US3445859A - Diffusion vacuum pump apparatus - Google Patents

Description

TEMPERATURE (F) J. D. COHOON 3,445,859

DIFFUSION VACUUM PUMP APPARATUS Filed July 1, 1965 E @U5O .02 .0: 0s .O(8 .|00 lzweizior:

I GAP I N JWD- 0050032 United States Patent 3,445,859 DIFFUSION VACUUM PUMP APPARATUS James D. Cohoon, Melrose, Mass., assignor to Dresser Industries, Inc., Dallas, Tex., a corporation of Delaware Filed July 1, 1965, Ser. No. 468,847 Int. Cl. F22b 29/00; H05b 3/02 US. Cl. 219-275 8 Claims ABSTRACT OF THE DISCLOSURE The pump comprises a cylindrical housing having a stainless steel base plate forming the lower portion of t e pump boiler. A heater plate, which is bolted to the underside of the base plate, is fabricated from cast iron; and electrical conductors are embedded in grooves in th heater plate. The base plate and heater plate are of different materials therefore there are different degrees of thermal deflection of the two plate members during operation. The thickness of the base plate is det rmined by a formula which takes into consideration the radius of the base plate, the normal design operating pressure of the pump, the normal design heat flux passing through the interface between the heater plate and base plate, and the physical characteristics of the base plate and heat plate materials. The formula establishes a base plate thickness which will result in a predetermined deflection of the base plate, resulting from the pressure differential across the base plate, which pressure deflection is just sufficient to compensate for the difference in the thermal deflection of the plates. The total deflection of the two plates is then the same to provide good surface contact over the entire interface area of the plates.

This invention relates generally to diffusion vacuum pumps and more particularly to a unique boiler assembly for such pumps.

The operation of diffusion vacuum pumps is generally well known. A pumping fluid is evaporated in a heated boiler of the pump and the resulting vapor directed at supersonic velocity through a nozzle system to be fully condensed on a cold surface. The vapor stream, while passing between the nozzle system and the condensing surface, accepts by diffusion gas molecules from the system being evacuated and compresses these molecules into a higher pressure area which normally communicates with a mechanical backing pump. The liquid produced on the condensing surface returns to the pump boiler to be reheated and re-evapo-rated.

There exist commercially two common boiler designs for supplying evaporation inducing heat to the pumping fluid of diffusion vacuum pumps. One arrangement provides heating elements within the pump casing and submerged in the pumping fluid pool so that heat is transferred directly from the heating elements to the pumping fluid. The other common boiler arrangement incorporates a heating plate attached outside the pump casing in intimate contact with the pump boiler bottom. In this type heat is transferred from the heating plate through the boiler bottom to the pumping fluid.

The latter arrangement, while offering the advantage of mechanical simplicity and low cost, suffers the disadvantage of susceptibility to heater plate failure. These failures normally result from overheating of the heater plate because of an insufficient heat transfer from the heater plate into the boiler bottom. The failures have become more prevalent with the advent of modern high performance diffusion pumps which require significantly higher heat flux between the heater plate and the boiler bottom.

The object of this invention therefore is to provide a diffusion vacuum pump which utilizes the mechanically Patented May 20, 1969 "ice simple external heater plate, exhibits the high pumping speeds obtainable with higher heat fluxes, and is not susceptible to excessive incidence of pump heater burn out.

One feature of this invention is the provision of a diffusion vacuum pump having a boiler base plate and attached external heater plate constructed of dissimilar materials and wherein the mechanical characteristics of the boiler base plate and contacting heater plate are such that during normal operation of the pump the combined thermal and pressure deflection of the boiler base plate will be substantially equal to the thermal deflection of the heater plate.

Another feature of this invention is the provision of a diffusion vacuum pump of the above featured type wherein the boiler base plate is circular and of such thickness as to be substantially deflected by the pressure differential across the base plate during normal operation of the pump thereby permitting an equalization of deflection experienced by the boiler base plate and external heater plate.

Another feature of this invention is the provision of a diffusion vacuum pump of the above featured types wherein the boiler base plate has a uniform thickness which is less than a certain calculable thickness dependent on the dimensions of the pump, the materials of construction and the normal pressure and temperature operating conditions for which the pump is designed.

Another feature of this invention is the provision of a diffusion vacuum pump of the above featured types wherein the pump is designed for normal operation with a heat flux across the interface between the heater plate and boiler base plate which is greater than 15,000 B.t.u. per hour per square foot of interface area.

Another feature of this invention is the provision of a diffusion vacuum pump of the above featured types wherein the thermal expansion coefficient of the boiler base plate material divided by its thermal conductivity is greater than the thermal expansion coefficient of the heater plate material divided by its thermal conductivity.

Another feature of this invention is the provision of a diffusion vacuum pump of the above featured types wherein the heater plate comprises a circular block having its surface interrupted by indentations and including elect-rically conductive coils positioned in and conforming to the indentations so as to be in close contact with the circular heater block.

Another feature of this invention is the provision of a diffusion vacuum pump of the above featured types wherein the boiler base plate is constructed of stainless steel and the heater plate is constructed of iron.

These and other features and objects of the present invention will become apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a partial schematic sectional drawing of a simplified diffusion pump according to the present invention;

FIG. 2 is a partial schematic drawing illustrating the thermal mismatch between boiler base plate and heater plate which contributes to heater plate failure; and

FIG. 3 is a diagram which plots for a particular pump the outside face temperature of the heater plate versus the resulting air gap existing between the heater plate and boiler base plate.

Referring now to the drawings, FIG. 1 shows a cylindrical pump housing 11 having its bottom end gas tightly closed by the circular base plate 12. Supported by the base plate 12 is a jet assembly 13 which is partially submerged in the pumping fluid bath 14.

Also attached to the base plate 12 and extending therefrom in a direction opposite to the pump housing 11 is the 3 cylindrical skirt 15. Within the skirt is the heater plate 16 which is attached in intimate contact with the base plate 12. The heater plate 16 is maintained in position by the securing nuts 17 screwed upon the threaded shanks of the studs 18 which extend from the base plate 12 through apertures in the heater plate 16.

The bottom surface of the circular block heater plate 16 is indented by a spiral groove which accommodates the electrically conductive heater wire 21. The heater wire 21 is forced into intimate contact with the surfaces of the groove 19 by, for example, peenin'g.

During operation of the diffusion pump shown in FIG. 1 an electrical current from a suitable power source (not shown) is conducted through the heating wire 21. The resistance heating produced is conducted into the heater plate 16 and across the interface 22 into the boiler base plate 12. Heat is subsequently conducted into the pumping fluid pool 14 causing evaporation and the well known diffusion pumping effect within the pump housing 11.

Referring now to FIG. 2 there is shown an exaggerated illustration of the physical phenomena that produces many of the heater plate failures experienced in pumps of this type. The heat flux generated by the heater wires '21 causes the circular base plate 12 and the circular heater plate 16 to deform spherically. However as shown in FIG. 2 the degree spherical deflection in the base plate 12 and heater plate 16 are not equal. This inequality of deflection can result from various causes. For example, in the preferred pump embodiment shown in FIG. 1 the base plate 12 is constructed of stainless steel while the heater plate 16 is constructed of cast iron. Accordingly the two components exhibit different coefficients of thermal conductivity and expansion causing them to assume spherical shapes of different radii. Also in the embodiment shown the attached pump housing 11 will exert a restoring moment on the base plate 12 which moment is not exerted on the heater plate 16.

FIG. 2 illustrates uneven deflection for a hypothetical situation in which the heater plate 16 is unbolted or unrestrained. In actual practice, with the heater plate bolted as shown in FIG. 1, the bolts 18- attempt to prevent the formation of the gap 23 but are stressed beyond their yield point. The bolts subsequently yield and a gap is formed reducing the area of contact between the base plate 12 and the heater plate 16. The heat flux across the reduced contact area in the center portion of the base plate 12 causes an increase in the spherical deflection mismatch between the two plates. This catastrophic elfect will continue raising the temperature on the face of the heater plate 16 until melting and destruction of the heater element occur.

FIG. 3 is a diagram showing for a typical pump the relationship between the temperature on the face of the heater plate 16 and the gap 23 between the heater plate 16 and the base plate 12. The pump used had a cast iron heater plate 16 and a stainless steel boiler plate 12 with a diameter of about 6 in. The calculated values plotted in the initial portion of the curve of FIG. 3 illustrate the catastrophic effect on heater plate temperature of an increasing air gap 23. It was calculated that failure of the heater element for the particular pump would occur if the gap 23 reached a magnitude of about .005 inch. The length of gap 23 of course decreases radially toward the center of the plates but the gaps considered were the maximum gaps existing at the circumference of heater plate 16. Furthermore the forces generated by the thermal mismatch are so extreme that it was found impractical to mechanically solve the heater failure problem by increasing the bolt strength.

The present invention uniquely solved the problem by changing the structural features of the pump to exploit a physical phenomena existing in the operating pump. During pump operation there is a pressure differential across the base plate 12 because of the reduced pressure existing within and the atmospheric pressure existing outside the pump housing 11. This pressure differential produces an inward deflection of the base plate 12 in opposition to the outward spherical deflection caused by the heat flux. A base plate 12 thickness can be calculated which under normal operating conditions will result in a certain base plate pressure deflection. Accordingly a combined thermal and pressure deflection can be established for the base plate 12 which is substantially equal to the thermal deflection of the heater plate 16 under normal pump operating conditions. In this way the stress on the securing bolts 18 can be reduced to a minimum and the above described heater failure eliminated.

Matching the base plate and heater plate deflections for a given pump entails calculation and use of a base plate bottom thickness such that thermal deflection P of the base plate as restrained by the attached pump housing plus the pressure deflection 5 of the restrained base plate will substantially equal the thermal deflection P of the unrestrained heater plate with the pump operating at rated pressure differential AP across the base plate and heat flux Q across the interface between heater plate and base plate. Under these conditions the stress on the restraining bolts will be substantially zero and the failure problems eliminated.

I Some degree of base plate pressure defection will obviously exist regardless of its thickness. Practically, however a substantial degree of pressure defection must be provided if a real thermal deflection mismatch is to be solved. For example, if the thermal deflection inequality is significant a pressure deflection of at least 1.4 10- times the diameter of the circular base plate must be provided if the mismatch is to be adequately compensated. This magnitude of pressure deflection is measured at the center of the circular base plate.

The restoring moment exerted on the plate 12 by the pump housing 11 in many pump embodiments will be negligible. This is especially true where a thermal gradient exists in the pump housing 11 during operation of the pump. Such a gradient tends to reduce any restoring moment exerted by the pump housing 11. In such instances the above equalization of heater plate and base plate deflections can be obtained by calculating a base plate thickness in accordance with the following equation:

2 II QLEZ 2Em Q k2 where AP represents the normal operating pressure differential for which the pump is \designed across the base plate 12 in pounds per square inch (p.s.i.), E represents the modulus of elasticity of the base plate 12 in p.s.i., m represents the inverse of Poissons ratio, a represents the outside radius of the base plate 12 in inches, Q represents the normal operating heat flux in B.t.u.s per hour per square ft. (B.t.u./hr.-ft. across the interface between the heater plate 16 and the base plate 12, a represents the thermal expansion coefficient of the base plate 12 in inches per inch per F. (in./in./ F.), k represents the conductivity of the base plate 12 in B.t.u.s per foot per hour per square foot per F. (B.t.u.-ft./hr.-ft. F.), 00 represents the thermal expansion coefficient of the heater plate 16 in in./in./ F. and k represents the thermal conductivity of the heater plate 16 in B.t.u.-ft./hr.-ft. F. Tests with such pumps have shown that use of base plate thickness 10% greater than indicated by the above formula will result in a significant incidence of heater failure.

Another important characteristic of the invention is the provision of a securing nut 17 and a threaded stud 18 through the center of the heater plate 16. This feature permits the operation of a pump designed as described above at lower than rated heat flux. Under such a condi tion the pressure deflection 6 of the base plate 12 will more than compensate for the difference in thermal deflections P P of the base plate 12 and the heater plate 16.

Thus an undesired separation gap will tend to form at the center of these elements. However, the restraining force of the centrally located nut and stud will prevent the formation of this gap. It will be understood that the mechanical force necessary to prevent this separation is substantially less than that required to prevent the thermally induced circumferential separation described above. For this reason the use of conventional bolting techniques are adequate.

Because the deflection caused by the pressure gradient across the base plate 12 inherently can be obtained in only one direction the present invention is of particular value in a diffusion vacuum pump wherein the temperature induced deflection of the base plate 12 is greater than that of the heater plate 16. This condition will exist if the 6/ k of the base plate material is greater than that of the heater plate material where 6 represents the thermal expansion coefficient in in./in./ F. and k represents the thermal conductivity in B.t.u.-ft./hr.-ft. F.

It will also be recognized that the problem solved and therefore the solution itself is uniquely related to diffusion vacuum pumps adapted to operate with a relatively high heat flux across the interface between the heater plate and the base plate. By relatively high is meant a heat flux greater than 15,000 B.t.u./hr.-ft. Such high heat fluxes have come into extensive use only recently with the advent of diffusion pumps designed to provide increased performance characteristics.

Thus, the present invention provides a much improved diffusion vacuum pump having high performance capabilities and exhibiting a strong resistance to heater failure.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A diffusion vacuum pump apparatus comprising a cylindrical pump housing adapted for connection with a chamber to be evacuated, a circular substantially uniform thickness base plate composed of a certain heat conducting metal and gas tightly sealed to said cylindrical pump housing so as to close the bottom end thereof forming a pump boiler portion adapted to contain a pumping fluid, a heater plate composed of a heat conducting metal dissimilar in composition to said certain metal and attached to the exterior of said pump housing in close contact with said circular base plate, said heater plate being adapted to supply heat energy through said circular base plate to the pumping fluid in said pump boiler portion, and wherein the thickness of said circular base plate is such that under normal operating conditions the designed normal operating pressure differential existing across said circular base plate will cause a spherical deflection thereof which deflection as measured by the total linear deflection perpendicular to and at the center of said circular base plate will have a magnitude of at least 1.4 10 d where d represents the diameter of the circular base plate.

2. A diffusion vacuum pump apparatus according to claim 1 wherein the 6/k of said base plate material is greater than that of said heater plate material where 5 represents thermal expansion coeflicient in in./in/ F. and k represents thermal conductivity in B.t.u.-ft./hr.- ft. F.

3. A diffusion vacuum pump apparatus according to claim 2 wherein said heater plate comprises a circular block having a surface interrupted by indentations and electrically conductive means positioned in and conforming to said indentations so as to be in close contact with said circular block.

4. A diffusion vacuum pump apparatus according to claim 1 wherein said heater plate comprises a circular block having a surface interrupted by indentations and electrically conductive means positioned in and conforming to said indentations so as to be in close contact with said circular block.

5. A diffusion vacuum pump according to claim 1 in cluding bolt means acting at the center of said circular base plate and said heater plate to restrain relative parting movement between the center portions thereof.

6. A diffusion vacuum pump apparatus comprising a cylindrical pump housing adapted for connection with a chamber to be evacuated, a base plate composed of stainless steel and gas-tightly sealed to said cylindrical pump housing so as to close the bottom end thereof forming a pump boiler portion adapted to contain a pumping fluid, a heater plate defined by a circular iron block having a surface interrupted by indentations and having electrical- 1y conductive means positioned in and conforming to said indentations so as to be in close contact with said iron block, said heater plate being attached to the exterior of said pump housing in close contact with said base plate, said heater plate being adapted to supply heat energy through said base plate to the pumping fluid in said pump boiler portion, and wherein the mechanical characteristics of said cylindrical housing and attached base plate and the mechanical characteristics of said heater plate are such that 6 +P will substantially equal P with said diffusion vacuum pump operating at rated AP and Q", where P represents the thermal deflection of said base plate as restrained by said attached cylindrical housing, P represents the unrestrained thermal deflection of said heater plate, 6 represents the pressure deflection of said base plate as restrained by said cylindrical housing, AP represents the designed normal operating pressure differential across said base plate and Q presents the designed normal operating heat flux across the interface between said heater plate and said base plate.

7. A diffusion vacuum pump apparatus comprising a cylindrical pump housing adapted for connection with a chamber to be evacuated, a base plate composed of a certain heat conducting material and gas tightly sealed to said cylindrical pump housing so as to close the bottom end thereof forming a pump boiler portion adapted to contain a pumping fluid, a heater plate composed of a material dissimilar in composition to said certain material and attached to the exterior of said pump housing in close contact with said base plate, said heater plate being adapted to supply heat energy through said base plate to the pumping fluid in said pump boiler portion, wherein the mechanical characteristics of said cylindrical housing and attached base plate and the mechanical characteristics of said heater plate are such that oq+P will substantially equal P with said diffusion vacuum pump operating at rated AP and Q where P represents the thermal deflection of said base plate as restrained by said attached cylindrical housing, P represents the unrestrained thermal deflection of said heater plate, 1x represents the pressure deflection of said base plate as restrained by said cylindrical housing, AP represents the designed normal operating pressure differential across said base plate and Q" represents the designed normal operating heat flux across the interface between said heater plate and said base plate, and wherein said base plate has a uniform thickness in inches which is less than 2 H 9 Q [01 [6 where AP is equal to the normal operating pressure difference across said base plate in p.s.i., E is equal to the modulus of elasticity of said base plate in p.s.i., m is equal to the inverse Poissons ratio, ais equal to the outside radius of said base plate in inches, Q" is equal to the normal operating heat flux in B.t.u./hr.-PT across the interface between said heater plate and said base plate, 04 is equal to the thermal expansion coeflicient of said base plate in in./in./ F., k is equal to the thermal conductivity of said base plate'in B.t.u.-ft./hr.-ft. F., a is equal to the thermal expansion coefficient of said heater plate in in./in./ F., and k is equal to the thermal conductivity of said heater plate in B.t.u.-ft./hr.-ft. F.

7 8 8. A diffusion vacuum pump apparatus according to FOREIGN PATENTS claim 7 wherein the oz/k, of said base plate material is 449 820 7/1948 Canada greater than that of said heater plate material where at 6:819 3/1956 Germany represents thermal expansion coefiicient in in./in./ F.

and k represents thermal conductivity in B.t.u.-ft./ OTHER REFERENCES hr.-ft. F. 0 National Research Corporation Technical Bulletin References Cited 0100-02, Dilfusion Pumps and Ultra-High Vacuum, pp. UNITED STATES PATENTS 045,098 7/1962 Norton 219538 X 10 RICHARD M. WOOD, Primary Examiner. 1879212 9/1932 Hamlen c. L. ALBRITTON, Assistant Examiner, 2,447,636 8/1948 Colaiaco 230101 I 2,668,005 2/1954 Wishart 2301 01 U S -C1, X R

29,03,181 9/1959 GiePen 230401 16564;219538,548;230101,"238; 23678

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