Vacuum Furnace With Cooling Means

The invention is an improvement in a single chambered vacuum furnace which is manufactured and sold by the Surface Division of Midland-Ross Corporation of Cleveland, Ohio, and especially in the cooling system of the furnace which is generally described in U.S. Pat. No. 3,565,410 that is typical of the many patents relating to such furnaces which normally employ low pressure/high volume radial flow fans in their cooling systems which are located interiorly of the furnace. It has been the thinking that, because of the high volume of gas being moved by such fans, it is necessary to provide in the heat treatment chamber, large centrally disposed, axially aligned inlet and exhaust openings through which cooled gas is circulated upwardly through the chamber. The flow patterns of the gas created by this porting arrangement are restricted to the center portions of the chamber and, in some instances, cause uneven cooling of a workpiece positioned in the chamber. Thus, there is a need for improving the flow of gas through the chambers to create a more uniform cooling process.

Some vacuum furnaces, presently on the market, utilize a series of small nozzles which are equally spaced circumferentially around the chamber for directing high velocity jets of cooled gas against a workpiece disposed in the chamber. The gas is left to exit the chamber as best it can through crevices or openings between adjacent loosely fitted components. These specific furnaces use high pressure/low volume fans and cooling systems which are located exteriorly of the furnace, thereby making the furnaces bulkier and requiring more room than the furnaces with the interiorly located cooling systems. The gas is circulated through the nozzles at extremely high velocities in the range of from 10,000 to 12,000 fpm (feet per minute) as compared to the low velocities of 1,000-3,000 fpm at which the gas is circulated through the large single inlet and exhaust porting arrangement. The disadvantages of using such high velocities are well known in the trade.

It was discovered that the uniformity and rate of cooling in the furnace employing the low pressure/high volume fans was dramatically improved by using numerous small inlet openings in place of the single, large inlet of the heat treatment chamber. For example, the velocities of the gas in various parts of the chamber were measured and it was found that there was a 230% variation from the mean velocity when the old, single inlet and exhaust porting arrangement was used, whereas in the new multiple small inlet porting system, the deviation was radically reduced to 97%.

Briefly stated, the invention is in a vacuum furnace having a low pressure/high volume radial flow fan which is located interiorly of the furnace for circulating cooled gas through the heat treatment chamber of the furnace. The enclosed chamber is formed within a thermally insulated double walled casing which is disposed interiorly of an outer shell of the furnace. An exhaust outlet for the chamber is provided in the vertically uppermost portion of the casing. An improved gas distribution system in the form of a plurality of small inlet openings strategically located in the vertically lower half of the casing, is provided to increase the uniformity of the flow pattern of gas through the chamber. Gas, removed through the exhaust outlet from the chamber during the cooling cycle, is circulated through the cooling coils disposed in fluid passageways formed between the inner casing and outer shell of the furnace. The cooled gas is then circulated through the novel gas distribution system for impingement against a workpiece positioned in the chamber.

The following description of the invention will be better understood by having reference to the annexed drawing, which a cross-section of a vacuum furnace made in accordance with the invention, as viewed from a vertical plane when the furnace is in a normally vertical operating position.

With reference to the drawing, there is shown a single chamber, vacuum furnace 5 which is similar in many respects to the furnace described in U.S. Pat. No. 3,565,410. The furnace 5 comprises a double-walled, liquid-cooled, outer cylindrical shell 6 which is supported horizontally on a plurality of vertically disposed legs 7,8. The cylindrical shell 6 has a pair of opposing ends which are normally sealed by covers, one of which is at least hinged so that it can be rotated out of position against the shell 6 to permit placement and removal of a workpiece 9 within the furnace 5.

A double-walled, thermally insulated casing 10 is disposed within the furnace 5 in spaced relation from the outer shell 6, and defines a heat treatment chamber 11 in which the workpiece 9 is supported on a plurality of horizontally disposed rails 12-14 that extend longitudinally of the chamber 11 and shell 6. The support rails 12-14 are supported, in turn, on a plurality of vertical posts 15-17 which extend through the bottom 18 of the casing 10 and are secured to framework 19 carried by the shell 6. A plurality of similar heaters 20 are disposed within the chamber 11 adjacent the inner periphery 21 of the casing 10. A large exhaust outlet 22 is centrally disposed in the top 23 of the casing 10. The exhaust outlet 22 is an elongated slot which extends longitudinally of the chamber 11 and has a transverse width of from 6 to 8 inches, depending on the size of the furnace 5. A baffle 24, spaced above the exhaust outlet 22 of the chamber 11, is disposed in a hood 25 which substantially covers the top 23 of the casing 10. A fan 26 is mounted in the hood 25 for circulating gas to and from the chamber 11. The fan 26 comprises an electrically operated motor 28 which is disposed exteriorly of the shell 6. The motor 28 is coupled by a drive shaft 29 to a blade or impeller 30 which is disposed internally of the shell 6 in communication with the hood 25. The fan 26 is a conventional low pressure/high volume radial flow fan which is capable of circulating gas, for example, at a volume of about 6,000 cfm (cubic feet per minute) and at a static pressure not exceeding one inch of water. The fan 26, during the cooling cycle and upon completion of the heat treatment of the workpiece 9, removes hot gas from the hood 25 and chamber 11 and circulates it in a radial direction into a pair of fluid passageways 31,32, formed between the inner casing 10 and outer shell 6. A pair of cooling coils 33,34, disposed in the fluid passageways 31,32, are designed to cool gas circulated therethrough by temperatures in the range of from 300.degree. F. to 500.degree. F.

A plurality of circular openings 35,36 are disposed in the lower sidewalls 37,38 of the casing 10 adjacent the bottom 18. The openings 35,36 are about 1-11/2 inches in diameter, and are spaced longitudinally of the casing 10 about six inches apart. A pair of annular openings 39,40 are disposed around each of the vertical posts 15-17 which extend through the casing 10. As previously indicated, the vacuum furnace 5 normally has a large centrally disposed inlet opening 41 in the bottom 18 of the casing 10. This inlet opening 41 is generally an elongated slot which extends longitudinally of the casing 10 and has a transverse width that varies from 6 to 12 inches, depending on the size of the furnace 5.

A specially configured metal plenum 42 covers the inlet 41. The plenum 42 may be permanently secured to the casing 10, or removably positioned in the inlet 41 by any suitable means, e.g. flanges depending from the plenum 40 for supporting engagement with the casing 10 adjacent the inlet 41. In any case, the metal plenum 42 is provided with a plurality of openings 43-47 to more uniformly distribute cooled gas in the chamber 11 for contact with the workpiece 9. The multiple inlet openings 35,36,39,40, 43-47 are properly sized and strategically located in the chamber 11 to provide a highly improved system for uniformly distributing cooled gas throughout the chamber 11 at comparatively low velocities not exceeding, for example, 3,000 fpm. This improved gas distribution system or porting arrangement, causes more uniformly cooling of the workpiece 9, especially at the lower outer corners 48,49 which have been found to be critical areas in the cooling of a workpiece. The openings 35,36 in the lower sidewalls 37,38 of the casing 10, are located on the level of the support rails 12-14 so as to direct cooled gas against the lower outer corners 48,49 of the workpiece 9. The cooled gas from the lower sidewall openings 35,36 collide with cooled gas emitting from the adjacent annular openings 39,40 to create sufficient turbulence in the areas of the support rails 12-14 so as to properly and uniformly cool the lower edges 48,49 of the workpiece 9. The cooled gas is more evenly dispersed in the chamber 11 by properly sizing the openings, so that the total gas discharge area of the openings 43-47 in the metal plenum 42 equals the total gas discharge area of the other inlet openings 35,36,39,40. This produces a well balanced cooling system.

It has been found that the multiple inlet openings and single exhaust opening porting arrangement is highly effective in uniformly cooling a workpiece which has been exposed during heat treatment to hot gas having very high temperatures in the range of from 1200.degree. F. to 2400.degree. F. Thus, the uniformity of the flow pattern of cooled gas through the heat treatment chamber of the vacuum furnace has been highly improved from what it was using, the single inlet and exhaust portion combination. It has been found highly effective to position the inlet openings in the lower half of the casing, rather than space them equally around the inner periphery of the casing as is done in other vacuum furnaces which employ high velocity nozzles for circulating cooled gas into the heat treatment chamber.