To increase the thermal efficiency coefficient and air heating efficiency, it is advisable to use OPT multi-pass recuperators that have one pass of the transfer medium and two or more passes of the heated medium. This allows incresing the thermal efficiency coefficient, or reduces the weight and size characteristics of the heat exchanger while maintaining the same thermal efficiency coefficient.
An example of such a heat exchanger is shown in this picture: one pass along the heat transfer medium (products of natural gas combustion), along with the useof smooth channels in which the heat transfer medium velocity reaches 20-30 m / s, allows for unimpeded passage of flue gas through the heat exchanger without settling on the walls. This is especially true for mediumwith high dust content, glass-melting waste and other polluted medium. In addition, it facilitates cleaning of the channels without the use of special tools and disassemble of the heat exchanger.
The use of several passes allows increasing the temperature of the air without significant increase in size of the heat exchanger.Each next pass is connected with the previous one through a connecting duct, with, if necessary, compensators to compensate linear temperature expansions of the metal in heat exchanger.
The use of several passes allows increasing the temperature of the air without significant increase in size of the heat exchanger.Each next pass is connected with the previous one through a connecting duct, with, if necessary, compensators to compensate linear temperature expansions of the metal in heat exchanger.
In some cases, the output of air ducts should have one or different directions to comply with production or technological requirements, and the number of passes must be even (exit in one direction) or odd (exit in different directions), for example, if it is planned toinstall a heat exchanger in the furnace flue.
It is necessary to take into account that the increase in the number of recuperator passes seriously affects its pressure drop, which significantly increases due to air rotation by 180 degrees, as well as by increasing the length of air passage in the recuperator channels. Aerodynamic resistance is directly related to the pressure drop when passing through the heat exchanger, which determines the choice of fan for air injection. Acceptable values of aerodynamic resistance largely determine the size and weight of the heat exchanger. A twofold increase in the allowable resistance value reduces weight and dimensions of the heat exchanger by 30-35%.
The task of an engineer is to select the optimal design of the heat exchanger, which will provide the required thermal efficiency coefficient, while not exceeding the permissible overall dimensions, as well as the allowable values of aerodynamic resistance both in the gas and in the air ducts.
The table shows an example of calculation of the heat exchanger for different number of passes for the following parameters:
- air inlet temperature is 0 ° С;
- flue gas inlet temperature 750 ° С;
- air flow rate of 4500 Nm3 / h;
- flue gas rate of 5,000 Nm3 / h;
OPT characteristics depending on number of passes
| Number of passes | Dimensions of heat transferring module, mmХmmХmm | Air outlet temperature, С | Flue gas outlet temperature, С | Thermal efficiency coefficient, % | Flue gas pressure drop, mm WG. | Air pressure drop, mm WG. | Mass, kg |
|---|---|---|---|---|---|---|---|
| 1 | 840х840х700 | 255 | 554 | 34 | 15 | 6 | 564 |
| 2 | 840х1680х700 | 389 | 444 | 52 | 16 | 25 | 1128 |
| 3 | 840х2520х700 | 472 | 373 | 63 | 34 | 26 | 1691 |
| 4 | 840х3360х700 | 525 | 325 | 70 | 44 | 38 | 2255 |
| 5 | 840х4200х700 | 565 | 290 | 75 | 53 | 49 | 2819 |
| 6 | 840х5040х700 | 589 | 268 | 78 | 63 | 59 | 3383 |
| 7 | 840х5880х700 | 612 | 247 | 82 | 72 | 71 | 3947 |
| 8 | 840х6720х700 | 629 | 231 | 84 | 81 | 83 | 4510 |
