Efficient heat recovery units

The task of creating efficient and inexpensive heat exchangers, in particular recuperators, is currently relevant throughout the world. Existing designs of heat exchangers have serious drawbacks, which often determine the absence of these devices in technological processes. These disadvantages include:

• poor weight and dimensions;
• high cost and long payback period;
• complex or impossible repair;
• low termoplasticity (i.e., the tendency to the appearance of thermal stresses, distortion and destruction);
• high pressure drop;
• tendency for slagging (overgrowing of working gaps by combustion products).

To solve these problems, a project to create a new generation of plate heat exchangers has been implemented. The basic element of such heat exchangers was the metallurgical semi-finished product, which is sheet ribbed panel (Fig. 1). The panel is a thin wall flat sheet, on the surface of which vertical ribs are longitudinally welded. The geometric dimensions of the structural elements are listed in Table. 1. The ratio of the thickness of the welded elements can take any value within these limits. The outer edge of the rib can also be corrugated to intensify convective heat exchange.

ribbed sheet panel.

Table 1. Geometrical characteristics of a sheet ribbed panel.

Thus, the ribbedpanel is an active compact heat exchanging surface, the parameters of which can be changed over wide limits to obtain optimal characteristics of the heat exchanger. A panel with an area of 1 m2, taking into account the surface of the welded ribs, can have a heat transfer surface of 2-10 m2, which makes it possible to obtain a developed surface in equal volumes, several times exceeding the heat transfer surface of tubular heat exchangers.

All known industrial methods to obtain the ribbed panels of the described design is almost impossible.This fact explains the absence of heat exchangers with equivalent characteristics, especially in the field of high-temperature heat transfer.

At the beginning of research, the high-frequency welding method was chosen among the existing methods, in which the edges of the parts being welded are heated by high-frequency currents. The method widely known in production of pipes, having significant advantages (primarily in terms of productivity), did not allow at that time to weld products having surface-edge joints, which include the welding process of the panels. There was a challenge to implement the process due to the fact that to obtain a welded joint of the “plane-edge” type of the elements listed in table. 1 (small thickness for welding long span welds) requires to solve a number of tasks, including:

• solution of issues related to the electromagnetic and temperature fields of elements welded in the high frequency field;
• solution of deformation problems during welding;
• solutionof the problems of stability of thin wall ribs when they are buckled to form a welded joint, and many others.

The research allowed developing the technology of high-frequency welding of fins with a speed of up to 80 m / min. This ensures stability of the quality of the welded joint over the entire length of the seam, the process does not require a shield atmosphere, filler metals, preliminary cleaning of the welded surfaces. Due to the features of thewelding method, the seam almost does not differ from the base metal, it is of equal strength, plastic, allows bends and local deformation, and is not prone to corrosion (including integranular corrosion).

The developed process makes it possible to manufacture panels from corrosion-resistant (including heat-resistant) steels and alloys with operating temperature of up to 1100-1250: С. It is also possible to manufacture bimetallic welded joints (for example, a sheet of one, and ribs of another steel or alloy) and the production of recuperators respectively, made of a heat-resistant metal on the hot side, and of ordinary stainless or low carbon steel on the cold side. These features can be useful especially in case of the high prices of stainless and especially heat-resistant steels.

High productivity of the process, which is ensured by high-frequency welding, is extremely important in production of new heat exchangers, since per 1 ton of weight up to 2-3 km of welding seams are involved, while the weight of such structures can reach up to 80 tons or more. No other type of welding can provide such performance.

Design features of new heat exchangers.
The new heat exchangers are called ribbed plate heat exchangers (OPT). Their design is quite simple. It can be described as a «layer cake», made in the form of alternating cavities of the heating and heated media. To ensure the required power of the heat exchanger, the certain number of modules is assembled and their arrangement is selected. At the inlet and outlet of the gas paths the diffusers and confusors are installed. If necessary, compensators for thermal expansion are also installed between the modules at the inlet or outlet of the heating path. It should be noted that due to the design of such modules, internal compensation of thermal expansion takes place, therefore OPT-type heat exchangers are oftenallowedto be used without a compensator.

The modular design in combination with a relatively low weight makes it possible to simplify the processes of installation, maintenance and repair of the heat exchanger. Open access to the channels of the gas paths allows inspecting and cleaning of the channels, while relatively small pressure drop guarantees high gas flow rates and self-cleaning speed (over 11 m / s), i.e. the design of the heat exchanger OPT allows avoiding deposits (slagging) in the channels, which have negative effect for the parameters of the heat transfer.

Depending on the direction of media motion, the design of the heat exchanger OPT can be performed with direct flow, counter flow or cross flow motion scheme. It is also possible to use a combination of these options. In each case, certain technological and design results are achieved.

Recuperators with cross flow motion and especially combined cross and counterflow motion are the most technologically advanced and have easy-to-use design.

Designs can have one or several passes in a heated environment. The change in the number of passes strongly influences the parameters of the heat exchanger and its pressure drop (sharply increasing the resistance with increasing number of passes). At the same time, an increase in the allowable pressure drop of two times makes it possible to reduce the weight and dimensions of the heat exchanger by 30-35 %.

Studies have shown that new recuperators due to the small thickness of the elements have additional advantages: low inertia; high Termoplasticity.

Comparison of the technical characteristics of traditional and new heat exchangers shows that having equal and the same performance, the weight and dimensions will be several times higher than those shown by the conventional heat exchaners. For example, below are the comparative characteristics of a real shell-and-tube heat exchanger and an OPT heat exchanger.

Applications

The processes in which heat recuperators are used (such as «gas-gas») can be divided into the following categories:

• with a low temperature of the coolant — 20-200 ° C (for example, for large volumes of gases — ventilation of the premises);
• with an average temperature of the coolant — 200-600 ° C (for example, in case of excess heat of boilers, including the preheated air supplied for combustion);
• with a high temperature of the coolant — 600-1000 ° C (for example, metallurgy, gas and oil refining, chemical production, etc.).

OPT heat exchangers can solve all the above problems being also most effective in high temperature processes.

The table shows the results of the economic effect calculations of use of heat exchangers in the chemical industry (calculations are made for real heat exchangers).

The recuperators developed by Termo Nord Stream can successfully replace existing devices, and also due to their characteristics can be installed at the places where conventional recuperators cannot be installed or impractical due to their technical and economic characteristics. Currently, the OPT heat exchangers are already being used in various industries, which provide significant savings of energy resources and reduced capital costs for implementation of heat recovery systems.

Conclusion

• The defining advantages of OPT recuperators include:
  • low weight and size parameters (2-10 times less than that of conventional recuperators);
  • easy maintenance, transportation, installation and repair:
  • low pressure drop;
  • low inertia;
  • high termoplasticity;
  • easy cleaning and self-cleaning of gas-air paths from combustion products;
  • short payback period (calculated in months);
  • internal compensation for thermal expansions, thanks to which they can often be used without compensators.
• The technical characteristics of OPT heat exchangers may vary to optimize the design over a wide range.
  Determining factors are:
  • speed of gas medium (heated and heated);
  • fin pitch and fin height of the base element of the heat exchanger (fin panel); the direction of gas medium motion;
  • number of air duct passes;
  • layout of the modules.
• Technical and economic efficiency of recuperators is higher, the higher the temperature of the coolant and the more about (remove heat exchange media.
  Table: Calculation of the economic effect of OPT heat exchanger installationin a chemical plant (plastic production; gas is heated using electrical energy).

  Indicators	Value
  Flue gas outlet temperature of the furnace, ° C	90
  The temperature of air taken from the workshop, ° C	30
  Volume of gas, Nm3	3000
  Volume of air, Nm3	3000
  The temperature of the air heated in the heat exchanger, ° С	50
  Heat flow returned to the furnace, kW	20
  The cost of the heat exchanger with VAT (mild steel), thousand rubles	247
  Savings per month on air heating (with two-shift operation), thousand rubles	28,8
  Payback period of the heat exchanger, months	7
  The economic effect of the use of a heat exchanger for a service life of 10 years, ths. Rub.	3456

• The higher the temperature of the coolant, the greater the pitch of the welded fin is. At temperatures of 900-1000 ° С and more, the fining of flue gas side becomes ineffective.
• The dimensions and weight of the heat exchanger OPT are most affected:
  • by volumes and temperatures of heat exchange media (volume and temperature ratios);
  • by admissible speeds of heat transfer medium and admissible pressure drop.

Ph.D. V.E. Zlotin