Patent Publication Number: US-2012037347-A1

Title: Method of controlling tube temperatures to prevent freezing of fluids in cross counterflow shell and tube heat exchanger

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to apparatus and methods for controlling temperatures in heat exchanger fluids and, more particularly, to apparatus and methods for controlling tube temperatures in cross counterflow shell and tube heat exchangers to prevent freezing of fluids therein. 
     Generally, a heat exchanger is a device that may be used for efficient heat transfer between multiple mediums. For example, a heat exchanger may take in a first medium at a low temperature and a second medium at a high temperature. Within the body of the heat exchanger, the first and second mediums may come into contact, either directly or indirectly via a solid wall. When the two mediums come into contact, the high temperature of the second medium may cause the first medium to raise its temperature, while the low temperature of the first medium may also cause the second medium to lower its temperature. In other words, heat is exchanged between the two mediums, causing the first medium to increase in temperature and the second medium to decrease in temperature. 
     A cross counterflow tubular heat exchanger is used to heat a cold gas (for example, −100° F.) using a liquid with a freezing point at roughly 12° F. In the last pass of the heat exchanger, the cold inlet gas at −100° F. (inside the tubes) transfers heat to the warm liquid which has already been cooled somewhat in the previous passes. The wall temperature must be kept above the freezing point of the liquid to prevent the liquid from freezing on the tubes. Freezing of the liquid must be prevented so flow on the liquid side is not reduced due to the frozen blockage and no frozen liquid can be allowed to spill off and migrate downstream. Performance of the overall system will be impaired if any freezing occurs. 
     As can be seen, there is a need for a heat exchanger apparatus and heat exchange methods that may prevent freezing of the heat exchange media. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a heat exchanger comprises a first pass of tubes carrying a cold fluid through the heat exchanger; a second pass of tubes carrying the cold fluid from the first pass of tubes, a warm fluid passing first over the second pass of tubes and then the first pass of tubes, wherein tubes of the second pass of tubes have at least one of a decreased diameter and an increased tube spacing, relative to tubes of the first pass of tubes. 
     In another aspect of the present invention, a cross counterflow shell and tube heat exchanger comprises a cold fluid inlet delivering a cold gas to a first pass of tubes; a second pass of tubes receiving the cold gas from the first pass of tubes; a warm liquid passing across the second pass of tubes and then the first pass of tubes, wherein tubes of the second pass of tubes have at least one of a decreased diameter and an increased tube spacing, relative to tubes of the first pass of tubes, and wherein the warm liquid remains in a liquid state as it passes through the heat exchanger. 
     In a further aspect of the present invention, a method of heat exchange between a warm liquid and a cold gas without freezing the warm liquid comprises passing the cold gas through a first pass of tubes through the heat exchanger; and passing the cold gas from the first pass of tubes through a second pass of tubes, wherein tubes of the second pass of tubes have at least one of a decreased diameter and an increased tube spacing, relative to tubes of the first pass of tubes, and wherein the warm liquid remains in a liquid state as it passes through the heat exchanger. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is cross-sectional view of a heat exchanger according to an embodiment of the present invention; 
         FIG. 2  is a cross-section view taken along line  2 - 2  of  FIG. 1 ; and 
         FIG. 3  is a flow chart describing a method for preventing freezing in a heat exchanger according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Various inventive features are described below that can each be used independently of one another or in combination with other features. 
     Broadly, embodiments of the present invention provide methods and apparatus for controlling tube temperatures to prevent freezing of fluids in cross counterflow shell and tube heat exchangers. Cold gas may flow inside the tubes of the heat changer and warm liquid may flow outside the tubes. The tube diameter and tube spacing may be varied through the tube passes through the heat exchanger in order to provide a hot side conductance to cold side conductance ratio which results in the tube temperature being safely above the liquid freezing point. The heat exchanger may be used in, for example, the aerospace industry as a fuel oil cooler or as a preconditioner for reactants in spacecraft propulsion systems. 
     As used herein, the modifier “cold”, as used to refer to a cold gas or a cold fluid, is a not meant to refer to any particular temperature, but as a temperature relative to a “warm fluid” or a “warm liquid”. Consequently, as used herein, the modifier “warm”, as used to refer to a warm fluid or a warm liquid, is a not meant to refer to any particular temperature, but as a temperature relative to a “cold fluid” or a “cold gas”. 
     Referring to  FIG. 1 , a cross counterflow shell and tube heat exchanger  10  may receive a cold fluid, such as a cold gas, through a cold fluid inlet  12 . The cold gas may be between about −80° F. to about −120° F., typically about −100° F. The cold gas may flow from the cold fluid inlet  12  to tubes  14  of a first cold fluid pass  16 . The cold gas may flow from the first cold fluid pass  16  to a second cold fluid pass  18 . The cold gas may flow from the second cold fluid pass  18 , through a third cold fluid pass  20  and a fourth cold fluid pass  22  before exiting out a cold fluid outlet  24 . While  FIGS. 1 and 2  show four cold fluid passes  16 ,  18 ,  20 ,  22 , the heat exchanger  10  may include at least two cold fluid passes and may include more than four cold fluid passes. 
     The heat exchanger  10  may receive a warm fluid, such as a warm liquid, through a warm fluid inlet  26 . The warm liquid may have a freezing point above the temperature of the cold gas flowing through the tubes  14 . For example, the warm liquid may have a freezing point from about 0° F. to about 20° F. In one embodiment, the warm liquid may have a freezing point of about 12° F. The warm fluid may pass in a cross counterflow arrangement through the heat exchanger  10 . Baffles  28  may be present inside the heat exchanger  10  to provide structural support of the tubes. While the heat exchanger  10  is shown as a cross counterflow heat exchanger, the tube design according to an exemplary embodiment of the present invention, as described in greater detail below, may be applied to other heat exchanger designs, such as cross parallel flow heat exchangers. 
     Referring now to  FIG. 2 , as the cold gas enters the first cold fluid pass  16 , the number of tubes  14  and tube diameter is such that the inside gas side  32  heat transfer is low relative to the liquid side  34 . The tube spacing in the first cold fluid pass  16  may be tighter (as compared to subsequent passes) so that the outside liquid side  34  heat transfer is high relative to the inside cold gas side  32 . As used herein the tube spacing refers to the non-dimensionalized tube spacing and may be defined as the center to center spacing (between adjacent tubes) divided by the diameter of the tubes. 
     In the second cold fluid pass  18 , after the cold gas within the tubes  14  has warmed some, the tube diameter can be decreased to improve the heat transfer on the inside gas side  32 . The tube spacing in the second cold fluid pass  18  may be increased relative to the first cold fluid pass  16 . In optional subsequent passes on the liquid side (such as third cold fluid pass  20  and fourth cold fluid pass  22 ), the tube spacing can be increased in order to reduce the liquid side pressure drop. Using this tube diameter and spacing design, the wall temperature of the tubes  14  may be controlled to prevent freezing of the liquid on the tubes  14 . In addition, the tube diameter and spacing design, in a cross counterflow arrangement, can be used to reduce the overall size of the heat exchanger, relative to conventional cross parallel flow arrangements. 
     Referring to  FIG. 3 , a method  40  for preventing liquid from freezing in a heat exchanger (e.g., heat exchanger  10 ) may include a first step  42  of passing the cold gas through a first pass of tubes through the heat exchanger. The first tube diameter and the first tube spacing may be designed such that the gas flow area is large (relative to subsequent tube passes through the heat exchanger). Such a design may provide a low heat transfer coefficient between a cold gas in the tubes and a warm liquid outside of the tubes. A second step  44  may include passing the cold gas from the first pass of tubes through a second pass of tubes, wherein tubes of the second pass of tubes have at least one of a decreased diameter and an increased tube spacing, relative to tubes of the first pass of tubes, and wherein the warm liquid remains in a liquid state as it passes through the heat exchanger. Optional third and fourth passes of tubes, as shown in steps  46  and  48 , may have at least one of a decreased diameter and an increased tube spacing, relative to the immediately preceding tube pass. The increased tube spacing may help limit the pressure drop on the liquid side of the heat exchanger. 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.