Heat exchange – Non-communicating coaxial enclosures – With communicating coaxial enclosure
Reexamination Certificate
2001-03-06
2002-05-21
Leo, Leonard (Department: 3743)
Heat exchange
Non-communicating coaxial enclosures
With communicating coaxial enclosure
C165S141000, C165S154000
Reexamination Certificate
active
06390185
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to concentric tube heat exchangers. More particularly, it relates to an improved recuperator for recovering exhaust heat from a Brayton Cycle gas turbine engine, Ericsson Cycle engine, or similar recuperated engine.
2. Description of Prior Art
The thermodynamic efficiency and resulting fuel economy of a gas turbine (Brayton Cycle) engine can be greatly increased by using an exhaust gas heat exchanger to recover heat from the low pressure exhaust stream to preheat the high pressure air between the compressor and combustor. The heat thus recovered in the preheating process, which would otherwise be wasted in the exhaust, does not have to be supplied by the combustor. As a result, the cycle efficiency is typically doubled from about 15% without a heat exchanger to 30% with a heat exchanger. Newer types of engines, such as the Afterburning Ericsson Cycle of my U.S. Pat. No. 5,894,729 (1999), make even better use of an exhaust gas heat exchanger and can achieve cycle efficiencies of over 40%.
There are two types of exhaust gas heat exchangers: recuperators and regenerators. Although the names are frequently used interchangeably, a recuperator usually refers to a heat exchanger where the high pressure compressor flow and the low pressure exhaust flow are continuously separated by walls and the heat transfer takes place through those walls. A regenerator usually refers to a heat exchanger where the same walls are alternately exposed to the high and low pressure flows. Although a regenerator is usually smaller than a comparable recuperator, the seals and moving parts needed for the flow switching causes mechanical complexity, flow leakage, lower heat recovery and higher cost. Therefore, recuperators are becoming the preferred type of exhaust gas heat exchanger.
A recuperator has requirements that are unique from other types of heat exchangers. First, it must be able to capture the maximum percentage of the available exhaust heat (have high effectiveness). The higher the effectiveness, the more efficient the engine becomes. However, high effectiveness generally requires more pressure drop in both the high pressure compressor outlet flow and the low pressure exhaust flow. The flow work represented by these pressure drops reduces the engine efficiency and can offset the gain from a higher effectiveness. Therefore, the pressure drop through the recuperator must be maintained as low as possible while still obtaining the highest thermal effectiveness. In addition, the recuperator should be easily and economically fabricated, be able to withstand the pressure load from the high pressure flow, allow for thermal growth during heating and cooling transients, be tolerant of fouling from exhaust products, and be capable of withstanding high exhaust temperatures.
Prior art recuperators have compromised one or more of those requirements. The most common approach has been to use a plate-fin heat exchanger. This type of recuperator is generally made of flat sheets interleaved with corrugated sheets that are furnace brazed or welded together. A leading prior art plate-fin heat exchanger is documented in U.S. Pat. No. 5,983,992 (1999). This type of recuperator can attain fairly high effectiveness but is expensive to make because of the large number of parts, many of which are thin wall plates subject to damage during manufacture. Furthermore, since recuperators operate at temperatures where creep strength is low, the pressure loads from the high pressure side can cause the thin plates to distort and shorten the life of the recuperator. Finally, the large number of highly stressed welds increases manufacturing cost and provides potential locations for failure and leakage.
The U.S. Army M1A1 main battle tank is powered by a gas turbine having an annular plate recuperator and is described in U.S. Pat. No. 5,388,398 (1995). This recuperator consists of many annular plates that are also very complex to manufacture and maintain leak free.
The spiral recuperator has been developed in an effort to avoid the complexity of plate type heat exchangers while also using the curved surfaces to hold pressure with less material stress. U.S. Pat. No. 4,883,117 (1989) proposes a typical spiral type recuperator. Spiral recuperators have a significant problem with thermal “short circuiting” that prevents them from achieving high thermal effectiveness. The spiral path puts cold fluid and warm fluid in the same flow in direct or close contact and causes the flow to have a driving potential to become the same temperature. Since the objective is to obtain the maximum temperature difference between the inlet and outlet, the “short circuit” effect can greatly reduce the thermal effectiveness.
A recent advance in spiral recuperators is the Rolls-Royce heat exchanger of U.S. Pat. No. 6,115,919 (2000). Although of spiral construction, both the high and low pressure flows are true counterflow and run along the axis of the spiral with no possibility for “short circuits”. The recuperator is intended for mass production by being formed of continuous strips that are rolled in a spiral to form the recuperator. Nevertheless, the recuperator is still quite complex and requires many welds of thin material at the header holes and sheet edges. The manufacturing cost will remain high due to the number of complex welds and the need to accurately align the spiral sheets. As with the plate-fin heat exchanger, the weld joints are always a potential location for failure and leakage.
The previously described recuperators make use of corrugated surfaces, wavy surfaces, fins or similar devices to increase turbulence and heat transfer. These devices are indeed effective in raising effectiveness, but they also increase pressure drop. It has been common practice in compact heat exchanger design to operate in the turbulent flow range and to avoid laminar flow. However, with small hydraulic diameters, significant heat transfer coefficients can be achieved with laminar flow. Just as important, with laminar flow, the overall heat transfer rate can be increased while the pressure drop is decreased. This is because the heat transfer coefficient in laminar flow is dependent only on passage geometry and not on the flow velocity. Additional flow paths can be added to proportionately increase the overall heat transfer conductance while at the same time proportionately decreasing the pressure drop. This characteristic is exactly what is needed for a high performance recuperator.
U.S. Pat. No. 5,725,051 (1998) describes a heat exchanger that is successfully used as a laminar flow recuperator. Although not currently used as an engine recuperator, a plastic version used in home ventilation systems achieves 94% effectiveness with a pressure drop of only 0.16 inch of water. The recuperator allows fresh outside air to be exchanged with inside air with only a 6% of the un-recuperated air conditioning or heating load. The disadvantage of this heat exchanger is that it has a very complex header system to distribute the two streams to their respective heat exchange flow paths.
Concentric tube heat exchangers are used frequently in applications other than recuperators. Although possessing the advantage of being able to use simple tubular construction, prior art concentric tube heat exchangers have had several limitations for use as recuperators. U.S. Pat. No. 6,012,514 (2000) is a simple concentric tube heat exchanger that is easy to manufacture and maintain. However, it uses gasketed construction that is not suitable for the high temperatures and pressures of a recuperator. More importantly, like other concentric tube heat exchangers such as in U.S. Pat. No. 4,204,573 (1980), U.S. Pat. No. 4,254,826 (1981), and U.S. Pat. No. 4,440,217 (1984), only two tubes are used in the concentric tube assemblies. With this arrangement, one flow passage is within the circular passage of the center tube and the other is in the annular region between the center and outer tube. It is difficult to achieve
LandOfFree
Annular flow concentric tube recuperator does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Annular flow concentric tube recuperator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Annular flow concentric tube recuperator will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2848815