optimizing the air cooling system, and the output power at the generator end under the condition that the water replenishment rate is 0%. The power can also be used to ensure the heat and steam consumption rate of power.
Guaranteed heat consumption rate assessment conditions (THA conditions)
Under the designed back pressure condition, the thermal and steam consumption rates of 0% water filling rate and rated power output are guaranteed.
Full open power of valve (VWO condition)
The steam intake of the turbine when the regulator is fully open and the power output at the motor end under the TMCR definition conditions. Generally, the admission volume under VWO is 1.03-1.05 times of the rated admission volume. This flow should be a guaranteed value.
Cardiac obstruction back Pressure (Choking Bask Pressure)
The blocking back pressure of the turbine refers to the back pressure when the steam flow velocity at the outlet of the turbine’s final stage is close to the sonic level (Mach number is about 0.95). Under normal circumstances, it is related to the steam intake of the turbine. Different steam intake has different blocking back pressure values. In order to regulate the technical conditions of the steam turbine, here refers to the blocking back pressure under the condition of TMCR flow.
Subcooling: the difference between the saturated steam temperature of the exhaust steam of the turbine and the condensation water temperature at the inlet of the condensing tank.

3.2 performance curves of water ring vacuum pumps for condensers in thermal power stations are mainly divided into two types. One is used for the start-up stage, which is the curve in the 2BEX sample. The other is a normal curve, which is shown in the 2BWD sample.

3.2.1 selection of vacuum pump at start-up stage
The purpose of the start-up phase is to pump the air in the condenser to a pressure of 20 kPa to 40 kPa to start the turbine before it starts.
Meeting the predetermined vacuum time of the power station is the main basis for choosing the vacuum pump at this stage. Time is usually less than 30 minutes.
The extraction time can be calculated by the following formula
T — extraction time min
V — volume m3 of the vacuum system
Q — the extraction rate of the vacuum pump m3/min
P1 — initial pressure of the vacuum system
P2 — pressure at the end of pumping in a vacuum system
3.2.2 during the normal operation stage, that is, after the steam turbine is in full operation, the gas pumped by the water ring vacuum pump includes air and water vapor, not the same air. The proportion of components in the mixture is related to the amount of air leakage and other factors.
Selection basis of water ring vacuum pump during normal operation:
MG: mass flow rate of leaking air from condenser
P1:2BWD series complete set of device inlet pressure
T1: inlet temperature of 2BWD series complete set
Tw1: system cooling water temperature
MG is generally provided by the design institute, P1 and t1 are obtained by checking the condenser performance curve through tw1 and load rate.
The selection of water ring vacuum pump in normal operation stage is based on the requirement of removing the leaking air from condenser. This stage of selection involves relevant knowledge of thermal engineering, limited in length, this article is not written. The selected vacuum pump must meet the requirements of two operating conditions at the same time.

Vacuum pump basic knowledge and selection guidance

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