Comparative analysis of recuperator efficiency

The case presents a comparative analysis of the design parameters of several types of recuperators and OPT recuperators.

The design of recuperators existing today have serious disadvantages, which often do not allow using recuperators in technological processes.

The disadvantages of conventional heat exchangers:

  • Excessive weight and size;
  • Complex or impossible repair ;
  • Structures are subjected to buckling and fracture due to thermal stresses followed by the significant difference in thickness of the elements and uneven heating and cooling. In tube recuperators, local overheating zones often arise due to uneven distribution of air flow through the pipes. More air is supplied to short pipe loops due to lower resistance, and less air is supplied to long loops. As a result, due to the uneven distribution of air flow in the weld zone, additional thermal stresses occur.
  • In conventional design, there is an uneven distribution of gas flows over the cross section of ducts, which also significantly affects the process of heating the heat transfer surfaces directly in the recuperator itself. In this case, there is an uneven thermal expansion of the elements of the nodes, i.e. thermal stresses occur and, as a result, welded joints and heat exchange surfaces are damaged.

A significant disadvantage of conventional heat exchangers is low compactness factor (the ratio of the heat transfer surface area to volume, m2 / m3)

This indicator reflects the rational layout of the device and, as a result, the efficiency of heat transfer between gas heating and heated medium.

Further, we consider the compactness factor, as comparative analysis of various designs of conventional recuperators and OPT recuperators.

Design of heat transfer surfaces of OPT recuperators provide uniform distribution of incoming flows (both on the flue and on the air sides). The gas flows at the inlet are evenly distributed into numerous streams, and flow motion occurs in narrow slot channels.
These conditions increase the degree of uniform heating of heat transfer surfaces, both from the side of the heating medium and from the side being heated. This, in turn, reduces thermal stresses and, accordingly, structural damage.

OPT recuperators are made of stainless sheet steel, the thickness of all heat transferring surfaces is usually 1.5 mm.

Advantages of OPT recuperators

  • Low weight and dimensional parameters (2-8 times less than traditional recuperators)
  • Ability to create any thermal capacity for heat utilization.
  • Easy maintenance, transportation, installation and repair.
  • Uniform distribution of gas flows directly into recuperator.
  • Low thermal inertia of heat transferring surfaces.
  • High thermal plasticity.
  • Operation in aggressive gas conditions.
  • Cleaning and self-cleaning of gas-air ducts from the combustion products due to straight channels, and low pressure drop ensuring significant gas speeds.
  • Access to each module during installation, repair and inspection.
  • Different steel grades for each module of the heat exchanger, depending on operating conditions.
  • Internal compensation of thermal expansions, OPT recuperators can be mounted without compensators.
  • Low cost, as well as low operating costs and, accordingly, short payback period (usually less than a year).

Technical data of OPT recuperators

Technical data of OPT recuperators can be selected optimally for specific customer requirements and can vary over a wide range depending on a number of parameters:

  • the speed of gas medium (heating, heated);
  • the fins pitch and height of fins (finned panel);
  • the direction of motion of gas medium;
  • the number of passes of air ducts; modular sections layout.

Analyzing the above information on existing recuperators and OPT recuperators, we set forth the technical parameters of the above recuperators in the form of tables, where we indicate the initial and calculated parameters according to technical literature sources *. Calculations of OPT recuperators were carried out according to a software developed by Termo Nord Stream LLC – TERMO (version 4.1).

Comparative analysis of U-shaped tube recuperator and OPT recuperator


TUBULAR U –
SHAPED RECUPERATOR

OPT
RECUPERATOR
Inlet flue gas temperature, °С
1 000
1 000
Outlet flue gas temperature, °С
845
834
Flue gas flow rate, Nm3/hour
14 500
14 500
Inlet air temperature, °С
0
0
Outlet air temperature, °С
400
430
Air flow rate, Nm3/hour
7 000
7 000
Approximate dimensions, mm*mm*mm
1750x
4513x
1115
1680x
840x
1000
Approximate volume of construction, m3
8,8
1,4
Approximate weight of tubing, kg
3 940
1 080
Number of flue gas passes
1
1
Number of air passes
2
2
Air pressure drop, mm WG
117
164
Flue gas pressure drop, mm WG
5
4
Heat transfer surface area, m2
75
78
Heat flow, kW
1 032
1 090

The technical parameters for the existing structures of metal recuperatorsin terms of the source and design data are taken in the following information and technical sources:

1 “Recuperators for industrial furnaces”. Author: Tebenkov B.P., Metallurgy Publishing House, Moscow, 1975

2 “Modern heating and thermal furnaces” Authors: Gusovsky VL, Ladyhechev MG, Usachev AB, Mashinostroenie, Moscow, 2001

3 “Design Book of Rolling Production Furnaces”. Authors: Tymchak V.M. Gusovsky V.L. other. Metallurgy Publishing House, Moscow. 1969

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Comparative analysis of straight tube recuperator (with two tube plates) and OPT recuperator


STRAIGHT TUBE RECUPERATOR

OPT RECUPERATOR
Inlet flue gas temperature, °С
1 100
1 100
Outlet flue gas temperature, °С
690
787
Flue gas flow rate, Nm3/hour
5 300
5 300
Inlet air temperature, °С
20
20
Outlet air temperature, °С
500
500
Air flow rate, Nm3/hour
4 200
4 200
Approximate dimensions, mm*mm*mm
1100x
1620x
2200
848x
840x
570
Approximate volume, m3
3,92
0,4
Approximate weight of tubing, kg
2 700
630
Number of flue gas passes
1
1
Number of air passes
4
2
Air pressure drop , mm WG
231
289
Flue gas pressure drop, mm WG
21,4
15
Heat transfer surface area, m2
56,2
36
Heat flow, kW
756
755
Analysis results:
  1. Dimensions of the four pass tube recuperator is almost 10 times greater than OPT recuperator.
  2. Mass of the tube recuperator with two tube plates is 4 times greater than OPT recuperator. The tube recuperator is supplied with a compensator on hot side to compensate temperature expansions of the structure having height of 2200 mm.
  3. The compactness factor of the OPT recuperator is 13 m2 / m3 = 36 / 0.4 = 90, while for a tube recuperator is 13 m2 / m3 = 56.2 / 3.92 = 14, i.e. OPT recuperator is 6 times more compact than a tube type recuperator.
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The technical parameters for the existing structures of metal recuperatorsin terms of the source and design data are taken in the following information and technical sources:

1 “Recuperators for industrial furnaces”. Author: Tebenkov B.P., Metallurgy Publishing House, Moscow, 1975

2 “Modern heating and thermal furnaces” Authors: Gusovsky VL, Ladyhechev MG, Usachev AB, Mashinostroenie, Moscow, 2001

3 “Design Book of Rolling Production Furnaces”. Authors: Tymchak V.M. Gusovsky V.L. other. Metallurgy Publishing House, Moscow. 1969

Comparative analysis of tube type radiation heat exchanger and OPT heat exchanger


TUBE TYPE RADIATION RECUPERATOR


OPT RECUPERATOR

Inlet flue gas temperature, °С
1 200
1 200

Outlet flue gas temperature, °С
916
962

Flue gas flow rate, Nm3/hour
2 200
2 200

Inlet air temperature, °С
20
20

Outlet air temperature, °С
350
352

Air flow rate, Nm3/hour
2 000
2 000

Approximate dimensions, mm*mm*mm
960x
960x
1800
420x
840x
440

Approximate volume, m3
1,2
0,16

Approximate weight of tubing, kg
270
228

Number of flue gas passes
1
1

Number of air passes
1
1

Air pressure drop , mm WG
drag
drag

Flue gas pressure drop, mm WG
1

Heat transfer surface area, m2
7,3
13

Heat flow, Kw
243,5
244

Analysis results:
  1. Dimensions of the tube recuperator exceed dimensions of the OPT recuperator by 7.5 times. The compactness factor of the tube recuperator is 6, while the compactness factor for OPT is 81, i.e. OPT is 13 times more compact.

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The technical parameters for the existing structures of metal recuperatorsin terms of the source and design data are taken in the following information and technical sources:

1 “Recuperators for industrial furnaces”. Author: Tebenkov B.P., Metallurgy Publishing House, Moscow, 1975

2 “Modern heating and thermal furnaces” Authors: Gusovsky VL, Ladyhechev MG, Usachev AB, Mashinostroenie, Moscow, 2001

3 “Design Book of Rolling Production Furnaces”. Authors: Tymchak V.M. Gusovsky V.L. other. Metallurgy Publishing House, Moscow. 1969

Comparative analysis of a pin fin recuperator of 800 mm single pin fin tubes and OPT recuperator

table_1
PIN FIN RECUPERATOR

table_2
OPT RECUPERATOR

Inlet flue gas temperature, °С
750
750

Outlet flue gas temperature, °С
470
477

Flue gas flow rate, Nm3/hour
1 330
1 330

Inlet air temperature, °С
20
20

Outlet air temperature, °С
320
332

Air flow rate, Nm3/hour
1 340
1 340

Approximate dimensions, mm*mm*mm
3200x
880x
510
848x
840x
310

Approximate volume, m3
1,44
0,22

Approximate weight of tubing, kg
3 740
1 470

Number of flue gas passes
1
1

Number of air passes
4
2

Air pressure drop , mm WG
92
91

Flue gas pressure drop, mm WG
2
1

Heat transfer surface area, m2
9
19

Heat flow, kW
147,5
154

Analysis results:
  1. Weight and size parameters of the pin fin recuperator, especially weight, are minimum 3.5 times greater than mass of the OPT recuperato.
  2. The compactness factor of the pin fin recuperator is equal to 6.25 m2 / m3 and in the OPT recuperator the factor is equal to 95 m2 / m3, i.e. the OPT recuperator is approximately 19 times more compact than the pin fin recuperator.
  3. The pin fin recuperators have significant disadvantage in terms of gas tightness of the whole structure. Low gas tightness significantly decreases recuperator efficiency as a result of leakage of air into flue gas stream. In this example the pin fin recuperators has a 4-pass air flow arrangement and the chance of overflow is very high. According to technical literature (1) the degree of overflow can reach 15%.

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The technical parameters for the existing structures of metal recuperators in terms of the source and design data are taken in the following information and technical sources:

1 “Recuperators for industrial furnaces”. Author: Tebenkov B.P., Metallurgy Publishing House, Moscow, 1975

2 “Modern heating and thermal furnaces” Authors: Gusovsky VL, Ladyhechev MG, Usachev AB, Mashinostroenie, Moscow, 2001

3 “Design Book of Rolling Production Furnaces”. Authors: Tymchak V.M. Gusovsky V.L. other. Metallurgy Publishing House, Moscow. 1969

Comparative analysis of termoblock type recuperator (cast iron recuperator) and OPT recuperator


TERMOBLOCK TYPE RECUPERATOR


OPT RECUPERATOR

Inlet flue gas temperature, °С
1 000
1 000

Outlet flue gas temperature, °С
785
808

Flue gas flow rate, Nm3/hour
220
220

Inlet air temperature, °С
20
20

Outlet air temperature, °С
215
215

Air flow rate, Nm3/hour
268
268

Approximate dimensions, mm*mm*mm
400x
600x
400
210x
210x
230

Approximate volume, m3
0,1
0,01

Approximate weight of tubing, kg
600
17

Number of flue gas passes
1
1

Number of air passes
1
2

Air pressure drop , mm WG
44
43

Flue gas pressure drop, mm WG
0,55
1

Heat transfer surface area, m2
1,92
1

Heat flow, kW
19
19

Installation of OPT recuperator requires a flue gas exhaust pipe (a mini stack) after the recuperator to provide a pressure head and create stack affect. A stack 2.5 meters high will provide a level of stack effect equal to approximately 2 mm. water gauge, which will be sufficient to overcome resistance of the flue gases along the way to the OPT recuperator.

Analysis results:
  1. Weight and size parameters of the “termoblock” recuperator are at least 35 times greater than the mass of the OPT recuperator.
  2. The compactness factor for the “thermoblock” is 11 m2 / m3 and for the OPT 100 m2 / m3, i.e. the OPT is approximately 9 times more compact than the “thermoblock” recuperator. It is also necessary to note, that in modern technical literature (2) the design of “termoblock” recuperators is not considered at all, since this type of recuperators is not used in new industrial furnaces due to low thermal efficiency, complex manufacturing using casting technology, especially for “termoblock” recuperators of large dimensions.

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The technical parameters for the existing structures of metal recuperators in terms of the source and design data are taken in the following information and technical sources:

1 “Recuperators for industrial furnaces”. Author: Tebenkov B.P., Metallurgy Publishing House, Moscow, 1975

2 “Modern heating and thermal furnaces” Authors: Gusovsky VL, Ladyhechev MG, Usachev AB, Mashinostroenie, Moscow, 2001

3 “Design Book of Rolling Production Furnaces”. Authors: Tymchak V.M. Gusovsky V.L. other. Metallurgy Publishing House, Moscow. 1969