Working principle of Roots pump- water ring pump vacuum unit

Working principle of Roots pump- water ring pump vacuum unit

For the high vacuum Roots vacuum pump (mechanical booster pump) cannot be directly discharged to atmosphere, such as air direct discharge will cause the Roots vacuum pump suction port and exhaust pressure is too large, so that the Roots vacuum pump overload, such as increase the motor power Roots vacuum pump will cause a small gap between the Roots vacuum pump overheating that Roots vacuum the pump rotor quickly because of thermal expansion card die. In order to ensure the Roots vacuum pump can achieve high vacuum must ensure that the clearance between the rotor of Roots vacuum pump. So the use must be equipped with Roots vacuum pump before the pump, Roots vacuum pump before the pump will start again with the pressure inside the system is pumped to a certain range, so you can avoid the Roots vacuum pump overload. Before the pump can be selected (liquid) water ring vacuum pump and rotary vane vacuum pump, vacuum pump, reciprocating vacuum pump and vacuum pump can be directly discharged to atmosphere.

In the general case, Roots vacuum units select water ring pump as a backing pump than the other vacuum pump is more favorable, which is mainly due to its ability to pumping a lot of condensable vapors, especially when gas seal mechanical vacuum pump to exclude coagulation steam ability is not enough, or use of solvent can to pump the oil deterioration in performance, or is vacuum system does not allow oil pollution is more obvious.

Roots pump and water ring pump unit widely used in chemical, food sublimation drying, and altitude simulation test of the vacuum pumping system. The combined unit is roughly in the following types.

(1)The Roots pump and water ring pump: unit of water ring pump is caused by preparative vacuum Roots pump. Therefore, the water ring pump maximum allows the exhaust pressure, that is to say, on the one hand, we should try to raise the water ring vacuum pump limit, on the other hand, also want to try to improve the maximum permissible discharge pressure of Roots pump.

In general, single-stage water ring vacuum pump limit degree is not high, and the requirement of China's production of Roots pump vacuum and high, so in fact not single-stage water ring vacuum pump as the Roots pump before the pump, and can raise limit vacuum degree of two-stage water ring pump as the pump before use, the two-stage water ring pump, can also improve the ultimate vacuum unit.

A Roots pump limit vacuum degree is low, especially when it and the water ring pump combination operation, use the limit, the entire unit ultimate vacuum may be lower, but if the two-stage Roots pumps in series with water ring pump combination, it can greatly improve the ultimate vacuum of the unit.

So in this type usually see is two-stage Roots pumps in series and then two-stage water ring pump for first stage pump is shown in Figure 1 is composed of units.

(2)Roots pump and water ring pump and air pump unit: even use two-stage water ring pump, the increase of ultimate vacuum only within a certain range. This is because of water saturated vapor pressure limitation. Water ring pump theoretical ultimate pressure is water saturated vapor pressure. If consider gas reflux and other factors influence, the actual water ring pump’s ultimate pressure is significantly higher than the temperature of saturated vapor pressure. In order to improve the ultimate vacuum of the backing pump, the water ring pump and air pump can be combined for use.

As shown in Figure 2。After connecting the first-stage atmosphere, the ultimate vacuum is up to 20~30Torr. If a water ring pump and secondary pump combination, the ultimate vacuum is up to 2~10Torr.

(3)Roots pump - water ring pump and mechanical vacuum pump in parallel: this unit is mainly used for the vacuum pumping system, which needs to dealing with large amount of water vapor and higher requirements on ultimate vacuum, such as in a vacuum drying.

Requirements for handling a large number of water vapor in vacuum system, using the water ring pump is more appropriate, but because the ultimate vacuum is not high, resulting in ultimate vacuum of the whole unit is low. Although in the requirements of higher vacuum degree of the vacuum pumping system and need to higher ultimate vacuum of Mechanical vacuum pump as a former level using a pump. But because of the water ring pump power consumption, efficiency is very low, high noise, in need of long time vacuum drying system, water ring vacuum pump is as the Roots pump before the pump very economic.

In these situations, the gas ballast mechanical vacuum pump and water ring pumps are connected in parallel, as the Roots pump’s backing pump. When vacuum drying, do pre-pumping by using water ring pumps, until a large number of water vapor reduced, then start the gas ballast mechanical vacuum pump, and cut off the water ring pump. If it needs a long time to complete drying, cooling water and power need less, as shown in Figure 3.

Operation of Roots pump-water ring pump unit

(1)Pre-installed condenser of the unit

In order to try to make the unit volume smaller, manage to make the pumping of the steam to be condensed before entering the pump unit, so that the remaining is the non condensable gas and a trace amount of residual steam. Gas cooling in the same pressure volume also decreases. So after condensation required pumping quantity decreases, the pump can also be selected small correspondingly.

Which way is more economical? It depends on the specific circumstances, for example:

Condensation of steam in two ways: one is to install a cooling device and another is in units of the high-pressure stage installed in a condenser, so that you can use ordinary water cooling.

The system needs to be pumped out of the amount of water 50kg per hour, the volume flow is 50000m3/h when the pumping pressure is 1Torr.

1) To suction the amount of water vapor, need three Roots pumps in series, and a water ring pump for the pre formed the unit, the unit total power 90kW.

2) In order to make steam be condensed before the arrival of the vacuum pump, a condenser is needed to be installed at Position A and a cooling device in power of 30000kcal/h, as shown in Figure 4. In 1Torr suction pressure, water vapor condensing temperature is -19℃, in order to ensure continuous work, the condensing temperature of the condensing device should be -25℃ and install two condensers in parallel. Calculate according to the part of the non-condensation gases, pumping volume of vacuum pumps can be reduced to 1000~2000m3/h. The power of the total unit (including the consumption power of the condenser) is also 90kW.

3)First use Roots pumps to pump out water vapor, and condense the water vapor under 45 Torr pressure; under this pressure, the condensing temperature is about 36℃, so that the condensing temperature of the condenser can maintain at 30~35℃, common cooling water can be used for cooling. The condenser is in Position B when the total power consumption is about 75kW.

Through the comparison of the above three groups of means, the third scheme is the best, which can reduce the power consumption of 15kW.

To sum up, only non-condensable gases are left after the water vapor being cooled. When the pressure is very low, steam specific volume is quite large, these condensable vapor condensation, pump pumping quantity is obviously big for reduced. In addition, regardless of the steam is condensed, under the same pressure only to gas temperature reduce, reduce the volume of traffic will. For example in chemical process gas in 200~300℃ temperature is common. If cooling from 300℃ to 50℃, the volume of air drying decreases about 45%, so that you can choose pumping vacuum pump unit with smaller capacity.

(2)Operation sequence of the unit:

1)If the unit doesn’t have bypass valves, air in the pumped system first flows into the Roots pump (gas driving the Roots pump rotor to rotate on their own, like flowmeter) and then flows into the water ring pump, finally is discharged into the atmosphere; until the water ring pump’s suction pressure (such as series of air pump, large pump suction pressure) reaches the Roots pump initially specified value (namely the allowable exhaust pressure), start the Roots pump and the unit officially operates, to begin to work.

2)If the unit has a by-pass valve, as shown in Figure 5, start the water ring pump at first, and then start the Roots pump, at this time, intake and exhaust pressure difference of the Roots pump is larger, the bypass valve automatically starts, was pumping gas in the container part by a by-pass valve into the water ring pump and another part in the role of Roots pump by the pump also entered the water ring pump, obviously increased pumping rate, so quickly reach the Roots pump vacuum, intake and exhaust pressure difference is small, the valve closes automatically (or manually shut down), the unit officially work. This method can greatly shorten the pre drainage time, but the equipment is complex.

(3) Performance relation of Unit- Roots pump- the backing pump

Closely related to the performance and performance of unit Roots pump, and the performance of Roots pump with the pump before vary.

1) As gaps exist among rotors of Roots pump, between the rotor and the shell gaps, there are return flows, which affected by the inlet pressure and the outlet pressure, even with the same Roots pump, when using different backing pump, pumping speed will vary.

Roots pump’s pumping rate can be determined by the following formula:

δ=δ₀(P₂/P₁/K)

Where:δ₀-the designed pumping rate;

P1-inlet pressure;

P2-outlet pressure;

K-intrinsic constant, which is determined by the shape of the pump rotor, the amount of clearance, the circumferential velocity of the rotor and the outlet pressure.

From the type known, the exhaust volume by the effects of ratio of the pressure of export and import, i.e., if increased to backing pump pumping rate, then Roots pump pumping rate will also increase.

(2)The ultimate pressure is determined by the pump's pumping rate, the return flow of each gap, the leakage of the pump body and the amount of gas in the high vacuum side:P0=(Q₁+Q₂+Q₃)/δ　　　Where：P0-ultimate pressure;

δ-pumping rate;

Q₁-return flow;

Q₂-leakage;

Q₃-discharge volume。

In these parameters, Q1 is influenced greatly by exhaust pressure, namely the backing pump’s ultimate pressure, in water ring pump for pre - pump, Roots pump’s ultimate pressure changes with water ring’s saturated vapor pressure.

Figure 6 is the performance comparison compares the performance of the same Roots pump equipped with different backing pump

It can be seen from the above figure, before the ultimate vacuum pump unit is higher, the ultimate vacuum is increased; two Roots pump in series, it can improve the ultimate vacuum unit (it is the fact that the front-stage Roots pump is the fist-stage pump of the next-stage Roots pump), and the performance curves slowly expand, namely its usage range is expanded (namely drawn by comparison of curves 1 and 2, 3 and 5). The curves of units 1 and 2 are roughly the same. Similarly, the curves of units 3,4 and 5 also have the same place. However, curves of units 1 and 2 and curves of units 3,4 and 5 are completely different. This shows that for the same Roots pump, if choose different backing pumps, the performance curve of units have essential differences. Thus, the backing pump has a considerable effect on the unit performance.

(4)Selection of the water ring pump

Increasing the maximum allowable limit as down the so-called water ring pump is to use water as the liquid ring of liquid ring pump, water as liquid ring has many advantages, such as cheap, easy to get, will not pollute the environment, but also has a significant drawback due to water saturation vapor pressure, the water ring pump suction pressure is high. At this time such as the use of saturated vapor liquid ring, can improve the pump vacuum. If a unit of a Roots pump exhaust pressure is 10 Torr, water as liquid ring must pump can be used as the Roots pump before the pump, if the switch to mineral oil liquid ring does not increase the pump can be used as a backing pump, which can simplify the device.

(5)The unit performance and allowable Roots pump discharge pressure

And the performance of the unit in a large extent depends on the discharge pressure permitted by Roots pump. This allows lower values, possibility of water ring pump as a first stage pump is smaller. If the Roots pump’s allowable value is 1~10Torr, And both of single and multi-stage water ring pump ultimate pressure is greatly higher than the value of the number of, so it would not be alone with this combination of Roots pump is used, and the need to add secondary air pump. If the Roots pump discharge pressure allowable value is above 100Torr, the first-stage water ring pump can also be used as the backing pump of a Roots pump, which greatly expands scope of application of the backing pump.

(6)Application examples

The production process of a chemical fiber products: low molecular polymer - made belt - Section - drying (use of Roots pump and water ring pump unit for vacuum drying) - spinning drafting, heating textile products.

Among them an important process is to dry sections in size of 5 × 5 × 5 (mm) for spinning, spinning process and the ideal condition is the moisture content of zero actually the failure to achieve this purpose. So moisture content needs to be less than 0.02%; if the water over the requirements to snag under high temperature and high pressure, it will make the polymer decomposition, the effect of the product strength.

In the drying process, the used Roots pump - water ring pump pumping system device is shown as Figure 7. The technical parameters of different pumps in the unit are as shown in the following table.

Brief introduction to the function of the main equipment is as follows:

(1) Vacuum valve

Close the vacuum valve, the unit to stop running, can keep a certain degree of vacuum drying system.

(2)Automatic pneumatic safety valve

In order to prevent the water ring pump system in the water flow to the Roots pump and vacuum drying system in unit time of a sudden stop.

(3)Pressure difference valve

Unit starts running, start the water ring pump, the pressure difference at both ends of the valve has the effect of differential pressure, reaches a certain value, the valve automatically opens, most of the gas in the system by the valve flow of water ring pump. When the atmosphere gradually through the air pump flow water ring pump, pressure differential pressure across the valve and reduce gradually, so as to close the valve, so air pump immediately began work, differential pressure valve is to shorten the normal work of the air pump before the pre drainage time.

(4)Water level meter

The role of the check valve prevents water pump flow back.

The sequence of operations is as follows: the first to start the water ring pump, gas in the system size is made up of Roots pump (at this time drive the Roots pump rotors to rotate) into the water ring pump and composed of a steam water separator row into the atmosphere. When pumped to the absolute pressure is 50Torr start Roots pump 2, when the absolute pressure to 20 restart Roots pump 1. Finally system pressure can reach 0.088Torr, under normal circumstances can be maintained below the 1Torr.

The performance parameters of Roots water ring vacuum unit please refer to:

JZJ2B series Roots water ring vacuum unit