THE PROPER OPERATION OF THE HIGH VACUUM PUMPING SYSTEM
PRAVILNO DELOVANJE VISOKOVAKUUMSKEGA ČRPALNEGA
SISTEMA
JOŽE GASPERIČ1
Jožef Štefan Institute, Jamova 39, 1001 Ljubljana, Slovenia
Prejem rokopisa - received: 1997-10-01; sprejem za objavo - accepted for publication: 1997-10-21
The proper operation of high vacuum pumping systems with oil diffusion pumps and backing rotary putnps is very important, otherwise the contamination of vacuum chambers by backstreaming oil vapours is extremely high as well as the oxidation and the loss of diffusion pump oil. The forepressure svvitching point, vvhere the diffusion pump is put into the operation by opening the high vacuum and forepressure valves is discussed. There are three possible situations: the pumping throughput is just adapted, or it is to low, or to high. What is going on, when the pumping troughput is adapted, it is illustrated using the troughput-pressure diagrams. At the end some practical advices are done, how to recognize that the operation of the HV pumping system is correct.
Key words: high vacuum pumping system, contamination, loss of diffusion pump oil, tolerable forepressure, switching point
Pravilno upravljanje in delovanje visokovakuumskega črpalnega sistema z oljno difuzijsko črpalko in rotacijsko predčrpalko je zelo pomembno, kajti v nasprotnem primeru pride do močno povečanega povratnega toka oljnih par iz difuzijske črpalke v vakuumsko komoro in s tem do njene kontaminacije. Izguba olja v difuzijski črpalki in njegova pospešena oksidacija sta spremljajoča pojava. Pri ročnem ali avtomatiziranem upravljanju črpalnega sistema je zelo pomemben trenutek oz. točka preklopa, ko prenehamo črpati s predčrpalko po obtočnem vodu in odpremo visokovakuumski in predvakuumski ventil in začnemo črpati z difuzijsko črpalko. Možna so tri delovna stanja: delovanje obeh črpalk je usklajeno, predčrpalka ima premajhno ali preveliko črpalno hitrost. Usklajeno stanje smo ponazorili na diagramu. Podani so tudi praktični napotki, kako spoznamo, ali je črpalni sistem pravilno upravljan.
Ključne besede: visokovakuumski črpalni sistem, kontaminacija, izguba olja difuzijske črpalke, mejni predtlak, preklopna točka
1	INTRODUCTION
Most usual, the high vacuum pumping systems con-sist of diffusion pumps and corresponding rotary backing pumps. The basic sheme is shovvn on Figure 1. Nor-mally, ali producers of such high vacuum systems give the prescription how to vvork vvith them at the begining, vvhen the vacuum chamber is pumped from atmospheric pressure, down to 104 and 10 5 mbar, respectively, vvhen the technological process is started (e.g. evaporation). In practice this procedure is never exactly followed by us-ers. Only in the čase, when the high vacuum system is fully automatized, the users can avoid the mistakes in the pumping procedure. Usually it happens that not skilled persons change the programme by moving the pressure svvitching points of valves. The results of that action are:
-	by backstreaming oil vapours from the diffusion pump the vacuum chamber as vvell as ali material in it are contaminated
-	the oxidation, deterioration and loss of the diffusion pump oil are very intensive.
The service and cleaning vvork of the system is re-quired.
2	EXPERIMENTAL
The procedure of pumping from atmospheric pres-
sure to the high vacuum is well known. The action of
both pumps has to be adjusted to minimize the pumping
time, which is very important in the industrial produc-tion. If shorter time is required ("The time is money!"), we have to know ali effects vvhich influence on it. One of them is the time svvitching programme of valves, vvhich
Slika 1: Visokovakuumski črpalni sistem
is also depended on the pumping characteristics of both pumps.
The most evident way, how to explain the interrela-tion between diffusion and backing pumps, are their throughput-pressure characteristics. As an example let us use the throughput characteristics of DI 12000 diffusion pump (Leybold) and corresponding backing pump (a combination of Roots pump WA 250 and the one stage rotary pump S 60A). From the diagram (Figure 2) it is evident that both throughput characteristics have the cross-section point M at the throughput QM and pressure pt. The pumps are selected so that the end of the diffusion pump curve is just on the throughput curve of backing pump. The pressure p, is called the tolerable forepressure. Practically, this forepressure is a limit of the diffusion pump operation. The total air flow from the vacuum chamber is now depended only on the backing pump and conductivities of connected lines, valves etc. When the tolerable forepressure takes plače the oil va-pours which are generated in the diffusion pump boiler and ejected through the jet system do not reach the water cooled walls of the pump, vvhere the condensation of them is expected, but one part escapes in the direction of vacuum chamber, vvhere it condenses (contamination!), the other part of oil vapours is pumped by the backing pump (the loss of the oil from diffusion pump). The oxi-dation and deterioration of the oil in the boiler of the pump and on the surface of jet system (temp. approx. 200°C) is accelerated.
Čase 1: When pumping of the chamber is started through the by-pass valve V2 from the atmospheric pressure (VI and V3 are closed) the air throughput Q and the pressure are slowly reducing (the direction from point A to M in diagram, Figure 2). When the point M on the
Figure 2: Throughput Q vs. pressure p Slika 2: Pretok zraka Q v odvisnosti od tlaka p
throughput curve is reached, the theoretical posstbility is done to pump with the diffusion pump. If in this moment the by-pass valve is closed, and VI and V3 are open, from the point M we are "driving" the pumping action of the chamber in the direction of DI 12000 (upper curve) to the next point C, where the pressure is Pc (M2) and forepressure pc (Ml) at the throughput point C'. The pressure pc is lower than tolerable one pt, and the diffusion pump can normally operate. After a longer time of pumping we "come" to the point D, where the throughput is Qd, pressure on high vacuum side Pd and forepressure pd (point D'). We can continue the pumping procedure as long as the operating (or ultimate) pressure is achieved. The diagram on Figure 2 is an example, how the diffusion and backing pumps are well adjusted.
Čase 2: Supposing that the by-pass valve (V2) is closed and other valves (VI, V3) are open in the point A, vvhere the pressure pa is higher then the tolerable forepressure pt. The pump capacity of the diffusion pump is totally lost. The diffusion pump is novv like "a cooking pot" for oil. Bad results vvere described above, although the pumping action of backing pump is conttnued. The situation is normalized, vvhen the point M is achieved and after that, as described above. This means that on such a way vve can not shorten the total pumping time.
Čase 3: If the by-pass valve (V2) is closed later, e.g. at the point E, and valves VI, V3 are open in next moment, the diffusion pump as vvell as backing pump throughput suddenly "jump" to the value at E' and the forepressure (Ml) increases from pe to pe". If in our pumping procedure is such a čase, vve had lost the time, vvhich is needed to pump the chamber from E" to E vvith backing pump.
We have described three characteristical situations in the operating procedure, and novv vve can ask ourselves, how to recognize that the high vacuum pumping system is operating correct or not. Normally, vve have not the
Figure 3: Pumping time characteristics of the adjusted high vacuum pumping system: a) inproper operation, b) proper and optimal operation, c), d) proper but not optimal operation Slika 3: Črpalna karakteristika usklajenega visokovakuumskega sistema: a) nepravilno delovanje, b) pravilno in optimalno delovanje, c), d) pravilno, vendar ne optimalno delovanje
pumping diagram (like that shown on Figure 2) at the hand.
The best way is monitoring of the both pressures readings Ml and M2. If the forepressure (Ml) increases to the values, vvhich are higher than the torelable forepressure, vvhen vve close V2 and open VI and V3, the operation of the diffusion pump is stopped (oil back-streaming and contamination of vacuum system, the oxi-dation and loss of the oil begin) (Figure 3a). The situ-ation is normalized, vvhen the pressure (Ml) is lovver then the tolerable forepressure.
The optimal čase is, vvhen after svvitching of the valves, as desribed above, the forepressure (Ml) increases just to the value p, (tolerable forepressure), Figure 3b. Ali other cases, as vve can see on the pumping characteristics, Figure 3c, d, are also proper but not op-
timal, vve are loosing time, vvhich is very important, especialy in the production in the industry.
It is also very simply to practically identify the tolerable forepressure. For most types of oil diffusion pumps it is betvveen 0,1 and 0,5 mbar. Many other cases exist, vvhere the pump combinations are not adapted (e.g. the pumping speed of backing pump is to high or to lovv) vvhich vvill be described later in a separate paper.
3 CONCLUSION
It is very simple to estimate that a high vacuum sys-tem is operating optimaly or not by observing the pressure changes in the vacuum chamber as vvell as the forepressure of diffusion pump during the svvitching period of values.