阅读说明：本技术 液态制冷剂泵 (Liquid refrigerant pump ) 是由 J·迪克曼 于 2019-08-02 设计创作，主要内容包括：所提供的泵包括入口、出口和涡旋泵送元件。涡旋泵送元件包括具有一个或多个固定涡旋的固定涡旋部件和具有用于泵送流体的一个或多个沿轨道运动的涡旋的沿轨道运动涡旋部件。固定涡旋部件和沿轨道运动涡旋部件被布置成能够向止推轴承提供加压流体并且防止固定涡旋部件和沿轨道运动涡旋部件之间的接触。(A pump is provided that includes an inlet, an outlet, and a scroll pumping element. Scroll pumping elements include a fixed scroll member having one or more fixed scrolls and an orbiting scroll member having one or more orbiting scrolls for pumping fluid. The fixed scroll member and the orbiting scroll member are arranged to provide pressurized fluid to the thrust bearing and to prevent contact between the fixed scroll member and the orbiting scroll member.)

1. A pump, comprising:

an inlet, an outlet, and a scroll pumping element, wherein the scroll pumping element comprises:

a fixed scroll member having one or more fixed scrolls and an orbiting scroll member having one or more orbiting scrolls for fluid pumping, wherein the fixed scroll member and the orbiting scroll member are arranged to provide pressurized fluid to a thrust bearing and to prevent contact between the fixed scroll member and the orbiting scroll member.

2. The pump of claim 1, wherein the one or more orbiting scrolls have a wrap length of 0.75 to 1.25 turns long and the one or more fixed scrolls have a wrap length of 1.75 to 2.25 turns long.

3. The pump of claim 1, wherein the one or more orbiting scrolls have a wrap length of 1 turn and the one or more fixed scrolls have a wrap length of 2 turns.

4. The pump of claim 1, further comprising an Oldham coupling associated with said orbiting scroll member.

5. A pump according to claim 4, wherein the Oldham coupling comprises a wear resistant coating.

6. The pump of claim 5, wherein the wear resistant coating is hard anodized aluminum.

7. The pump of any of claims 1-6, further comprising an orbital drive mechanism in fluid communication with the outlet, the orbital drive mechanism comprising a drive shaft, two drive shaft bearings supporting the drive shaft, and a radially compliant crank mechanism associated with the drive shaft.

8. The pump of claim 7, wherein the drive shaft bearing is a radial ball bearing with ceramic balls.

9. The pump of claim 7, further comprising an orbiting scroll bearing located between the radially compliant crank mechanism and the orbiting scroll member.

10. The pump of claim 9, wherein the orbiting scroll bearing is a radial ball bearing with ceramic balls.

11. The pump of claim 7, further comprising an electric drive motor mechanically coupled to the orbital drive mechanism.

12. The pump of claim 11, wherein the electric drive motor is hermetically enclosed within a fluid space.

13. The pump of claim 11, further comprising a second fluid outlet allowing fluid to flow through the electric drive motor.

14. The pump of any of claims 1-6, wherein both the fixed scroll member and the orbiting scroll member have wear resistant surfaces.

15. The pump of claim 14, wherein the wear resistant surface comprises or consists of hard anodized aluminum.

Technical Field

The present invention relates to pumping low viscosity non-lubricating fluids and more particularly to a pump based on scroll technology for refrigerants, such as liquid fluorocarbons and similar fluids used in various heat transfer, refrigeration and space conditioning applications.

Background

It is well known that low viscosity, non-lubricating liquids (such as fluorocarbon refrigerants) are difficult to pump, particularly when the application requires the pump to increase the liquid pressure significantly, while the liquid at the pump inlet is minimally subcooled (i.e., has the least net positive suction head, commonly abbreviated as NPSH, in pump industry terminology) and the pumped liquid is not mixed with or contaminated by the lubricant alone. The challenge of pumping in such situations is well known. For example, the liquid viscosity of common fluorocarbon refrigerants and heat transfer fluids is generally low and does not provide boundary lubrication type lubricity, making the mechanical design of pumps handling these fluids challenging in terms of mechanical friction losses and wear life of rotating shafts, bearings and pump components. In addition, many potential applications of refrigerant pumps require that the liquid be minimally subcooled, thereby minimizing the net positive suction head. Under these conditions, many types of pumps are highly susceptible to cavitation, resulting in rough operation, poor performance, corrosion and eventual failure of pump components, particularly centrifugal pump impellers.

However, there are several potential applications for this type of liquid pump. For example, in a two-phase heat transfer loop, liquid refrigerant is pumped to one or more heat exchangers to cool various components. A part of the liquid is evaporated in each heat exchanger, and the heat of evaporation of the evaporated liquid absorbs the cooling load of the heat exchanger. Depending on the flow rate of the heat transfer fluid and at a constant temperature, more heat can be carried. The two phases (vapor and remaining unevaporated liquid) pass through a condenser before returning to the pump, usually as the least subcooled liquid returning to the pump. The amount of pressure rise required to force liquid to flow in the circuit depends on the overall size and length of the circuit and the type of flow control device used.

With the development of the internet and information storage, the energy consumption for cooling data centers has become enormous, which is comparable to the power consumption of the servers themselves. The so-called liquid refrigerant economizer cycle can save a large amount of cooling energy, especially in colder climates. The liquid refrigerant economizer cycle is implemented by adding a liquid refrigerant pump to a typical data center cooling device. When the outdoor temperature is sufficiently cold, typically below 45 degrees Fahrenheit (F.), refrigerant vapor from the evaporator bypasses the refrigerant compressor and is directly condensed in the condenser. The condensed refrigerant (also typically having a minimum subcooling degree) must be pumped back to the evaporator by sufficient pressure to force the refrigerant through the expansion device.

Another potential application is in supermarket refrigeration systems where liquid refrigerant is pumped with as low a head as is allowed by a fully floating outdoor ambient temperature. In the case of extremely low condensing temperatures and pressures, it is necessary to use a liquid pump to raise the liquid pressure high enough to pass through a normal size expansion valve. With this arrangement, the wattage of pump power will save about 25 watts of compressor power as compared to conventional head pressure control which does not allow the condensing temperature to drop below 75 ° F.

Accordingly, there is a need for new pumps or pumping systems suitable for addressing the challenges and uses described above.

Disclosure of Invention

The following summary is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.

In a first aspect a1, a pump includes an inlet, an outlet, and a scroll pumping element. The scroll pumping element includes a fixed scroll member having one or more fixed scrolls and an orbiting scroll member having one or more orbiting scrolls for pumping fluid, wherein the fixed scroll member and the orbiting scroll member are arranged to provide pressurized fluid to a thrust bearing and prevent contact between the fixed scroll member and the orbiting scroll member.

A second aspect a2 includes the pump of the first aspect a1 wherein the orbiting length of the one or more orbiting scrolls is 0.75 to 1.25 revolutions long and the orbiting length of the one or more fixed scrolls is 1.75 to 2.25 revolutions long.

A third aspect A3 includes the pump of the first aspect a1, wherein the orbiting length of the one or more orbiting scrolls is 1 turn long and the orbiting length of the one or more fixed scrolls is 2 turns long.

A fourth aspect a4 includes the pump of the first aspect a1 and further includes an Oldham coupling associated with the orbiting scroll member.

A fifth aspect a5 includes the pump of fourth aspect a4, wherein the oldham coupling includes a wear resistant coating.

A sixth aspect a6 includes the pump of the fifth aspect, wherein the wear resistant coating is or includes hard anodized aluminum.

A seventh aspect a7 includes the pump of any one of the first aspect a1 to the sixth aspect a6, further comprising an orbital drive mechanism in fluid communication with the outlet, wherein the orbital drive mechanism includes a drive shaft, two drive shaft bearings supporting the drive shaft, and a radial compliant crank mechanism associated with the drive shaft.

An eighth aspect A8 includes the pump of the seventh aspect a7, wherein the drive shaft bearing is a radial ball bearing with ceramic balls.

A ninth aspect a9 includes the pump of the seventh aspect a7, further including an orbiting scroll bearing located between the radially compliant crank mechanism and the orbiting scroll member.

A tenth aspect a10 includes the pump of the ninth aspect a9 wherein the orbiting scroll bearing is a radial ball bearing with ceramic balls.

An eleventh aspect a11 includes the pump of any one of the first aspect a1 to the tenth aspect a10, further including an electric drive motor mechanically coupled to the orbital drive mechanism.

A twelfth aspect a12 includes the pump of the eleventh aspect a11, wherein the electric drive motor is hermetically enclosed within the fluid space.

A thirteenth aspect a13 includes the pump of the eleventh aspect a11, further including a second fluid outlet that allows fluid to flow through the electric drive motor.

A fourteenth aspect a14 includes the pump of any one of aspects a1 through a13, wherein the orbital drive mechanism includes a drive shaft with an electric drive motor mechanically coupled to the drive shaft and located outside the pressure housing.

A fifteenth aspect a15 includes the pump of any one of the first to fourteenth aspects a 1-a 14 wherein the fixed scroll member and the orbiting scroll member each have wear resistant surfaces.

A sixteenth aspect a16 includes the pump of the fifteenth aspect a15 wherein the wear resistant surface comprises or consists of hard anodized aluminum.

Drawings

The aspects set forth in the drawings are illustrative and exemplary in nature and are not intended to limit the subject matter defined by the claims. The following detailed description of illustrative aspects can be understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals, and in which:

FIG. 1 illustrates a plan view of an orbiting scroll member in accordance with aspects provided herein;

FIG. 2 illustrates a plan view of a fixed scroll component in accordance with aspects provided herein;

FIG. 3 illustrates fixed and orbiting scroll members engaged together in accordance with aspects provided herein;

FIG. 4 illustrates an exemplary axial pressure balance on an orbiting scroll member in accordance with aspects provided herein; and

fig. 5 illustrates an exemplary cross-section of a pump assembly according to aspects provided herein.

Detailed Description

The scroll pump provided in the present invention satisfies the need for a pump that can handle liquid refrigerant and similar heat transfer fluids with minimal subcooling. Scroll machines, and particularly scroll refrigerant compressors, have had a good history of ease of handling two-phase flows, for example, in the form of slugs (slugs) of liquid refrigerant drawn into the scroll refrigerant compressor. They pass harmlessly through the swirl element. A scroll pump is provided herein which utilizes relative motion between a pair of meshing scrolls that move in an orbiting manner, wherein one scroll orbits relative to the other at a small, low speed circumference. At these lower relative velocities, it was found that the potential for damage due to cavitation was minimal.

The basic advantages of the scroll pump provided herein include self porting at the inlet and outlet, thus eliminating the need for intake and exhaust valves. In addition, the circular orbital motion of the moving vortex can be dynamically balanced. The pump is capable of providing a continuous flow of fluid. In the provided pump, the fluid cavity formed by the vortex forms a trailing hydrodynamic wedge providing robust hydrodynamic lubrication, although the provided pump design typically uses a low viscosity liquid. Further, the scroll machines used in connection with the provided pumps are scalable over a wide range of flow rates, and thus a family of scroll pumps developed for this purpose will be able to meet a wide range of demands.

The scroll pump design provided in the present invention is capable of reliable operation under the above conditions and has a long service life. The disclosed scroll pump is capable of pumping fluid with minimal subcooling (NPSH). In the prototype test, there was no reduction in fluid flow as the inlet subcooling of the fluorocarbon fluid R-134a was reduced from 20 ° F to 2 ° F. Even with only 1 ° F subcooling, the pump flow is only reduced by 20%.

The essence of the mechanical design challenge is to drive the orbital motion of the orbiting scroll member in a manner that maintains a tight clearance between the active fluid pumping surfaces of the two scroll elements while avoiding or minimizing the contact that causes friction and wear. According to certain aspects, there are five sets of mechanically loaded relatively moving surfaces within the scroll pumping device:

axially facing surfaces of vortex elements (involving greater fluid pressure)

Radial contact between the vortex side walls (i.e. flanks) (involving moderate pressure and inertia forces)

Sliding contact between keys and grooves of the Oldham coupling (involving moderate pressure and inertial forces) provided that the correct angular alignment of one vortex relative to the other is maintained using the Oldham coupling

Rotary bearing for converting the rotary motion of the drive shaft into orbital motion of an orbiting scroll member (a "scroll drive bearing", heavy duty)

Rotary bearing for supporting the drive shaft (heavy load)

The axial (tip) clearance between the two scroll elements is minimized by applying pump discharge pressure to the space containing the orbital drive mechanism, causing the same pressure to be applied to the back of the orbiting scroll member, pushing it against the fixed scroll member. The tip clearance is then reduced to that created by the dimensional tolerance of the axially facing surfaces of the two scrolls that have been manufactured. However, this can result in a large contact force between the two scroll elements, resulting in excessive frictional losses and wear. A film of the liquid being pumped is provided by joining a second pair of scroll elements on both scroll members, the sole purpose of the second pair of scroll elements being to pump sufficient fluid to the thrust bearing regions of both scroll members to provide the film. The fluid film thickness is typically less than 0.001 inch (25 microns) and the average axial gap between the two pumping vortices is similar.

The moderate radial contact force between the two scrolls is controlled by the hydrodynamic wedge presented above at each flank contact point between the two scrolls.

The fluid film generated by the sliding motion and load reversal is sufficient to handle moderate loads on the euler's key.

The bearings and scroll drive bearings are heavily loaded. Ceramic ball bearings with hardened steel races and silicon nitride ceramic balls are used in the non-lubricated, low viscosity fluid environment of orbital drive mechanisms. The metal balls in standard non-lubricated bearings tend to fail due to debris falling off the balls and welding to the races as wear occurs, whereas the silicon nitride ceramic balls do not have this failure mechanism.

To minimize the disruption of the occasional break in the fluid film, the critical component may optionally have a wear resistant layer, optionally a self-lubricating coating or surface treatment. Optionally, the two scroll elements and the Oldham's ring are made of an aluminium alloy, the surface being hard anodised and impregnated with Polytetrafluoroethylene (PTFE).

FIG. 1 shows a plan view of an orbiting scroll member 1 including an inner pumping scroll 2 and an outer thrust bearing scroll 3. The orbiting scrolls 2 and 3 project from a base surface 4, the base surface 4 also serving as one surface of a fluid film thrust bearing. The through-hole 5 provides fluid communication between the liquid being pumped and the space surrounding the orbital drive mechanism (fig. 5).

FIG. 2 shows a plan view of fixed scroll member 11, including inner pumping scroll 12 and outer thrust bearing scroll 13. The tips of the scrolls 12 and 13 are coplanar with each other and with the thrust bearing surface 14. The orifice (inlet) 15 is a liquid inlet and the orifice (outlet) 16 is a liquid outlet. The grooves 17 distribute the pressurized fluid evenly over the thrust bearing surface 14.

Referring to fig. 1 and 2, the inner pumping scroll set includes an inner pumping scroll 2 of the orbiting scroll part 1 (fig. 1) and an inner pumping scroll 12 of the fixed scroll part 11 (fig. 2), and the outer thrust bearing scroll set includes an outer thrust bearing scroll 3 of the orbiting scroll part 1 (fig. 1) and an outer thrust bearing scroll 13 of the fixed scroll part 11 (fig. 2). The inner pumping scroll 2 and/or the outer thrust bearing scroll 3 of the orbiting scroll member 1 (FIG. 1) are optionally one lap long. The inner pumping scroll 12 and/or the outer thrust bearing scroll 13 of the mating fixed scroll part 11 are optionally 2 turns long. The inner pumping scroll 2 and/or the outer thrust bearing scroll 3 of the orbiting scroll member 1 are commonly referred to as "orbiting scroll(s)". Also, the inner pumping scroll 12 and/or the outer thrust bearing scroll 13 of the fixed scroll part 11 are generally referred to as "fixed scroll(s)".

This asymmetric configuration minimizes the flow variation at the rail location and the axial pressure variation experienced by the thrust bearing. In certain aspects, the orbiting length of the inner pumping scroll 2 and/or the outer thrust bearing scroll 3 of the orbiting scroll member 1 (FIG. 1) is 0.75 to 1.25 revolutions long, and the orbiting length of the inner pumping scroll 12 and/or the outer thrust bearing scroll 13 of the fixed scroll member 11 (FIG. 2) is 1.75 to 2.25 revolutions long.

FIG. 3 illustrates a plan cross-section of orbiting scroll member 1 (FIG. 1) assembled with fixed scroll member 11 (FIG. 2) in one exemplary representative orbital position. Orbiting scroll member 1 (not shown in figure 3) orbits in a counter-clockwise direction. This orbiting motion within the inner pumping scroll 12 of the fixed scroll member 11 transports fluid from the inlet 15 to the outlet 16 of the inner pumping scroll 2, which moves in response to the movement of the orbiting scroll member 1. The uniform orbital motion of the outer orbiting scroll 3 along orbiting scroll member 1 (fig. 1) within the outer thrust bearing scroll 13 of fixed scroll member 11 results in fluid being pumped to thrust bearing surface 14 (fig. 2) providing a thin fluid film separating base surface 4 (fig. 1) of orbiting scroll member 1 (fig. 1) and fixed scroll surface 14 by about 0.0005 inches (12 microns) to prevent wear and minimize friction.

FIG. 4 illustrates a cross-sectional view of orbiting scroll member 1 as illustrated in FIG. 1. The illustration in FIG. 4 depicts an axial pressure balance generated on orbiting scroll member 1 in accordance with aspects provided herein. Orbiting scroll member 1 is positioned by the net effect of fluid pressure acting on a first side 51 towards the orbital drive mechanism and a second side 52 towards the scroll pumping elements. The pump outlet pressure enters the orbital drive unit through the orifice 5 (fig. 1) and acts on all parts of the side 51. Force line 110 depicts the force generated as a result of pump outlet pressure acting on the first side 51 of the orbiting scroll member 1. On side 52 of orbiting scroll member 1, the fluid pumping region (fig. 2 and 5) formed by the inner pumping scroll set comprising inner pumping scroll 2 of orbiting scroll member 1 and inner pumping scroll 12 of fixed scroll member 11 generates a pump inlet pressure, depicted by force line 103, acting on a portion of orbiting scroll member 1, and a pump outlet pressure acting on the remaining region defined by force line 103. Outside the pumping scroll, the outer thrust bearing scroll set, which includes outer thrust bearing scroll 3 of orbiting scroll member 1 and outer thrust bearing scroll 13 of fixed scroll member 11 (fig. 2 and 5), pumps sufficient fluid into the thrust bearing gap between the base surface 4 (fig. 1) surface and thrust bearing surface 14 (fig. 2) to generate a pressure (i.e., the pressure depicted by force lines 102 and 104) sufficient to balance the outlet pressure 110 acting on side 51.

Fig. 5 illustrates a horizontal cross-section of pump 30 according to aspects provided herein. The pump 30 is configured to pump a liquid, such as, but not limited to, a low viscosity, non-lubricating liquid, such as a fluorocarbon refrigerant. The pump 30 is not limited to pumping refrigerant. In some embodiments, the pump 30 may be described by three general components: a scroll pumping element, an orbital drive mechanism, and an electric drive motor. Each component, scroll pumping element, orbital drive mechanism and electric drive motor are mechanically, rotationally and/or fluidly connected to each other.

The scroll pumping elements are connected to inlet 15 and outlet 16, the inlet 15 and outlet 16 being formed by scroll end covers 33. The scroll pumping elements are further connected to an orbital drive housing 31. The orbital drive housing 31 may be configured as a pressure housing connected to the scroll end cover 33 and the fixed scroll member 11. The scroll pumping element comprises a fixed scroll member 11 for pumping fluid and an orbiting scroll member 1. The fixed scroll part 11 and the orbiting scroll part 1 are arranged to be able to supply a pressurized fluid to a thrust bearing and prevent contact between surfaces of the fixed scroll part 11 and the orbiting scroll part 1. In some embodiments, orbiting scroll member 1 and fixed scroll member 11 are disposed within a liquid tight housing comprising an orbiting drive housing 31, a motor housing 32, a scroll end cover 33, and a motor end cover 34. The second outlet 35, which is configured, for example, with the motor end cover 34, provides a means for partially pumping the liquid flow around the motor stator 41 and through the gap between the motor stator 41 and the motor rotor 42, thereby cooling the electric drive motor.

The orbital drive mechanism is in fluid communication with the outlet 16 of the scroll pumping elements. The orbital drive mechanism includes a drive shaft 21, one or more drive shaft bearings 25 and 26 that support the drive shaft 21. The orbital drive mechanism may also include a radially compliant crank mechanism 23, the crank mechanism 23 being connected to the drive shaft 21 by a pivot pin 27, the crank mechanism 23 in turn driving the orbital motion along the orbiting scroll member 1 through an orbital scroll bearing 24. An Oldham coupling 22 may also be included in the orbital drive mechanism. The oldham coupling 22 maintains the correct angular alignment of the orbiting scroll member 1 relative to the fixed scroll member 11.

As mentioned above, the oldham coupling may have a wear resistant coating, such as hard anodized aluminum. Further, the drive shaft bearings 25 and 26 and/or the orbiting scroll bearing 24 may be radial ball bearings. The radial ball bearings may be ceramic balls, such as silicon nitride ceramic balls, or the like disposed within a hardened steel race. In some embodiments, the fixed scroll member 11 and/or the orbiting scroll member 1 may each have a wear resistant surface. The wear resistant surface may comprise hard anodized aluminum.

Still referring to the pump in fig. 5, the electric drive motor includes a motor stator 41 and a motor rotor 42. The motor rotor 42 may be connected to the drive shaft 21. That is, when the motor stator 41 induces an electromagnetic field, the motor rotor 42 is driven to rotate, thereby rotating the drive shaft 21. The electric drive motor is mechanically coupled to the track drive mechanism. In some embodiments, the motor housing is fluidly connected to the orbital drive mechanism and/or the scroll pumping elements. For example, the electric drive motor can be closed in a fluid-tight manner. The second outlet 35 may distribute (i.e., output) fluid flowing through the motor housing 32 and/or the electric drive motor from the orbital drive mechanism and/or the scroll pumping element. This may provide cooling for the electric drive motor and components within the motor housing 32.

It should now be appreciated that the present invention relates to a scroll pump capable of meeting the need for a pump capable of handling liquid refrigerant and similar heat transfer fluids with minimal subcooling. A scroll pump provided herein utilizes relative motion between a pair of meshing scrolls that orbit, with one scroll orbiting at a low speed and a small circumference relative to the other scroll. For example, but not limited to, the liquid pump of the present invention comprises a pressure housing, an inlet, an outlet, a scroll pumping element, and an orbital drive mechanism. Scroll pumping elements include a fixed scroll member having one or more fixed scrolls and an orbiting scroll member having one or more orbiting scrolls for pumping fluid. The fixed scroll member and the orbiting scroll member are arranged to provide pressurized fluid to the thrust bearing and prevent contact between the fixed scroll member and the orbiting scroll member, wherein the orbiting drive mechanism is in fluid communication with the outlet.

The disclosed embodiments may be embodied in many different forms and the disclosure should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout.

It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a "first element," "component," "region," "layer" or "portion" discussed below could be termed a second element, component, region, layer or portion without departing from the teachings herein.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, including "at least one", unless the content clearly indicates otherwise. "or" means "and/or". As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

Unless defined otherwise, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention.