US2630266A - High-vacuum device - Google Patents

Description

March 3, 1953 R. B. LAWRANCE 2,530,266

HIGH-VACUUM DEVICE Filed April 15, 1951 INVENTOR. Richard B. Law/"once ATTORNEY FIG. I

UNITE fii'i'tZS 'iilN'l' @FFHCE- HIGH-VACUUM DEVICE Richard B. Lawrance, Cambridge, Mass., assignor to National Research Corporation, Cambridge, Mass, a corporation of Massachusetts Application April 13, 1951, Serial No. 220,831

7 Claims. (01. 230-101) l 2 This invention relates to a high-vacuum device tend as far as possible into the region of high and more particularly to an improved highinlet pressures. The inlet pressure depends on vacuum diffusion pum the amount of gas being pumped, on the inlet di- A principal object of the present invention is to mensions, and other characteristics of the top jet provide an improved diffusion pump which is b of the pump. As long as the next lower jet (the rugged in construction and cheap to manufacture. bottom jet in the embodiment shown) is ef- Another object of the invention is to provide fectively removing the pumped gases from the a diffusion pump which has high pumping speeds region between the top jet and the next lower jet,

against high forepressures. the volumetric pumping speed of the pump is es- Still another object of the invention is to prosentially independent of inlet pressure. Back vide a diffusion pump of the above type which leakage through the top jet, and a consequent is particularly useful as a booster pump, and reduction of volumetric pumping speed, will ocwhich is capable of employing relatively cheap cur when the pressure in the region between the pump oils with a high degree of emciency. top jet and the next lower jet exceeds a certain Other objects of the invention will in part be amount. This undesirable condition can always obvious and will in part appear hereinafter. be produced by increasing the flow through the The invention accordingly comprises the appump excessively, and constitutesa primary limiparatus possessing the construction, combinatation on the pump performance at high inlet tion of elements and arrangement of parts which pressures; it can also be produced in the normal are exemplified in the following detailed disclo- 20 workin range of flow if the pumping speed of the sure, and the scope of the application of which next lower jet is intentionally or accidentally will be indicated in the claims. made too small. In the case of pumps having For a fuller understanding of the nature and three or more stages the same considerations objects of the invention, reference should be had apply for each successive pair of stages. to the following detailed description taken in con- The over-all forepressure characteristics of the nection with the accompanying drawing wherepump depend primarily on the attributes of the in: bottom jet. In order to obtain operation against Fig. 1 is a diagrammatic sectional view of one high forepressures, the gap between the bottom preferred embodiment of the invention; and jet umbrella and the inner wall of the pump body Fig. 2 is an exaggerated enlarged view of a por- 30 must be relatively small, on the order of A inch tion of Fig. 1. or less. The dimensions of this gap, for a pump In general, the present invention is directed to of a given diameter, are fairly critical. If the an mp ved high-vacuum diffusion pump and gap is too small it restricts the flow of the gas particularly to a diffusion pump which is capable which is being pumped by the top jet, resulting of high pumping speeds against high forepres- 5 in unsatisfactory pumping speeds at high inlet SureS- In a preferred embodiment of the invenpressures. If the gap is too large it permits'back tion the pump comprises a vertical pump body leakage of the gas which is being pumped by the With at least two oil vapor jets positioned therebottom jet. It can thus be seen that the res p p y has foreline fl quirements for this gap are essentially contrafrom the lower portion thereof, and includes a 40 dictory, and the optimum design thereof is a boiler section at the bottom thereof. Heating compromise falling within fairly narrow limits. means are provided inside of the boiler section for In the cooling of diifusion pumps several convaporizing the pump oil inv the boiler section, siderations are involved. It is desired (a) to n a li 0 1 i arranged to pr v de oncondense substantially all of the oil vapors densation of the oil vapors on the inner wall of the 45 which strike the wall of the pump body, (7)) to pump body, the oil flowing down this inner wall trap any back-streaming oil vapors, and (c) to on its return path tothe boiler. maintain the temperature of the oil, to which In operating diffusion pumps of the general the undersides of the respective jets are exposed, type described above several. considerations apsufficiently low so that the oil vapor pressure does ply. When the pump is tobe used as a booster 0 not materially interfere with the proper operapump, i. e., one which has blank-of)": pressures tion of the jets. Since the viscosity of the oil on the order of 18" to 16* l-Ig abs, and is drastically increases as its temperature decreases, to operate against forepressures in the neighborthe cooling requirements must be balanced hood of approximately 1 mm., it is desired that against the other requirements of the pump. On the pumping speed of the pump be high and ex- 00 the one hand, the oil flowing back to the boiler must be cool enough so that it does not have an excessive vapor pressure. On the other hand, the oil must be sufficiently warm so that it flows in a thin film down the pump body. This latter requirement is particularly important in the light of the critical limitations placed on the gap between the bottom jet umbrella and the pump body.

Other requirements for a pump of this general type are that it be cheap to manufacture and cheap to operate. These requirements impose, on the other considerations, limitations such as the use of metal tubing (preferably with straight sections), ease of disassembly, the elimination of unduly complicated piping, and the use of a relatively cheap pump oil.

In the present invention the desired operating characteristics of a booster pump, of the general type described above, are obtained by careful control of the cooling conditions prevailing in the pump. This is true despite the use of pump oils which have radical variations in vapor pressure and viscosity for relatively small changes in temperature. K

These results are achieved in the present invention by applying maximum cooling to those portions of the pump where excessive oil'viscosities cannot interfere with the operation of the pump, and by warming the oil where low viscosity thereof is of the utmost importance. In the physical embodiment illustrated this variation in cooling and warming effects is accom: plished in a simple, dependable manner by so placing the cooling coil that it operatesb 'fih as a coolingmeans and as a warming means,at different portions of the pump. 'With' this arrangement maximum cooling of the oil takes place above and at the top of the jet, while warming of the oil takes place adjacent and above the gap between the bottom jet umbrella and the pump body.

In the simple construction shown a single cooling coil is employed, and its positioning on the pump body is such that the water flowing therethrough serves both of-thesecooling and warming functions. This single cooling coil, which obviously does not have to be made from a single piece of tubing, is arranged so that it has two sections of maximum cooling, the first of these sections being at the top of a pump body, above the top jet and in the region where the vapors from the top jet strike the pump body. The second maximum cooling section is on the foreline of the pump. The cooling coil then includes a warm-up section, in which the cooling water is warmed to the desired temperature. The cooling coil finally includes an oil-warming section adjacent the gap between the bottom jet umbrella and the interior of the pump body. This oil-warming section preferably extends up the pump body to just below the region where the vapors from the top, jet are condensed. The cooling water flowing through the coil thus first chills the top of the pump, then chills the foreline, then is warmed by the heat generated in the pump boiler, and finally warms the pump oil flowing down the interior of the body from the top jet. By controlling the speed of flow of water through this cooling coil the temperature of the various portions of the pump body may b accurately controlled;

As outlined briefly above, the critical size of the gap between the bottom jet umbrella and the inner wall of the pump body requiresan equilibrium temperature for the oil. which is 4 sufliciently high to permit this oil to be freeflowing. This temperature is preferably such that the pump oil has a vapor pressure less than about 10 microns and has a viscosity less than about 50,000 Saybolt Universal seconds. When operating within this temperature range the pump oil will flow smoothly down the inner wall of the pump body, and will remain as a thin film as it passes through the gap between the bottom jet umbrella and the inner wall of the pump body. If the pump oil is too cold the film of oil will have an excessive thickness and will reduce the area of this gap at the bottom jet umbrella sufiiciently so as to drastically reduce the pumping speed of the pump. The viscosity of the oil must be such that the oil film passing through this gap at the bottom jet be thin enough so that it obstructs no more than about 30% of this gap. It preferably reduces this gap by about only 10%.

Referring now to Figs. 1 and 2 it can be seen wherein the above described features of the invention are embodied in the illustrated pump construction. In Fig. 1 the pump body to is open at the top, as indicated at l I, for connection to a system to be evacuated. Inside of the pump body there is provided a jet assembly generally indicated at l2. In the preferred form shown the jet assembly preferably comprises a lower cylindrical portion M which terminates at its top in a jet :6. The jet umbrella 10, for the lower jet l6, defines a narrow gap II with the inner wall of the pump body It. It is this gap II which has such critical limitations as to its size. With a 4 inch pump body diameter this gap is about 0.150 inch when the pump is used as a booster pump.

The lower cylindrical portion M of the jet tube is preferably concentrically positioned with respect to the pump body In and defines a cylindrical lower pumping chamber [8 with the pump body. The jet assembly also includes a second jet tube 20 which terminates in a top jet 22. This second jet tube is preferably of smaller diameter than the first jet tube l4, and defines an upper pumping chamber with the pump body. The foreline 24 is connected to the pump body adjacent an opening 26 therein, this foreline in operation being preferably connected to a suitable mechanical vacuum pump.

At the bottom of the pump body there is provided a boiler section 28 which is arranged to confine a quantity of pump oil 30. This pump oil is preferably heated by a plurality of heaters, one of which is schematically indicated at 32. The pump oil 30, in its return from the pumping chambers, fiows back into the boiler by means of openings 34 cut in the bottom of the lower jet tube H. A helical coil 36 is preferably wound around the lower jet tube [4 and substantially completely bridges the gap between this lower jet tube and the inner face of the pump body so as to provide a long path for the return of the oil to the boiler section. This construction is described and claimed in the copending application of Lawrance and Middleton, Serial No. 189,444, filed October 10, 1950, now Patent No. 2,585,139.

A cooling coll, indicated at 38, starts at the top of the pump body where a few turns constituting a first portion 38a thereof extend downwardly substantially past the region where the vapors from the top jet strike the inner wall of the pump body. The cooling coil has been illustrated as a continuous coil with a single designating numeral 38. For simplicity of functional description the component portions of the cooling coil have been given alphabetical subscripts such as 38a, 381), etc. The cooling fluid is preferably water and this cooling coil 38 preferably comprises a copper tubing. Since relatively cold water is employed, the top portion of the pump body, surrounded by'the portion or section 38a of cooling coil 38, is maintained relatively cold by the cold water which is flowing therein. The cooling coil next extends to the foreline and is wrapped around this foreline, as at 381), until it approaches the pump body.

Ihe cooling coil then conducts the slightly warmed water to the hot part of the pump, i. e., the boiler section thereof. That portion of the cooling coil surrounding the top of the boiler section of the pump is indicated at 33c and is wrapped quite closely around this portion of the pump. The turns 380 of the cooling coil proceed up the outside of the pump body. The greater number of turns per inch at the lower section of the pump body is for the purpose of permitting the cooling water to be heated by hot oil flowing downwardly on the inside of the pump body and by the heat emanating from the heaters and the hot vapors inside of the pump. By the time the water in the cooling coil, proceeding in an upward direction, reaches portion 38d of the cooling coil adjacent the bottom jet of the pump, it has attained a temperature which is within the desired range of pump oil temperatures. This heating of the water in coil 380 also has the decided advantage of maintaining at a low temperature that oil which boiler past the spiral 3B. This oil temperature is preferably sufficiently low so that the bottom jet is not subjected to an excessive oil vapor pressure in the pumping chamber i8.

By the time that the cooling water has traveled upwardly beyond the foreline opening 2'5 it has attained a temperature such that the inner wall of the pump is hot enough to permit the pump oil to flow freely, but is cool enough so that the oil has a vapor pressure less than about microns. At this point the water differs in temperature by a few degrees from the pump oil, due to the heat transfer gradient through the oil film and the pump body. A valve ii} is provided, preferably on the inlet side of the cooling coil, to control the flow of cooling water therethrough so that its outlet temperature (which is a measure of the water temperature in coil 38d) may be adjusted to fall within the desired temperature range.

In Fig. 2 there is shown an enlarged exaggerated sectional view of a portion of the Fig. 1 pump adjacent the bottom jet. As can be seen from Fig. 2 the vapors 3t generated in the boiler are directed downwardly and outwardly as they leave the bottom jet IS. The pump oil 35, condensed from the top jet, flows down the inside of the pump body and somewhat reduces the size of the gap I! between the outside of the bottom jet umbrella i9 and the inside of the pump body iii. As long as this layer of oil is maintained freefiowing, the thickness thereof can be readily controlled and kept below about 0.05 inch. When the oil has a viscosity less than about 50,000 Saybolt seconds, it operates satisfactorily and does not unduly restrict this gap ll. However, as the viscosity of the oil 3!] rises above about 50,009 Saybolt seconds it tends to flow more thickly and sluggishly and to form large ripples which may completely block portions of the gap i'l. When this occurs the top jet, of course, can do no is flowing down to the pumping whatsoever-since there is no place for the pumped gases to escape.

When operating with a pump oil such as a pentachlorinated'diphenyl the water temperature at the outlet end of the cooling coil is preferably maintained between about 77 to F. Between these temperatures the vapor pressure of the pump oil is sufficiently low so that it cannot interfere with the pumping of the top jet, while the viscosity of the oil is sufficiently low so that it does not appreciably block the 0.15 inch gap il adjacent the bottom jet. Since the inlet water temperature will be about i070 F., the vapors from thetop jet are substantially completely condensed when they strike the pump body. Even though the oil from the top may be chilled too much when first condensing on the wall, it is soon warmed-up by the oil-warming portion 38d so that it has the proper low viscosity as it approaches the gap l'l.

In one preferred embodiment of the invention the pump body it is made of a four-inch stainless steel tube, while the interior portions of the jet assembly are made of aluminum. With a fourinch pump of this type the gap i1 is referably about 0.150 inch. The cooling coil 33 preferably comprises a copper tubing. The boiler section of the pump also preferably includes an additional cooling coil, not shown, which is operated to cool down the pump oil in the boiler when the operation of the pump is stopped and it is desired to open the pump interior to atmospheric pressure. 1

While the preferred embodiment of the inven-- tion utilizes a cooling coil, which has greatadvantages from the standpoint of economy of construction, a cooling jacket may be equally employed. Separate heating of the cooling water may equally be utilized, but obviously this will add additional expense and will not have the important advantage of controlling the oil temperature as the oil approaches the boiler.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: y

l. Anoil diffusion pump comprising a vertical pump body with at least a bottom oil vapor jet and a top oil vapor jet positioned therein, the bottom jet including a jet umbrella spaced by a gap from the pump body, pump cooling means mounted exteriorly of said pump body through which a cooling fluid is circulated, a foreline extending from the lower part of said pump body, an oil boiler section at the bottom of said pump body, and heating means associated with said boiler section for vaporizing pump oil in said boiler section, said pump body being open at the top for connection to a system to be evacuated, the cooling means including several portions, a first portion of said cooling means providing maximum cooling to the top of the pump body and that portion of the pump body Where vapors from said top jet strike the pump body, said cooling means including a cooling-fluid warming portion surrounding the lower portion of said pump body adjacent said boiler section, said cooling means extending upwardly from said warming portion past the gap between the-pump body and the bottom jet umbrella and up to about the first cooling portion, said portions of said cooling means being connected together so that the flow of cooling fluid through said cooling means is first adjacent the top of the pump, then to the lower part of the pump body where the cooling fluid is heated by the lower part of said pump body, and then upwardly past the bottom jet, and mean for controlling the rate of flow of cooling fluid through said cooling means so that the cooling fluid, when reaching said gap, has a temperature between that temperature at which the pump 011 is free-flowing and that temperature at which the pump oil has an appreciable vapor pressure.

2. An oil diffusion pump comprising a vertical pump body with at least a bottom oil vapor jet and a top oil vapor jet positioned therein, the bottom jet including a jet umbrella spaced by a gap from the pump body, a cooling coil mounted exteriorly of said pump body through which a cooling fluid is circulated, a foreline extending from the lower part of said pump body, an oil boiler section at the bottom of said pump body, and heating means associated with said boiler section for vaporizing pump oil in said boiler section, said pump body being open at the top for connection to a system to be evacuated, the cooling coil including several portions, a first portion of said cooling coil providing maximum cooling to the top of the pump body and that portion of the pump body where vapors from said top jet strike the pump body, said cooling coil including a coo1ing-fluid warming portion surrounding the lower portion of said pump body adjacent said boiler section, said cooling coil extending upwardly from said warming portion past the gap between the pump body and the bottom jet umbrella and up to about the first cooling portion, said portions of the cooling coil being connected together so that the flow of cooling fluid through said cooling coil is first adjacent the top of the pump, then to the lower part of the pump body where the cooling fluid is heated by the lower part of said pump body, and then upwardly past the bottom jet, and means for controlling the rate of flow of cooling fluid through said coil so that the cooling fluid, when reaching said gap, has a temperature between that temperature at which the pump oil is free-flowing and that temperature at which the pump oil has an appreciable vapor pressure.

3. An oil diffusion pump comprising a vertical pump body with at least a bottom oil vapor jet and a top oil vapor jet positioned therein, the bottom jet including a jet umbrella spaced by a gap from the pump body, pump cooling means mounted exteriorly of said pump body through which a cooling fluid is circulated, a foreline extending from the lower part of said pump body, an oil boiler section at the bottom of said pump body, and heating means associated with said boiler section for vaporizing pump oil in said boiler section, said pump body being open at the top for connection to a system to be evacuated, the cooling means including several portions, a first portion of said cooling means providing maximum cooling to the top of the pump body and that portion of the pump body where vapors from said top jet strike the pump body, said cooling means including a cooling-fluid warming portion surrounding the lower portion of said pump body adjacent said boiler section, said cooling means extending upwardly from said warming portion past the gap between the pump body and the bottom jet umbrella and up to about the first cooling portion, said portions of said cooling means being connected together so that the cooling fluid flows through said cooling-fluid warming portion where it is heated by the lower part of the pump body before flowing past the bottom jet, and means for controlling the rate of flow of cooling fluid through said cooling means so that the cooling fluid, when reaching said gap, has a temperature between that temperature at which the pump oil is free-flowing and that temperature at which the pump oil has an appreciable vapor pressure.

4. An oil diflusion pump comprising a vertical pump body with at least a bottom oil vapor jet and a top oil vapor jet positioned therein, the bottom jet including a jet umbrella spaced by a gap from the pump body, a cooling coil mounted exteriorly of said pump body through which a cooling fluid is circulated, a foreline extending from the lower part of said pump body, an oil boiler section at the bottom of said pump body, and heating means associated with said boiler section for vaporizing pump oil in said boiler section, said pump body being open at the top for connection to a system to be evacuated, the cooling coil including several portions, 2, first portion of said cooling coil providing maximum cooling to the top of the pump body and that portion of the pump body where vapors from said top jet strike the pump body, said cooling coil having a portion thereof surrounding said foreline to apply a relatively high degree of cooling thereto, said cooling coil including a cooling-fluid warming portion surrounding the lower portion of said pump body adjacent said boiler section, said cooling coil extending upwardly from said warming portion past the gap between the pump body and the bottom jet umbrella and up to about the first cooling portion, said portions of the cooling coil being connected together so that the flow of cooling fluid through said cooling coil is first adjacent the top of the pump, next to the foreline, then to the lower part of the pump body where the cooling fluid is heated by the lower part of said pump body and then upwardly past the bottom jet, and means for controlling the rate of flow of cooling fluid through said coil so that the cooling fluid, when reaching said gap, has a temperature between that temperature at which the pump oil is free-flowing and that temperature at which the pump oil has an appreciable vapor pressure.

5. The pump of claim 3 wherein the gap between the pump body and the bottom jet umbrella is on the order of 0.15 inch and the temperature of the pump oil flowing therepast is such that the pump oil flows in a film not substantially greater than 0.05 inch thick.

6. The pump of claim 3 wherein the gap between the pump body and the bottom jet umbrella is on the order of 0.15 inch and the temperature of the pump oil flowing therepast is such that the pump oil has a viscosity less than about 50,000 Saybolt Universal seconds.

"I. The pump of claim 3 wherein the gap between the pump body and the bottom jet umbrella is on the order of 0.15 inch, the pump oil is a pentachlorinated diphenyl and the temperature of the pump oil flowing therepast is between about '77 F. and F.

RICHARD B. LAWR-ANCE.

N 0 references cited.

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