Patent Publication Number: US-6988542-B2

Title: Heat exchanger

Description:
FIELD OF THE INVENTION 
   This invention relates to heat exchangers generally, and more particularly, to a heat exchanger that may serve as a water heater and a gas cooler. 
   BACKGROUND OF THE INVENTION 
   Ozone layer and/or global warming problems have focused considerable attention on the nature of refrigerants employed in refrigeration systems of various sorts. Some such systems, particularly those that do not have sealed compressor units as are commonly found in vehicular air conditioning systems, are prone to refrigerant leakage. Older refrigerants, HFC 12, for example, are thought to cause depletion of the ozone layer while many of the replacements, HCFC 134a, for example, are believed to contribute to the so-called “greenhouse effect” and thus global warming. 
   As a consequence, a considerable effort is underway to develop refrigeration systems employing transcritical refrigerants such as carbon dioxide. Carbon dioxide is plentiful in the atmosphere and may be obtained therefrom by conventional techniques and employed as a refrigerant in such systems. Should the systems leak the CO 2  refrigerant, because it was originally obtained from the atmosphere, there is no net increase of the refrigerant in the atmosphere, and thus no increase in environmental damage as a result of the leak. 
   Transcritical refrigeration systems, such as CO 2  systems, operate at relatively high pressures and require, in lieu of a condenser in a conventional vapor compression refrigeration system, a gas cooler for the refrigerant. 
   The heat rejected by a gas cooler can be employed for various useful purposes and one such use is for heating potable water for residential, commercial, or industrial usages. The present invention is primarily directed at providing a combination water heater and gas cooler. 
   SUMMARY OF THE INVENTION 
   It is the principal object of the invention to provide a new and improved heat exchanger. More specifically, it is an object of the invention to provide a new and improved heat exchanger that can be used with efficacy in a refrigeration system for cooling gaseous refrigerant while heating potable water. 
   An exemplary embodiment of the invention achieves the foregoing object in a heat exchanger intended for use as a water heater/gas cooler that includes first and second generally parallel, spaced, tubular water headers. A plurality of water tubes extend in spaced, generally parallel relation between the water headers and are in fluid communication therewith. A water inlet is provided in one of the water headers and a water outlet is provided in one of the water headers. 
   A plurality of gas tubes, at least one for each water tube, are helically wound about corresponding ones of the water tubes in heat transfer facilitating contact therewith and each gas tube has opposed ends. First and second, generally parallel spaced gas headers are connected in fluid communication with the respective ones of the opposed ends of the gas tubes and a gas inlet is provided in one of the gas headers and a gas outlet is provided in the other of the headers. 
   In a preferred embodiment, there is at least one additional outlet in one of the water headers. 
   A preferred embodiment also contemplates that there may be at least one baffle in at least one of the water headers. 
   In one embodiment of the invention, a non-straight turbulator wire is disposed in the water tubes. More preferably, the turbulator wire is a helical or spirally shaped wire. 
   One embodiment of the invention contemplates that the water tubes are generally straight and the water headers are remote from one another. 
   In another embodiment of the invention, the water tubes are bent to bring the water headers into proximity to one another. 
   One embodiment of the invention contemplates that the tubes be formed of a metal selected from the group that consists of copper and stainless steel. 
   In one embodiment of the invention, the interior of the water tubes is grooved. 
   One embodiment of the invention contemplates that the exteriors of the water tubes have helical grooves and that the gas tubes are wound in the grooves. 
   In a preferred embodiment, each gas tube includes an inside diameter in the range of about 0.04 inches to 0.10 inches and is helically wound to a pitch in the range of about 0.20 inches to 2.0 inches. 
   In a highly preferred embodiment, the inside diameter of the gas tubes is about 0.08 inches and the pitch is about 0.30 inches. 
   A preferred embodiment of the invention contemplates that the water tubes have an inside diameter in the range of about 0.10 inch to 0.50 inches. 
   According to the embodiment mentioned immediately preceding, the water tubes include a helical internal spring wire turbulator having a diameter in the range of about 0.03 inches to 0.08 inches and a pitch in the range of about 0.20 inches to 1.0 inches and the water tube inner diameter is in the range of about 0.10 inches to about 0.40 inches. 
   In this embodiment, it is preferred that the water tubes be smooth walled. 
   In another embodiment of the invention, the water tubes each have a helical groove in which a corresponding one of the gas tubes is snugly received and each helical groove has a pitch in the range of about 0.20 inches to 2.0 inches. More preferably, the internal diameter of this embodiment of the water tubes is in the range of about 0.14 inches to 0.50 inches and includes a grooved inner wall surface. 
   Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of one embodiment of a heat exchanger made according to the invention; 
       FIG. 2  is a side elevational of an alternative embodiment; 
       FIG. 3  is an enlarged, fragmentary view of a water tube employed in one embodiment of the invention; 
       FIG. 4  is a fragmentary view of a water tube employed in another embodiment of the invention; 
       FIG. 5  is a sectional view of still another embodiment of the invention, and specifically the water tube in gas tube relationship in such embodiment; and 
       FIG. 6  is a perspective view of another embodiment of a heat exchanger made according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will be described as being useful in the environment of a refrigeration system employing a transcritical refrigerant such as CO 2 . However, it is to be understood that the heat exchanger may be used in other heat exchange applications that do not involve refrigeration and/or water heating and may find use in refrigeration systems using nontranscritical and/or conventional refrigerants. Accordingly, no limitation to a water heater/gas cooler in a transcritical refrigeration system is intended except insofar as expressly stated in the appended claims. 
   Referring to  FIG. 1 , a heat exchanger made according to the invention includes a pair of spaced, cylindrical, tubular headers,  10  and  12 , which are generally parallel to one another. Smaller diameter cylindrical, water tubes  14  extend between the headers  10 , 12  and are in fluid communication with the interior thereof. 
   In the embodiment illustrated in  FIG. 1 , the header  10  has an inlet at an end  16  with the opposite end  18  being plugged by any suitable means. The header  12  includes an outlet  20  with the opposite end  22  being suitably plugged. However, desired, a so-called multipass unit can be utilized wherein both the inlet  16  and outlet  20  are in the same header  10  or  12  with the passage of water through the tubes  14  being caused to occur in a serial fashion as by the conventional use of interior baffles  24  and  26  respectively, in the headers  10 , 12 , as shown in  FIG. 1 . However, it is to be specifically noted that either or both of the baffles  24  and  26  are purely optional and if desired, flow through each of the tubes  14  could be in a hydraulically parallel fashion or, in some instances, could be a combination of hydraulically parallel and hydraulically serial flow, as desired. 
   Regardless of the particular flow pattern used, the invention contemplates that one or both of the headers  10  and  12  may be provided with at least one outlet in addition to the outlet  20  from the header  12 . Thus, an outlet conduit  28  is located in the header  10  between the baffle  24  and the end  18  while a similar outlet conduit  30  is located in the header  12  between the baffle  26  and the outlet  20 . The additional outlets provide a means whereby water flowing through the tubes  14  may be outletted to a point of use at different temperatures. For example, when the baffles  24  and  26  are present, water passing to the outlet  30  will pass through all three runs of the tubes  14  illustrated and thus be more subjected to heating than water passing to the outlet  28  which only passes through two of the tubes  14  which, in turn, will be hotter than water passing out of the outlet  20  which has passed through only one of the tubes  14 . 
   The heating of the water in the tubes  14  is obtained by wrapping a cylindrical tube  32  of smaller diameter than the tubes  14  about each of the tubes  14 . Each of the helical tubes  32  is wrapped tightly about the corresponding tube  14  to be in good heat transfer contact therewith and preferably, will be metallurgically bonded to the associated water tube  14  by brazing or soldering. 
   The tubes  32  are gas tubes with opposed ends  34  and  36  adjacent, respectively, the headers  10  and  12 . The ends  34  extend to and are in fluid communication with a gas header  40  while the ends  36  extend to and are in fluid communication with the interior of a second gas header  42  which is spaced from and parallel to the header  40 . The header  40  is capped at an end  44  and thus the opposite end  46  provides a gas outlet where countercurrent flow is desired in the case where the baffles  24  and  26  are omitted. The gas header  42  has an open end  46  which serves as an inlet and a capped end  48 . 
   In the embodiment illustrated in  FIG. 1 , the water tubes  14  are straight tubes. However, in some cases, for spatial reasons, the tubes  14  may be bent intermediate their ends to be, for example, U-shaped as illustrated in  FIG. 2  to bring the headers  10  and  12  into proximity with one another. 
     FIG. 3  illustrates a preferred construction for the water tubes  14 . A spring wire turbulator  50  extends generally the length of each of the tubes  14 . The spring wire turbulator  50  is basically a wire helix with spaced convolutions and induces turbulence in the water flowing within the water tubes  14  which in turn will enhance heat transfer. 
   As an alternative to the use of a turbulator such as the spring wire turbulator  50 , the inner wall of the water tubes  14  may be provided with a conventional heat transfer enhancement in the form of multiple, small grooves  52  formed on the interior of the tube wall. This embodiment is illustrated in  FIG. 4 . 
   In some cases, where improved heat transfer between the gas tubes  32  and the water tubes  14  is desired, the latter are provided with a helical pattern of grooves  54  which receive corresponding convolutions of the helical part of each of the gas tubes  32  as shown in  FIG. 5 . Again, it is preferred that the gas tubes  32  be metallurgically bonded to the water tubes  14  within the grooves  54 . 
   The embodiment of the invention shown in  FIG. 5  contemplates that both the water tubes  14  and the gas tubes  32  have a basically circular cross section and as a consequence, it will be appreciated that very nearly 180° of the periphery of each convolution of the gas tube  32  will be in contact with the exterior wall surface of the corresponding water tube  14  thereby maximizing the area over which heat transfer may occur. 
   In general, the water tubes  14  can be of three types. In the embodiment shown in  FIG. 1 , a smooth walled tube (both inner and outer wall surfaces are smooth) with the internal spring turbulator  50  is employed. The tube  14  will typically have an inside diameter in the range of about 0.10 inches to 0.40 inches. The helically formed spring wire turbulator  50  will have a diameter of 0.03 inches to 0.08 inches. The pitch of the convolutions of the turbulator  50  will be in the range of 0.20 inches to 1.0 inch. 
   Where water tubes such as that shown in  FIG. 5  are employed, the same dimensions are employed and may include the spring turbulator  50  although the same is not illustrated in  FIG. 5 . 
   When the embodiment illustrated in  FIG. 4  is used for the water tubes  14 , the tube  14  has a smooth exterior wall and an inside diameter in the range 0.14 inches to 0.50 inches. 
   The gas tubes  32  are preferably smooth walled (both inner and outer wall surfaces are smooth) with an inside diameter of 0.04 inches to 0.10 inches. The pitch of the helical section of the gas tubes  32  will be in the range of 0.20 inches to 2.0 inches. Of course, in the  FIG. 5  embodiment, the pitch of the grooves  54  in the tube  14  will be the same as the pitch of the helically wound part of the gas tubes  32 . 
   In one example of a heat exchanger made according to the invention and used as a water heater/CO 2  cooler, for an incoming water temperature of 50° F. and an incoming CO 2  temperature of 250° F. and at a pressure of 1600 psia, a heat transfer effectiveness of 95% can be obtained with a construction employing a water tube  14  having an inside diameter of 0.19 inches, a spring wire turbulator diameter of 0.051 inches, a spring wire turbulator pitch of 0.25 inches with the water entering at a Reynolds number of about 1,000. The gas tube or CO 2  tube  32  will have an inside diameter of 0.08 inches and a pitch of 0.30 inches. CO 2  flow entering the tubes  32  should be at a Reynolds number of about 130,000. 
   It should be appreciated that while the embodiments discussed above describe one preferred arrangement wherein there is a one-to-one correspondence between the gas tubes  32  and the water tubes  14 , in some applications it may be desirable to have one or more of the gas tubes  32  helically wound about each of the water tubes  14 . This can be desirable, for example, when a lower pressure drop is desired for the gas flow through the gas tubes  32  and/or an increased amount of gas flow is required through the gas tubes  32  to improve the performance of the water heater/gas cooler. One example of this construction is shown in  FIG. 6  wherein there are two of the gas tubes  32  for each of the water tubes  14 , with the second set of gas tubes  32  shown by dashed lines for purposes of clarity. In all other respects, the heat exchanger of  FIG. 6  is identical to the exchanger of  FIG. 1  as described above. It should be understood that such a construction can be applied to any of the above-described embodiments, such as for example, the embodiment shown in  FIG. 2 , wherein one or more additional gas tubes  32  can be wound about the water tube  14 . 
   From the foregoing, it will be appreciated that a relatively simple design of a heat exchanger is provided which allows assembly by brazing and/or soldering. Wall thickness of the gas tubes  32  will be dependent upon the pressure that they must withstand for any given inside diameter in the specified ranges. Suitable fixturing can be readily brazed or soldered to the ends of the header tubes servicing as inlets and/or outlets as well as to the additional outlets provided. As a consequence, heated potable water may be readily supplied relatively inexpensively by capturing the heat that would ordinarily be rejected from the hot gas and utilizing the same to heat water. The use of plural outlets at different locations allows the desired water temperature to be selected without affecting the operation parameters on the gas side of the system.