Heat exchanger of the type of a falling-film evaporator having refrigerant distribution system

Abstract: A horizontal shell-and-tube heat exchanger operable as a falling-film evaporator and refrigerant distribution system is disclosed. The system includes an inline heat exchanger tube bank (15) operating in a falling-film evaporation mode, including a plurality of unstaggered rows of heat exchanger tubes, wherein the rows are substantially vertically aligned with spaces separating the rows, and a supply for providing refrigerant. At least one refrigerant distribution mechanism (12) in fluid communication with the supply and having a refrigerant exit in proximity to the plurality of rows of heat exchanger tubes (15) is provided with a plurality of baffles (14) positioned at the refrigerant exit. Each of the baffles (14) is in substantial alignment with one of the spaces, extending over the one of the spaces to direct refrigerant from the exit of the distribution mechanism (10) away from the spaces and onto the substantially vertical aligned rows of heat exchanger tubes (15).

Description: Technical Field This invention is directed to a horizontal shell-and-tube heat exchanger according to the preamble of claim 1. Such a heat exchanger is known from US- A-5 645 124. Background Art In the horizontal shell-and-tube falling-film heat exchangers of vapor compression chillers, the heat exchanging fluid is sprayed on the top of the first layer of tubes forming the tube assembly, with the use of spray nozzles. In these situations, a staggered tube pitch is normally used. The staggered tube pitch tends to cause larger pressure drops on the vapor side of the tube, as the vapor moves up through the stack, particularly with vapor compression chillers. To rectify this, manufactures will use liberal tube pitches in order to reduce the pressure drop. This results in a larger shell diameter leading to a higher cost for equipment. Additionally, in order to arrange tubes in an in-line pattern so as to minimize the pressure drop penalty, other manufacturers use complicated dripper distribution systems which are heavily susceptible to machine leveling and costly in design and structure. An additional problem occurs in two phase refrigerant distribution, wherein the spray nozzles used often do not provide a uniform liquid distribution, leading to inferior heat exchanger performance. An additional weakness of staggered tube bank layout is that available fluid flow is distributed over a larger number of tubes. This leads to a relatively lower film flow rate and a lower local recirculation rate (defined as film flow rate divided by vapor production rate), increasing the potential for dry-out and significant reduction in heat transfer coefficients. An in-line tube bank arrangement allows to operate at a higher film flow rate to improve the above mentioned weakness. However, while doing so, it is possible that certain portion of distributed liquid may not reach heat transfer surface, an object of improvement for the present invention. An example of a recent falling film evaporator design, having a refrigerant distribution system, is shown in US Patent 5,645,124. Several embodiments of refrigerant distributors are shown in this patent, the preferred one uses a mesh screen interface between a distributor and the tubes and another with a tube-in-tube arrangement for distributing the refrigerant onto the screen and heat exchange tube assembly. As shown in FIG. 7, the refrigerant travels through and inner tube and is released through a top orifice into an outer tuber. The refrigerant drips down the sides of the inner tube through a bottom opening onto the screen and then the evaporator tube. An alternative evaporator tube design is shown whereas instead of a true cylindrical structure, the tube includes a V-point on the bottom surface for directing the drips of refrigerant onto the under-positioned evaporator tube. In another embodiment, along with the v-point, the evaporator tube includes a pooling area for collecting and then distributing the refrigerant for a more controlled distribution. The mesh screen is placed in touch with or in close proximity to the top most evaporator tubes of the tube assembly. The mesh screen is corrugated or waved, forming peaks and valleys, such that each valley is generally parallel with and located directly above the longitudinal axis of one of the upper most evaporator tubes. In this manner, pools of liquid refrigerant grow axially along the valleys of the screen until gravity overcomes the surface attraction force which suspends the liquid refrigerant on the mesh screen surface. The refrigerant then falls and drops onto the upper most evaporator tubes. When the screen is touching the tubes, the refrigerant flows onto the tube. While the use of the screens may minimize refrigerant splashing and the amount of refrigerant sucked into the compressor, the screen acts as a substantial impediment to the flow of refrigerant to the evaporator tubes. There exists a need therefore, for an improved refrigerant distribution system for a falling-film heat exchangers, which system allows for an in-line tube pattern, uniform distribution of the refrigerant and minimal pressure drop over the evaporator tubes. Disclosure of Invention The primary object of this invention is to provide an improved distribution system for falling-film heat exchangers. Another object of this invention is to provide an improved distribution system for falling-film heat exchangers, wherein an in-line heat exchanger tube system can be used with a simple distribution system, thereby decreasing pressure drop and improving refrigerant distribution efficiency. Still another object of this invention is to provide an improved refrigerant distribution system for a falling-film heat exchanger, wherein spray nozzles are used for distribution and a mechanism is provided for accurately directing the refrigerant to the in-line stacked heat exchanger tubes for efficient distribution of the refrigerant. Still another object of this invention is to provide an improved refrigerant distribution system for a falling-film heat exchanger having increased local film flow rates by using in-line tube banks, wherein local recirculation rate (defined as film flow rate divided by vapor production rate) is increased to improve wettability and heat transfer coefficients. The foregoing objects and advantages set forth herein are achieved by the horizontal shell-and-tube heat exchanger according to claim 1. The system includes an inline heat exchanger tube bank operating in a falling-film evaporation mode, including a plurality of unstaggered rows of heat exchanger tubes, wherein the rows are substantially vertically aligned with spaces separating the rows, and a supply for providing refrigerant. At least one distribution mechanism in fluid communication with the supply and having a refrigerant exit in proximity to the plurality of rows of heat exchanger tubes is provided with a plurality of baffles positioned at the refrigerant exit. Each of the baffles is in substantial alignment with one of the spaces, extending over the one of the spaces to direct refrigerant from the exit of the distribution mechanism, away from the one of spaces and onto the substantially vertical aligned rows of heat exchanger tubes. Brief Description of the Drawings FIGS. 1a is a frontal schematic view of the spray distribution system and heat exchanger of the present invention, using distributor baffles and an in-line tube bank in accordance with the principles of the present invention; FIG. 1b is a side view taken along line 1b-1b of FIG 1a; FIG. 2 is a schematic representation of a staggered tube bank, for a heat exchanger design of the prior art; FIG. 3a is an enlarged frontal of the baffles of the spray distribution system shown in FIG. 1; and FIG. 3b is an enlarged side view taken along line 3b-3b of FIG 3a, of the baffles of the spray distribution system shown in FIG. 1. Best Mode for Carrying Out the Invention Referring to FIG. 1, a schematic representation of the preferred embodiment of the improved refrigerant distribution system of the present invention is shown, which is designated generally as 10. The system generally includes spray nozzles 12, baffles 14, and a tube bank 15. Spray nozzles 12 are designed and function in a manner known in the art, and spray refrigerant downwardly on the heat exchange tube system, arranged in a vertically aligned, non-staggered manner. As shown in FIG. 1, each spray nozzle 12 covers an area of spray A, over which area a refrigerant is sprayed downwardly onto the heat exchanger tubes. In a typical arrangement, and with the heat exchanger tubes 16 in a non-staggered positioning, refrigerant would flow in substantial volume between the spaced apart heat exchanger tubes, thereby effecting the efficiency of the tube coverage by the available refrigerant. This is typically the reason why a staggered heat exchanger tube arrangement is used, which staggered arrangement is shown, for example, in FIG. 2. In this manner, any refrigerant falling between heat exchanger tubes is used by the next row of tubes, which are offset in a staggered manner from the previous and subsequent rows. However, as indicated in the background section, the staggered arrangements suffer from an undesirable pressure drop, among the other things described. In accordance with the principles of the present invention, baffles 14 are provided at the edge of the spray nozzles 12, and are spaced such that they are positioned over the spaces between heat exchanger tubes. That is, with the heat exchanger tubes arranged in the preferred un-staggered, vertically stacked manner, the baffles 14 are aligned with the spaces between the heat exchanger tubes such as, for example, baffle 14a over space 18. To facilitate directing the refrigerant toward the heat exchanger tubes between which each baffle is placed, the baffle preferably has a chevron shape, having a narrow or pointed end up toward the spray nozzles, such that the legs 22 of the baffles, and surfaces 24 thereof, are pointed substantially towards the longitudinal upper surface center 23 of the two heat exchanger tubes between which the chevron shaped baffle is positioned. In addition, the bottom edge 25 of each baffle is serrated, as shown in FIG. 3, such that the serrations face the in-line tube stacks, as shown. In the preferred embodiment, the distance between the tips 26 of the serrations 28 is a predetermined pitch P. The pitch P is selected to be less than the natural pitch of the refrigerant, measured as it drips in liquid columns from the horizontal tubes of the heat exchanger. The natural pitch of the liquid column is given by the following equation, (1) 2*π* [(2*sigma) / (rho*g)] 0.5 , where, sigma = surface tension, rho =liquid refrigerant density, and g = acceleration due to gravity. In one embodiment, the preferred pitch of the tips of the serrations will be in the range of sixty to eighty five percent (60-85%), and more particularly seventy five percent (75%) of the value determined by equation (1). However, the type of refrigerant used and other factors, including the type of chiller and other equipment used, may effect the optimum percentage chosen. In all cases, the pitch of the serrations should be less than the value determined by Equation (1). By having the pitch of the serrations less than the natural pitch of the liquid column, quick distribution of the liquid refrigerant is facilitated. In addition, the serrations provide passage for vapor to escape the tube bank without significant pressure drop, which would otherwise cause reduction in suction pressure and reduction in chiller efficiency. In operation, as refrigerant is sprayed downwardly from the nozzles, the refrigerant which exits the nozzles in alignment or on a collision course with spaces 18, flows down the back edge 24 of legs 22 of baffle 14, over the edge 25 and through the serrations 28 and is directed to the top center 23 of the surrounding heat exchanger tubes. Refrigerant then flows around the heat exchanger tubes, over the outside surface of the tube until it reaches the under surface of the tube and gravity breaks the surface bond between the refrigerant and the tube to cause the remaining, non-vaporized refrigerant to move to the underlying heat exchanger tubes. While a chevron shaped baffle is shown, it is possible that other shapes can be used such c-shaped, v-shaped and half octagonal shaped, to name a few. The primary advantage of this invention is than an improved distribution system is provided for falling-film heat exchanger. Another advantage of this invention is that an improved distribution system is provided for falling-film heat exchanger, wherein an in-line heat exchanger tube system can be used with a simple distribution system, thereby decreasing pressure drop and improving refrigerant distribution efficiency. Still another advantage of this invention is that an improved refrigerant distribution system is provided for falling-film heat exchanger, wherein spray nozzles are used for distribution and a mechanism is provided for accurately directing the refrigerant to the in-line stacked heat exchanger tubes for efficient distribution of the refrigerant. Still another advantage of this invention is that an improved distribution system is provided which uses an in-line tube bank and exhibits increased film flow rate and local recirculation rate of distributed fluid.

Claims: A horizontal shell-and-tube heat exchanger having a falling-film evaporator and refrigerant distribution system, comprising: an inline heat exchanger tube bank operating in a falling-film evaporation mode, including a plurality of unstaggered rows of heat exchanger tubes, wherein said rows are substantially vertically aligned with spaces separating the rows; a supply for providing refrigerant; at least one refrigerant distribution mechanism in fluid communication with said supply and having a refrigerant exit in proximity to said plurality of rows of heat exchanger tubes; and characterized by a plurality of baffles positioned at said refrigerant exit, wherein each of said baffles is in substantial alignment with one of said spaces, extending over said one of said spaces to direct refrigerant from said exit of said distribution mechanism and away from said one of spaces and to the substantially vertical aligned rows of heat exchanger tubes. The heat exchanger according to claim 1, wherein each of said baffles has a wide end and a narrow end, said narrow end positioned closest to said refrigerant exit. The heat exchanger according to claim 2, wherein said baffles are chevron shaped for directing the refrigerant to the heat exchanger tubes. The heat exchanger according to claim 2 or 3, wherein said baffles have a serrated surface for contacting the refrigerant exiting said refrigerant distribution mechanism. The heat exchanger according to claim 4, wherein said serrated surface includes a plurality of serrations extending in the direction of refrigerant flow from said refrigerant distribution mechanism The heat exchanger according to claim 5, wherein said refrigerant flows off said tubes in liquid columns having a natural pitch and wherein each of said plurality of serrations has a tip the distance between the tips of the serrations being a predetermined pitch smaller in size than said natural pitch. The heat exchanger according to claim 6, wherein said predetermined pitch has a size in the range of 60-80% of said natural pitch. The heat exchanger according to claim 7, wherein said predetermined pitch is 75% of said natural pitch. The heat exchanger according to any preceding claim, wherein said refrigerant is a low surface tension refrigerant. The heater exchanger, according to any preceding claim, wherein said distribution mechanism comprises a system of spray nozzles.

Inventors: GUPTE NEELKANTH S, GUPTE, NEELKANTH S.
Applicants: CARRIER CORP, CARRIER CORPORATION
Application year: 2000
Main classification: F28D3
IPC codes: B01D1, B01D3, F28D3, F28F9