Patent Application
20110083619
The use of steam, for driving steam turbines for the purpose of generating electricity or for other processes such as desalination or enhanced mineral recovery, has been a common practice for many years. Various methods of generating steam have been employed making use of fossil fuels, nuclear fusion, and more recently, using solar energy. Generally, a heat exchanger having a primary liquid that is heated from the various sources set forth above heats a secondary liquid to generate steam.
As the demands for steam for generating electricity and other recovery processes increase, it has become necessary to improve the heat transfer efficiency between primary and secondary mediums passing through a heat exchanger. Additionally, for improved solar plant economics, the immediate use of available solar energy has become very important, requiring optimized transfer of heat from the primary fluid to the secondary fluid during periods of solar field start-up and normal operations. During the start up when the primary fluid is being heated to a temperature reaching the desirable range, and mid-day operation when the primary fluid may be heated to a temperature a bit above the optimum range it is important to maintain optimum heat transfer between the two fluids.
Known steam generating heat exchangers have made use of smooth tubes as a part of a shell and tube heat exchanger where the primary heating medium flows on the outside of the tubes and the secondary vapor generating medium flows on the inside of the tubes. When the temperature of the primary medium is maintained within a narrow window, the smooth tubes have been known to provide satisfactory heat transfer. However, vapor blanketing inside the tubes is known to occur in the transition phase tube where the two phase flow with high vapor fraction flows over hot metal surface resulting in scaling of deposits known to reduce the heat transfer efficiency of the heat exchanger. Further problems arise when the temperature of the primary medium cannot be controlled within a narrow range, which is known to occur in solar fields. When a solar field heats the primary medium to a temperature above optimum, scaling occurs inside the transfer tubes at an advanced rate due to the rapid transition of the phase of the secondary fluid from liquid to vapor. During start ups, when the primary medium temperature is still less than optimum, heat transfer is very poor in parts of the heat exchanger and it takes a long period for generating required vapor fraction in the heat exchanger tubes. Furthermore, prior art methods of generating steam have intentionally restricted the amount of steam generation to about 10 to 12% of the amount of liquid passing through a heat exchanger to avoid the problems set forth above.
Therefore, to meet the demands of high-efficiency steam generation processes, it has become necessary to improve the heat transfer efficiency of the steam generating system and the ability to generate higher percentages of steam to liquid. Furthermore, it is also necessary to reduce the frequency of downtime during which the steam generating system is not operating due to the required cleaning of the inside of the steam generator tubes and by reducing the period of time for requisite cleaning of the tubes.
A heat exchanger assembly is used to transfer heat from a first medium to a second medium to convert the second medium from liquid to vapor. The vapor, or steam, is used to drive a steam turbine for generating electrical energy or for other processes such as desalination or mineral recovery. A tube is disposed inside the heat exchange assembly and includes an inner wall and an outer wall the length of which define a longitudinal axis. The outer wall includes a plurality of spaced fins oriented in a generally perpendicular manner to the longitudinal axis of the tube. The inner wall defines a preheat zone and a dual phase zone. The preheat zone defines a helical rib configured to provide swirling motion and increased heat transfer surface area to liquid entering the tube. The dual phase zone is spaced from the preheat zone and defines a helical rib configured to provide a swirling motion to the steam and liquid passing through the dual phase zone. In each instance, the helical rib improves the efficiency of heat transfer from the first medium to the second medium to convert liquid to steam.
The configuration of the inventive heat exchanger and dual enhanced tube is believed to provide heat exchange benefit for the specific purpose of converting liquid to vapor while meeting the demands of high-efficiency solar heat collector assemblies. By providing helical ribbing to the entry of a phase conversion tube, the efficiency of the first zone of the tube is improved to provide enhanced heat transfer from the first medium to the second medium while the second medium is still primarily in the liquid phase. In the zone before the outlet of the tube, where the second medium consists of high vapor fraction, the helical rib provides swirling turbulent motion to the vapor preventing vapor blanketing on the tube wall, which is known to cause reduced heat transfer as well as rapid scaling and fouling with deposits further resulting in the decrease of the performance of the heat exchanger. The dual enhanced tube provides the ability to increase the steam to liquid percentage to 20% without causing scaling inside the tube. This increased percentage over prior art tubes provides significant efficiency benefits. Therefore, the dual enhanced tube not only improves heat transfer efficiency, but also reduces the amount of maintenance and cleaning required of known steam generating systems.
Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
A heat exchanger assembly of the present invention is generally shown at 10 of
The second medium, or liquid, enters the assembly 10 through a liquid inlet 16 and exits the assembly 10 through a two phase flow outlet 18 after having been converted partially to vapor. It should be understood by those of skill in the art that the two phase flow will exit the assembly through the outlet 18. The second medium contemplated by the inventors is water that is converted to steam in the assembly 10 to be delivered into an external steam drum either by natural circulation or forced circulation by pumps. It should be understood by those of skill in the art that other mediums may be used, such as, for example, an ammonia water mixture or the like. Steam from the steam drum is then superheated in a heat exchanger and supplied to drive a steam turbine (not shown) generating electrical energy or supply to processes such as desalination or mineral recovery, in a known manner.
As best represented in
Referring to
The dimensional aspects of the dual enhanced tube are shown in
Internally, the helical rib 44 has an average rib height (di−dp)/2 of between about 0.5 mm to 1.0 mm. The average thickness of the internal helical ribs 44 is between about 0.38 mm and 0.44 mm. The spacing, Ws of the helical ribs 44 at the inlet of the dual enhanced tube, as defined by the number of ribs per unit length of the tube is about 6 per centimeter length of tube at the inlet and 2 per centimeter length of tube at the outlet. The helical ribs 44 terminate in height at an apex having an apex angle of about ten to thirty degrees while along the axis of the tube forming a helix angle of ten to forty five degrees with respect to the tube axis.
During operation, liquid entering the dual enhanced tube 24 is initially heated in a first zone 46 which extends the length required to raise the temperature of a liquid to its boiling point. This is believed to be between about 15% and 33% of the length of the dual enhanced tube 24. Therefore, the actual length of the first zone 46 could vary depending upon the temperature of the heating medium. Furthermore, the length of the first zone 46 is shortened by the inventive dual enhanced tube due to the swirling turbulence generated by the helical ribs 44 and the improved heat transfer between mediums by the extended surface area generated by the outer fins 42. By shortening the length of the first zone 46, the phase transition zones of the dual enhanced tube 24 are lengthened providing enhanced phase transition to the liquid entering the dual enhanced tube 24.
A second zone 48 is contemplated to be in the central region of the dual enhanced tube 24 where the liquid, now raised to a temperature of its boiling point begins a transition from liquid to steam, or vapor. The flow velocity of the second medium increases in the second zone 48 due to a lower density attributable to the generation of some vapor. The lower density is believed to cause flow turbulence in the middle zone. Therefore, it is not believed that a substantial amount of vapor film is built up in this middle zone 48 that would cause scaling reducing heat transfer efficiency of the assembly 10. Therefore, a reduced cost dual enhanced tube is contemplated to have a smooth inner surface in the central region or middle zone while helical ribbing 44 is only swaged at the opposing ends of the dual enhanced tube 24.
A third zone 50 is located at the opposite end of the dual enhanced tube 24 from the first zone 46. The third zone 50 is a location in which more liquid is converted to vapor, or steam, known to result in vapor film coating the inner surface 40 of the dual enhanced tube 24. As set forth above, vapor film is known to reduce the heat transfer efficiency of the tube and also results in a build-up of deposits requiring frequent cleaning of the assembly 10. The helical ribbing provides swirling and turbulent motion to the vapor preventing a vapor film from occurring and subsequent build-up of deposits on the inner surface 40.
An alternative embodiment is shown in
An alternate dual enhanced tube 124 is shown in
While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
