Claims:of the invention as claimed.
We Claim:

1. A heat transfer sheet assembly for a rotary air-preheater, comprising:
a. a heat transfer sheet (14) made of series of repeat patterns (15) of length P, said repeat patterns (15) comprising:
i. a pair of notches (16, 17) extending parallel to each other along the flow direction spaced at a distance of Pn, each of said notches (16, 17) comprising a pair of lobes (18,19) projecting outwardly on the opposite sides of the sheet (14) having peak to peak height of Hn which forms the opening for said notches (16, 17); and
ii. undulations (20) extending on both sides and between said notches (16, 17) at an angle Au to the flow direction, wherein opening of said undulations (20) Hu is from the top of one undulation to the bottom of the neighbouring undulation on the same sheet,
wherein Pn should be in the range of 25 mm to 150 mm, the ratio Pn/P should be less than 0.5, Hu should be in the range of 2.5 mm to 5 mm, Hn should be in the range of 6 mm to 12 mm, the ratio Hu/Hn should be in the range of 0.20 to 0.50, the pair height (Hu+Hn) should be in the range of 9 mm to 15 mm and Au should be in the range of 20 o to 40 o to attain the optimum performance for the heat transfer sheet (14) through heat transfer assembly.

2. The heat transfer sheet assembly as claimed in claim 1, wherein said projection of said lobes (18, 19) on either sides of the sheet forms a crest (21) and a trough (22) that hold the adjacent sheets at a pre-determined spacing forming the passage way for the fluid flow.

3. The heat transfer sheet assembly as claimed in claim 1, wherein said repeat patterns (15) between adjacent sheets are placed in a zigzag arrangement.

4. The heat transfer sheet assembly as claimed in claim 1, wherein said heat transfer sheets (14) are stacked together in a spaced relationship to provide the passageways for the flow between the adjacent sheets.

5. The heat transfer sheet assembly as claimed in claim 1, wherein said lobes (18, 19) are in the form of a U-shaped groove.
, Description:FIELD OF THE INVENTION:

The present invention relates generally to heat transfer sheets or heat transfer elements used in rotary regenerative heat exchangers. More particularly, it relates to an optimized design of heat transfer sheets for better thermal performance and reduced pressure drop.

BACKGROUND OF THE INVENTION:

Regenerative air pre-heaters are heat recovery systems used in boilers which recover the heat from the hot flue gas and pre-heats the incoming combustion air. This recovered heat energy would lead to significant cost savings by means of reduced fuel consumption, better combustion and cleaner emissions.

FIG. 1 represents a typical tri-sector type rotary air pre-heater (1) that has a cylindrical rotor (2) mounted on a housing (3). The housing (3) defines both the inlet (4, 5) and the outlet ducts (6, 7, 8) and the direction of arrows indicates the stream of flue gas (11), primary (12) and secondary air (13) through the rotor (2). The cylindrical rotor (2) is divided into several sectors by radial diaphragm plates (9). The diaphragm plates (9) support the baskets which hold the heat transfer elements or sheets. Maximum number of sheets are closely stacked in each basket in a spaced relationship which define the space for the flow of gases through baskets. The center section assembly (10) and the sealing system define the air and gas side compartments. The air side compartment is bifurcated into primary air (PA) side and secondary air (SA) side. The rotor (2) is rotated at a constant speed by drive motor which causes the heat transfer sheets to be alternatively exposed to the flue gas and the air sides.

Heat transfer sheets play an important role in the transfer of heat. When the sheets are exposed to the flue gas side, it absorbs the heat from the gas and then when exposed to the air side it transfers the absorbed heat to the air. The heat transfer capability of sheets purely depend on the assembly pattern, profile and dimensions. Apart from the heat transfer, the sheets create resistance to the flow leading to the pressure drop across the air preheater.

The flow passage for the hot gas and the cold air through the heating sheets are defined by the notches and the undulations. Notches which are bilobed with the lobes extending above and below the heating sheet at staggered intervals regulate the spacing between the adjacent sheets. The sheets are provided with a series of undulations (corrugations) between the notches that extend obliquely at an angle to the line of flow. These undulations create turbulence in the flow thus enhancing heat transfer.

Depending upon the dimensional relationship between the notches, corrugations and the assembly pattern between the adjacent sheets the performance of the heating sheets (in-terms of heat transfer rate and pressure drop) can vary widely. The performance of the heating sheets are verified by means of computational fluid dynamics simulations.

Many heat transfer sheets comprising notches separated by undulation, corrugation or wavelike portion are known in the existing art. For example,

United States Patent No. 8,622,115 to James David Seebald, entitled “Heat transfer element for a rotary regenerative heat exchanger” discloses a rotary regenerative heat exchanger employing heat transfer elements shaped to include notches, providing spacing between adjacent elements and undulations (corrugations) in sections between the notches. Between notches first and second undulations are present (Hu1 and Hu2), where Hu2 less than Hu1 less than Hn and the ratio of Hu/Hn is Hu2/Hn> 0.06, Hu2/Hn<0.72, Hu1/Hn>0.30 and Hu1/Hn<0.90.

United States Patent No. 4,744,410 to James A. Groves, entitled “Heat transfer element assembly” discloses a rotary regenerative heat exchanger for transferring heat from a hot fluid to a cold fluid by means of an assembly of heat transfer element which is alternately contacted with the hot and cold fluid. The heat transfer element assembly is comprised of a plurality of heat transfer plates stacked alternately in spaced relationship. The spacing between adjacent plates is maintained by spacers which comprise notches in the form of bilobed folds crimped in the plates at spaced intervals to prevent nesting between adjacent plates.

Non-patent literature “Basketed Element Heating Element”, Paragon Technology discloses a DN (Double Notched) series of element profiles (ranging from DN2.9 to DN7) offering reduced fouling and better cleanability when compared with the equivalent DU elements, while maintaining similar pressure drop and heat transfer characteristics. Unlike DU, DN element contains notches as well as undulations on all of the element sheets.

However, none of the prior art discloses heat transfer sheets with unique pattern and assembly having optimized length and height of undulations and notches.

Hence, there is a need for an optimized design of heat transfer sheets for better thermal performance and reduced pressure drop.

SUMMARY OF THE INVENTION:

An objective of the present invention is to provide a profile and an assembly pattern of the heat transfer sheets for better thermal performance and reduced pressure drop.

To achieve the above mentioned objective, the present invention provides a single heat transfer sheet that is made of series of repeat pattern of length P. Each repeat pattern has a pair of longitudinal notches spaced by a distance Pn. Each notches has lobes which project outwards from the plane of the sheet in opposite sides of heat transfer sheet. The notch opening Hn defines the flow passage between the sheets. Oblique undulations with opening Hu are provided on either sides and between the notches at an angle Au.

According to the present invention, to obtain the optimum performance, Pn should be in the range of 25 mm to 150 mm , the ratio Pn / P should be less than 0.5, Hu and Hn should be in the range of 2.5 mm to 5 mm and 6 mm to 12 mm respectively, the ratio Hu/Hn should be between 0.2 mm and 0.5 mm, the pair height (Hu+Hn) should be between 9 mm and 15 mm and Au should be in the range of 20o to 40o.

The projection of the lobes and on either sides of the sheet forms a crest and a trough which hold the adjacent sheets at a pre-determined spacing thus forming the passage way for the fluid flow. The sheets are staked to form bundles and the repeat patterns between adjacent sheets do not overlap but are placed in a zigzag arrangement.

Other objects, advantages and features of the present invention will become more apparent from the following detailed description and claims, taken in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed for the purpose of illustration only and are not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS:

The objective of the present invention will now be described in more detail with reference to the accompanying drawings, in which:

FIG. 1 illustrates a perspective view of a typical tri-sector rotary regenerative air preheater;

FIG. 2 illustrates an end view of a single heat transfer sheet made of series of repeat patterns;

FIG. 3 illustrates an end view of the repeat pattern;

FIG. 4 illustrates an end view of a pair of heat transfer sheets to illustrate the flow passage available between the sheets;

FIG. 5 illustrates a perspective view of the heat transfer sheet assembly in accordance with the present invention;

FIG. 6 illustrates an end view of heat transfer sheets stacked in a basket; and

FIG. 7 is a graph showing the effect of undulation angle Au on Colburn-j factor and friction f factor.

REFERENCE NUMERALS:

1 - Rotary air pre-heater
2 - Cylindrical rotor
3 - Housing
4, 5 - Inlet ducts
6, 7, 8 - Outlet ducts
9 - Diaphragm plates
10 - Center section assembly
11, 12, 13 - Hot Flue Gas, Primary and Secondary air flow
14 - Heat transfer sheet
15 - Repeat patterns
16, 17 - Notches
18, 19 - Lobes
20 - Undulations
21 - Sheet forming crest
22 - Trough
23 - Passage area for the fluid flow
24, 25, 26 - Sheets

DETAILED DESCRIPTION OF THE INVENTION:

The present invention relates to an optimized design of heat transfer sheets for better thermal performance and reduced pressure drop.

Referring now in detail to the appended drawings, FIG. 2 represents a single heat transfer sheet (14). The heat transfer sheets are thin sheets of metal on which the desired configuration is achieved by rolling or stamping. Heat transfer sheet (14) comprises a series of repeat patterns (15) which in turn comprise a pair of notches (16, 17) and obligue undulations (20). Each notch (16) and (17) has a pair of lobes (18, 19) that projects outwardly from the sheet surface. Lobes (18, 19) may be in the form of a U-shaped groove.

The height and the distance between the notches (16, 17) are designated as Hn and Pn respectively. Oblique undulations (20) on either sides and between the notches (16, 17) extend at an angle Au to the flow direction. Hu indicates the height of the undulations (20). The undulations induce turbulence in the fluid flow hence improving heat transfer rate.

FIG. 3 illustrates an end view of the repeat pattern (15). The projection of the lobes (18, 19) on either sides of the sheet forms a crest (21) and a trough (22). The crest (21) and trough (22) hold the adjacent sheets at a pre-determined spacing thus forming the passage way for the fluid flow.

FIG. 4 illustrates an end view of a pair of sheets where the notches between the adjacent sheets do not overlap and the shaded region (23) defines the passage area for the fluid flow. The sheets are staked to form bundles as shown in FIG. 5 such that the repeat patterns (15) between adjacent sheets do not overlap but are placed in a zigzag arrangement. The sheets (24), (25) and (26) are identical but sheet (25) differs from sheets (24) and (26) in the orientation. The orientation of the sheet (25) can be achieved from either of the sheets (24) or (26) by rotating the sheet by 180 o in the direction perpendicular to the plane of the sheet followed by flipping the sheet along the direction of flow. This stacking arrangement is followed in filling the baskets as shown in FIG. 6. The following parameters should be maintained in the range as prescribed below in order to achieve the optimum thermal performance and resistance to flow.

25 mm < Pn < 150 mm
Pn / P < 0.5
2.5 mm < Hu < 5 mm
6 mm < Hn < 12 mm
0.20 < Hu / Hn < 0.5
9 mm < (Hu + Hn) < 15 mm
20o< Au < 40o

Apart from heat transfer and resistance to flow, the choice of parameters for the heat transfer sheets depends on the type of combustion fuel, nature of flue gas, operating temperature ranges of air preheater and the load condition of the power plant. The heat transfer sheets in air pre-heaters of the power plants operated either with coal containing sticky-ash and/or heavy oil or below full load conditions for long duration of time are susceptible to fouling and choking. In this case larger notch opening Hn, smaller undulation opening Hu and smaller Au angle are the preferred choice as this configuration reduces the hindrance to the fluid flow and also increases the ease of cleanability during soot blower operation, hence reducing the possibility of choking.

For a specific case, the effect of undulation angle Au on the heat transfer (Colburn-j factor) and the pressure drop (friction factor f) is studied through computational fluid dynamics simulations as shown in FIG. 7. The parameters taken are sheet thickness = 0.6 mm, Hu = 3.73 mm, Hn = 9.29 mm, Pn = 36 mm, P = 144 mm. The graph shown in FIG. 7 shows the increase in Colburn-j factor accelerates for Au above 20o whereas the increase in friction f factor is very steep for Au above 40o. Henceforth optimum range for Au is chosen as 20o to 40o.

Thus, the present invention provides better thermal performance and reduced pressure drop.

While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope of the invention as claimed.