Work Principle of the JZJ series Package Units

Normally, to choose the water ring vacuum pump as the fore pump is more advantaged than to choose others for the water ring vacuum pump can pump plentiful coagulable steam. And this advantage is more obviously when to use the gas-down and oil seal mechanical vacuum pump as the fore pump but it is low ability of pumping the coagulable steam, or to use the solvent as the operating liquid but making the oil bad, or the system are not allowed the pollution.
The Roots pump and water ring vacuum pump unit is extensively used in many areas, such as, the chemical industry,the foodstuff sublimation and vacuum dryness,the simulation experiment in the high vacuum etc..
This kind of unit are classified approximately to several types as bellow:
（1）Roots pump－water ring pump：the function of the water ring vacuum pump is to form the pre-vacuum for the Roots pump. One side, the limited vacuum of the water ring vacuum pump should be increased to the best of its ability; another side, the max discharge pressure of the Roots pump should be also improved by many measures.
In general, the single-grade vacuum pump is not able to be used as the fore pump to the unit for two reasons. One, the limited vacuum of the single-grade water ring vacuum pump is not high; Two, the pre-vacuum requirements by the most Roots pump produced in our country at present is higher. So the dual-grades water ring vacuum pump with high vacuum can be selected as the fore pump in order to improve the whole set vacuum.
Notes: One set Roots pump's limited vacuum is very low, especailly assembled with the water ring vacuum pump. Because its usage scope is limited, the whole unit's limited may be less. But to assemble two set Roots pumps in series with the water ring vacuum pump can improve the whole unit's limited vacuum largely.
So this kind of unit is usally composed of two Roots pumps and one set dual-grades water ring vacuum pump.
　
（2）the Roots pump－water ring pump with gas ejector unit： Even to adopt the dual-grades water ring vacuum pump as the fore pump, to improve the whole unit's limited vacuum is just in definite scope for being limited by the water saturation pressure. The theory limited pressure of the water ring vacuum pump refers to the water saturation pressure. If considering the reback influence of the gases, the real limited pressure of the water ring pump is obvious higher than the wate saturation pressure at the same temperature. In order to improve the fore pump's limited vacuum, it can be assembled with the ejector. (See Picture2) So the unit's limited vacuum can arrive at 20~30Torr after coupling with the ejector. If the water ring vacuum pump coupling with two ejectors, the unit's limited vacuum can arrive at 2~10Torr。

(3）Roots pump－water ring vacuum pump with the machinery vacuum pump in parallel connection. This unit is mainly used for pumping mass steam and the higher vacuum is required by the vacuum system. Such as, in the vacuum dryness.
When pumping mass steam, to select the water ring vacuum pump as the fore pump in the unit is more proper. But for its low vacuum, the whole unit limited vacuum is also very low. And the water ring vacuum pump's power is large, its efficiency is very low, and with high noise, so to use it as the fore pump as fore pump working in a long period is not much economical.
For the above situation, the gas-down machinery vacuum pump and the water ring vacuum pump can be used in parallel connection as the fore pump.　In vacuum dryness, to use the water ring vacuum pump firstly till the steam decreases largely, then to start the gas-down machinery vacuum pump and turn off the water ring vacuum pump. If the longer time is required in the dryness, both the supply water and power is less. See. Picture 3.

The operation of the JZJ series Package Units

（1）load the condenser before the unit
In order to make the vacuum unit's cubage be smaller, may cool down the steam before they enter the unit. Thus the remainder only contains the uncoagulable gases and little steam. After reducing the temperature,the gases volume is reduced too under the same pressure. So by adding the condenser, the suction capacity becomes low, accordingly the pump can be also chosen a small one.

Which type is more economical? It is according to the real condition.
For example：

There are two methods to cool the steam:one is to load a cool device, the other is to add a condenser in the high pressure stage of the unit in order to cool liquid by the normal water.
The unit requires pumping steam volume about 50kg per hour. That is to say, the liquid flow is 50,000m3/h when the suction pressure is 1Torr.
　　1)In order to pump the above steam amount, it is necessary to fix three Roots pumps together in series and use one water ring pump as the fore pump,. The total output is 90kW。
　　In order to cool down the steam before they enter the vacuum pump, must load a condenser and a cool device which power is 30,000kcal/h at the position"A"(like picture4). When the suction pressure is at 1Torr，the steam coagulation temperature is -19℃. In order to keep the pump work continually, the cool device should be set at temperature -25℃，and connected two condensers parallelly. According to the part of non-coagulated steam to caculate that the suction capaciry may reduce to 1000~2000m3/h，the whole set's output（includes the condenser's power）is also 90kW.
　　3）At first, pump the steam by using the Roots pump and cool the steam at 45Torr pressure, the coagulated temperature is about36℃. So the cool temperature of the condenser always keeps 30~35℃ and the normal water can be used to cool. Load the condenser in position "B".The total output at the moment is about 75kW.
　　Comparing the three types above, we realize the third is the best.It can save 15% drive consumption.
In summary,the non-coagulated gases are left after cooling the steam.
When the pressure is very low, the steam volume is very large. After these coagulated steam is been cooled, the suction quatity that intaking by the pump reduces more. Otherwise,no matter the steam is cooled or not, only reduce the air temperature under the same pressure, its volume flow must be decrease , too. For example: the gases with 200~300℃ are very common in the chemical industry. If cool it from 300℃ to 50℃，the drying gases volume will reduce about45%，thus the smaller suction capacity vacuum unit can be chosen.
　　（2）The operation sequence of the unit：
　　1）When without the pass-by valve in the unit, the water ring pump must be started firstly. The gases in the pumping system get through the Roots pump(the gases pull the rotor of the Roots pump to circumgyrate automatic like the flowmeter),then enter the water ring pump, finally pump out to the atmosphere. When the suction pressure of the water ring pump(if fixe with ejector in series, then refers to the ejector's suction pressure) reach the initial rated data of the Roots pump(i.e. the allowed discharge pressure)，to start the Roots pump, then the unit begins to work.
　　2)If with the pass-by valve(like the picture5)，to start the water ring pump firstly,then to start the Roots. At the moment, the pressure difference between the inlet and outlet side of the Roots is larger.The pass-by valve opens automatically. Pars of the gases in the container pass the pass-by valve to enter the water ring pump, and another parts also enter the water ring pump under the effect of the Roots. It is obvious that the inhaling speed increases. By this way, it can reach the preparation vacuum requires by the Roots pump rapidally. While the pressure difference between the inlet and outlet side is smaller, the valve shuts off by itself(or by hand). The unit beginns running. Although this method can shorten the preparation vacuum period, the equipment is more complex.
　　（3）The performance relation among the Roots unit, Roots pump, and the fore pump

The capacity of the unit is related with the Roots ability closely. But the Roots has different function according to the fore pump's different type.
　　1）Owing to the gap exists between the rotors, the rotor and the casing, so the return flow exists. Morever, the return flow is influenced by the inlet and outlet pressure, even if the same Roots pump but with different fore pump, the pumping speed is also various .
　　The Pumping Speed of Roots is definited by the below formula：
　　 δ＝δ₀(P₂/P₁/K)
in formula：δ₀－Design Pumping Speed；
　　　　　P₁－Inlet pressure；
　　　　　P₂－Outlet pressure；
　　　　　K－connatural constant，the constant is definited by the Rotor shape,the gap,the ring speed of the Rotor and the outlet pressure.
According the situation above, the suction capacity is influeced by the rate betweent the inlet and outlet pressure. That is to say if to increase the fore pump's pumping speed, the Roots' pumping speed will also increase.
　　（2）the limit pressure is definited by the return flow volume in every gap, leakage of the casing,and the discharge capaciey.It is ：
　　　P0＝(Q₁+Q₂+Q₃)/δ　　　in formula：P0－limit pressure；
　　　　　 δ－Pumping Speed；
　　　　　 Q₁－return flow；
　　　　　 Q₂－leakage quatity；
　　　　　 Q₃－discharge capacity.
In those parameters,Q_{1 is inflenced by the discharge pressure(is the limit vacuum of the fore pump) very greatly. By using the water ring pump as the fore pump, the limit pressure of the Roosts is different according to the differnet saturated steam} pressure of water.

　　Picture6 the Performance Comparison when the Roots fit the different fore pump together

It is obvious from the picture 6, the higher the fore pump limited vacuum, the higher the unit limited vacuum,too. To use two Roots pumps in series can improve the vacuum unit limit vacuum.(In fact, the foremer Roots pump acts as the fore pump of the second). And the smooth part of its performance curves will enlarge too, that is to say the usage scope is extensive(the conclusion is obtained by comparing with the curve 1 and 2, the 3 and 5). The curve of Roots unit 1 is as same as of Roots unit 2. Likely, the curves of the 3,4,and 5 are also samiliar. Morever, the curve of the Roots unit 1 and 2 is different with of the Roots unit 3,4,5 completely. So it illuminates to the same Roots pump, when to select the different fore pump, its performance curve has essential difference. As a result, the fore pump fluences the Roots unit's the performance largely.
　　（4）To select the water ring pump
The so-called water ring pump is a kind of liquid ring pump which using the water as the operating liquid. The usage of the water has many advantages. Such as, low price, easy to obtain, no enviroment pollution. But it also has a big shortage that the high water saturated steam temperature leads to the high suction pressure of the water ring pump. If using the liquid which has low saturated steam temperature as the operating ring, the limited vacuum of the pump can be improved efficiently. For example, the max permitted discharge pressure of a Roots vacuum unit is at 10Torr, it is necessary to add a ejector with the water ring vacuum pump as the fore pump of the Roots vacuum unit. But to use ore oil as the liquid ring, the ejector is not needed, so the combination will be simply.
　　（5）The unit ability and the permitted discharge pressuer of the Roots pump
The unit ability lies on the Roots pump's permitted discharge pressure mostly. The lower the permitted pressure, the smaller possiblity to use the water ring pump as the fore pump. If the permitted pressure of the Roots pump is 1~10Torr，no matter the single or dual-grades water ring pump isn't able to be connected alone with the Roots pump for their limited vacuum is much higher than the permitted pressure. So it is necessary to add the second Roots pump. If the discharge pressure of the Roots pump is allowed above 100Torr, the former water ring vacuum pump is also able to be the fore pump as the Roots pump, this can improve the usage scope of the fore pump.
　　（6）Application example
The production process of a chemical fibre product:low molecule－high molecule－make banding－slice－dry（using the Roots－water ring pump vacuum unit to dry them in vacuum）－raw silk from cocuuns－extending and heating－spin－finished product.
The important working procedure of the process is drying the sheets which cubage is 5×5×5(mm) in order to raw the silk from cocoons. In the process the ideal station is without any moisture content. In fact, it is impossible. So the water content reqires not to exceed 0.02%，if it is higher than the requirament and to raw the silk under the high temperature and pressure, it will decompose the high molecule, even influences the products' intensity.
During the drying process, the usage of the Roots and Water ring vacuum unit's vacuum pumping system installation is shown like the picture7.The technology parameter of every pump in the unit is bellow:

Name	pumping speed (m3/h)	DN of Rotor (mm)	speed (r/min)	power (kW)
Roots pump1	400	160	1450	2.2
Roots pump2	200	102	2900	1.3
Dual-stage water ring pump	100	the fore impeller's dimeter is half of the latter	1450	5.5

　　The brief function of the main equipment is bellow：
　　（1）valve　　shut off the valve, the unit stops running and keeps a peroid vacuum of the dry equipment.
　　（2）automatic pneumatic safy valve　To prevent the returns water in the water ring pump system rushes into the vacuum drying system suddenly when stop the unit accidently.
　　（3）differential pressure valve　When the unit beggins to run, start the water ring pump first.The differential pressure at the two sides of the valve is become big gradually, when reaches the definit date, the valve turns on automaticly. At the moment, the most air of the system passes through the valve to entrance the water ring pump. When the air passes through the ejector pump to enter the water ring pump,the two sides differential pressure would be smaller and smaller gradually untill shut off the valve.Thus the ejector start to work normally, the function of the differential pressure valve is to shorten the preparation pumping time before the ejector pump's normal work.
　　（4）water gauge　　has the funtion as the non-return valve and prevents the water returns back from the water ring pump.
　　The operation sequence is bellow：At first, start the water ring pump,the air of the system passes through the Roots pump(pushing the Rotor of the Roosts pump to circumdyrate）to enter the water ring pump then through the seperator to discharge outside.When pumping ultimated pressure reaches the50Torr,to start the Roots pump2;when the ultimated pressure reaches to 20Torr, then start the Roots1，at last the whole system's pressure can reach the 0.088Torr，in general, can keep under the 1Torr.

　　the capacity parameter about the JZJ series Package Units refers to bellow：

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