GB2090959A

GB2090959A - Air Preheater

Info

Publication number: GB2090959A
Application number: GB8200294A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-01-12
Filing date: 1982-01-06
Publication date: 1982-07-21
Also published as: IT8119092A0; IT1210975B

Abstract

The delivery duct (2) of a blower (1) communicates with heat exchangers (10, 20, 30) located in series relationship within a stack conveying flue gases arriving e.g. from a boiler. Air is introduced into the blower from pivot and second ducts (40, 41), the second duct introducing ambient air preheated in an inner tube of heat exchanger (30). The amount of air entering the blower from the two ducts is controlled by distributing valve means (51) driven by a temperature sensor located in the coolest zone of the stack. The air passes through the heat exchangers (10, 20, 30) and exits through outlet duct (60) to the utilising apparatus. Further valve means (62) controls the amount of air passing to the outlet duct to be mixed with ambient air preheated in the inner tube of heat exchanger (30). <IMAGE>

Description

SPECIFICATION Air Preheater Background of the Invention The present invention relates to an air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimneys and the like.

A presently much felt problem is that of recovering heat from the discharge flue gases or smokes from combustion processes in gerieral. In fact it is well known that the discharge flue gases contain a great amounts of useful power, in the form of heat, which, in the practice, is lost by discharging.

In order to recover heat from the discharge flue gases several systems have already been designed which are generally formed by a plurality of heat exchangers, of any suitable shapes and size. which are contacted by the discharge flue gases or smoke.

This approach, however, is not completely satisfactory since it involves great draft problems in the stack and, moreover, it may cause an excessive lowering of the flue gas temperature, which, in turn, originates highly corrosive condensates susceptible to seriously damage the systems.

Yet another drawback of the known systems is that they are not able of controlling and adjusting the thermal exchange process, thereby the efficiency of the overall system is a poor one.

Summary of the Invention Accordingly the present invention provides an air preheater with recovery of heat from the discharge flue gases or smokes in stacks and the like, which, in addition to providing very good thermal exchange conditions, is also effective to control the thermal exchange process peformance, in such a way as to prevent corrosions from occurring, as due to the so-called "Dew-point" in the coldest zone of the preheater.

Within the scope of the above task it is a main object of the present invention to provide such an air preheater which is ofsimple structure and effective to be quickly and easily installed.

Yet another object of the present invention is to provide such a preheater which, in addition to having high performance characteristics, is also effective to give high reliability and use safety characteristics.

Yet another object of the present invention is to provide such an air preheater which can be easily constructed starting from easily available materials and elements and which, furthermore, is very flexible in operation and of reduced cost maintenance.

The task and objects thereinabove mentioned, as well as yet other objects which will become more apparent hereinafter, are achieved by an air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimneys and the like, characterized in that it comprises an exhauster the delivery duct thereof is coupled to a plurality of heat exchangers, located in succession in a discharge smoke conveying path, the air inlet to said exhauster consisting of a first outside-air receiving duct and a second duct effective to receive preheated air from the air preheated in said heat exchangers, as controlled by distributing valve means which latter are driven by a temperature sensor included in the coldest zone of said stack therein said plurality of heat exchangers are located.

Brief Description of the Drawings Further characteristics and advantages of the invention will become more apparent from the following detailed description of a preferred, though not exclusive, embodiment of an air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimneys and the like, being illustrated by way of an indicative and not limitative example in the accompanying drawings, where: Fig. 1 is a schematic view illustrating a possible embodiment of the subject air preheater; Fig. 2 is a schematic view illustrating a heat exchanger with the counter-current air passage; Fig. 3 illustrates a heat exchanger with a cocurrent type of air passage; Fig. 4 illustrates a heat exchanger provided with a central core; and Fig. 5 is a cross-section view, as taken along the line V-V of Fig. 4, which practically corresponds to the cross-sections of the heat exchangers illustrated in Figs. 2 and 3.

Description of the Preferred Embodiments With reference to the number references of the figures of the accompanying drawings, the air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimneys and the like, comprises an exhauster 1, the delivery duct 2 thereof communicates with a plurality of heat exchangers, located in succession Or series relationship in the inside of the stack conveying the flue gases arriving, for example, from a boiler or the like.

The arrangement and shape of the heat exchangers may be any suitable ones, depending on the needs.

With reference to Fig. 1 and only by way of an indicative example, the plurality of heat exchangers comprises a first heat exchanger 10, which is better illustrated in Fig. 2, which is provided with an inner pipe or duct coaxially extending with respect to a main pipe 12, which latter is contacted on the outside thereof by the flue gases or smokes.

More specifically, the main pipe 12 is provided, on the inside face thereof, with longitudinally extending fins 13, whereas, on the outer surface thereof, it is provided with helically extending fins 14.

This design of the pipe 12 presents the advantage of conveying air in the inside of the pipe on the exchange surface consisting of said main pipe 52, thereby improving the heat exchange process.

Moreover the provision of the helically extending fins 14 practically improves or enhance the heat amount which may be conveyed from the main pipe 12 to the inside.

Another feature is the fact that the main pipe 12 is closed. at one end thereof, in such a way that the air entering the inner pipe 11 has, in the practice, to reverse its flow, thereby exiting again from the inlet portion.

The air flow entering the inner pipe 11 is previously preheated by the air which, with an opposite flow direction, contacts it during its passage through the main pipe 12.

The air exiting the first heat exchanger 10 is introduced into a second heat exchanger 20, which is illustrated in detail in Fig. 4, which is provided with a main outer pipe 21, which latter is structurany equal to said main pipe 12.

In the inside of the main outer pipe and, coaxially extending therewith, there is provided an inner closed pipe 22, which acts as a core, and effective to allow for air to contact the longitudinally extending fins 23 as provided on the inner surface of the outer main pipe 21, thereby exploiting in a very satisfactory degree the exchange surface of said longitudinally extending fins 23 and of a helically extending finnings 24 provided on the pipe and corresponding to the thereinabove mentioned fins or finning 14.

Downstream of the second heat exchanger 20 there is provided a third heat exchanger 30, therein there is conveyed the air exiting said second heat exchanger 20; this latter is provided with an outer helically extending finning 32 and an inner longitudinally extending finning 33, corresponding to the hereinabove mentioned finnings.

In the inside of the outside pipe 31 there is provided an inner throughgoing pipe 34, open at both ends thereof, and performing two functions, which will be described in a more detailed way thereinafter.

The air is introduced into the exhauster 1 by means of a first outside-air receiving duct 40 and a second duct 41, effective to pick up the air preheated by the preheated air in said heat exchangers.

The second duct 41 communicates with the inside throughgoing pipe 34, in such a way that it is able of conveying air to the exhauster, which air is preheated by the air previously heated by the device according to the present invention.

This aspect is a very important one, since, on one hand, the air entering the inner pipes, contained by the main pipes, as it occurs with respect to the throughgoing pipe 34, acts as a silencing medium for the overall apparatus.

The second aspect, also of great importance, consists of the fact, that by means of the thereinabove described device, it is possible to preheat the outside air to the environment temperature, by using air already preheated by the preheater device itself.

This approach permits to reduce the dangerous effect of the corrosion due to the so-called "dew point", since the critical temperature point is never reached thereunder the corrosive condensate formation occurs.

In order to carry out this controlling operation, there are provided distributing valve means 50 which operate at the meeting point of the first duct 40 and the second duct 41, and which valve means are driven or controlled by a temperature sensor 51, located in the coldest zone of the stack, therein said plurality of heat exchangers are located.

In the practice, said valve means are effective to permit only environment air to enter the exhauster, which air arrives from the duct 40, or the preheated environment air arriving from the second duct 41, or, possibly, a mixture thereof, in such a way as to reach optimum temperature values, effective to provide a highly efficient thermal exchange.

This aspect is the core of the invention, since the new concept is introduced of supplying the preheater with air which is preheated by the same air arriving from said preheater.

This result is achieved due to the provision of the throughgoing inner pipe 34 which, in the practice, carries out a co-current thermal exchange with the preheated air circulating between said pipe 34 and the outer pipe 31.

The preheated air, passed through the heat exchangers, is introduced into an outlet duct 60, coupled to the using apparatus.

Moreover, in order to enhance the flexibility use of the preheater, there is provided a communication port 61, controlled by shut off valve means 62, effective to provide for, if it is necessary, a communication between the outlet duct 60 and the second duct 41, in such a way as to introduce the environment air, arriving from the inner throughgoing pipe or duct 34, directly into the-outlet duct 61 in order to mix the preheated air and obtain the desired thermal value.

The operation of the preheater according to the present invention is a greatly simple one, as one may deduce from the above description.

In fact, the air introduced by the exhauster 1 into the delivery duct 2 passes, in succession, through the heat exchangers 10, 20 and 30, by carring out the desired thermal exchange.

Then it is introduced into the outlet duct 60, which is coupled to the intended using apparatus.

In the case in which critical points are detected, the distributing valve means 50 operate which by shutting off, for example, the air inlet directly from the first duct (41), introduce into the exhauster 1 air, as previously preheated by the preheater in such a way as to prevent temperature critical points to occur in the coldest zone of the preheated.

This fact, as thereinabove mentioned, reduces the deleterious effect of the corrosion due to the dew point, since it immediately carries out a temperature increasing due to the fact that the preheater according to the present invention is used.

This possibility of introducing preheated air from the same air preheated by the preheater can be achieved owing to the provision of a heat exchanger provided with a throughgoing inner pipe communicating, at one end, with the environment air and, at the other end thereof, with the inlet of the exhauster.

Said inner throughgoing pipe 34 is located in the inside of an outer pipe, suitably finned, which is contacted by the discharge flue gases.

This embodiment also affords the advantage of acting as a silencing medium for the overall apparatus.

The invention, as described, is susceptible to several modifications and variations, all falling within the scope of the inventive idea.

Thus, for example, the heat exchangers can be realized in a different way or located with a different series arrangement.

Experimental tests have revealed that optimum results could be obtained by locating in series arrangement two heat exchangers, like the heat exchanger 20, conveying into a third heat exchanger, of the type of the exchanger 30.

Likewise, optimum results have been obtained by designing the third heat exchanger in such a way as to directly introduce into the inner throughgoing pipe 34 the air preheated during the passage through the preheater and as optionally mixed with outside air.

Obviously it is also possible to carry out several other variations, without departing from the scope of the invention concept, based mainly on the idea of using, for supplying the exhauster, the air already preheated by the preheated air of the preheater.

It should also be noted that all of the constructional details can be changed, without departing from the scope of the invention.

Moreover the used materials, though they are compatible to the intended use, as well as the contingent shapes and sizes can be any, depending on the needs.

Claims

1. An air preheater comprising an exhauster whose output is coupled to a plurality of heat exchangers located in a hot gas discharge path and whose inlet comprises a first duct for induction of atmospheric air and a second duct coupled through valve means for receiving preheated air from said heat exchangers; said valve means being controllable by a temperature sensor locatable in a relatively cool zone of said path.

2. An air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimmneys and the like, according to the preceding claim, characterised in that at least one of said heat exchangers is provided with an inner pipe coaxially extending with respect to a main pipe, and provided with longitudinally extending inner fins and helically extending outer fins, said main pipe being located in said stack and being closed at one end thereof in such a way as to reverse the flow of air entering said heat exchangers, to cause said air to enter said heat exchanger and exit therefrom at the same side thereof.

3. An air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimneys and the like, according to the preceding claims, characterized in that at least one of said heat exchangers is provided with an inner pipe, closed at the ends thereof, and coaxially extending in the inside of a main pipe, provided with an inner longitudinally extending finning and a helically extending outer finning.

4. An air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimneys and the like, according to the preceding claims, characterized in that at least one of said heat exchangers is provided with a throughgoing inner pipe, open at both ends thereof, and coaxially arranged in the inside of an outer pipe, provided with an inner longitudinally extending finning and an outer helically extending finning.

5. An air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimneys and the like, according to one or more of the preceding claims, characterized in that said throughgoing inner pipe is effective to preheat air introduced into said exhauster by means of preheated air from said preheater.

6. An air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimneys and the like, according to one or more of the preceding claims, characterized in that the air introduced through said inner pipe is effective to act as silencing medium for said preheater.

7. An air preheater with recovery of heat from the discharge flue gases or smokes in stacks chimneys and the like, according to one or more of the preceding claims, characterized in that said distributing valve means are effective to introduce into said exhauster only the air arriving from said first duct and/or only the air arriving from the second duct, and/or a mixture of air arriving from said first and second ducts.

8. An air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimneys and the like, according to one or more of the preceding claims, characterized in that it comprises a passage port between said second duct and the outlet duct of said preheater, said passage port being controlled by closure or shut off valve means and being effective to allow for preheated air to be mixed with air arriving from said throughgoing inner pipe, in order to introduce into the user device the preheated air from said preheater.

9. An air preheater with recovery of heat from the discharge flue gases or smokes in stacks, chimneys and the like, according to one or more of the preceding claims, characterized in that said preheated air exhauster is supplied-with the preheated air from said preheater.

10. An air preheater substantially as hereinbefore described with reference to the accompanying drawings.