Before selecting the type, we must make clear several basic concepts about the vacuum pump. Vacuum: the degree of thinness of a gas in a vacuum state, usually expressed as vacuum. The reading from a vacuum gauge is called vacuum. The vacuum value represents the actual value of the system pressure lower than the atmospheric pressure. The value expressed from the table is also known as the table pressure, which is also known as the ultimate relative pressure. The absolute pressure limit of the rotary vacuum pump is about 10Pa. Indicates that the actual system pressure is lower than the atmospheric pressure. The internal pressure is always lower than the external pressure because the air in the container is pumped. So when you represent it in terms of relative pressure or table pressure, you have to put a negative sign in front of the value, which means that the pressure inside the container is lower than the pressure outside. Extreme absolute pressure: how much pressure is higher than the measured internal pressure ratio "theoretical vacuum" (theoretical vacuum pressure value is 0Pa). The object it compares is the absolute vacuum pressure of the theoretical state.
Due to the limitations of the process, we cannot draw the internal pressure to the absolute vacuum of 0Pa in any case. Therefore, the vacuum value of the vacuum pump is higher than the theoretical vacuum value. So in absolute vacuum, there's no negative sign in front of the value. Pump: the pump is a measure of the pump speed. The common units are L/S and m cubed /h. It's a parameter that compensates for the leak rate. It's not hard to understand why, in theory, pumping a container of the same volume, a vacuum pump with a large amount of air could easily get the vacuum we need, while a vacuum pump with a small amount of air could not even get the vacuum we want. Since it is always impossible for pipelines or containers to be absolutely free of air leakage, the large amount of air pumped makes up for the factor of vacuum reduction caused by air leakage. Therefore, it is very easy for the large amount of air to be pumped to the ideal vacuum value. It is suggested here that, in the case of calculating the theoretical amount of pumping, we try to choose the vacuum pump with a higher level of pumping. The specific calculation formula of the amount of pumping is described below.
After the basic parameters of vacuum, absolute pressure and relative pressure are clear, we can enter the formal selection of vacuum pump.
1. The degree of vacuum required by the process
The working pressure of the vacuum pump should meet the requirements of the process working pressure, and the vacuum degree should be half to one order of magnitude higher than the vacuum degree of the vacuum equipment when selecting the type. (for example, vacuum process requires the vacuum degree of 100pa(absolute pressure), and vacuum pump should be selected at least 50pa-10pa). Generally, if the absolute pressure is required to be higher than 3300Pa, the water ring vacuum pump is preferred as the vacuum device. If the absolute pressure is lower than 3300Pa, the water ring vacuum pump cannot be selected, and the rotary vacuum pump or the vacuum pump of higher vacuum level is selected as the vacuum device.
2. Amount of extraction required by the process (extraction rate)
The vacuum pump requires the extraction rate (that is, the capacity of the vacuum pump to discharge gas, liquid and solid under its working pressure), and the general unit is m3/h, L/S,m3/min. The specific calculation method can be calculated by referring to the following formula. Of course, the selection of vacuum pump is a comprehensive process, involving relevant experience and other factors.
S = (V/t) by ln (P1 / P2)
Where, S is the pumping rate of vacuum pump (L/ S).
V is the volume of vacuum chamber (L)
T is the time required to achieve the required vacuum degree (s).
P1 is the initial pressure (Pa).
P2 is the required pressure (Pa).
3. Determine the components of the object being pulled
First, the object to be extracted is a gas, liquid or particle. If the extracted gas contains water vapor or a small amount of impurities such as particle and dust, the rotary vacuum pump should be carefully selected.
Second, it is important to know whether the object is corroded (acid or alkaline, what is the PH value?). If the acid base corrosion or organic corrosion and other factors of the gas, should be filtered or neutralization treatment can choose rotary vacuum pump.
Thirdly, is the object being pumped contaminated with rubber or oil? The corresponding vacuum equipment should be selected for different pumped media. If the gas contains a lot of steam, particles, and corrosive gases, it should be considered to install corresponding auxiliary equipment, such as condensers, filters, etc. (contact our corresponding technical engineers for details).
Fourth, the vacuum pump noise, vibration, aesthetic impact on the factory.
Fifth, as the saying goes, cheap goods are not good. The purchase of vacuum pumps and vacuum equipment should also give priority to the quality of equipment, transportation and maintenance costs.