Vacuum
Vacuum, in theory, does not refer to any substance within the volume. (In reality, true vacuum does not exist.) Typically, any gas pressure less than normal atmospheric pressure (101325 Pa) inside a container is called a vacuum state.
Vacuum degree
The degree of vacuum indicates the degree of gas dispersion under vacuum conditions, generally expressed in terms of pressure values.
In practical applications, there are two types of vacuum degrees: absolute vacuum and relative vacuum. The value read on the vacuum gauge is called the degree of vacuum.
The vacuum degree value represents that the actual pressure value of the system is lower than the atmospheric pressure value, and the value displayed on the manometer is also called gauge pressure.
The industry also refers to this as final relative pressure, that is, degree of vacuum = atmospheric pressure – absolute pressure (atmospheric pressure is generally regarded as 101325Pa, the final absolute pressure of the water ring vacuum pump is 3300Pa, and the final absolute pressure of the rotary vane vacuum pump is about 10Pa).
Final Relative Pressure
Relative pressure refers to the degree of gas dispersion within a container compared to atmospheric pressure. It represents the actual value of the system pressure that is lower than the atmospheric pressure value.
As the air inside the container is pumped out, the internal pressure is always lower than the external pressure.
Therefore, when using relative pressure or gauge pressure to represent it, the value must be preceded by a negative sign to indicate that the internal pressure of the container is lower than the external pressure.
Ultimate Absolute Pressure
Final Absolute Pressure refers to the difference between the absolute pressure inside a container and the theoretical vacuum pressure (which has a pressure value of 0 Pa).
As a result of technical limitations, it is impossible to pump the internal pressure of a container up to the absolute vacuum value of 0 Pa.
Therefore, the vacuum value achieved by a vacuum pump is always higher than the theoretical vacuum value. When using absolute vacuum to express this value, there is no need for a negative sign.
For example, if the degree of vacuum of a device is marked as 0.098 MPa, then in reality it is -0.098 MPa.
Pumping capacity
Pumping capacity is a factor measuring the pumping speed of a vacuum pump, usually expressed in units of L/s and m³/h.
Compensates for system leakage rate. It is easy to understand why a vacuum pump with a high pumping capacity can easily achieve the desired degree of vacuum, while a vacuum pump with a low pumping capacity may be slow or unable to achieve the desired degree of vacuum when pumping the same volume. of container.
This is because it is impossible for the pipeline or container to completely prevent gas leakage, and the high pumping capacity compensates for the decrease in vacuum due to leakage.
Therefore, a high pumping capacity vacuum pump can easily achieve the ideal vacuum degree.
It is recommended that, when calculating the theoretical pumping capacity, you choose, if possible, a vacuum pump with a higher pumping capacity. The formula for calculating pumping capacity will be presented below.
The conversion methods between Pa, KPa, MPa, mbar, bar, mmH2O, Psi are shown in the following table:
Conversion table for pressure units commonly used in laboratories
| Units | Father | KPa | MPa | Pub | amber | mmH 2 O | mmHg | psi |
| Father | 1 | 10 -3 | 10 -6 | 10 -5 | 10 -2 | 101.97×10 -3 | 7.5×10 -3 | 0.15×10 -3 |
| KPa | 10 3 | 1 | 10 -3 | 10 -2 | 10 | 101.97 | 7.5 | 0.15 |
| MPa | 10 5 | 10 3 | 1 | 10 | 10 4 | 101.97×10 3 | 7.5×10 3 | 0.15×10 3 |
| Pub | 10 5 | 10 2 | 10 -1 | 1 | 10 3 | 10.2×10 3 | 750.06 | 14.5 |
| amber | 10 2 | 10 -1 | 10 -4 | 10 -3 | 1 | 10.2 | 0.75 | 14.5×10 -3 |
| mmH2O | 10 -1 | 9.807×10 -3 | 9.807×10 -6 | 98.07×10 -6 | 98.07×10 -3 | 1 | 73.56×10 3 | 1.42×10 -3 |
| mmHg | 9.807×10 -3 | 133.32×10 -3 | 133.32×10 -6 | 1.33×10 -3 | 1.33 | 13.6 | 1 | 19.34×10 -3 |
| psi | 133.32×10 -3 | 6.89 | 6.89×10 -3 | 68.95×10 -3 | 68.95 | 703.07 | 51.71 | 1 |
Selection of vacuum pumps
1. The degree of vacuum required for the process
The working pressure of the vacuum pump must meet the process requirements, and the selected vacuum degree must be half to an order of magnitude higher than that of the vacuum equipment. (For example, if the required degree of vacuum at absolute pressure is 100 Pa, the degree of vacuum of the selected vacuum pump must be at least 50-10 Pa.)
If the absolute pressure requirement is greater than 3300 Pa, a water ring vacuum pump should be given priority as the vacuum device. If the absolute pressure requirement is less than 3300 Pa, a rotary vane vacuum pump or a vacuum pump with a higher vacuum level must be selected as the vacuum obtaining device.
2. The pumping capacity required for the process
The vacuum pump requires a pumping speed (i.e. the ability of the vacuum pump to discharge gaseous, liquid and solid substances under its working pressure), usually expressed in units of m³/h, L/s and m³/min.
The specific calculation method can be calculated based on the following formula for selection. Of course, vacuum pump selection is a comprehensive process involving related experience and other factors.
S=(V/t)×ln(P1/P2)
- S – vacuum pump pumping speed (in L/s).
- V – the volume of the vacuum chamber (in L).
- t – the time required to reach the required degree of vacuum (in s).
- P1 – the initial pressure (in Pa).
- P2 – the required pressure (in Pa).
3. Determination of the composition of the pumped object
First, it is necessary to determine whether the pumped object is gas, liquid or particles.
If the pumped gas contains impurities such as water vapor or a small amount of particles and dust, a rotary vane vacuum pump must be selected with care.
If a high degree of vacuum is required, a filtering device must be added to filter out impurities before using a rotary vane vacuum pump.
Secondly, it is important to know whether the pumped object is corrosive (acidic or alkaline, what is the pH value?). If the gas contains corrosive factors such as acids and bases or organic corrosion, it must be filtered or neutralized before selecting a rotary vane vacuum pump.
Third, consider whether the pumped object will contaminate rubber or petroleum products. Different vacuum equipment must be selected for different pumped media. If the gas contains a large amount of vapor, particles and corrosive gases, an appropriate auxiliary device must be installed in the pump inlet piping, such as a condenser, filter, etc. (specifically, contact our technical engineering team).
Fourth, consider whether the noise, vibration and appearance of the vacuum pump have an impact on the factory.
Fifth, as the saying goes, you get what you pay for. When purchasing a vacuum pump or vacuum equipment, priority should be given to the quality of the equipment, transportation costs and maintenance and upkeep fees.
Pumping speed and vacuum system configuration
Different vacuum systems require different vacuum levels. Therefore, a set of vacuum units must be used to complete the process, connecting in series vacuum pumps that work in different pressure ranges.
The high vacuum pump achieves the required vacuum degree of the system, while the low vacuum pump discharges directly into the atmosphere.
Of course, the simplest vacuum unit is a direct vent vacuum pump. However, a high vacuum system usually requires a three-stage unit, and a medium vacuum system usually requires a two-stage unit.
It is difficult to create an effective high vacuum unit using just a high vacuum pump and a low vacuum pump. There are several reasons for this.
One of the reasons is the continuity of the flow.
High vacuum pumps have restrictions on the pressure they can handle in the front stage. When the pre-stage pressure is higher than a certain pressure, the pump does not work properly.
When the pre-stage pump reaches this critical pressure, the pumping speed may decrease, so that the exhaust flow rate of the pre-stage pump may be lower than that of the main pump, which breaks the flow continuity requirement and will inevitably cause the vacuum unit to not work properly.
However, if a medium vacuum pump is connected between the high and low vacuum pumps, it can play the role of filling the gap, ensuring flow continuity, and all pumps can work in their ideal state. Roots pumps can work in the medium vacuum range and are the most suitable, which is why they are also called Roots booster pumps.
Due to its low compression ratio, it can be connected to a range of several Pa to several hundred Pa. When a three-stage high vacuum unit enters a higher vacuum level, since the exhaust flow rate of the main pump slows down significantly, only a small pre-stage pump is needed to maintain pumping continuity. This method is often adopted in real applications, which can reduce the power consumption of the unit.
Another reason why a high vacuum unit often requires a three-stage unit is the limitation of the suction pressure of the high vacuum pump. The pump has an initial working pressure and traditional high vacuum pumps are in the range of several Pa. Therefore, the pre-stage pump must pre-pump to this pressure before the main pump can start running.
However, the prestage pump that vents directly to the atmosphere often takes a long time to pump to this pressure because as the pressure decreases, the pump's pumping speed also decreases. Especially for vacuum units with periodic pumping, the time required to reach the working vacuum degree is important.
The longer the pre-pumping time, the longer it takes to get up to working pressure, so adding a medium vacuum pump in combination with a low vacuum pump can reach the pressure at which the main pump can operate in a longer time. short, which can ensure the efficiency of equipment use.
Roots pumps and oil booster pumps can be used as medium vacuum pumps. Molecular booster pumps have a very high compression ratio, which allows them to achieve a clean vacuum and excellent high vacuum performance.
They also have super strong pumping ability in the mid-vacuum range. This makes Molecular Boost Pumps currently the only vacuum pump that combines medium and high vacuum performance. Therefore, it can be combined with a low vacuum pump to form a high vacuum unit with performance comparable to a three-stage unit.
Specifically, due to the high resistance of molecular booster pumps, the prestage pump can easily be in a high flow state; and the high suction pressure of the molecular booster pump reduces the pre-pumping load of the pre-stage pump.
Molecular booster pumps can work in the range of 100-50Pa, and the prestage pump from atmosphere to this pressure basically follows the rule that the pressure drops by an order of magnitude each time the unit passes. Therefore, the unit can have a high pumping efficiency.
Simplifying high vacuum units and removing Roots pumps is another advantage of molecular booster pumps. For larger high vacuum application equipment, the pre-pumping capacity of the pre-stage pump can be suitably boosted to further reduce pumping time.
Because the pre-pumping time is much shorter than the entire exhaust process, the pre-stage pump can also be used as the pre-pumping function of various devices, which is often very practical. This greatly simplifies the vacuum unit for large-scale applications.
In some medium vacuum applications it is necessary to get into the 10-1Pa range, which is often difficult to achieve with a two-stage Roots pump unit.
However, using a series-connected three-stage Roots pump unit can increase the vacuum level by an order of magnitude to reach 10-1Pa. Therefore, three-stage units are also commonly used in medium vacuum applications.
Since molecular booster pumps can achieve a total pumping speed of 10-1Pa, they can also replace two-stage Roots pumps in a three-stage medium vacuum unit.
Generally speaking, Roots pumps that operate continuously in the low pressure to medium vacuum range can be completely replaced by molecular booster pumps.
On the other hand, Roots pumps that operate continuously in the mid-vacuum tip pressure range should be relatively smaller because pre-stage pumps generally have strong pumping speed in this pressure range.
