The present invention relates to a heat exchanger of the kind which comprises a primary side in which the primary fluid, which may be air or some other gas, is cooled by a secondary fluid, which may also be air or some other gas or a liquid, to an extent which will allow conden¬ sation to take place.

Problems occur on the primary side when the fluid on the primary side of the heat-exchanger contains substances such as sulphur, chlorine, or like substances which together with water form compounds which have a corroding effect on the heat transfer surfaces of the heat-exchanger or on other parts thereof. Such corrosion can quickly impair the efficiency of the heat-exchanger.

This problem has long been known to the art and many solutions to the problem have been proposed, these solutions primarily requiring the heat-exchanger to be constructed from corrosion-resistant materials, and may or may not include rinsing or flushing of the heat transfer surfaces with water or some other appropriate liquid.

In those cases when corrosion is believed likely, the heat-exchanger assembly has been manufactured from a particular type of stainless steel, and a number of such steels have been produced. Although such stainless steel heat-exchangers have been found successful to a given extent, they are less successful in overcoming conden- sation problems occurring with flue gases for instance.

Examples of the steels produced in this regard include SMO 254 ( Avesta) , R 460 (Fagersta) , and a new steel material produced by Sandvik and designated 2304. It is not known whether or not any of these steels will be able to resist corrosive environments over long periods of time, when the gas on the primary side is flue gas and the gas on the secondary side is air and con¬ densation of substances in the flue gas takes place. As is well known, the penetration of corrosive condensate into narrow crevices will give rise to so-called cavita- tional corrosion, which is still more troublesome than the corrosion of free heat-transfer surfaces.

Flue gases always contain larger or smaller amounts of sulphur, which in combination with water (condensation) will finally form sulphuric acid. This compound of sulphur is able to affect the material of the heat transfer surfaces at relatively low concentrations, in a manner to break down said surfaces. It is possible that this attack can be withstood for longer periods when the heat transfer surfaces are made of titanium, but this material is highly expensive and would therefore be excluded in the majority of cases, because of its high cost.

All high-alloyed, so-called stainless materials contain, inter alia, significant quantities of chromium and nickel. These elements are becoming progressively scarce and it can be feared that in the future the price of alloys which contain these elements will drastically increase. Consequently, there is a particular need for other, less expensive technical solutions.

Corrosion which results from, e.g., sulphuric acid in different degrees of concentration is strongly dependent

on temperature and exhibits a pronounced maximum at temperatures between 110-120°C, at which the effect of the acid is ten times greater than a slightly higher or slightly lower temperature. Consequently, this range of temperature should be avoided with respect to heat transfer surfaces or in the case of sensitive materials in general. Problems caused by corrosion can also be reduced, through avoidance of condensation on the heat transfer surfaces, by ensuring that the dew point at which condensation takes place is not reached.

The problems which need to be overcome in heat- exchangers which work with flue gases are essentially those of:

1. reducing or completely avoiding condensation, even at low flue-gas temperatures.

2. reducing or completely avoiding dirt on the heat transfer surfaces. Dirt will quickly lower the heat-exchange yield.

3. lowering the temperature of the exiting gas as much as possible without creating corrosion problems as a result thereof. This has significant bearing on the economony of the system.

These problems can be solved by the present invention, which relates to the heat exchange between a primary fluid, such as air or some other gas, on the primary side of the heat exchanger, and a secondary fluid, such as gas or liquid on the secondary side of said heat- exchangerassembly in at least three heat-exchanger- assembly stages or sections. In the first of these stages, the temperature is lowered to a value in the

region of the corrosion maximum of the gas and this section or stage of the heat-exchanger assembly can be constructed from a less expensive material, such as aluminium or a low-alloy steel. The gas is then cooled rapidly in a space whose defining surfaces are not sensitive to corrosion, whereafter the gas is passed through the final stage of the heat-exchangerassembly, either while flushing with water or cooling with the aid of ambient air introduced into stage two of the heat exchange process, or a combination of both measures. In the latter case, the heat transfer surfaces may comprise a relatively low-alloyed material, whereas in the former case there is used a higher alloyed material.

When cooling water is used in stage two of the heat- exchange process, the water preferably finely divided into the form of small droplets, so as to effect rapid cooling of the flue gas, while in stage three of said process the condensate is diluted in conjunction with flushing or rinsing clean the heat transfer surfaces.

When air is introduced by means of the bypass fan into stage two is cold, the air will have a low moisture content, due to its low temperature. This cold air will mix rapidly with the slightly chilled air arriving from stage one, this air having a relatively high moisture content, and by varying the amount of air introduced via the bypass fan it is possible to adjust or adapt the total moisture content of the gas mixture such as to prevent precipitation of the moisture in the third stage of the heat-exchangerassembly, in other words the mois¬ ture content of the gas does not reach saturation point in the heat-exchanger assembly.

The temperature of the gas leaving stage three of the heat-exchanger assembly can be detected with the aid of a thermostat, so as to enable the temperature of the gas to be maintained above its calculated dew point. The bypass fan can be controlled accordingly, either to increase or to decrease the flow of air generated there¬ by, with the aid of fan-speed regulating means or a throttle function. If condensation takes place, the condensate is drained into a separate vessel provided therefor and equipped with a level sensor which when the level in the vessel rises above a given value, sends a signal to the fan causing the same to increase the air flow, therewith providing an immediate indication of the occurrence of condensation.

Naturally, the air introduced into the system via the bypass fan will cool the gas arriving from stage one, but a large part of this heat loss is recovered, how¬ ever, as a result of the increased flow in stage three.

The invention will now be described in more detail with reference to the accompanying drawing, which illustrates schematically an exemplifying embodiment of the inven¬ tive apparatus.

The heat-exchanger assembly comprises three stages or sections referred to as stage 1, stage 2 and stage 3 respectively, and is equipped with a primary fan 19 and a secondary fan 9. Stage 2 of the heat-exchanger assembly includes a space or chamber 4, to which there is connected a bypass fan 5. Stage 2 also includes a further space or chamber 12, said chambers 4 and 12 being separated from one another by an effectively sealed wall 13.

Flue gas is introduced into a heat exchange device 7 mounted in stage 1 of the heat-exchanger assembly, through a conduit 10 and inlet connection 11 and, subse¬ quent to passing through the heat exchange device 7, passes into the space 4 in stage 2, where, in the case of the illustrated embodiment, the gas is either mixed with air from the fan 5 or with water from a pump 5, and enters stage three of the heat-exchanger assembly in a chilled state. Stage 3 includes a further heat exchange device, referenced 3, through which the gas passes. The gas exits from stage 3 of the heat-exchange assembly through an outlet connection 18, aided by a fan 19, and is passed from said connection 18 to a smoke stack or some like device (not shown) .

As will be seen from the drawing, connected to the third stage on the outlet side thereof is a conduit 1 and a fluid inlet connection 2, through which fluid, such as air, some other form of gas, or liquid is introduced into stage 3 of the heat-exchanger assembly. This fluid passes through the heat exchange device 3 and into the space or chamber 12 of stage 2 , from where said fluid passes through the heat exchange device 7 in stage 1 of the assembly and exits through an outlet connection 8, aided by a fan or pump 9. The thus heated fluid is then passed to the process for which it is intended.

The heat exchange devices 3 and 7 consist of one or more modules having planar or particularly profiled plates, in accordance with Swedish Patent No. 8000118. The throughput capacity and heat exchange efficiency of the latter system is many times greater than the former system with planar heat transfer plates and also pro¬ vides a smaller and more compact heat-exchanger assembly. In one of the spaces formed between the

plates, the primary fluid passes into the adjacent secondary fluid. Thus, in the heat exchange device of stage 3 the channels carrying the primary fluid are connected to the outlet connector 18 and the space 4, whereas in stage 1 of the heat-exchanger assembly the heat exchange device is connected to the inlet connection 11 and also to the space 4. In this way, the channels carrying the secondary fluid are connected to the inlet connection 2 for the medium to be heated, and to the space or chamber 12 and the outlet connection 8.

The air or water delivered by the fan or pump 5 will have the lowest possible temperature. This air or water is passed through a conduit 6 to the space 4 of stage 2 and is mixed in counterflow with the gas arriving from the heat exchange device 7 of stage 1, preferably with the aid of nozzles (not shown). Cooling is controlled either by a thermostat 17 or a level sensor 15, both of which influence the flow generated by the fan or pump 5, through the agency of signal conductors.