# Corrugated Tube Regenerator for an Expansion Engine

A regenerative heat exchanger for transferring heat from the exhaust gas to the intake working fluid of a prime mover. Application includes gas turbines for both motor vehicles and distributed electric generation. The heat exchanger employs a rotating matrix, which circulates through working fluid exhaust and intake channels while absorbing and rejecting heat between the two channels. Features include corrugated tubes for enhanced heat transfer, minimally welded low stress construction, quick-detach assembly of standard components, and purge flow sealing using recovered heat. Effectiveness exceeding 95% increases thermal efficiency of low-pressure ratio gas turbines.

**Description**

**BACKGROUND OF THE INVENTION**

The present invention relates to counter flow regenerative heat exchangers for heat recovery in low capacity prime movers. This includes distributed electric generation and vehicle use, and pertains particularly to an improved regenerator for small gas turbine engines. Low capacity gas turbines are generally considered to be impractical, especially in variable speed automotive use, due to very high turbine speed, inefficient turn-down during deceleration and idling, and high exhaust temperature. The rotary regenerator of the present invention with heat transfer enhancing features resolves these issues, enabling efficient low compression operation with effectiveness greater than 95%. Low stress and compact low cost metallic construction withstands high turbine outlet temperature associated with low compression. As a result, cycle efficiency is high within turbine stress limitations imposed by the pressure-speed relation, wherein rotor speed is directly proportional to working fluid flow rate and compression ratio and indirectly proportional to turbine diameter. Additional benefits of enhanced heat transfer in low compression application are reduced leakage of intake air into exhaust gas, improved flow distribution through the heat transfer surface and closer balance between intake air and exhaust gas pressure drop. Estimated cost is 40 $/kW engine capacity

The regenerator of the present invention employs a rotating matrix of corrugated heat transfer tubes, which absorb heat on both inner and outer surfaces from the lower pressure exhaust gas side for transfer to the pressurized intake air side of the regenerator. Heat transfer in the laminar flow range provides a compact and high effectiveness design. Compactness is further improved using either pre-fabricated honeycomb or packed tubular cell construction in a hexagonal array. Each cell contains one or more corrugated tubes with enhanced heat storage and heat transfer capability. Further compactness is achieved in the tubular type matrix by meshing the ridges and grooves of corrugated tubes in hexagonal groups within the cells. This arrangement provides positive tube positioning in a relatively low stress unconstrained matrix. Heat transfer may be further enhanced in both matrix types by insertion of longitudinal strip-fins within the corrugated tubes. The stainless steel or nickel alloy matrix operates well within recommended service temperature limits approaching normal micro-turbine inlet gas temperature of 1150 K in this low stress application. The honeycomb matrix may be mass produced using a relatively inexpensive automatic welding process and the alternate cell tube matrix is non-welded. The corrugated tubes are readily available and installed without welding at minimal cost.

The matrix is supported on bearings at each end of a central shaft and rotates through seals having minimal working fluid leakage. Two factors lower seal leakage; low compression ratio of the application and an elongated matrix due to enhanced heat transfer. Matrix length to diameter is reduced from a ratio of about 5 to a ratio of 1 in the regenerator of the present invention. Seal leakage may be further reduced by a purge system, drawing fluid from a turbine bearing air supply or a rotor cooling water supply for distribution along matrix support bars. An electric motor provides rotation of the matrix via a pinion and ring gear.

Current practice for small gas turbines utilizing heat recovery to increase thermal efficiency is to employ recuperators with fixed surface area for stationary use and rotary regenerators for automotive use. In the former case micro-turbines for distributed electric generation are gaining wide acceptance, while in the latter case gas turbine development is ongoing and limited to constant speed proto-types. The state-of-the-art micro-turbine heat exchanger is a counter-flow recuperator, which operates in the laminar flow range for acceptable heat transfer and effectiveness in a plate type matrix with numerous parallel flow passages fitted with strip-fins. It is the most expensive component of the gas turbine system, constructed of high temperature alloys with a large number of closely spaced brazed joints and complex header arrangements. Efforts are ongoing to develop a less expensive heat exchanger. The state-of-the-art automotive heat exchanger is a more advanced rotary regenerator, which also relies on laminar flow, but in a ceramic disk matrix, It must withstand thermal cycling to nearly turbine inlet gas temperature during deceleration and idling. Both fixed and rotary heat exchangers are subject to design compromise to limit thermal stresses. The fixed metallic recuperator is constrained by thermal expansion and maximum service temperature is limited to about 950 K. Estimated cost is 160 $/kW engine capacity. The ceramic rotary regenerator matrix can withstand elevated turbine exhaust temperature, but off-design operating conditions may impose excessive thermal stress. In addition, the latter is subject to erosion/corrosion due to seal leakage and is not conducive to heat transfer enhancement geometry and ring gear attachment. Estimated cost is 80 $/kW engine capacity

**SUMMARY AND OBJECTS OF THE INVENTION**

Accordingly, objects and advantages of the rotary regenerator of the present invention are:

(a) to provide a rotary regenerator for increasing thermodynamic cycle efficiency of expansion engines;

(b) to provide a rotary regenerator having high effectiveness;

(c) to provide a rotary regenerator with a low constraint metallic heat transfer matrix to withstand thermal stresses at highest service temperature;

(d) to provide a rotary regenerator having a compact assembly using a hexagonal matrix;

(e) to provide a rotary regenerator having low seal leakage or increased pressure capability;

(f) to provide a rotary regenerator having heat recovering purge flow for matrix lubrication and low seal leakage without loss of engine efficiency;

(g) to provide a rotary regenerator having uniform matrix flow distribution;

(h) to provide a rotary regenerator constructed of readily available components including enclosure and heat transfer cells; and

(i) to provide a rotary regenerator with tube matrix accessibility contained in a quickly detachable enclosure.

Further objects and advantages are to provide an inexpensive and reliable regenerator, which will enable widespread application of expansion engines including low capacity gas turbines. Still further objects and advantages will become apparent from a consideration of the following description and drawings.

**BRIEF DESCRIPTION OF THE DRAWINGS**

In the drawings, closely related figures have the same number but different alphabetical suffixes.

**DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS**

**100** of a prime mover of the present invention. Arrows indicate flow direction of working fluid from a compressor discharge line **102** through a pressurized regenerator channel **104** and discharging to a combustor intake line **106**, while working fluid exhaust from a turbine discharge line **108** continues through a depressurized regenerator channel **110** to atmosphere. Heat is transferred from the turbine exhaust to pressurized working fluid within a rotating heat transfer matrix **112**. The matrix is contained and supported in a containment vessel **114** constructed of two tee fittings **116** held together by a bolted clamp **118**. Clamped stainless steel tee fittings are available from Victaulic Company of Easton, Pennsylvania. Semi-circular baffle plates **120**, welded to the fittings and abutted to a colder matrix end support bar **122** and to a hotter matrix end support bar **124**, divide the pressurized and depressurized channels. Each bar is fitted with shaft bearings **126**, which support a central rotational shaft **128** of the matrix. The matrix is driven by a geared electric motor **130** via a ring gear **132** attached to the matrix. Radial leakage of working fluid across the ends of the matrix is limited by appropriate surfacing of the bars, while a circumferential seal **134** limits longitudinal leakage of working fluid past the matrix and insulation **136** limits heat loss from the vessel.

**138** are in a hexagonal honeycomb pattern constructed of longitudinally welded cells to facilitate a low leakage and compact matrix. Honeycomb matrix of stainless steel is available from Benecor, Inc. of Wichita, Kans. The matrix is held in place in the center by shaft **128** and at the periphery by a circular duct **140**. **142**, one of which isE inserted in each cell. The corrugated tubes are retained in the cells by retainer plates **144** having an appropriate perforation pattern and flow area greater than flow area through and along the corrugated tubes. Corrugated tubing of longitudinally welded stainless steel is available from Hose Master Inc. of Cleveland, Ohio and in open seam form from George Risk Industries of Kimball, Nebraska. Components in contact with the matrix are shown including the two shaft support bars with bearings, the ring gear and the seal.

The corrugated heat transfer tubes, with both inside and outside active surfaces, enable low hydraulic diameter of the matrix and high conductive heat transfer coefficient in the laminar flow range. Heat transfer coefficient and friction factor are comparable to that of a fixed plate type recuperator operating in similar flow conditions, but at about one-third of the cost. This is accomplished by elimination of headers and associated welds in conjunction with automated honeycomb matrix production. Performance of the exemplary regenerator is estimated at operating conditions applicable to a compact motor vehicle at a cruising speed of 120 km/h (75 mph). Turbine inlet gas temperature is 1110 K (2000 R) and compression ratio is 3 with exhaust and pressurized side losses limited to 2.5% and 1%, respectively. At these conditions cycle efficiency and regenerator effectiveness are approximately 30% and 92%, respectively. The regenerator is configured as a hexagonal group of 7 corrugated tubes per cell, with sizing based on turbine exhaust temperature of 900 K (1620 R) and heat duty of 460,000 kJ/h (436,000 Btu/h). Heat duty is based on the assumption that a portion of the exhaust is bypassed around the regenerator to avoid surface area penalty during infrequent high power operation. The resulting matrix geometry is; surface area per cell=300 cm^{2}(46 in^{2}), flow area per cell=0.65 cm^{2}(0.10 in^{2}), total cells=230, hydraulic diameter=0.21 cm (.084 in.), cell and corrugated tube length=30.5 cm (12 in.), and matrix mass per cell=0.045 kg (0.10 lb.).

**212** of the regenerator of the present invention. **238** in a hexagonal pattern. The cell arrangement forms a non-welded matrix held in line contact by compression imposed by a duct **240**. Smaller diameter filler tubes **241** complete fitting of the hexagonal matrix to the circular duct. **242** inserted in a cell tube. **244** for holding the corrugated tubes in the matrix. Two plates are held within the duct between the matrix support bars **224**, **226** and the ends of the cell tubes. Each plate has a flow area through the perforations greater than the flow area through the matrix. **246** of adjacent corrugated tubes illustrating meshing of annular corrugations **248** of the corrugated tubes. Nearly full engagement of the corrugations is expected to decrease the matrix and vessel diameters by about 20% with little effect on hydraulic diameter and heat transfer rate of the matrix. Overall sizing of the tubular cell matrix is comparable to the honeycomb matrix of ^{2}), flow area per 7 tube cell=4.6 cm^{2}(0.72 in^{2}), total cells=43, hydraulic diameter=2.5 cm (0.10 in.), cell and corrugated tube length=30.5 cm (12 in.), and matrix mass per 7 tube cell=0.20 kg (0.45 lb.).

**346** inserted in a corrugated tube **342**. The strip decreases hydraulic diameter of the tube by an estimated 33% while increasing heat transfer coefficient and friction factor in approximately the same proportion.

**400** is shown in relation to prime mover components including a compressor **450**, a combustor **452** with a fuel tank **454**, and a turbine **456**. Open arrows and dashed lines indicate flow and direction of purge water and steam. Purge flow is from a water tank **458** through a recovery evaporator **460** of the compressor from which a portion of steam is diverted and injected into the tips of a colder end matrix support bar **422**. The remaining portion then continues through a recovery superheater **462** of the combustor for superheating and injection into the tips of a hotter end matrix support bar **424**. **422** or **424** connected to water or steam lines of the purge flow supply. Purge flow distribution nozzles **425** are oriented to discharge toward the pressurized channel of the matrix. A shaft bearing **426** is shown oriented at right angles to the steam discharge.

Working fluid leakage across a non-purged matrix is low because of low compression ratio and high length to diameter ratio of the matrix. The purge system is adaptable in high temperature gas turbines employing a water cooled turbine rotor while reducing surface area of the matrix. This is because of two factors; zero working fluid leakage and enhanced heat transfer with non-luminous water vapor radiation.

While I have illustrated and described my invention by means of specific embodiments, it is to be understood that numerous changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. For example, prime mover heat input may include solar, the regenerator may be oriented with downward exhaust requiring only one tube retainer plate at the bottom, and fin strips with various cross-section configurations may be inserted in the corrugated tubes.

## Claims

1. Rotary regenerator heat recovery means of a prime mover comprising heat transfer matrix means with corrugated heat transfer tubes inserted longitudinally in working fluid flow cells of said matrix means, wherein circumferential surface ridges and grooves of said corrugated tubes provide heat transfer enhancement, while said matrix provides transfer of exhaust gas heat of said prime mover to the higher pressure intake working fluid of said prime mover during rotation of said matrix about a central shaft of said regenerator means.

2. The matrix means of claim 1 comprising a one piece array of working fluid honeycomb cells containing said corrugated tubes for providing compact heat transfer surface means.

3. The matrix means of claim 1 comprising a packed array of working fluid tubular cells containing said corrugated tubes, wherein said array is retained at the periphery by essentially circular duct means for providing compact heat transfer surface means.

4. The matrix means of claim 3 comprising essentially line contact between said tubular cells, wherein said tubular cells are held in essentially hexagonal arrangement by said duct means,

5. The heat transfer cells of claim 1 comprising strip fin means disposed longitudinally through said corrugated tubes for providing further heat transfer enhancement of said matrix means.

6. The regenerator means of claim 1 comprising working fluid containment means constructed of first conduit fitting means and second conduit fitting means, wherein each said fitting means comprises a main branch with support means for said matrix and connection means for attachment of said first fitting means to said second fitting means, a pressurized branch for channeling compressed working fluid, and an exhaust branch for channeling exhaust working fluid.

7. The support means of claim 6 comprising purge flow distribution means, wherein pressure of purge flow to said distribution means is provided by a purge system with compression work means selected from the recovered energy group consisting of solar, building amplified wind, motor vehicle momentum, motor vehicle draft loss and motor vehicle shock.

8. The purge system of claim 7 comprising purge flow heating means wherein purge flow temperature is increased by recovered heat selected from the group consisting of combustor waste heat, compressor heat, motor vehicle transmission heat and stored solar heat.

9. A method for constructing a heat transfer matrix of a rotary regenerator of a prime mover comprising the steps of:

- enclosing said matrix within an essentially circular duct disposed in essentially parallel relation to working fluid flow cells of said matrix,

- inserting and retaining corrugated heat transfer enhancement tubes into said cells, and

- inserting a matrix rotation shaft along the area centroid of said matrix, wherein said matrix provides transfer of exhaust gas heat of said prime mover to the higher pressure intake working fluid of said prime mover during rotation of said matrix about said shaft.

10. The method of claim 9 comprising the first step of sequentially packing cell tubes in essentially line contact within an essentially hexagonal arrangement to provide an array of tubular working fluid flow passages of said matrix.

11. The method of claim 10 comprising the next step of inserting filler tubes in essentially line contact with said cell tubes and with said duct for providing an essentially circular periphery of said matrix.

12. The method of claim 11 comprising the next step of attaching the periphery of at least one perforated retainer disk to said duct for retention of said corrugated tubes in said cell tubes, wherein perforations of said disk or disks provides for essentially unrestricted flow of working fluid through said disks.

13. The method of claim 12 comprising the next step of attaching said cell tubes to said disk or disks by a process selected from the group consisting of gasketing, interference fitting, welding and brazing, for limiting leakage of working fluid between ends of said cell tubes and said disk or disks.

14. The method of claim 10 comprising working fluid leakage limiting means, wherein material of said cell tubes and of said duct means are selected to limit the difference of diametral thermal expansion between said cell tubes and said duct.

15. The method of claim 14 comprising the first step of grinding or machining the outside diameter of said cell tubes, wherein line contact between said cell tubes is maintained within a specified tolerance range for limiting leakage of working fluid between said tubular cells.

16. A rotary regenerator of a prime mover comprising: wherein circumferential surface ridges and grooves of said corrugated tubes provide enhanced transfer of exhaust gas heat from a combustor and a turbine of said prime mover to the higher pressure intake working fluid from a compressor of said prime mover, during rotation of said matrix about a central shaft mounted to said bearings.

- a heat transfer matrix containment vessel,

- a rotating heat transfer matrix within said vessel, p1 corrugated working fluid tubes inserted within working fluid flow passages of said matrix,

- colder matrix end support bars within said containment,

- hotter matrix end support bars within said containment, and

- matrix shaft bearings within said bars,

17. The support bars of claim 16 comprising purge flow distribution nozzles disposed radially with respect to said shaft and in direct fluid communication with at least one end of said bars, for passage and direction of purge flow from a purge system toward pressurized working fluid of said prime mover.

18. The purge system of claim 17 comprising a purge flow evaporator in contact with said compressor and a purge flow superheater in contact with said combustor, for providing steam from said evaporator to said colder matrix end support bars and steam from said superheater to said hotter matrix end support bars.

19. The corrugated tubes of claim 16 comprising strip fins inserted in said corrugated tubes for providing further heat transfer enhancement within said matrix.

20. The corrugated tubes of claim 16 comprising corrugation geometry, wherein the width of said grooves exceeds the width of said ridges to provide meshing of said tubes disposed in adjacent relation, for reducing the diameter of said matrix.

**Patent History**

**Publication number**: 20140216687

**Type:**Application

**Filed**: Feb 7, 2013

**Publication Date**: Aug 7, 2014

**Inventor**: Jay Stephen Kaufman (Kingston, NH)

**Application Number**: 13/761,209

**Classifications**

**Current U.S. Class**:

**Movable Heating Or Cooling Surface (165/86);**Casing Or Tank Enclosed Conduit Assembly (165/157); Heat Exchanger Or Boiler Making (29/890.03)

**International Classification**: F28D 11/04 (20060101); B21D 53/02 (20060101); F28D 1/053 (20060101);