Abstract:
An improved atomizer, in which the liquid to be sprayed is circulated around the nozzle tip to prevent degradation of the liquid in hot environments. The circulation is controlled by a valve, which permits the liquid to circulate even when no liquid is being sprayed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/640,612 filed on Dec. 30, 2004, entitled “Atomizer Cooling by Liquid Circulation Through Atomizer Tip Holder”, which is incorporated herein by reference in its entirety. 

   TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to liquid spray devices, and more particularly to an improved atomizer. 
   BACKGROUND OF THE INVENTION 
   An “atomizer” is a dispenser that turns a liquid into a fine spray. For some applications, atomizers are used to spray a fuel or other liquid into a hot environment. 
   In the case of fuel atomizers, the fuel can undergo chemical changes leading to carbonaceous dry materials that plug the atomizer if the fuel temperature is not maintained below the thermal oxidation temperature, typically in the range of 200° C. to 300° C. This chemical degradation of the fuel due to thermal oxidation is often referred to as fuel “coking.” 
   Similarly, in spraying urea-water mixtures into the exhaust of engines as part of a selective catalytic reduction (SCR) system for control of nitric oxide (NO) emissions, the atomizer can sometimes overheat and cause the water to vaporize, leaving behind solid urea particles that plug the atomizer. 
   In the design of fuel atomizers or other atomizers, the liquid flowing through the atomizer is also used to cool the atomizer and to avoid chemical changes in the liquid that can lead to atomizer plugging. However, in some applications, such as fuel injection atomizers, the atomization is intermittent. The atomizer remains in place in the hot environment when no liquid is flowing through the atomizer. Overheating of the liquid in the atomizer under these conditions can cause atomizer plugging and failure. 
   A solution to this problem can be achieved if the atomizer temperature can be maintained below the temperature at which the liquid undergoes thermal degradation. To cool the atomizer and avoid thermal decomposition, water or engine coolant is often used. However, routing cooling water to the atomizer is often difficult, expensive, or impractical. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
       FIG. 1  illustrates a pressure-swirl atomizer with bypass. 
       FIG. 2  illustrates the atomizer of  FIG. 1  modified in accordance with the invention. 
       FIG. 3  is second embodiment of an atomizer, modified in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The concept discussed herein is directed to an atomizer design that reduces the chance of atomizer plugging, whether the liquid being sprayed is fuel, urea-water mixtures, or some other liquid or liquid mixture that is subject to thermal degradation. This concept may be implemented as an improvement to an existing, commercially available atomizer. The concept reduces or eliminates the probability of thermal degradation of the liquid being sprayed, while extending the flow range of the atomizer. 
     FIG. 1  illustrates an example of the type of atomizer with which the invention may be used. This atomizer  10  is the commercially available pressure-atomized Variflo™ bypass nozzle, available from Delavan Spray Technologies. Atomizer  10  comprises a nozzle  10   a  screwed into an adapter  10   b . In accordance with the bypass design of atomizer  10 , with constant supply pressure at inlet  10   c  and with bypass channel  14  closed, the nozzle  10   a  operates as a simplex nozzle with the liquid being delivered via spray channel  13  and sprayed out from orifice  11   a . With the bypass channel  14  open, part of the liquid is allowed to return to a source reservoir (not shown), with the result being reduced discharge flow. 
   The atomization principle of atomizer  10  is based on swirling the liquid in a swirl chamber just upstream of an orifice disk  11 . As a result of the swirling, a thin sheet of liquid flows along the outer edges of the orifice disk  11 . The liquid is then atomized as it leaves the orifice  11   a . The swirling flow is created by narrow slots cut at an angle in the distributor  12 . 
   As discussed in the Background, a limitation to any atomizer for applications in a hot environment is that when the spray is turned off, that is, when flow is stopped in the atomizer, the liquid remains in the tip and may be subject to thermal degradation. If the atomizer is then turned back on, the atomizer may then be clogged or if not clogged, the atomized liquid may be degraded. In an atomizer such as the example of  FIG. 1 , the problem is exacerbated by the fact that the slots in the distributor  12  are quite small and easy to plug. 
     FIG. 2  illustrates the concept proposed herein. The modified atomizer  20  is cooled with the same fluid that is to be sprayed from the atomizer. For cooling, fluid from reservoir  25  is directed to a cooling channel  21 , which leads to a annular channel  21   a  machined into adapter  10   b  in the region where the nozzle  10   a  screws into the adapter  10   b . The fluid in the annular channel  21   a  cools the adapter in the area near the orifice. 
   As indicated by the dotted lines, if desired, after a heat exchange occurs, the liquid may be directed out of the atomizer via the bypass channel  24 . To this end, the annular channel can be made to be in liquid communication with the bypass channel. The circulated liquid flows back into the liquid supply reservoir  25 . 
   The atomizer&#39;s normal valve, used to turn off and on liquid flow to the atomizer, and located upstream from the atomizer, is replaced with a 3-way valve  22  which directs flow from a supply pump  26  to either the atomizer spray path  23 , in the normal way, or to the cooling channel  21  when the spray is turned off. Thus, depending on the setting of valve  22 , the liquid flows in a spray path”  23  when the atomizer is on (spraying), and a cooling path  21  when the atomizer is off (not spraying). 
   When the spray is stopped (off) and the liquid is circulating within nozzle  10   a  via the cooling channel  21 , the atomizer remains relatively cool, below the liquid thermal decomposition point, by its thermal contact with the adapter  10   b . The standard liquid pump  26  that supplies pressure to the atomizer may be used to cool the atomizer even when the atomizer is not spraying liquid. 
   In this way, a standard atomizer nozzle  10   a  can be used and replaced as necessary. If the atomizer already has a bypass channel  24 , the only modification is to the adapter  10   b  that holds the nozzle  10   a . In practice, the cooling channel  21  could be bored into the adapter body, or it can be external to the adapter. 
   If the atomizer does not already have a bypass channel for permitting liquid to exit the circulation chamber, the atomizer may be modified to have an exit channel. 
   If a high-pressure boost pump (not shown) is used to increase the pressure from a supply pump to improve atomization quality, the 3-way valve  22  may be placed upstream of the high-pressure pump, as even low pressure is sufficient for cooling the atomizer. A check mechanism may be necessary as part of bypass valve  27  to avoid liquid flow backward through the bypass line if the drain is arranged as shown in  FIG. 2 . 
   Some existing fuel injectors provide fuel flow through the injectors even when they are not spraying fuel. However, in these injectors, the purpose of the fuel flow is not to cool the injector, but rather, to provide fuel in a convenient location to be injected when required. In those injectors, an expensive solenoid control valve must be built into the fuel injector, greatly increasing the cost. 
   In general, the modification discussed above is to an atomizer having a housing surrounding the nozzle. In the example of  FIGS. 1 and 2 , the housing is a removable adapter  10   b . The housing has an annular channel  21   a  for containing the liquid delivered from the reservoir via the circulation channel. The liquid may enter (or remain in) this annular region even when the spray is turned off and is pressurized by the same pressure used for providing the spray. 
     FIG. 3  illustrates an alternative embodiment of the invention. A circulation cylinder  31  has been added in the region of the atomizer nozzle. Cylinder  31  permits liquid that is normally sprayed through the atomizer (when the atomizer is “on”) to flow through the cylinder  31  on one side of the nozzle and to exit on the other side. The flow of liquid inside cylinder  31  can take many forms. As another example, the liquid can flow around the nozzle. 
   Cylinder  31  may be easily attached to an existing housing, such as adapter  10   b . In fact, for purposes of generality, both the embodiment of  FIG. 2  and the embodiment of  FIG. 3  could be described as having a housing (adapter  10   b  or cylinder  31 ) having an annular bore around the nozzle  10   a.    
   As illustrated in  FIG. 3 , the discharged liquid can flow out through an existing bypass line  14 . Alternatively, for atomizers not already having a bypass line, an exit line can be provided. 
   Other elements of  FIG. 3  are similar to those of like numbering in  FIG. 2 . 
   For the embodiments of  FIGS. 2 and 3 , the modifications described herein allow the atomizer to be used for intermittent operation in a hot environment. Without the modification, the atomizer would suffer from thermal degradation of the liquid in the atomizer and eventual atomizer plugging. 
   The thermal degradation point for fuels like diesel fuel is above 200° C., so maintaining the atomizer temperature lower than that value should prevent degradation. For urea-water mixtures, the temperature is lower, probably less than 70° C.