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

Abstract

Methods and apparatus for controlling tube temperatures may 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 a spacecraft propulsion system.

We claim: 
     
         1 . A heat exchanger comprising:
 a first pass of tubes carrying a first fluid at a first temperature through the heat exchanger;   a second pass of tubes carrying the first temperature fluid from the first pass of tubes, through the heat exchanger;   a second fluid at a second temperature passing 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.   
     
     
         2 . The heat exchanger of  claim 1 , wherein the first temperature is colder than the second temperature. 
     
     
         3 . The heat exchanger of  claim 1 , wherein tubes of the second pass of tubes have both a decreased diameter and an increased tube spacing, relative to tubes of the first pass of tubes. 
     
     
         4 . The heat exchanger of  claim 1 , further comprising a third pass of tubes, wherein tubes of the third pass of tubes have at least one of a decreased diameter and an increased tube spacing, relative to tubes of the second pass of tubes. 
     
     
         5 . The heat exchanger of  claim 4 , further comprising a fourth pass of tubes, wherein tubes of the fourth pass of tubes have at least one of a decreased diameter and an increased tube spacing, relative to tubes of the third pass of tubes. 
     
     
         6 . The heat exchanger of  claim 1 , wherein the heat exchanger is a cross counterflow shell and tube heat exchanger. 
     
     
         7 . The heat exchanger of  claim 6 , further comprising baffles to provide structural support of the tubes. 
     
     
         8 . The heat exchanger of  claim 1 , wherein a gas flow area of the first pass of tubes is greater than a gas flow area of the second pass of tubes. 
     
     
         9 . The heat exchanger of  claim 2 , wherein the first fluid is a cold gas and the second fluid is a warm liquid. 
     
     
         10 . The heat exchanger of  claim 9 , wherein the cold gas has a temperature of about −100° F. and the warm liquid is a warm liquid having a freezing point of about 12° F. 
     
     
         11 . A cross counterflow shell and tube heat exchanger comprising:
 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; and   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.   
     
     
         12 . The heat exchanger of  claim 11 , wherein tubes of the second pass of tubes have both a decreased diameter and an increased tube spacing, relative to tubes of the first pass of tubes. 
     
     
         13 . The heat exchanger of  claim 11 , further comprising a third pass of tubes, wherein tubes of the third pass of tubes have at least one of a decreased diameter and an increased tube spacing, relative to tubes of the second pass of tubes. 
     
     
         14 . The heat exchanger of  claim 11 , further comprising a fourth pass of tubes, wherein tubes of the fourth pass of tubes have at least one of a decreased diameter and an increased tube spacing, relative to tubes of the third pass of tubes. 
     
     
         15 . A method of heat exchange between a warm liquid and a cold gas without freezing the warm liquid, the method comprising:
 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.   
     
     
         16 . The method of  claim 15 , wherein the warm liquid passes over the tubes in a cross counterflow arrangement. 
     
     
         17 . The method of  claim 15 , further comprising passing the cold gas from the second pass of tubes through a third pass of tubes, wherein tubes of the third pass of tubes have at least one of a decreased diameter and an increased tube spacing, relative to tubes of the second pass of tubes. 
     
     
         18 . The method of  claim 17 , further comprising passing the cold gas from the third pass of tubes through a fourth pass of tubes, wherein tubes of the fourth pass of tubes have at least one of a decreased diameter and an increased tube spacing, relative to tubes of the third pass of tubes. 
     
     
         19 . The method of  claim 15 , wherein the pressure drop across the second set of tubes is less than the pressure drop across the first set of tubes.