HK1028099B - Heat exchanger including falling-film evaporator and refrigerant distribution system - Google Patents

Description

Heat exchanger comprising a falling film evaporator and a refrigerant distribution system

Technical Field

The present invention is directed to improving the performance of heat exchangers, and in particular to an improved distribution system for falling film heat exchangers.

Background

In a horizontal shell and tube falling film heat exchanger with a vapor compression cooler, a heat exchange fluid is sprayed with spray nozzles on top of a first layer of tubes forming a tube assembly. In these cases, a staggered tube pitch is typically used. The staggered tube pitch tends to create a large pressure drop on the vapor side of the tubes as the vapor passes upwardly through the stacked tubes, particularly in vapor compression coolers. To correct for this, manufacturers use free tube pitch to reduce pressure drop. This results in a larger housing diameter and thus higher equipment costs. In addition, in order to arrange the tubes in an in-line manner to minimize pressure drop losses, other manufacturers have employed complex trickle flow distribution systems that are highly susceptible to machining levels and costly to design and construct. Another problem occurs in two-phase refrigerant distribution, where the nozzles used often do not provide uniform liquid distribution, thereby reducing the performance of the heat exchanger. Another disadvantage of staggered tube bundle arrangements is: the available fluid flow is distributed over a large number of tubes. This results in a relatively low membrane flow rate and a low local recirculation rate (defined as membrane flow rate divided by steam production rate), thereby increasing the likelihood of air drying and significantly reducing the heat transfer coefficient. An in-line tube bundle arrangement can be operated at higher membrane flow rates to ameliorate the above-mentioned weaknesses. However, while this can be done, some portion of the dispensed liquid still does not reach the heat transfer surface, which is where the present invention is desired to be improved.

An example of a recent falling film evaporator design having a refrigerant distribution system is shown in U.S. patent 5,645,124 assigned to U.S. standards corporation. Several embodiments of refrigerant distributors are shown in this patent, one preferred embodiment employing a mesh screen type interface between a distributor and each tube, and another preferred embodiment employing a sleeve-type arrangement to distribute refrigerant to the mesh screen and heat exchange tube assemblies.

As shown in fig. 7, the refrigerant flows through an inner tube and into an outer tube through a top hole. The refrigerant drips down the side of the inner tube, through a bottom opening onto the mesh screen, and then onto the evaporator tubes. The figure also shows another design of evaporator tubes, which are no longer a true cylindrical structure, which tubes comprise a V-point on the bottom surface to direct refrigerant droplets onto the evaporator tubes arranged below. In another embodiment, the evaporator tubes include a cooling zone in addition to the V-point for collecting refrigerant and then distributing the refrigerant to provide more control over the distribution.

The mesh screen is placed in contact with or in close proximity to the topmost evaporator tube of the tube assembly. The screen is corrugated or undulating in shape with peaks and valleys such that each valley is generally parallel to and directly above the longitudinal axis of the uppermost one of the evaporator tubes. In this manner, a pool of liquid cryogen is formed axially along the valleys of the mesh until gravity overcomes the surface attraction forces that cause the liquid cryogen to be suspended above the mesh surface. The refrigerant then falls and drips onto the topmost evaporator tube. When the mesh screen is in contact with each tube, refrigerant flows over the tubes. Although the use of a mesh screen minimizes splashing of refrigerant and the amount of refrigerant drawn into the compressor, the mesh screen significantly impedes the flow of refrigerant to the evaporator tubes.

Accordingly, there is a need for an improved refrigerant distribution system for a falling film heat exchanger that allows for in-line arrangement of tubes, uniform refrigerant distribution, and minimal pressure drop across the evaporator tubes.

Disclosure of Invention

It is a primary object of the present invention to provide an improved distribution system for a falling film heat exchanger.

It is another object of the present invention to provide an improved distribution system for a falling film heat exchanger wherein an in-line heat exchange tube system can be used with a simple distribution system to reduce pressure drop and improve refrigerant distribution efficiency.

It is a further object of the present invention to provide an improved refrigerant distribution system for a falling film heat exchanger wherein the nozzles are available for distribution and a mechanism is provided to accurately direct refrigerant to the in-line heat exchange tubes for efficient refrigerant distribution.

It is a further object of the present invention to provide an improved refrigerant distribution system for a falling film heat exchanger that provides local film flow rates by using in-line tube bundles wherein the local recirculation rate (defined as film flow rate divided by vapor production rate) is increased to improve wettability and heat transfer coefficient.

The foregoing objects and advantages of the present invention are achieved by a horizontal shell and tube heat exchanger which can be used as a falling film evaporator and refrigerant distribution system. The system comprises: an in-line heat exchange tube bundle operating in falling film evaporation mode comprising a plurality of non-staggered rows of heat exchange tubes, wherein said rows are in substantial vertical alignment with spaces separating said rows; and a supply pipe for supplying the refrigerant. At least one distribution mechanism is in fluid communication with the supply tube and has a refrigerant outlet adjacent the plurality of rows of heat exchange tubes, and a plurality of baffles are disposed at the refrigerant outlet. Each baffle aligned with one of said spaces and extending above one of said spaces to direct refrigerant from the outlet of said distribution mechanism and exiting one of said spaces onto the substantially vertically aligned columns of heat exchange tubes; wherein each of said baffles has a wide end and a narrow end, said narrow end being disposed closest to said refrigerant outlet; the baffle has a toothed surface for contacting refrigerant exiting the spray mechanism.

Drawings

FIG. 1a is a schematic front view of the spray distribution system and heat exchanger of the present invention employing a distributor baffle and in-line tube bundle in accordance with the principles of the present invention;

FIG. 1b is a side view taken along line 1b-1b of FIG. 1 a;

FIG. 2 is a schematic view of a staggered tube bundle for use in a prior art heat exchanger;

FIG. 3a is an enlarged front view of the baffles of the spray dispensing system shown in FIG. 1; and

FIG. 3b is an enlarged side view of the baffles of the spray dispensing system of FIG. 1 taken along line 3b-3b of FIG. 3 a.

Detailed Description

Referring to figure 1, there is shown a schematic diagram of a preferred embodiment of the improved refrigerant distribution system of the present invention, generally indicated by the numeral 10. The system generally includes nozzles 12, baffles 14, and a tube bundle 15.

The nozzles 12 are designed and function in a manner known in the art and spray refrigerant downwardly onto the heat exchange tube system arranged in a vertically aligned non-staggered manner. As shown in fig. 1, each nozzle 12 additionally overlies a spray area a over which refrigerant is sprayed downwardly onto the heat exchange tubes. In a typical arrangement, with the heat exchange tubes 16 in a non-staggered arrangement, the refrigerant can flow in a substantial space between the spaced heat exchange tubes, thereby utilizing the available refrigerant to affect tube coverage. This is the primary reason why staggered heat exchange tube arrangements are employed, such as shown in fig. 2. In this way, the next row of tubes, which are staggered from the front and rear rows of tubes, can utilize the refrigerant falling between the heat exchange tubes. However, as indicated in the background section, this staggered arrangement has one such disadvantage: in addition to many of the others described, undesirable pressure drop phenomena can occur.

In accordance with the principles of the present invention, baffles 14 are positioned at the edges of the nozzle 12 and spaced from each other so that they are positioned over the spaces between the heat exchange tubes. That is, the heat exchange tubes are arranged in this preferred non-staggered, vertical arrangement with baffles 14 aligned with the spaces between the heat exchange tubes, such as the baffle 14a above the space 18. To facilitate directing the refrigerant toward the heat exchange tubes between which each baffle is disposed, the baffles are preferably chevron-shaped and have a narrow end or tip directed upwardly toward the nozzles so that the legs 22 of the baffles and their surfaces 24 are directed substantially toward the longitudinal upper surface centers 23 of the two heat exchange tubes between which the chevron baffle is disposed. In addition, the bottom edge 25 of each baffle is serrated, as shown in FIG. 3, so that the serrated projections face toward the in-line bundle, as shown. In the preferred embodiment shown, the spacing between the tips 26 of the teeth 28 is a predetermined pitch P. The pitch P is selected to be less than the natural pitch of the refrigerant, which is the distance measured when the refrigerant drops as a liquid column off each horizontal tube of the heat exchange tubes. The natural pitch of the liquid column is given by the following equation,

(1) 2*π*[(2*sigma)/(rho*g)]^0.5

in the formula (I), the compound is shown in the specification,

the sigma is the surface tension force,

rho ═ liquid refrigerant density, and

g is the gravitational acceleration.

In one embodiment, the preferred pitch of the tips of the tooth-like projections is in the range of sixty percent to eighty-five percent (60-85%), more preferably 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 affect the optimum percentage. In all cases, the pitch of each serration should be smaller than the value determined by equation (1). By making the pitch of the tooth-like projections smaller than the natural pitch of the liquid column, a rapid distribution of the liquid refrigerant can be facilitated. In addition, the serrations provide a passage for steam to escape from the tube bundle without a significant pressure drop that would otherwise reduce suction pressure and cooler efficiency.

In operation, as refrigerant sprays downwardly from the nozzles, refrigerant exiting or colliding with the nozzles directed at the spaces 18 flows downwardly onto the back edges 24 of the legs 22 of the baffle 14, past the edges 25 and through the teeth 28, and is directed to the top center 23 around the heat exchange tubes. The refrigerant then flows around each heat exchange tube, over the outer surface of the heat exchange tube until it reaches the lower surface of the heat exchange tube and gravity breaks the surface adhesion between the refrigerant and the heat exchange tube to move the remaining unvaporized refrigerant to the bottom heat exchange tube. Although a chevron baffle is shown, other shapes, such as C-shaped, V-shaped, and semi-octagonal, may be used.

The main advantages of the invention are: it provides an improved distribution system for a falling film heat exchanger. Another advantage of the present invention is: it provides such an improved distribution system for a falling film heat exchanger wherein an in-line heat exchange tube system can be used with a simple distribution system to reduce pressure drop and improve refrigerant distribution efficiency. Yet another advantage of the present invention is: it provides an improved refrigerant distribution system for a falling film heat exchanger wherein nozzles are employed for distribution and a mechanism is provided to accurately direct refrigerant to the in-line heat exchange tubes for efficient refrigerant distribution. Yet another advantage of the present invention is: it provides an improved dispensing system that utilizes an in-line tube bundle and can increase the membrane flow rate and the local recirculation rate of the dispensed fluid.

While the invention has been shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and various other changes in forms and details may be made therein without departing from the spirit and scope of the invention.

Claims (7)

1. A horizontal shell and tube heat exchanger having a falling film evaporator and refrigerant distribution system comprising:

an in-line heat exchange tube bundle operating in falling film evaporation mode comprising a plurality of non-staggered rows of heat exchange tubes wherein said rows are in vertical alignment with spaces separating said rows;

a supply pipe for supplying a refrigerant;

at least one refrigerant distribution mechanism in fluid communication with said supply tube and having a refrigerant outlet adjacent said plurality of rows of heat exchange tubes; and

a plurality of baffles disposed at said refrigerant outlet, wherein each baffle is aligned with one of said spaces and extends above one of said spaces to direct refrigerant from said distribution mechanism outlet and exiting one of said spaces to each of the vertically aligned columns of heat exchange tubes;

wherein each of said baffles has a wide end and a narrow end, said narrow end being disposed closest to said refrigerant outlet; the baffle has a toothed surface for contacting refrigerant exiting the spray mechanism.

2. The heat exchanger of claim 1, wherein the baffle is in the shape of a chevron to direct refrigerant to each heat exchange tube.

3. The heat exchanger of claim 1, wherein said toothed surface comprises a plurality of toothed projections extending in a direction of refrigerant flow out of said spray mechanism.

4. The heat exchanger as recited in claim 1 wherein said refrigerant flows away from said heat exchange tubes in a liquid column having a natural pitch, each of said plurality of serrations having a tip with a predetermined pitch that is less than said natural pitch.

5. The heat exchanger of claim 4, wherein the predetermined pitch has a dimension in the range of 60-85% of the natural pitch.

6. The heat exchanger of claim 4, wherein the predetermined pitch is 75% of the natural pitch.

7. The heat exchanger of claim 1, wherein the distribution system comprises a nozzle system.