Patent Application
20060108107
1. Field of the Invention
This invention relates generally to tube configurations used in heat exchangers and their methods of manufacture.
2. Background Art
In many chemical, electronic, and mechanical systems, thermal energy is transferred from one location to another or from one fluid to another. Heat exchangers allow the transfer of heat from one fluid (liquid or gas) to another fluid. Conventionally, the reasons for transferring heat energy are:
Regardless of the function the heat exchanger fulfills, in order to transfer heat, the fluids in thermal contact must be at different temperatures to allow heat to flow from the warmer to the cooler fluid according to the second principle of thermodynamics.
Traditionally, for round tube plate fin heat exchangers there is no direct contact between the two fluids. Heat is transferred from the fluid to the material isolating the two fluids and then to the cooler fluid.
Some of the more common applications of heat exchangers are found in the heating, ventilation, air conditioning and refrigeration (HVACR) systems, electronic equipment, radiators on internal combustion engines, boilers, condensers, and as pre-heaters or coolers in fluid systems.
All air conditioning systems contain at least two heat exchangers—usually an evaporator and a condenser. In each case, the refrigerant flows into the heat exchanger and transfers heat, either gaining or releasing it to the cooling medium. Commonly, the cooling medium is air or water.
A condenser accomplishes this by condensing the refrigerant vapor into a liquid, transferring its phase change (latent) heat to either air or water. In the evaporator, the liquid refrigerant flows into the heat exchanger. Heat flow is reversed as refrigerant evaporates into a vapor and extracts heat required for this phase change from the hotter fluid flowing on the outside of the tubes.
Tubular heat exchangers include those used in an automotive heat exchanger environment, such as a radiator, a heater coil, an air cooler, an intercooler, an evaporator and a condenser for an air-conditioner. For example, a hot fluid flows internally through pipes or tubes while a cooler fluid (such as air) flows over the external surface of the tubes. Thermal energy from the hot internal fluid transfers by conduction to the external surface of the tubes. This energy is then transferred to and absorbed by the external fluid as it flows around the tubes' outer surfaces, thus cooling the internal fluid. In this example, the external surfaces of the tubes act as a surface across which thermal energy is transferred.
Traditionally, longitudinal or radial fins may be positioned in relation to the external surface of the tubes to turbulate the externally flowing fluid, increase the area of the heat transfer surface and thus enhance the heat transfer capacity. One disadvantage, however, is that fins add to material and manufacturing cost, bulk, handling, servicing and overall complexity. Further, they occupy space and therefore reduce the number of tubes that can fit within a given cross sectional area and they collect dust and dirt and may get clogged, thereby diminishing their effectiveness.
Densely configured external fins tend to constrict external fluid flow. This promotes an increase in the pressure drop of the external fluid across the heat transfer surface and may add to heat exchanger costs by requiring more pumping power. In general, expense related to pumping is a function of the pressure drop.
Fin-less, tube heat exchangers are known. See, e.g., U.S. Pat. No. 5,472,047 (Col. 3, lines 12-24). Conventionally, however, they are made of tubes having a relatively large outside diameter. Often, tubes are joined with wires, such as the steel coils found at the back of many residential refrigerators.
The U.S. references identified during a pre-filing investigation were: US 2004/0050540 A1; US 2004/0028940 A1; U.S. Pat. Nos. 5,472,047; 3,326,282; 3,249,154; 3,144,081; 3,111,168; 2,998,228; 2,828,723; 2,749,600; and 1,942,676.
Foreign references identified during a pre-filing investigation were: GB 607,717; GB 644,651; and GB 656,519.
The invention includes a wound tube heat exchanger, which receives a heat exchange fluid that flows within the exchanger. The exchanger has one or more layers of a one or more small diameter (preferably with an OD<5 mm), tubes. In one embodiment, the tube surface is bare. In other embodiments, the outside tube surface is enhanced to produce turbulence and convective heat transfer. Each layer is wound around and is separated by a spacer members. At least some of the layers have an ovate, oblong or racetrack-like configuration with a pair of opposing linear runs that are connected by a pair of opposing curved sections. The elongate spacer member has forwardly and rearwardly facing edges. The edges define engagement surfaces that detachably retain the opposing linear runs. In some embodiments, the layers are circular, oval or rectangular with radiused corners. Spacer members may act as support members, fixtures and/or thermal communication devices between tubes and may become part of the refrigerant circuit. Furthermore, the spacer member may promote condensate drainage from evaporative heat exchangers.
FIGS. 1 & 3-4 depict a tube heat exchanger 10 for receiving a heat exchange fluid that flows within the heat exchanger. In one embodiment the tube surface is bare. In other embodiments, the outside tube surface is enhanced to disturb air flow and promote convective heat transfer. The heat exchanger has one or more layers 12 of a single, long, continuous, tube 14. The tube 14 has an outside diameter (OD), an inside diameter (ID) along which the heat exchange fluid passes, and a wall thickness (T=(OD−ID)/2)). It will be appreciated that the tube 14 need not be circular or annular in cross section. For some applications, for example, the tube 14 may usefully have an oval configuration or other non-circular cross section which may be helpful in directing incident air flow and promoting local turbulence.
At least some of the one or more layers 12 have an ovate, oblong, or racetrack-like configuration 15 (
As shown in
Although in
In
The invention includes a continuous tube having several windings. In practice, the windings are prepared by conforming the tubes' outside diameter with a tool such as a mandrel that typically is relatively flat and long. Conventional working operations produce a series of tube windings that are composed of layers of coiled sections that are generally ovate, oblong, oval or racetrack-like in shape. A rounded corner lies at each end of the oval configuration. The rounded corners are connected at opposite ends of each oval by linear, relatively straight runs.
In one manufacturing process, the mandrel has an outside surface in which one or more continuous helical grooves are defined. During the winding steps, the tube becomes accommodated by the helical groove.
By using rounded corners, kinks and sharp changes in bend radii are avoided. In general, the bend radius (R) is large (about 10:3) in relation to the outside diameter (OD) of the tube.
The spacer member 24 serves to position interposed tube layers. Detents, preferably frusto-circular if round tubes are used, 30 are defined within edges 26,28 of the spacer. These detents 30 terminate at the spacer edges in a position that is slightly offset from a major diameter of a detent, which may be circular, or noon-circular. In this way, the outside diameter of a linear tube run is engaged by a snap fit within the spacer. The distance between consecutive detents (center-to-center of the grooves) influences the heat transfer properties of the heat exchanger. In one embodiment, this distance is twice the outside diameter (OD) of the tube.
When successive layers of the coil are engaged by the spacer 24, their overall orientation is relatively flat, as shown in
One consequence of a staggered (as opposed to an in-line) configuration as shown is that there are relatively few spaces through which fluid flowing outside the tubes and through the heat exchanger can pass without interruption. Because of the relatively tight packing density of the tube configuration depicted, fluid flowing around the outside of the tubes is in thermal contact for a protracted period (“dwell time”) with the tube runs 16,18 that are positioned above and below the spacer 24.
No headers are needed at the inlet or the outlet side of the heat exchanger. Nor are there any serpentine fins or louvers. Accordingly, in a preferred embodiment, the heat exchanger effectively is a wound layered tube apparatus. Hence, it is less expensive to manufacture and maintain than conventional round tube plate fin heat exchangers.
The spacing member 24 serves to position adjacent tubes in a given layer and to separate the layers within a given coil (
Where the heat exchanger serves as an evaporator, a liquid refrigerant flows into the inlet. Following heat transfer, its temperature rises so that it vaporizes inside the tube. This lowers the temperature of the tube, which in turn lowers the temperature of a fluid such as air that is in thermal contact with the outside of the tube. In practice, it is sometimes desirable to adjust the flow of the incoming liquid refrigerant so as to produce 100% of vapor at the outlet that is not superheated; i.e., it exits at around its boiling temperature.
Conventionally, a control system is adapted in order to accomplish this thermodynamic state. In practice, the vaporized refrigerant will enter a compressor, which will increase the pressure of the vaporized refrigerant. Its temperature then rises, just as the temperature of the barrel of a bicycle pump rises when a bicycle tire is inflated. Pressurized vaporized refrigerant then enters a condenser, which may be formed from a wound layered tube, such as the embodiments described herein. The condenser effectively changes the state of the compressed and warmed refrigerant fluid so that it becomes preferably completely-liquified to a lower temperature. In turn, the refrigerant fluid in that state is delivered to an evaporator, which again can be formed from a wound layered tube heat exchanger such as the embodiments depicted.
The heat exchanger tubes can be made from any heat-conducting material. Metals, such as copper or aluminum are preferred, but plastic tubes having a relatively high thermal conductivity may also be used.
The practical relationships between the tube inside diameter (ID), outside diameter (OD), and wall thickness (T) are somewhat limited by the manufacturing techniques used to form the tube. Clearly, the selection of suitable dimensions will influence the pressure-bearing capability of the resulting heat exchanger. In general, it can be stated that as the outside diameter (OD) decreases, the thinner the wall section (T) can be. Preferably, the outside diameter (OD), inside diameter (ID) and thus wall thickness (T) should be selected so that the tube can hold the pressure of a refrigerant without deformation of the tube material. When the outside diameter decreases, there is more tube outer surface as compared to the internal volume of the tube. As a consequence, there is more heat transfer area per refrigerant volume.
Environmentally benign consequences of using carbon dioxide as a refrigerant fluid often occur. Operating pressures are higher than normal refrigerants. Small diameter tube heat exchangers are beneficial when using carbon dioxide as a refrigerant as carbon dioxide has low viscosity and thus the pressure drop within a tube is small. In addition, the tube wall can be kept thin in spite of high operating pressures. If there is any leakage, the consequences to ambient atmosphere do not present significant environmental risks.
As is apparent from the drawings, the spacer member 24 prevents tube migration. Preferably, the spacing of grooves 30 within the spacer member 24 is such as to cause the runs of consecutive layers to lie closely together and in parallel. This results in a packing density that presents a resistance to the passage of ambient heat exchange fluid, induces local turbulence, diminishes laminar flow, and thereby promotes the efficiency of heat transfer.
One drawback of conventional evaporators is that water condensate tends to accumulate at various locations within the heat exchanger. This tends to block the air flow. By positioning the invention in a vertical orientation (
An additional attribute of the spacer member 24 is that it supports the three-dimensional shape of the tube heat exchanger. Although one spacer member 24 is depicted in
If desired, the embodiments of
The embodiment depicts two layers on both sides. Typically, this configuration is suitable for such application as an air conditioning heat exchange unit's condenser. In such applications, ambient air flows radially under the influence of a fan that may be located on the top or bottom of the heat exchanger. Conditioned air thereafter flows outwardly axially.
If desired, any of the tubes depicted in
Where the tube is seamless, the surface enhancements are generally axial. Where the tube is welded, internal enhancements may be axial, helical, or a combination thereof. It will be appreciated that the geometry of the internal enhancements can include incursions that are cross-hatched, disposed in a herringbone or V configuration, or otherwise in the form of a turbo-spiral surface texture.
Referring now to
Mention was made earlier of external surface enhancements in the form of annular fins. In such embodiments, a surface enhancement that extends up to 1.0 mm from the outside tube surface tends to promote heat transfer. Other forms of surface enhancement could be provided, such as needles that may extend up to 1.0 mm or more into the fluid (such as air) that flows outside the tubes.
In
In an alternate embodiment of the invention, the spacer member 24 in
During the manufacturing steps, a spacer member 24 that is configured as a manifold may itself serve as a mandrel or holder for a tube that is wrapped therearound. In such manufacturing steps, the spacer member 24 serving as a mandrel also serves as a fixture that assists in forming a heat exchanger having a desired configuration.
Mention was made earlier that the embodiments of
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.
