Low pressure drop heat exchanger

Abstract: A two pass heat exchanger (10) is provided. The first pass includes a plurality of tubes (30) located in liquid refrigerant (60) when employed as an evaporator, whereby the liquid refrigerant (60) draws heat from the water flowing through the tubes (30) causing the water to be cooled and the liquid refrigerant (60) to evaporate. The second pass is a single pipe (40) which need not be located in the liquid refrigerant (60). The two pass heat exchanger can also be used as a condenser.

Description: This invention relates to a heat exchanger evaporator comprising a shell and a pair of end pieces sealed to said shell; a first tube sheet coacting with a first one of said pair of end pieces to define an intermediate water box, a second tube sheet coacting with a second one of said pair of end pieces and a divider plate to define an inlet water box and an outlet water box; said first and second tube sheets coacting with said shell to define a chamber, said chamber having inlet and outlet ports in communication therewith for fluid to be evaporated in said chamber, a first pass including a plurality of heat transfer tubes extending from said inlet water box through said chamber to said intermediate water box; and a second pass extending from said intermediate water box through said chamber to said outlet water box whereby a water circuit is serially defined by said inlet water box, said first pass, said intermediate water box, said second pass and said outlet water box. A heat exchanger of this type having a plurality of heat transfer tubes in both passes is disclosed in "Heat Exchanger Design Handbook" published 1983 by VDI-Verlag, Hemisphere Publishing Corporation (4.2.3. - Types of shell-and - tube heat exchangers - by E.A.D. Saunders). Another heat exchanger type is disclosed in US-A-4,289,196 in which primary water flows downward over steam generator tubes and secondary fluid flows up in the tubes in counter flow relationship with the primary water for evaporation in the tubes. A single feedwater tube extends down along the steam generator tubes to flow the feedwater or secondary fluid to a lower header and into the steam generator tubes. Shell and tube heat exchangers, of the kind where water flows through a plurality of tubes in both passes in heat transfer relationship with a refrigerant on the shell side, are often used as evaporators, along with at least one compressor and other components to create an assembled water chilling unit. As an assembly, the changing of one component often has an impact on the other structure. For example, the evaporator may serve as the support for the compressor or condenser. Another general constraint in chiller design is to have an even number of passes on the waterside so that all of the water connections can be located at one end of the heat exchanger shell, thus permitting the cleaning or servicing of the tubes from the other end without disturbing the water connections. There are occasions where it is desired to reduce heat exchanger size to meet a given set of thermal and pressure drop requirements, yet such a reduction of the exchanger shell may not be possible due to the interrelationship of the various components of the chiller. For example, to match desired performance characteristics, it may be desirable to use a short length condenser shell in combination with a long length cooler shell, but the chiller assembly would be compromised as a result. The reduced heat exchange requirement for a heat exchanger is addressed by providing a two pass design with essentially all of the required heat transfer taking place in one pass. The one pass employs tubes having the desired diameters and surface characteristics for the desired heat transfer and pressure drop while the second or return pass employs a single large diameter tube or pipe. Specifically, the second pass of a two pass shell and tube heat exchanger has the normal compliment of tubes replaced with a return pipe. This allows a drastic reduction in the total number of heat exchanger tubes, when very high heat transfer performance is not a requirement, without the usual accompanying increase in water side pressure drop. Additionally, this configuration allows the maintenance of relatively high water side velocities in the tubes of the first pass for the effective use of the heat transfer surface. In an evaporator, because the second pass would have only nominal heat transfer due to its limited heat transfer surface area, the second pass need not be located within the liquid refrigerant which permits the lowering of the refrigerant level and thereby the refrigerant charge in the system. It is an object of this invention to permit the removal of substantial members of heat exchanger tubes without sacrificing waterside pressure drop and pumping power. It is another object of this invention to make cost effective use of enhanced heat transfer tubing by keeping waterside velocities relatively high without the usual increase in overall heat exchanger waterside pressure drop. It is a further object of this invention to allow for the optimization of heat exchangers for use in water chiller units without compromising the design of the other chiller components. It is another object of this invention to reduce the refrigerant charge in a refrigeration system. These objects, and others as will become apparent hereinafter, are accomplished by the present invention in its preferred embodiment at least. In accordance with the invention, there is provided a heat exchanger evaporator according to claim 1. Figure 1 is a sectional view of a heat exchanger employing the present invention; and Figure 2 is a sectional view taken along line 2-2 of Figure 1. In the Figures, the numeral 10 generally designates a two pass shell and tube heat exchanger evaporator. Heat exchanger 10 has a generally cylindrical shell 12 with end pieces 13 and 14, respectively. End piece 13 coacts with tube sheet 15 to define intermediate water box 20. End piece 14 coacts with tube sheet 16 and divider plate 18 to define inlet water box 21 and outlet water box 22, respectively. Heat exchanger 10 has a first pass heat exchanger extending from inlet water box 21 to water box 20 and includes a plurality of small diameter heat transfer tubes 30. Typically, the tubes 30 are internally and/or extemally enhanced to promote heat exchange. The second pass heat exchanger of heat exchanger 10 is a large diameter pipe or tube 40 extending from intermediate water box 20 to outlet water box 22. Tubes 30 and pipe 40 are located in a generally cylindrical chamber 50 defined by shell 12 and tube sheets 15 and 16. Chamber 50 receives liquid refrigerant 60 from the condenser (not illustrated) via inlet 12-1. Because pipe 40 is generally not relied on for providing heat transfer, the level of the liquid refrigerant 60 need only be above tubes 30, and need not cover pipe 40. The heat transfer area of pipe 40, as compared to the total of tubes 30 will be small. In operation, liquid refrigerant 60 is supplied from the condenser (not illustrated) via inlet 12-1 to chamber 50 where it extracts heat from and thereby cools the water passing through tubes 30 while the liquid refrigerant 60 evaporates. The gaseous refrigerant passes from chamber 50 via outlet 12-2 to the suction of the compressor (not illustrated). Water from the closed loop cooling circuit of the refrigeration system (not illustrated) is supplied from the building cooling system to inlet water box 21. The water then passes through tubes 30 in heat exchange relationship with the liquid refrigerant 60. The liquid refrigerant draws heat from and thereby cooling the water while the liquid refrigerant 60 is evaporated. The heat transfer takes place in the first pass defined by tubes 30 with only a small amount of heat transfer being available through pipe 40, whether or not pipe 40 is located in liquid refrigerant 60. The water passing through the second pass defined by pipe 40 enters outlet water box 22 from which it flows into the closed circuit building cooling system to provide cooling.

Claims: A heat exchanger evaporator (10) comprising: a shell (12) and a pair of end pieces (13, 14) sealed to said shell, a first tube sheet (15) coacting with a first one (13) of said pair of end pieces to define an intermediate water box (20); a second tube sheet (16) coacting with a second one (14) of said pair of end pieces and a divider plate (18) to define an inlet water box (21) and an outlet water box (22); said first and second tube sheets coacting with said shell to define a chamber (50), said chamber (50) having a liquid inlet port (12-1) and an outlet port ( 12-2) in communication therewith for fluid to be evaporated in said chamber (50); a first pass including a plurality of heat transfer tubes (30) extending from said inlet water box through said chamber to said intermediate water box; and a second pass extending from said intermediate water box through said chamber to said outlet water box whereby a water circuit is serially defined by said inlet water box, said first pass, said intermediate water box, said second pass and said outlet water box, characterized in that liquid refrigerant (60) is located in said chamber, said first pass is located in said refrigerant, and said second pass is defined by a single large diameter pipe (40) located above said refrigerant (60). The heat exchanger of claim 1, characterized in that said shell (12) is of a generally cylindrical shape and is horizontally oriented; one of said ports (12-1) is located at the bottom of said shell (12), and the other port (12-2) is located at the top of said shell (12).

Inventors: HUENNIGER EDWARD ALLEN, HUENNIGER, EDWARD ALLEN
Applicants: CARRIER CORP, CARRIER CORPORATION
Application year: 1997
Main classification: F28D7
IPC codes: F25B39, F28D7, F28B1