More particularly, the present invention relates to oil sump heaters for use with refrigeration equipment particularly including compressors.

In refrigeration equipment, refrigerants have an affinity for oil and therefore tend to mix with oil. This mixing is accentuated during shutdown, when the oil cools off.

This has the undesirable effects of reducing the utility of both the oil and the refrigerant. To minimize the mixing of oil and refrigerant, the oil is heated to reduce the amount of refrigerant dissolved into the oil. A common system for heating the oil is disclosed in, for example, U. S. Patent No.

4,399,663 to Hesler. In such a system, an oil sump contains an oil reservoir and an oil heater. The oil heater intersects the oil reservoir and is placed in direct contact with the oil.

This type of oil heater, where the oil heater protrudes into the oil reservoir, is an immersion heater; i. e., the heater is immersed in oil located in the reservoir.

The above-described conventional immersion oil heater has the undesirable results of: oil-coking on and near the oil heater, the creation of an oil leak path where the oil heater enters the oil reservoir, and the creation of a non- uniform temperature throughout the oil reservoir. Oil-coking is the process by which heated oil decomposes and leaves a carbon residue. Oil-coking on an immersion oil heater causes a build-up of carbon residue around the heater, and the carbon residue accumulates to create an insulator around the heater.

A more uniform temperature is important to avoid nuisance trips of the temperature protection equipment. Also, this insulator absorbs some of the heat the oil heater gives off, thereby decreasing the oil heater's effectiveness. Contamination problems are also a potential with this type of heater. An additional drawback of the immersion oil heater is that, since the heater penetrates the oil reservoir wall, the oil must be drained in order to change a heating element when the element needs replacement.

Belly heaters have also been used to heat oil in refrigeration equipment, but the use of belly heaters causes undesirable results. For example, belly heaters create a hazard, particularly to service technicians, because belly heaters generate great heat on the refrigeration equipment's outer surface. Additionally, belly-type heaters do not have a structure for localizing heat output from the oil heater.

Instead, belly heaters are strapped onto refrigeration equipment and heat up the oil sump's outer surface and/or a great number of oil sump components in addition to the oil and oil reservoir. This results in the belly heaters being inefficient.

BRIEF SUMMARY OF THE INVENTION One object of the invention is to eliminate leak paths from the oil reservoir.

Another object of the invention is to minimize oil- coking in the oil reservoir and hence to minimize oil contamination.

An additional object of the invention is to provide a more uniform temperature throughout the oil reservoir.

Still another object of the invention is to provide an oil sump heater which has a heating element that can be replaced without having to drain the sump's oil.

One or more of the preceding objects, or one or more other objects which will become plain upon consideration of the present specification, are satisfied at least in part by the invention described herein.

The invention, which satisfies one or more of the above objects, is a refrigeration apparatus containing an oil sump. The oil sump houses an oil reservoir defined by a wall.

The apparatus also contains at least one oil sump heater in thermal contact with the wall. At least one oil sump heater is positioned to be in a substantially non-contacting relationship with any oil in the sump. Additionally, there is a structure for localizing the heater's output heat. This structure for localizing heat output may contain a cavity. This cavity may contain the oil sump heater (s). The oil sump heater is in a substantially non-contacting relationship with the oil in order to avoid oil-coking and to eliminate an oil-leak path from the reservoir. At the same time, a more uniform temperature is provided throughout the oil sump.

The invention further provides a method of heating an oil reservoir having a wall disposed around the reservoir for a centrifugal compressor. The method comprises the steps of: forming a passageway in the wall where the passageway is

not open to the oil reservoir; heating the passageway; transferring heat from the passageway to the wall; and transferring heat from the wall to the oil reservoir to thereby heat any oil disposed therein.

Additionally, the invention provides an oil sump heater for a centrifugal compressor having an oil sump. The invention includes a compressor wall shaped to form the oil sump; a first passageway in the wall where the first passageway is not open to the oil sump; and a heating mechanism in the passageway for transferring heat to the wall and subsequently to the oil sump.

The invention still further provides a method of heating an oil sump in a centrifugal compressor having an exterior housing wall. The method comprises the steps of: shaping a portion of the exterior housing wall to form the oil sump; forming a passageway in the wall portion where the passageway does not connect with the oil sump; heating the interior of the passageway; transferring heat from the passageway interior to the wall; and transferring heat from the wall to oil in the oil sump.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS Figure 1 is a side view of a refrigerant compressor and oil sump, and oil sump heater according to the present invention.

Figure 2 is a side view of the heater and oil sump taken along line 2-2 of Figure 1.

Figure 3 is a side view of the oil sump and the heater taken along lines 3-3 of Figure 1.

Figure 4 is a perspective view of the oil sump and heater of the present invention.

Figure 5 is an alternative embodiment of the heater and oil sump as shown in Figure 4.

DETAILED DESCRIPTION OF THE INVENTION A refrigeration apparatus 10 is surrounded by a casing 12 and includes an oil sump 30. A compressor 31 is located above and connected to the oil sump 30. The compressor 31 is preferably a gear driven centrifugal compressor as shown but can also be implemented in other forms such as the direct drive centrifugal compressor sold by The Trane Company under the trademark CenTraVac, a heliroter compressor sold by The Trane Company under the trademark Series R, or the like.

The oil sump 30 itself is located in a lower- portion 14 of the refrigeration apparatus's casing 12. The oil sump 30 contains an oil reservoir 32 and cavities 34-35. The cavities 34-35 contain oil sump heaters 36-37. The cavities 34-35 are positioned so that the heaters 36-37 contained by the cavities 34-35 are in a substantially non-contacting relationship with oil 38 in the oil sump 30. A relationship where the heaters 36-37 do not substantially contact the oil 38 occurs when the heaters 36-37 are not directly immersed in the oil 38 of the reservoir 32.

The oil sump 30 and the compressor 31 are preferably directly connected and open to each other as is shown in Figure 1. The oil sump 30 and compressor 31 do not have to be open to each other; however, some connection is required if they are not. Such a connection could include a drain line and a return or vent line (not pictured) located between the oil sump 30 and compressor 31.

The oil reservoir 32 is surrounded and formed by a wall 40, which wall 40 is also a part of the oil sump 30. The oil sump heaters 36-37 are positioned within the cavities 34-35 and are in thermal contact with the wall 40. As a result of being in thermal contact with the wall 40, the oil sump heaters 36-37 can heat oil 38 in the oil reservoir 32. The oil reservoir 32 is surrounded by the wall 40 which has an internal surface 44 for containing oil 38. The oil sump heaters 36-37

do not intersect or penetrate the internal surface 44; i. e., the oil sump heaters 36-37 are placed outside of the internal surface 44, so that no heater 36-37 portion is located within the reservoir 32. Because there is no such intersection or penetration into the reservoir 32, oil-coking on the oil sump heaters 36-37 is avoided.

The oil reservoir 32 is adapted to contain both oil 38 and a refrigerant, as are many other oil reservoirs used with refrigerant compressors. This adaptation is primarily unique to refrigerant compressors, where the oil and refrigerant are not completely separable, and the present invention is applicable to all such adaptations.

The cavities 34-35 are preferably each located on opposing sides 16,18 of the oil reservoir 32. The cavities 34-35 are also preferably machined into the oil sump 30 and, in particular, into the oil reservoir wall 40. Although the cavities 34-35 are cylindrically shaped, this is not necessary to the invention and the cavities 34-35 could have star, triangular, rectangular or other shapes. Additionally, the cavities 34-35 may contain conductive grease (not shown) surrounding the oil sump heaters 36-37, although the use of such grease is not necessary to the invention. While the cavities 34-35 are a structure for localizing heat output from the oil sump heaters 36-37, other such structures exist where those structures are in thermal contact with the oil sump heaters 36-37. Also, although the cavities 34-35 are symmetrical in the Figures, a symmetrical arrangement is not necessary to the invention.

The oil sump heaters 36-37 are in thermal contact with the cavities 34-35 and generate enough heat to vaporize the refrigerant mixed with the oil. The oil sump heaters 36-37 contain heating elements 46 that may be clothed with sheathes 45, although sheathing is not necessary. The heating elements

46 may be electric elements, although this is not necessary to the invention. The most common type of heating elements are electric and resistance. Typical heating elements are sold by the Indeeco Company of St. Louis, Missouri as immersion heaters.

The oil sump heaters 36-37 are adapted to heat oil 38 in the sump 30 in order to reduce the amount of refrigerant dissolved into the oil 38. It should be noted that, although the oil sump heaters 36-37 are the same size and symmetrically arranged in the Figures, this is not necessary to the invention.

Figure 5 shows an alternative embodiment 70 of the present invention essentially similar to Figure 4 but where the cavities 34,35 are formed as a single looped passageway 72 from an external inlet aperture 74 to an external exit aperture 76. A heated fluid is passed from a source of heated fluid 78 via conduit 80 into the passage 72 through the inlet aperture 74. The heated fluid should be hot enough to vaporize the refrigerant mixed with the oil and should be moving sufficiently to transfer significant heat to the wall 40. The heated fluid transfers heat to the oil reservoir wall 40 which in turn transfers heat to the oil reservoir 32 and the oil 38 contained therein. The heated fluid then exits through the exit aperture 76 and may be returned to the source 78 of heated fluid.

While the invention is described in connection with one or more embodiments, it will be understood that the invention is not limited to those embodiments. One the contrary, the invention includes all alternatives, modifications, and equivalents as may be included within the spirit and scope of the appended claims.