Patent Application
20120305229
Heat exchangers are used for many purposes, including drying of agricultural and forestry products, grasses, foods, hops; goods made from plant matter or animal products in a chamber, shed, or building.
Conventional joining techniques for metal tubing structures, such as heat exchangers and similar devices that must withstand high-temperature or high-thermal-gradient environments consist of welding, brazing, soldering, and similar assembly processes. These conventional techniques usually result in a rigid, monolithic structure that is relatively inflexible. Thermal stresses induced by the high-temperature and high-thermal-gradient environments reduce service life by causing material failure due to differential expansion, joinery failures due to thermal fatigue, etc. Once failure occurs, field service becomes very costly for maintenance, such as on-site welding, and often the entire unit must be removed in order to accomplish repairs.
In conventional heat exchangers that use a fluid exchange medium, the working-fluid is forced through an inlet of the device, creating a motive gradient that causes flow through the device toward an outlet. In the case of flue-gas heat exchangers, for example, this creates a condition where the flue gases inside the tubing of the device are at a higher operating pressure than the ambient-fluid (e.g., air) that is being heated, outside the tubing of the device. In the event of leakage in the heat exchanger's tubing or working-fluid path, potentially toxic flue gases may escape to be introduced into the air or other ambient-fluid. In certain circumstances, such as when drying of agricultural products or the like in a closed building, this escape of flue-gas can lead to contamination of the product being dried and an unsafe environment for human workers.
The conventional way to increase the safety of such devices is to use more expensive grades of metallic tubing and materials that are capable of operating in high-temperature environments. When flame impingement on metallic surfaces in direct-fired heat exchangers and similar devices occurs, however, the corrosion-resistant steels and even more exotic alloys are subject to significant corrosion and wear. Long-term exposure to such environments severely shortens the service life of these materials.
What is needed is a heat exchanger that is more resistant to high-temperature and high-thermal-gradient environments, and methods to maintain such heat exchangers at a much lower cost.
A heat exchanger and related methods are provided. An example heat exchanger has moveable or flexible joints between tubular members that contain flowing heating medium. The moveable joints allow the overall structure of the heat exchanger to adapt to expansion, contraction, and other stresses of high-temperature, high thermal-gradient operation. The moveable joints also provide breaks in the structure, enabling repair or replacement of parts. A negative internal pressure is maintained to prevent flue gases and heat from escaping through the moveable joints, and prevents contamination of the ambient environment and products being dried. Moveable support measures also allow each tier of tubes to expand or contract independently of each other to accommodate thermal gradients within a given tier of tubes, within a given manifold, and between tiers of tubes, so that virtually all of the thermal stresses associated with conventional heat exchangers are mitigated or eliminated.
This summary section is not intended to give a full description of an example heat exchanger and related methods. A detailed description with example implementations follows.
This disclosure describes heat exchangers and related methods. In one implementation, an example heat exchanger has moveable or flexible joints between tubular members that contain a flowing heating medium (“working-fluid”). Moveable, as used herein, means that connected members are allowed some play between each other, rather than a completely rigid conventional connection through welding and such means. Tube or tubular, as used herein, mean hollow or tube-like, to contain a flowing heating medium, such as heated air.
Tubular members connected at a moveable joint may be able to move longitudinally, transversally, rotationally, etc., with respect to each other. The moveable joints allow the overall structure of the heat exchanger to adapt to expansion, contraction, and other stresses of high-temperature, high thermal-gradient operation. In one implementation, the moveable joints allow hinge-like, rotational, and longitudinal (in-and-out) movements or play between tubular members of the heat exchanger during heating and cooling cycles. The moveable joints also provide breaks in the structure, enabling a way to dismantle the heat exchanger for repair or replacement of parts, without replacing the entire heat exchanger. This is preferable to conventional maintenance and repair of a permanently rigid and monolithic conventional structure, which is cumbersome. A conventional structure may be “totaled” if one section of the heat exchanger goes bad.
In conjunction with the moveable connection joints between tubular parts, an example heat exchanger may include a negative pressure system or device for moving a heating medium, such as hot air, through the example heat exchanger. Thus, a blower, vacuum pump, impeller (“vacuum device”), etc., may be connected to pull the fluid heating medium through the heat exchanger. Although the moveable connection joints are designed to provide a tight seal and to be leak-proof, maintaining an internal negative pressure with respect to the ambient environment being heated, aims to ensure that flue gases and heat do not leave from the example heat exchanger. This prevents contamination of the ambient environment, including both products being dried by the heat exchanger and humans. Negative-draft induction rather than a forced-draft system thus provides an additional benefit for the example heat exchanger.
While the moveable individual joints between adjoining tubes provide flexibility for thermal expansion and contraction, moveable support measures also allow each tier of working-fluid tubes to expand or contract independently of each other. Working in combination, this arrangement accommodates thermal gradients within a given tier of working-fluid tubes, within a given manifold, and between tiers of tubes; so that virtually all of the thermal stresses associated with conventional heat exchangers are mitigated or eliminated.
In one implementation, a direct-fired implementation of the example heat exchanger uses refractory-lining in flame impingement areas (areas of direct exposure to a flame and entrained particulates).
In either implementation of the moveable tube connection joint illustrated in
While the moveable individual joints between adjoining tubes provide flexibility for thermal expansion and contraction, moveable support measures also allow each tier of working-fluid tubes to expand or contract independently of each other. Working in combination, this arrangement accommodates thermal gradients within a given tier of working-fluid tubes, within a given manifold, and between tiers of tubes; so that virtually all of the thermal stresses associated with conventional heat exchangers are mitigated or eliminated.
At block 502, tubular members are joined to make a grid of a heat exchanger.
At block 504, moveable joints are provided to accomplish the joining. The moveable joints enable the grid to expand and contract in response to heating and cooling cycles.
Features of the implementations described can be combined in different configurations within the scope of the subject matter. Thus, while there have been shown and described fundamental novel features as applied to the example implementations thereof, various omissions, substitutions, and changes in the form and detail of devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the description. Elements and techniques shown and described in connection with any disclosed form or implementation of the example heat exchanger may be incorporated in any other disclosed, suggested, or described form or implementation as a general matter of design choice.
Although exemplary systems and methods have been described in language specific to structural features or techniques, the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as example forms of implementing the claimed systems, methods, and structures.
