GEOTHERMAL HEAT EXCHANGER

Abstract

A one-piece, unitarily formed compact geothermal heat exchanger comprises a one-piece body having an external screw-type configuration including a cutting tip and a spiral thread extending from the cutting tip upwards towards the top of the body of the heat exchanger. The top face of the body includes a working fluid inlet and a working fluid outlet, each of which are in communication with an internal continuous helical channel inside of the one-piece body through which the working fluid travels during operation to transfer heat energy between the working fluid and the ground. A compact geothermal heat exchanger having such configuration may be installed by screwing the heat exchanger into the ground to the desired depth, without requiring prior digging or other excavation of the ground surface. The compact geothermal heat exchanger may then be connected to a conventional geothermal heating and cooling system for geothermal heating and/or cooling of a space, such as the interior of a building. The compact geothermal heat exchange may also be connected in series to provide expanded capacity for customized cooling/heating needs and cost consideration.

Claims

1. A geothermal heat exchanger, comprising: a unitary, single body shaft configured for threaded engagement with a section of ground material, said shaft having a helical, open fluid conduit extending downward through an interior of said body shaft from a top end of said shaft and back upward to said top end of said shaft; wherein said fluid conduit of said geothermal heat exchanger is configured for fluid communication with a working heat transfer fluid flowing through a geothermal heating and cooling system.

2. The geothermal heat exchanger of claim 1, said single body shaft further comprising a plurality of helical threads on an exterior of the shaft.

3. The geothermal heat exchanger of claim 2, said single body shaft further comprising a cutting tip at a bottom end of the shaft.

4. The geothermal heat exchanger of claim 3, wherein said helical threads extend from said cutting tip to the top end of said shaft.

5. The geothermal heat exchanger of claim 1, said single body shaft further comprising a working fluid inlet in the top end of the shaft, and a working fluid outlet in the top end of the shaft.

6. The geothermal heat exchanger of claim 5, wherein each of said working fluid inlet and said working fluid outlet is in fluid communication with said helical, open fluid conduit.

7. The geothermal heat exchanger of claim 6, wherein each of said working fluid inlet and said working fluid outlet is configured for attachment to fluid conduits carrying said working heat transfer fluid between said geothermal heat exchanger and a portion of said geothermal heating and cooling system.

8. A geothermal heating and cooling system, comprising: a heat pump configured for temperature conditioning of an enclosed space to a desired temperature; a geothermal heat exchanger; and fluid conduits extending between the heat pump and the geothermal heat exchanger to enable heat transfer between the geothermal heat exchanger and the heat pump; the geothermal heat exchanger further comprising: a unitary, single body shaft configured for threaded engagement with a section of ground material, said shaft having a helical, open fluid conduit extending downward through an interior of said body shaft from a top end of said shaft and back upward to said top end of said shaft; wherein said fluid conduit of said geothermal heat exchanger is in fluid communication with said fluid conduits extending between the heat pump and the geothermal heat exchanger to enable fluid communication of a working heat transfer fluid between said fluid conduit of said heat exchanger and said heat pump.

9. The geothermal heating and cooling system of claim 8, wherein said working fluid inlet and said working fluid outlet are in fluid communication with said fluid conduits.

10. The geothermal heating and cooling system of claim 8, said single body shaft further comprising a plurality of helical threads on an exterior of the shaft.

11. The geothermal heating and cooling system of claim 10, said single body shaft further comprising a cutting tip at a bottom end of the shaft.

12. The geothermal heat exchanger of claim 11, wherein said helical threads extend from said cutting tip to the top end of said shaft.

13. A method of forming a geothermal heating and cooling system, comprising the steps of: forming, using an additive manufacturing process, a geothermal heat exchanger, the geothermal heat exchanger further comprising: a unitary, single body shaft configured for threaded engagement with a section of ground material, said shaft having a helical, open fluid conduit extending downward through an interior of said body shaft from a top end of said shaft and back upward to said top end of said shaft; wherein said fluid conduit of said geothermal heat exchanger is configured for fluid communication with a working heat transfer fluid flowing through a geothermal heating and cooling system; screwing said geothermal heat exchanger into ground material; and placing said fluid conduit of said geothermal heat exchanger in fluid communication with a heat pump configured for temperature conditioning of an enclosed space to a desired temperature.

14. The method of forming a geothermal heating and cooling system of claim 13, said single body shaft further comprising a plurality of helical threads on an exterior of the shaft.

15. The geothermal heating and cooling system of claim 14, said single body shaft further comprising a cutting tip at a bottom end of the shaft.

16. The geothermal heat exchanger of claim 15, wherein said helical threads extend from said cutting tip to the top end of said shaft.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying drawings in which:

[0015] FIG. 1 is a side view of a one-piece, unitarily formed compact geothermal heat exchanger in accordance with certain aspects of an embodiment of the invention.

[0016] FIG. 2 is a perspective view of the one-piece, unitarily formed compact geothermal heat exchanger of FIG. 1.

[0017] FIG. 3 is a side view of the one-piece, unitarily formed compact geothermal heat exchanger of FIG. 1 showing an internal working fluid flow channel in phantom.

[0018] FIGS. 4-7 are top, front, side, and bottom views, respectively, of the one-piece, unitarily formed compact geothermal heat exchanger of FIG. 1 including exemplary dimensions for a small-scale prototype unit of the one-piece, unitarily formed compact geothermal heat exchanger of FIG. 1 (all dimensions shown being in inches).

[0019] FIG. 8 is a schematic view of simulated heating of working fluid passing through a one-piece, unitarily formed compact geothermal heat exchange formed in accordance with aspects of the invention.

[0020] FIG. 9 is a schematic view of simulated cooling of working fluid passing through a one-piece, unitarily formed compact geothermal heat exchange formed in accordance with aspects of the invention.

[0021] FIG. 10 is a schematic view of simulated velocity magnitude of working fluid passing through a one-piece, unitarily formed compact geothermal heat exchange formed in accordance with aspects of the invention.

[0022] FIG. 11 is a schematic view of a geothermal heating and cooling system for geothermal heating and/or cooling of a space employing a one-piece, unitarily formed compact geothermal heat exchanger in accordance with further aspects of an embodiment of the invention and showing operation in both cooling and heating modes.

DETAILED DESCRIPTION

[0023] The invention summarized above may be better understood by referring to the following description, claims, and accompanying drawings. This description of an embodiment, set out below to enable one to practice an implementation of the invention, is not intended to limit the preferred embodiment, but to serve as a particular example thereof. Those skilled in the art should appreciate that they may readily use the conception and specific embodiments disclosed as a basis for modifying or designing other methods and systems for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent assemblies do not depart from the spirit and scope of the invention in its broadest form.

[0024] Descriptions of well-known functions and structures are omitted to enhance clarity and conciseness. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, the use of the terms a, an, etc. does not denote a limitation of quantity, but rather denotes the presence of at least one of the referenced items.

[0025] The use of the terms “first”, “second”, and the like does not imply any particular order, but they are included to identify individual elements. Moreover, the use of the terms first, second, etc. does not denote any order of importance, but rather the terms first, second, etc. are used to distinguish one element from another. It will be further understood that the terms “comprises” and/or “comprising”, or “includes” and/or “including” 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.

[0026] Although some features may be described with respect to individual exemplary embodiments, aspects need not be limited thereto such that features from one or more exemplary embodiments may be combinable with other features from one or more exemplary embodiments.

[0027] In accordance with certain aspects of an embodiment, a one-piece, unitarily formed compact geothermal heat exchanger is shown in FIGS. 1 and 2 generally at 100. Geothermal heat exchanger 100 includes a unitary, single-body and one-piece shaft 102, with helical threads 104 wrapping around the shaft 102 from a top of shaft 102 to the bottom of shaft 102. A cutting tip 106 is formed at the bottom of heat exchanger 100 having the configuration of a cutting tip on a drill bit, including a plurality of, and preferably four, flanks 108. Cutting tip 106 and helical threads 104 enable easy placement of heat exchanger 100 into the ground via a simple screw-in installation into the ground, without requiring prior excavation of the site for heat exchanger 100. Each of a working fluid inlet 110 and a working fluid outlet 112 is provided in the top of heat exchanger 100, which inlet 100 and outlet 112 provide access to a working fluid flow channel 114 (shown in FIG. 3 and discussed in greater detail below) the extends downward through shaft 102 to transfer heat energy between a working fluid flowing through working fluid flow channel 114 and the surrounding ground in a geothermal thermodynamic process.

[0028] Working fluid inlet 110 and working fluid outlet 112 of heat exchanger 100 may be connected to a geothermal heating and cooling system for geothermal heating and/or cooling of a space, as shown schematically in FIG. 11.

[0029] FIG. 3 provides a simplified view of the working fluid flow channel 114 inside of unitary, single-body shaft 102. In the simplified view of heat exchanger 100 of FIG. 3 (in which external details of the body shaft 102, helical threads 104, and cutting tip 106 are not shown for clarity), heat exchanger 100 includes an outer cylindrical shell formed by unitary single-body shaft 102 and internal helical and spiral working fluid flow channel 114. More particularly, working fluid flow channel 114 comprises a helical and spiral winding hollow space inside of single-body shaft 102, extending from inlet 110, spiraling downward toward cutting tip 106 at the bottom of shaft 102, and from there extending vertically upward through the center of flow channel 114 to outlet 112. In this configuration, heat exchanger 100 is formed as a single, unitary piece of solid steel formed from an additive manufacturing process (discussed in detail below), with the working fluid flow channel 114 simply formed by the internal, helical void inside of shaft 102. In operation, the working fluid (e.g., water or other heat exchange fluid) flows from the inlet, downward through the spiral flow channel 114 inside of heat exchanger 100, then upward through the middle of shaft 102 to outlet 112. Inlet 110 and outlet 112 may in turn be connected to fluid conduits that extend between heat exchanger 100 and a heat pump inside of an enclosed space, which heat pump is in turn configured for temperature conditioning of that enclosed space to a desired temperature, which heat pump systems are well known to those of ordinary skill in the art and thus not further detailed here.

[0030] A heat exchanger 100 formed in accordance with the foregoing description may avoid the difficulties presented by the complex installation process and large space requirements of currently known in-ground geothermal heat exchangers as part of a geothermal heating and cooling system. A heat exchange 100 formed as discussed herein via an additive manufacturing process may be integrated with conventional geothermal heating and cooling systems for improved efficiency and easy installation. Such an additively manufactured heat exchanger 100 eliminates the need to excavate the soil and install long piping as has be conventionally required in prior known geothermal systems. Such an additively formed heat exchanger 100 may be formed using the direct metal laser sintering technique, and may be integrated with a heat pump of traditional configuration to exchange heat between a constant temperature water bath circulator and a water heat sink. Such an additively formed heat exchanger 100 is capable of operating in both geothermal heating and cooling modes, may perform its geothermal heat transfer functions in a more compact configuration than previously known geothermal heat exchangers (which both is easier to install and requires less physical space), and mitigates the risk of working fluid leakage into the ground due to its one-piece, unitary, solid steel construction, all leading to less maintenance and less operating costs during long-term operation.

Exemplary Implementation and System Performance Model

[0031] Following is a non-limiting exemplary embodiment of a heat exchanger incorporating certain aspects of the invention as described above. A prototype of heat exchanger 100 was produced through a direct metal laser sintering (DMLS) method, which builds metal parts from a CAD file using a laser source to selectively fuse fine metal powder. An EOSINT M280 type of DMLS machine was used at the University of the District of Columbia to print the 3D model. Such an EOSINT M280 machine can be used to seamlessly manufacture complex heat exchanger designs in order to improve performance and reduce weight while limiting the number of components required for the final product, as well as meeting conformal geometries for space-limited applications.

[0032] The proposed heat exchanger was made up of Maraging Steel MS1 powder. Maraging steels have a unique combination of features for materials that are printed from 3D technology. High strength, high fracture toughness, good weldability, and dimensional stability during aging are some features of Maraging steels. Due to those promising properties, Maraging steels are used extensively in high-performance industrial and engineering parts such as aerospace and motor racing applications. Table 1 below shows the operating conditions of the machine to fabricate the 3D model of the prototype heat exchanger 100.

TABLE-US-00001 TABLE 1 Conditions and materials applied to EOSINT M280 machine to generate prototype heat exchanger Fabrication Requirement EOS280 Operating Mode Metal Powder EOS Maraging Steel MS1 Process Gas Nitrogen Laser Power 400 W Layer Thickness 40 micrometer Software EOSTATE Magics RP (materialize) CAD Interface CREO, SolidWorks (STL file form)

[0033] The prototype of the heat exchanger 100 was built in the vertical orientation and a solid support structure was projected onto the build plate to act as a foundation. The STL file of the model developed by CREO was then imported into Materialise Magics software for fixing errors, editing the design, and preparing the platform. The Materialise Magics software is a tool that helps to guide on issues such as best part orientation and support generation. Once the file preparation was completed, the file was exported to the EOSINT M280 laser-based powder bed fusion system for printing.

[0034] In the design procedure of the prototype of ground-source heat exchanger 100, different considerations were taken into account in order to achieve the appropriate sizing of the system to make it thermodynamically and mechanically efficient. However, a tremendous emphasis was placed on a developing an approach that would enable easy installation of the proposed heat exchanger. Traditionally, ground-source heat exchangers are placed in the ground by removing the soil. However, a heat exchanger 100 configured in accordance with aspects of the invention employs a screw-type configuration, and is designed to be screwed into the ground without removal of the soil. FIGS. 4-7 depict different views of the exterior of the fabricated heat exchanger 100 with dimensions in a scaled-down prototype configuration suitable for testing of the thermodynamic properties of the invention. The internal working fluid flow channel 114 of the heat exchanger 100 was designed by combining helical and spiral contours. This design contributes to saving the building material (Maraging steel powder), decreasing the overall mass of the heat exchanger, and increasing the heat transfer surface area for the working fluid. The sizing of the heat exchanger 100 was determined by paying attention to both heat transfer considerations and manufacturing considerations. The heat transfer considerations were implemented by taking into account the amount of thermal load to be fitted for a building's space conditioning. Table 2 below lists the operating conditions used in the present study to specify the system sizing and analyze heat transfer performance, including thermal load, ground temperature, inlet water temperature, and desired outlet water temperature from the heat exchanger 100. Using these known parameters, an analytical approach was conducted to determine the required length of the heat exchanger channel. The working fluid flow channel's and helical coil's diameters were also chosen based on manufacturing considerations, consisting of space limitations in design, technical difficulties in additive manufacturing of the device, and the other considerations, which are employed to produce a compact heat exchanger with easy and cost-effective installation in the ground with a minimal space to be occupied.

TABLE-US-00002 TABLE 2 Operating conditions used to determine sizing and analyze heat transfer performance Operation Conditions Heating Mode Cooling Mode Working Fluid Water Water Thermal Load 1450 W 1450 W Mass Flow Rate 0.029 kg/s 0029 kg/s Ground Temperature 15° C. 15° C. Inlet Temperature  1° C. 32° C. Outlet Temperature 13° C. 20° C.

[0035] The following analytical approach is implemented to determine the system sizing, including the length of channel and heat transfer area while the channel's and helical coil's diameters are chosen based on manufacturing considerations as stated above.

[0036] The assumptions adopted to analyze the heat transfer performance of heat exchanger 100, either analytically or numerically, are as follows:

[0037] i) Quasi-steady state condition is maintained within operation of the heat exchanger;

[0038] ii) Thermal conductivity and specific heat of the materials remain constant at average temperature of inlet and outlet; and

[0039] iii) The ground temperature is taken constant, and temperature on the exterior surface of the heat exchanger is assumed uniform at the constant temperature of the ground (i.e. uniform wall temperature boundary conditions).

[0040] Using the energy balance for sensible heat exchange of a liquid single-phase flow, the mass flow rate of water can be simply calculated by having known values of thermal load, inlet and outlet temperatures:

{dot over (Q)}={dot over (m)}C.sub.p|(T.sub.out−T.sub.in)   (1)

[0041] After having the mass flow rate calculated, the flow regime is identified to be whether laminar or turbulent by calculating the Reynolds number expressed below and then comparing it with the critical Reynolds (i.e., Re.sub.cr=2300):

[00001]$\begin{array}{cc}\mathrm{Re}=\frac{4\stackrel{.Math.}{m}}{\pi d_i{\mu }_w}& \left(2\right)\end{array}$

[0042] According to the Newton's cooling law, the overall heat transfer coefficient (UA) can be calculated:

{dot over (Q)}=UA ΔT.sub.lm   (2)

where UA stands for the overall heat transfer coefficient by convection of the working fluid and conduction of the wall thickness, and ΔT.sub.lm accounts for the logarithmic mean temperature difference (LMTD) which is defined for uniform wall temperature conditions as follows:

[00002]$\Delta T_{\mathrm{lm}}=\frac{T_{\mathrm{out}}-T_{\mathrm{in}}}{\ln \left(\frac{T_{\mathrm{wall}}-T_{\mathrm{in}}}{T_{\mathrm{wall}}-T_{\mathrm{out}}}\right)}$

[0043] The overall heat transfer coefficient, in turn, can be expressed in terms of convective and conductive thermal resistances as follows:

[00003]$\begin{array}{cc}\mathrm{UA}=\frac{1}{R_{\mathrm{tot}}}=\frac{1}{R_{\mathrm{conv}}+R_{\mathrm{cond}}}=\frac{1}{\frac{1}{h\times A}+\left(\frac{\ln \left(\frac{d_o}{d_i}\right)}{2\pi L{K}_s}\right)}& \left(4\right)\end{array}$

in which heat transfer coefficient (h), length of the channel (L), and surface area (A) are still unknown.

[0044] Nusselt number is then expressed for a circular channel as below:

[00004]$\begin{array}{cc}{\mathrm{Nu}}_{\exp }=\frac{{\mathrm{hd}}_i}{K_w}& \left(5\right)\end{array}$

where Nu.sub.exp stands for the Nusselt number derived from the well-recognized experimental correlations available in the literature for helically coiled tubes.

[0045] Table 3 below lists two empirical Nusselt correlations used in the present study to estimate and compare heat transfer coefficients for the operating conditions introduced earlier in Table 2.

TABLE-US-00003 TABLE 3 Empirical correlations of Nusselt number for helically coiled tubes with their validity ranges. Authors Validity Ranges (Year) (geometry in mm) Correlations Boundary Conditions Pawar and Vivek D/d = 13.2, 15.6, 18.2; Nu = 0:0472 Dn.sup.0.8346(Pr).sup.0.4 Constant wall temperature, (2013) [17] d = 20.8; p = 29.15; where Dn = Re({square root over (d)}/D) based on average liquid t = 2.3; bulk temperature and L = 8195, 9688, 11180 average wall temperature D = 287.3, 337.3, 387.3 Dn = 586-4773 Pr = 3.83-7.3 Xin and Ebadian D/d = 11.3-37.5; Nu = (2.153 + 0.318 Dn.sup.0.643)(Pr).sup.0.177 Constant wall heat flux, (1997) [18] d = 10.16, 22.9; where Dn = Re({square root over (d)}/D) based on peripheral L = 5500; average and local bulk p = 62.5, 76.325, temperature 381; t = 1.27; D = 127, 259, 381 Dn = 20-2000; Pr = 0.7-175 5000 < Re < 100000 Pr = 0.7-175 [00005]$\mathrm{Nu}=0.00619{\left(\mathrm{Re}\right)}^{0.92}{\left(\mathrm{Pr}\right)}^{0.4}\left(1+3.455\frac{d}{D}\right)$ Constant wall temperature

[0046] Considering Eqs. (1)-(5) coupled with either of the empirical Nusselt correlations shown in Table 3, there is a system of six nonlinear equations which is solved by developing a code in Engineering Equation Solver (EES) to obtain six unknowns, including ni, Re, UA, h, Nu, and L. After computing the above, Table 4 below concludes the system sizing for the length of the helically coiled channel, inner diameter of the channel, and diameter of the helical coil.

TABLE-US-00004 TABLE 4 System sizing with dimensions of channel and helical coil Dimensions Value Inner Diameter of the Channel  6.35 mm Diameter of the Helical Coil 96.52 mm Length of the Channel    5 m

[0047] The heat transfer performance of the heat exchanger analyzed as above was evaluated, the results of which analysis are reported in Gemeda, Takele et al., “Design and Development of a Novel Additively Manufactured Geothermal Heat Exchanger,” Center for Advanced Manufacturing in Space Technology & Applied Research (CAM-STAR), University of the District of Columbia, Washington, D.C (2020), the specification of which is incorporated herein by reference.

[0048] The prototype heat exchanger described above and shown in FIGS. 1-3 was designed using CREO software, after which the model was imported to COMSOL-Multiphysics to conduct numerical analysis.

[0049] Three types of meshes were developed in the present study to analyze the simulation, including finer, fine, and normal. The size settings for each mesh are shown in Table 5 below. Comparing the maximum velocities of the fluid flow in the middle of the channel demonstrated that the three velocity profiles were close, especially with the finer and fine meshes. The fine mesh size was eventually chosen as it allows one to conduct a detailed analysis while still maintaining a reasonable computational time.

TABLE-US-00005 TABLE 5 Mesh types developed to conduct simulation Description Finer Fine Normal Calibrate for Fluid Fluid Fluid dynamics dynaimcs dynamics Maximum element size 0.148 0.212 0.4 Minimum element size 0.016 0.04 0.12 Curvature factor 0.4 0.5 0.7 Resolution of narrow 0.9 0.8 0.6 regions Maximum element 1.4 1.13 1.2 growth rate Maximum velocity 0.0197 0.0195 0.0187 value Computational time 24 min 27 sec 6 min 21 sec 2 min 46 sec

[0050] The following assumptions were adopted to conduct the simulation: no slip boundary condition, normal inflow velocity, constant ground temperature boundary conditions, negligible gravity, and thermophysical properties of the materials are set to remain constant at average temperature of inlet and outlet as listed in Table 6 below.

TABLE-US-00006 TABLE 6 Thermophysical properties of the materials to develop simulation Thermal conductivity of EOS MaragingSteel MS1 15 W/m° C. Specific heat capacity EOS MaragingSteel MS1 450 J/kg° C. Density EOS MaragingSteel MS1 8 g/cm.sup.3 Thermal conductivity of water 0.6 W/m° C. Specific heat capacity of water 4200 J/kg° C. Density of water 997 kg/m.sup.3

[0051] FIG. 8 shows temperature distribution of the working fluid for the heating scenario with the operating conditions and geometry introduced in Tables 2 and 4, respectively. As represented in simulation results, the bulk temperature of water increases gradually from the inlet temperature of 1 C (274 K) to the outlet temperature of 13 C (287 K) as a result of heat acquisition supplied by the ground with a higher constant temperature of 15 C (288 K), indicating a temperature increase of nearly 12 C in the heating mode based on the current design of heat exchanger 100.

[0052] Similarly, FIG. 9 represents temperature distribution of the working fluid for the cooling scenario with the operating conditions and geometry introduced in Tables 2 and 4, respectively. As illustrated in simulation results, the bulk temperature of water decreases gradually from the inlet temperature of 32 C (305K) to the outlet temperature of 20 C (293 K) as a result of heat rejection to the ground with a lower constant temperature of 15 C (288 K), indicating a temperature reduction of virtually 12 C in the cooling mode based on the current design of heat exchanger 100.

[0053] Further, FIG. 10 shows velocity distribution of the working fluid flowing through the internal working fluid channel 114. As clearly represented in this figure, the fluid velocity fluctuates between almost 0.9 m/s and 1.1 m/s along the channel while the analytical calculation of the fluid's mean velocity is found to be nearly 0.93 m/s for the operating conditions and geometry addressed earlier in Tables 2 and 4, respectively.

[0054] Such results show that an additively manufactured ground-source heat exchanger for geothermal systems configured in accordance with aspects of the invention may help to eliminate the need of excavating the ground during installation while maintaining an efficient heat exchanger ready for inclusion in conventional geothermal heating and cooling systems. Such a heat exchanger provides a more compact configuration than previously known systems, and mitigates the risk of working fluid leakage into the ground, leading to less maintenance and less operating costs during long-term operation.

[0055] Having now fully set forth the preferred embodiments and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. It should be understood, therefore, that the invention may be practiced otherwise than as specifically set forth herein.