Roots water ring vacuum unit is composed of Roots vacuum pump as the main pump and water ring vacuum pump as the front pump in series. Water ring pump is more advantageous than other vacuum pumps in roots water ring vacuum unit. It overcomes the limit pressure difference (the limit pressure of the unit is much higher than that of water ring pump) when a single water ring vacuum pump is used. At the same time, it retains the advantages of Roots vacuum pump, which can work quickly and has a larger pumping rate. In particular, it can be used to pump out a large number of condensable steam, especially when the capacity of the gas seal mechanical vacuum pump to remove condensable steam is not enough, or the solvent used can make the pump oil deteriorate and affect the performance, or the vacuum system does not allow oil pollution. Next, I will introduce how to install roots water ring vacuum pump system.
Condenser installed in front of the unit:
In order to make the unit as small as possible, try to make the steam to be pumped condense before entering the pump unit, so that the remaining is non condensable gas and trace residual steam. The volume of gas decreases under the same pressure after cooling. Therefore, after condensation, the required air extraction is reduced, and the pump can be selected to be smaller accordingly.
Which way is more economical? As the case may be, examples are as follows:
There are two ways to condense steam: one is to install a cooling device, the other is to install a condenser in the high-pressure stage of the unit, so that it can be cooled by ordinary water.
The system needs to pump out 50kg of water vapor per hour, and the volume flow is 50000m3 / h when the suction pressure is 1torr.
1) Three roots vacuum pumps are connected in series and one water ring vacuum pump is used as the unit composed of the front stage to pump the above water steam. The total power of the unit is 90kw.
2) In order to make the steam condense before reaching the vacuum pump, a condenser and a cooling device with a power of 30000 kcal / h should be installed at position A. under the suction pressure of 1torr, the condensing temperature of the water steam is – 19 ℃. In order to ensure continuous operation, the condensing temperature of the condensing device should be – 25 ℃, and two condensers should be installed in parallel. According to the calculation of the components of non condensing gas, the pumping capacity of vacuum pump can be reduced to 1000 ~ 2000m3 / h, and the power of the total unit (including the consumption power of condenser) is also 90kw.
3) First, the water vapor is pumped out by roots vacuum pump and condensed under the pressure of 45 Torr. Some of the condensation temperature under this pressure is about 36 ℃, so the condensation temperature of the condenser can be kept between 30 ~ 35 ℃, which can be cooled by common cooling water. The condenser is set at B. At this time, the total power consumption is about 75kW.
According to the comparison of the above three methods, the third scheme is the best, which can reduce the power consumption of 15kw.
To sum up, only non condensable gas is left after water vapor cooling. When the pressure is very low, the specific volume of water vapor is quite large. After these condensable vapor condenses, the air volume required by the vacuum pump is obviously greatly reduced. In addition, no matter whether the steam condenses or not, as long as the temperature of the gas decreases under the same pressure, its volume flow will decrease. For example, it is not uncommon for gases with a temperature of 200-300 ℃ in chemical process. After cooling from 300 ℃ to 50 ℃, the volume of dry air can be reduced by about 45%, so that a smaller capacity vacuum pumping unit can be selected.
Operation sequence of the unit:
1) When there is no bypass valve in the unit, the water ring vacuum pump shall be started first. The gas in the pumped system enters the water ring vacuum pump from the roots pump (the gas drives the rotor of the roots pump to rotate by itself, just like the flowmeter), and then it is discharged to the atmosphere. When the suction pressure of the water ring vacuum pump (if there is a large air pump in series, it is the suction pressure of the large air pump) reaches the initial specified value of the roots vacuum pump (i.e Allow the exhaust pressure), start the roots vacuum pump, the unit is officially running, start to work.
2) When there is a bypass valve in the unit, start the water ring vacuum pump first, and then start the roots vacuum pump. At this time, the pressure difference between the inlet and exhaust of Roots vacuum pump is large, and the bypass valve opens automatically. Part of the gas in the pumped container enters the water ring vacuum pump through the bypass valve, and the other part also enters the water ring vacuum pump through the pump under the action of Roots vacuum pump. Obviously, the pumping rate increases, so it will soon reach For the pre vacuum to roots vacuum pump, the air pressure difference between inlet and outlet is small, the valve is closed automatically (or manually), and the unit is in formal operation. This method can greatly shorten the pre pumping time, but the equipment is complex.
Water ring vacuum pump selection
The so-called water ring vacuum pump is a liquid ring vacuum pump with water as liquid ring. Water as liquid ring has many advantages, such as low price, easy to obtain, no pollution to the environment, etc. But there is also a big disadvantage, because the saturated steam pressure of water is high, the suction pressure of water ring vacuum pump is also high. At this time, if the liquid with low saturated steam pressure is used as liquid ring, the limit vacuum degree of vacuum pump can be improved. If the maximum allowable discharge pressure of Roots vacuum pump in a unit is 10torr, the air pump must be increased when water is used as the liquid ring to be the front pump of the roots pump. If mineral oil is used as the liquid ring instead of air pump, it can be used as the front pump, which can simplify the device.
Unit performance and allowable discharge pressure of roots pump
The performance of the unit largely depends on the allowable discharge pressure of Roots vacuum pump. The lower the allowable value is, the less likely the water ring vacuum pump is to be used as a pre pump. If the allowable value of Roots vacuum pump is 1 ~ 10torr, and the limit pressure of single or multi-stage water ring vacuum pump is much higher than this value, it is impossible to use the pump alone in combination with roots vacuum pump, but need to add two-stage large air pump. If the discharge pressure of roots pump is more than 100 Torr, the former water ring vacuum pump can also be used as the former pump of Roots vacuum pump, which greatly expands the application scope of the former pump.