FI71066C - RISING FILM LTV-AVDUNSTARE MED FOERBAETTRAD VAERMEOEVERFOERINGSMEKANISM - Google Patents

Description

1 71066

Rising film LTV evaporator with improved heat transfer mechanism

The invention relates to the concentration of various solutions in the process industry. In this case, the most commonly used evaporator structure is the so-called "rising film - LTV". This means a heat transfer surface formed of long vertical tubes. In these tubes, the liquid to be evaporated flows upwards from the liquid by the vapor evaporating. In such a generally known evaporator structure, the liquid to be evaporated is led to a liquid space from which the liquid rises to heat transfer pipes, outside which heating steam is conducted. As it rises into the tubes, the solution heats up and begins to boil. The steam formed raises the solution 15 with it, from which more steam evaporates, so that the amount of steam and the flow rate in the pipes increase. Most preferably, the heat transfer occurs when the velocity is such that the solution flows as a thin film along the wall of the tube and steam flows in the center of the tube. The evaporated vapor and the concentrated solution are separated in a separator.

This structure is commonly used because it is simple, the heat transfer is efficient due to the fast flow and no electrical power is required to flow the liquid over the heating surface as in other types of evaporators. 25 These evaporators are generally used in multi-stage series, in which the steam evaporated in the previous units acts as heating steam in the next unit operating at a lower pressure. Black-liquor evaporators in the pulp industry used to be typically 5-speed, but have recently been converted to 6-phase as energy prices have risen. Similarly, new evaporators to be built are usually 6-phase. When the evaporation capacity of a 5-stage evaporator is generally in the order of 17-20 kg / m h, there is about 12-14 2 35 kg / m h in a 6-stage evaporator, which is due to the reduction of the available 2 71066 temperature difference per unit. Thus, as the amount of evaporation decreases in a single heat transfer tube, the rate of volatile vapor in the tube decreases correspondingly, and in several cases the operating limit of the "rising film" mechanism 5 is already approached. This can be seen in the unstable operation of the evaporator when the 6-stage evaporator is operated at low capacity. The steam flow is not enough to lift the solution, which impairs heat transfer and boils coughing and unevenly in different tubes.

This problem can be eliminated by means of heat transfer tubes with a reduced flow cross-sectional area relative to the heat surface. The invention is characterized in that the heat transfer tubes, with the exception of their attachment points, are formed by inwardly directed, longitudinal corrugations of the tube. In this way, pipes of the same size (e.g. 51 mm in diameter, 8.5 m in length) can be shaped into a cross-flow surface suitable for different units for the operation of the "rising-film" mechanism. The installation of the pipes 20 can be carried out as for unshaped pipes, because the diameter of the shaped pipe does not exceed the original diameter of the pipe at any point.

When forming pipes, their wall material can also be stretched and thus increase the heat transfer surface by up to tens of percent. This allows the evaporator unit to accommodate more heat surface than unmodified tubes. In this way, it is also possible to not only improve the operation of the evaporator but also to increase the evaporation capacity by changing the pipes of the existing units.

An improved rising film LTV evaporator according to the invention is shown in Figure 1. The liquid to be evaporated 102 is introduced into a liquid space from which the liquid rises to heat transfer tubes 101, outside which heating steam 103 is passed.

3,71066

The heat transfer tubes used according to the invention can also be well applied to other types of tube heat exchangers. Figure 2 shows a liquid / liquid-heat exchanger in which the liquid flowing in the jacket is controlled by intermediate plates. In the improved structure shown in the figure, the flow cross-sectional area of the tubes has been reduced and the thermal surface has been increased by using tubes corrugated in the part between the spacers, whereby the points of the spacers are unmodified.