Abstract:
A cooling system for an agricultural vehicle, such as a tractor driven by an internal combustion engine includes a main cooling system containing a coolant. The cooling system includes a sorption cooling system which includes an evaporator for evaporating a refrigerant, a sorption chamber for the sorption of the refrigerant vapor, a desorption chamber for the desorption of the refrigerant from the sorbent, and a condenser for condensing the refrigerant. An exhaust gas stream from the engine is conducted to the desorption chamber to provide the heat necessary for the desorption. The evaporator is used for additional cooling of the coolant of the main cooling system and/or for cooling a second exhaust gas stream from the engine.

Description:
BACKGROUND  
       [0001]     The present invention relates to an engine driven agricultural tractor cooling system with a main cooling system containing a coolant.  
         [0002]     Increasing power output of engines and stricter emission control regulations increases the demands on a vehicle cooling system. The rate of thermal output that can be rejected by a conventional cooling system is essentially determined by parameters such as the size of the surface of a cooler, heat transmission coefficients, flow velocities of the coolant, and by temperature differences of the associated media (surroundings, coolant, etc.). In agricultural vehicles, particularly in tractors, the thermal output of conventional cooling systems is determined generally by the size of the available space of the configuration and the maximum allowable temperature difference. For a water-based cooling system, cooling this maximum temperature difference is a result of the maximum surrounding temperature and the highest allowable cooling water temperature.  
         [0003]     Various cooling systems are known which achieve more efficient cooling with maximum prevailing temperature differences. For example, published German patent application DE 198 54 544 describes a cooling system for a supercharged engine with an improved cooling capacity. This cooling system includes a high temperature cooling circuit and a low temperature cooling circuit. The high temperature cooling circuit includes in a main branch, the engine, a high temperature re-cooler, and a branch circuit with a high temperature charge air cooler. The low temperature cooling circuit includes a low temperature re-cooler in series with a low temperature charge air cooler. Furthermore, this system also includes an engine oil/gearbox oil heat exchanger and a heat exchanger for cooling electronic components. Thus, the heat rejected in the engine oil/gearbox oil heat exchanger is rejected at a relatively high temperature level, while the electronic components and the charge air is cooled simultaneously at a lower temperature. However, this cooling system is costly, and the resulting temperature levels are not appropriate for the cooling of a supercharged engine of an agricultural tractor.  
         [0004]     Other cooling systems are known that are based on the principle of the sorption cooling, in which cold is primarily generated from heat. Such systems are gaining importance, particularly in connection with air conditioned vehicles. Adsorption and absorption refrigeration machines are described in various publications, for example, Andreas Gassel, “Die Adsorptionskaeltemaschine-Betriebserfahrungen und thermodynamische Berechnung”-“The adsorption refrigeration machine—Operating experience and thermodynamic calculations”; Article draft for Ki air and refrigeration technology or York International “Prinzip Absorptionskaltemaschine”-“Principles of absorption refrigeration machine”, prospectus KK14300). DE 199 27 879 A1 describes a vehicle air conditioning system with an adsorption refrigeration arrangement. The system includes an adsorption refrigeration arrangement in which liquid refrigerant is evaporated, in order to extract the heat required for it from a liquid or gaseous medium for the generation of low temperatures. The evaporated refrigerant is conducted to a sorbent to be adsorbed. The sorbent loaded with refrigerant is heated in order to desorb the refrigerant again, and the refrigerant is subsequently liquified in order to make it available for renewed evaporation. Thus, the heat rejected by the engine is used to heat the sorbent. The system includes methanol as refrigerant and activated charcoal as sorbent. However, this absorption refrigeration system is configured for the air conditioning of a vehicle, and appears to be inappropriate for the cooling of individual components of a supercharged engine, particularly that of an agricultural tractor. Furthermore, other potential sources of heat, such as the engine exhaust, cannot be utilized for the high temperatures required for the desorption in the adsorption refrigeration machine disclosed by published German patent application DE 199 27 879 A1 because extensive design-engineering changes would be required.  
       SUMMARY  
       [0005]     Accordingly, an object of this invention is to provide a cooling system which reduces the thermal load on a conventional main cooling system.  
         [0006]     A further object of the invention is to provide such a cooling system for an engine block or a charge air cooling system.  
         [0007]     These and other objects are achieved by the present invention, wherein a cooling system includes a known sorption cooling system. The sorption cooling system includes an evaporator for evaporating a refrigerant, a sorption chamber containing an absorbent which absorbs the evaporated refrigerant, a desorption chamber for desorption of the refrigerant from the absorbent and a condenser for condensing the refrigerant. To supply the heat required for the desorption, a first exhaust gas stream of the engine is communicated to the desorption chamber. The evaporator provides additional cooling of the coolant of the main cooling system, and/or cools a second exhaust gas stream from the engine. Combining a conventional main cooling system with a sorption cooling system decreases the load on the main cooling system and the evaporator reduces heat rejection. The evaporator is positioned at the cylinder head and directly cools the cylinder head, so that the heat rejected there does not fully load the main cooling system.  
         [0008]     The heat removed by the sorption cooling system or the heat removed by the evaporator can be delivered to the surroundings at a considerably higher temperature level by the condensation of the refrigerant. Thus, higher temperature differences to the surroundings can be realized and hence a more compact configuration of coolers is possible.  
         [0009]     The evaporator can be positioned to extract heat from other components, such as, for example, the engine itself, parts of the mechanical power transmission, components of the power electronic system, electrical machines, the vehicle cab, the charge air, a recirculated exhaust gas stream or any other components that can be cooled contained in or on the vehicle. Moreover, this cooling system can be used to cool fluids such as engine oil or gearbox oil.  
         [0010]     The sorption cooling system extracts heat from the conventional main cooling system, at a cost in thermal output. Preferably, thermal output supplies the power for the sorption cooling system. Thereby in a sorption cooling system a “thermal compressor” is realized in contrast to the mechanically driven compressors which are widely used in refrigeration machines and in air conditioners.  
         [0011]     The thermal output required to drive the sorption cooling system is extracted from the vehicle, preferably from the exhaust gas of the engine. But, other sources of thermal energy are also conceivable. For example, the rejected heat of the engine or the engine cooling water or any other available source of heat in the vehicle could be used.  
         [0012]     The process where a material is taken up selectively by another material is known as absorption or adsorption. When the particular process is unknown, the process is known as sorption. The sorbing material is referred to as a sorbent. The material that is sorbed is the sorbate. “Desorption” is the regeneration or the separation of the material that was sorbed.  
         [0013]     Absorption is the process in which gases are taken up by fluids or solids, wherein the dissolved gas component is the absorbent and the fluid (solvent) is the absorbate. Desorption is the reverse of the absorption, wherein gas is driven off at increased temperature and/or reduced pressure and the solvent is regenerated.  
         [0014]     Adsorption is the deposition of gases and dissolved materials (adsorbate) on the surface of solids (adsorbent), for example, the binding of steam as adsorbate to an activated charcoal adsorbent. Adsorption takes place not only on the outer surface of the absorbent, but also in its pores, as long as these are accessible to the absorbate. During the adsorption process, the heat of adsorption is liberated. The heat of adsorption is approximately of the magnitude of the heat of condensation. Important adsorbents are activated charcoal, silica jell, aluminum oxide or even fullers earth.  
         [0015]     In general, desorption is the reverse of the absorption or adsorption process at higher temperatures or lower pressures where absorbed or the adsorbed material is regenerated.  
         [0016]     An absorption cooling system operates with a liquid solvent as sorbent or absorbent, and an adsorption cooling system operates with a solid sorbent. An absorption cooling system will include an absorbent and an absorbate, such as, for example, a solvent and a refrigerant, where the refrigerant is absorbed by the solvent and is again separated from it in the desorption process. For example, lithium bromide absorbs water, and water absorbs ammonia. The material absorbed functions as a refrigerant (absorbate), while the other material functions as a solvent (absorbent). The refrigerant and the solvent are together characterized as an operating pair. The solution of the materials is heated in order to separated them from each other again in the desorption chamber (boiler or separator). The refrigerant evaporates first because of its lower evaporation temperature. The evaporated refrigerant is freed from the rest of the solvent with which it had been evaporated by means of a liquid separator. Then, the refrigerant is cooled in the condenser (liquefier) and thereby liquified. The pressure of the refrigerant is reduced to an evaporation pressure corresponding to a predetermined temperature by a control valve. In the evaporator the refrigerant is evaporated by absorbing heat, and the heat absorbed provides the cooling effect. Next, the refrigerant vapor is conducted into the sorption chamber. After the separation from the refrigerant (absorbent) or after the desorption, the pressure of the solvent (absorbate) is reduced by a valve to the sorption chamber pressure, cooled and conducted to the sorption chamber. Thereby, the solvent takes up the refrigerant vapor in the sorption chamber. A solvent pump conducts the enriched solution back to the ejector, the circulation is thereby closed. The entire solvent circulation operates as a “thermal compressor”, and functions as a compressor of a compression refrigeration machine. As noted initially, the amount of heat required for the evaporation QO and the amount of heat required for the desorption QH can be derived from differing vehicle components, so that the amount of heat that is to be rejected by the main cooling system is reduced, and that the amount of heat required for the evaporation QO is derived from the main cooling system. ***The system operates more effectively with higher temperature of the cooling water of the main cooling system. The tendency to raise the pressure level in the main cooling circuit in order to attain correspondingly higher allowable temperatures therefore promotes the possibilities of the absorption cooling systems. The driving temperatures for the ejection lie between 90° C. and 140° C., where the medium or the components to be cooled can assume similar temperatures as with the cooling with an adsorption cooling system.  
         [0017]     An adsorption cooling system includes an adsorption chamber filled with a sorbent or an adsorbent, and a desorption chamber filled with an adsorbent, a condenser and an evaporator. As in the case of an absorption cooling system, in an adsorption cooling system various pairs of materials are possible. The adsorbent may be for example, silica gel, and the refrigerant may be adsorbate water. It is also known to use activated charcoal as adsorbent and methanol as adsorbate. The process is discontinuous and closed. During a cycle the following processes occur: The water (adsorbate) adhering to the silica gel (adsorbent) is driven out in the desorption chamber with the supply of an amount of heat QH by a heated water circulation associated with the desorption chamber. The water is liquified in the condenser and heat is carried away by the cooling water circuit associated with the condenser. The condensate is sprayed into the condenser and evaporated under strong negative pressure. An amount of heat QO is extracted from the surroundings or from a component that is to be cooled. The water vapor is adsorbed in the adsorption chamber and the resulting heat of adsorption is conducted to a cooling water circuit associated with the adsorption chamber. By simply reversing the heating and cooling water circuits of the desorption or the adsorption chamber between the two chambers the functions of desorption and adsorption are interchanged at the end of a cycle and the process is started anew.  
         [0018]     The desorption of the adhering water and the generation of pressure for the condensation occurs at low temperatures of 60° C.-70° C., so that this technology can be applied at lower temperatures than with an absorption cooling system.  
         [0019]     Such a sorption cooling system can be driven “at no cost” by heat sources available on the vehicle, for example, the exhaust gas, instead of mechanical power required by compressors. Furthermore, the cooling capacities of a conventional cooling system can be reduced, and the amount of heat that must be removed in a water-to-air heat exchanger from the engine is reduced, thereby reducing the load on the main cooling system, and it is possible to remove heat at a higher temperature than in a conventional cooling system. Moreover, the energy required for cooling of further components can be minimized and fuel consumption can be reduced. In addition, it is possible to use environmentally benign refrigerants. Furthermore, if an air conditioning installation exists, mechanical or electrical compressor drive units can be omitted or their size reduced. Furthermore, various fluids and tractor components can be cooled to temperatures below the temperature of the surroundings. In contrast to a cooling system with a compressor, fewer moving parts are required, and hence fewer components are subject to wear. Beyond that, the exhaust back pressure is not increased with the heat exchanger outside at the exhaust pipe of the vehicle.  
         [0020]     In a preferred embodiment of the invention, a first exhaust gas stream of the engine is an exhaust gas stream of an exhaust gas recirculation system, so that the heat required for desorption is extracted from the vehicle exhaust gas recirculation system. This advantageously cools the recirculated exhaust gas, and when the recirculated exhaust gas reaches the engine combustion chamber it results in a reduced heating of the charged air and thereby an improved charging or improved emission values can be obtained during the combustion.  
         [0021]     In another embodiment, the first exhaust gas stream is branched off from the main exhaust gas stream delivered to the environment. Thus, the heat of an exhaust gas stream can be used to drive the desorption chamber, thereby improving the total energy balance of the vehicle.  
         [0022]     In a further embodiment, the main cooling system is a charge air cooling system and that the evaporator of the sorption cooling system is used for additional cooling of the charge air cooling system. Thus, the charge air cooling system can be configured more efficiently, or the cooling performance for the charge air can be improved so that an increased amount of charge air reaches the engine combustion chamber. This improves emission values.  
         [0023]     In a further embodiment, the main cooling system is an engine block cooling system and that the evaporator of the sorption cooling system is used for additional cooling of the engine block cooling system. This reduces the load on the main engine cooling system. This can be utilized either to reduce the required cooler volume or, if necessary, to increase the entire cooling capacity.  
         [0024]     In a further embodiment, the main cooling system is an air conditioner cooling system and that the evaporator of the sorption cooling system is used for additional cooling of the cab air flow. This permits a reduced size of the air conditioner compressor, and less mechanical energy needs to be drained away from the vehicle. This, in turn, leads to an improved total energy balance of the vehicle and hence a lower fuel consumption.  
         [0025]     In a further embodiment, the second exhaust gas stream is an exhaust gas stream of an exhaust gas recirculation system, where the evaporator of the sorption cooling system cools the exhaust gas in the exhaust gas recirculation system. Thus, the heat of the exhaust gas is used to drive the desorption chamber, thus improving the total energy balance of the vehicle. Another advantage is that the recirculated exhaust gas is cooled, and upon the recirculation of the exhaust gas into the engine combustion chamber, heating of the charge air is reduced, thereby improving air charge or emissions during the combustion.  
         [0026]     Preferably, the condenser is connected with a cooler, which carries away the heat liberated in the condenser. The amount of heat generated by condensing the refrigerant (sorbent) can thereby be efficiently delivered to the surroundings, and heat is transferred at a higher temperature.  
         [0027]     In another embodiment of the invention, the sorption chamber is connected with a cooler which carries away the heat liberated in the sorption chamber. Thereby the amount of heat generated by the sorption of the refrigerant (sorbent) can be efficiently delivered to the surroundings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  is a schematic block diagram of a cooling system according to the invention with a sorption cooling system to reduce the load on a charge air cooling circuit;  
         [0029]      FIG. 2  is a schematic block diagram of a sorption cooling system which reduces the load on the charge air cooling system;  
         [0030]      FIG. 3  is a schematic block diagram of a cooling system according to the invention with a sorption cooling system which reduces the load on the cooling circuit for an internal combustion engine;  
         [0031]      FIG. 4  is a schematic block diagram of a cooling system according to the invention with a sorption cooling system which cools an exhaust gas recirculation system; and  
         [0032]      FIG. 5  is a schematic block diagram of a cooling system according to the invention with a sorption cooling system which reduces the load on a cab air cooling system. 
     
    
     DETAILED DESCRIPTION  
       [0033]      FIG. 1  shows a cooling system  10  for an internal combustion engine  12  with a charge air system  14 , a charge air cooling system  16 , an engine cooling system  18 , an exhaust gas recirculation system  20 , and a sorption cooling system  22 . The sorption cooling system  22  is applied in order to reduce the load on the charge air cooling system  16 .  
         [0034]     The engine  12  includes an intake system  24  which is supplied with super charged and cooled air  26  and recirculated exhaust gas  28 . The recirculated exhaust gas  28  is withdrawn from the exhaust gas stream  30  flowing out of the engine  12  and conducted to the intake system  24  of the engine  12 . By means of the exhaust gas recirculation system  20 , the amount of the exhaust gas delivered to the surroundings can be reduced and the emissions reduced.  
         [0035]     In order to cool the engine  12 , the engine cooling system  18  is connected to the engine  12  over coolant lines  32 . The coolant lines  32  are connected with an engine cooler  34 , so that a coolant (not shown) in the coolant lines  32  circulates between the engine cooler  34  and the engine  12  and carries away heat generated in the engine  12  to the engine cooler  34 .  
         [0036]     The charge air system  14  is used, among other uses, to compress the intake air taken in from the surroundings so that an increased amount of air flows into the intake system  24  of the engine  12 , resulting in improved combustion of the fuel and thereby reducing emissions in the exhaust gas. The charge air system  14  includes an air filter  36  that filters intake air taken in from the surroundings and from which the filtered intake air is conducted into a turbo-supercharger  38 . The turbo-supercharger  38  includes a drive side (not shown) and a compressor side (not shown). The drive side of the turbo-supercharger  38  is driven by a drive exhaust gas stream  40 , where the drive exhaust gas stream  40  is also branched off from the exhaust gas stream  30 . The intake air from the air filter  36  is compressed in the compressor side of the turbo-supercharger  38 . This compression increases the density of the intake air and increases the heat of the intake air. This heat, in turn, has a negative effect on the combustion of the fuel, for which reason the charge air system  14  as a rule is also connected to a charge air cooling system  16 . The charge air cooling system  16  includes a charge air cooler  42  which is connected over charge air coolant lines  44  with a heat exchanger  46 . The heat exchanger  46  is positioned between the turbo-supercharger  38  and the intake system  24  so that a coolant (not shown) in the charge air coolant lines  44  circulates between the heat exchanger  46  and the charge air cooler  42  and cools a flow of heat coming from the compressed charge air on the charge air cooler  42 .  
         [0037]     According to  FIG. 1 , the sorption cooling system  22  includes a desorption chamber  48  arranged in the recirculated exhaust gas stream  28 , a condenser  50  associated with the desorption chamber  48 , a condenser cooler  54  connected by condenser coolant lines  52  for removing the heat liberated in the condenser, an evaporator  56  arranged at the charge air coolant lines  44 , and a sorption chamber  58  with a sorption chamber cooler  62  connected by sorption chamber coolant lines  60  for removing the heat liberated in the sorption chamber  58 .  
         [0038]     At least some components similar to those shown in  FIG. 1  and with the same reference numbers are also included in  FIGS. 2-5 .  
         [0039]      FIG. 2  illustrates the principle of operation of a cooling system wherein the sorption cooling system  22  reduces the load on the charge air cooling system  16 . Several components shown in  FIGS. 1 and 3 - 5  are omitted from  FIG. 2  to better illustrate the operation of the cooling system  10 .  
         [0040]     The sorption cooling system  22  of  FIG. 2  includes a solvent circulation circuit  64  and a refrigerant circuit  66 . The solvent circuit  64  includes the desorption chamber  48  driven by the recirculated exhaust gas stream  28 , a first control valve  68 , the sorption chamber  58  and a solvent pump  70 . The refrigerant circuit  66  includes the desorption chamber  48  driven by the recirculated exhaust gas stream  28 , the condenser  50 , a second control valve  72 , the evaporator  56  and the sorption chamber  58 . Furthermore, the condenser  50  and the sorption chamber  48  are each connected with a cooler  54 ,  62  which delivers the thermal output to be carried away from the condenser  50  or the sorption chamber  58  to the surroundings. To reduce the load on the charge air cooling system  16 , the evaporator  56  of the sorption cooling system  22  is integrated into the charge air cooling circuit  74  of the charge air cooling system  16 .  
         [0041]     The cooling system  10  includes a two-material mixture (not shown) in the solvent circuit  64 , which is located in the supply line  76  directed at the desorption chamber  48 . The two-part mixture in the supply line  76  consists of a solvent (not shown) which is mixed with a refrigerant (not shown) for cooling and circulating in the refrigerant circuit  66  or which has sorbed this in the sorption chamber  58 . The two-part mixture is conveyed by the solvent pump  70  into the desorption chamber  48 . In the desorption chamber  48  the two-part mixture is heated by the heat from the recirculated exhaust gas stream  28 . The refrigerant taken up by the solvent has a lower evaporation temperature than the solvent, so that the refrigerant evaporates before the solvent evaporates. This causes the desorption of the refrigerant out of the solvent. The refrigerant vapor desorbed in the desorption chamber  48  or driven out, flows through a first connecting line  78  into the condenser  50 . The refrigerant vapor is liquified in the condenser  50  where the thermal flow is carried away at a higher temperature level compared to a conventional cooling system and with a higher temperature difference between the condenser cooler  54  and condenser  50 . The condensed or cooled refrigerant flows into the evaporator  56  over a supply line  80  controlled by the second control valve  72 . The refrigerant is evaporated in the evaporator while taking up heat from the charge air cooling system  16 . Heat is thereby effectively withdrawn from the charge air cooling system  16  by the evaporator  56  or by the heat taken up by the refrigerant. Thus the cooling system  10  reduces the load on the charge air cooling system  16 , thereby either improves the cooling capacity of the charge air cooling system  16  or reduces the dimensions of the charge air cooling system  16 .  
         [0042]     The refrigerant vapor flowing out of the evaporator  56  flows into the sorption chamber  58 . The solvent circulating in the solvent circuit flows over the first control valve  68  into the sorption chamber  58  and is available to take up the refrigerant vapor or for the sorption of the refrigerant vapor. In the sorption chamber the refrigerant vapor is taken up by the solvent or it is sorbed, thereby generating heat of solution that is carried away over the sorption cooler  62 . The refrigerant flowing out of the evaporator that has not been evaporated or is still liquid is conducted over a further supply line  82  over the desorption chamber  48  and driven into the condenser  50 . Thereby, both circuits are closed, that is, the refrigerant circuit  66  and the solvent circuit  64  are closed. The improved cooling performance relative to the charge air on the basis of lower combustion temperatures in the intake system  24  of the engine  12  improves emissions.  
         [0043]     The cooling system  10  and the sorption cooling system  22  of  FIG. 2  can also reduce the load on other main cooling systems or even for the cooling of the recirculated exhaust gas  48 . This is shown in  FIGS. 3-5 .  
         [0044]     In a second embodiment shown in  FIG. 3 , the evaporator  56  is integrated into the engine cooling system  18 . In this embodiment the heat removal of the engine cooling system  18  or the reduction of the load on the engine cooling system  18  is performed in the same way by the evaporator  56 , as is the case with the reduction of the load on the charge air cooling system  16  of  FIG. 1  or  2 . In the  FIG. 3  embodiment a separate exhaust gas stream  84  is branched off from the main exhaust gas stream  30  in order to drive the desorption chamber. The embodiment shown in  FIGS. 1 and 2  is also conceivable, in which the desorption chamber  48  is driven by the recirculated exhaust gas stream  28  as is shown in  FIG. 3  with a desorption chamber  48 ′ driven by a recirculated exhaust gas stream. Furthermore, an embodiment is conceivable even without the charge air cooling system  16  and without the exhaust gas recirculation system  20 . Similar to the charge air cooling system  16  of  FIGS. 1 and 2 , the  FIG. 3  embodiment reduces the load on the engine cooling system  18 , either by increasing the cooling capacity of the engine cooling system  18  or by reducing the size of the engine cooling system  18 .  
         [0045]     In the further embodiment of  FIG. 4 , the sorption cooling system  10  can be used to cool the recirculated exhaust gas stream  28 , to improve combustion performance of the engine  12  and thereby reduce the load on the engine cooling system  18  and the charge air cooling system  16 , or improve the entire energy balance of the engine  12 . If the recirculated exhaust gas stream  28  is conducted without any cooling into the intake system  24  of the engine  12 , the charge air that was previously cooled by the charge air cooling system  16  is heated. This worsens emission performance of the engine  12 . The evaporator  56  of  FIG. 4  is integrated into the exhaust gas recirculation system  20  and heat is withdrawn from the recirculated exhaust gas stream  28  during the evaporation of the refrigerant in the evaporator  56 . According to  FIG. 4 , as is also shown in  FIG. 3 , the separated exhaust gas stream  84  is also used to drive the desorption chamber  48 .  
         [0046]     In a further embodiment shown in  FIG. 5 , the evaporator  56  is integrated into an air conditioning cooling system (not shown). In this way a cab air stream  86  pre-cooled by a conventional, mechanically driven compressor (not shown), can be post-cooled or, in a reverse arrangement, also pre-cooled by the evaporator  56  and post-cooled by the compressor. The other components of the cooling system of  FIG. 5  can be arranged similarly to the embodiments of  FIGS. 1-4 . The reduced load on the air conditioning system by the desorption cooling system leads to smaller mechanically driven compressor and thereby reduces energy requirements for the air conditioning system.  
         [0047]     The sorption cooling systems of  FIGS. 1-5  can also be combined with each other. For example, several evaporators can be arranged in a parallel or a series circuit and used to cool the charge air, the recirculated exhaust gas,  58 , the engine cooling water and/or the cab air stream  86 .  
         [0048]     While the present invention has been described in conjunction with a specific embodiment, it is understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.