Boiler Plant Including Two Rotary Regenerative Air Preheaters

Abstract

In a boiler plant including two rotary regenerative air preheaters connected in parallel in conduits branched off from main air and gas ducts are provided flow distributing damper means controllable such as to keep the discharge temperatures of the gas flows from the preheater at the same value.

Description

This invention relates to boiler plants which include two rotary regenerative air preheaters connected in parallel in conduits branched off from main air and gas ducts.

Even though the two preheaters are of identical general construction they almost never have the same characteristics. For instance, they may present different resistances to flow and different tendencies to accumulate soot and ash. Further, the flow patterns in the ducts and branch conduits may also be different. All these differences result in different heat transfer conditions so that the temperature drop of the gases is not the same in the two preheaters.

Of course it is desireable to utilize the heat contents of the gases to the greatest possible extent. However, for certain reasons the gas temperature at the outlet from the preheater must not lie below a predetermined minimum value which may be different for different plants. On the other hand, the discharge temperature of the gas must not be much higher than this minimum temperature but lie at or closely above said temperature. It is evident that if the discharge temperatures of the gas flows from the two preheaters are different the heat contents of the gas flow of the higher temperature will not be sufficiently utilized.

According to the invention there is provided an adjustable damper means for controlling the distribution of the main flow of one of the fluids between the two preheaters. Such damper means makes it possible to keep the discharge temperatures of the gas flows from the preheaters at the same value under all operating conditions irrespective of differences in the characteristics of the preheaters, flow patterns etc. The magnitude of this value may be controlled in any suitable manner.

If the damper means is provided in the air system the control of the temperature level may be accomplished by by-passing a controllable portion of the total air flow. However, preferrably the damper means is provided in the gas system while each preheater is associated with means for by-passing at least part of the air flow passing through its air branch conduit.

The invention will now be described more in detail with reference to the accompanying drawing which diagrammatically illustrates an embodiment of a preheater section of a boiler plant according to the invention.

The preheater section shown in the drawing includes two air preheaters 10 of the well-known Ljungstrom type and of identical design each comprising a rotatable matrix and stationary air and gas conduit connections.

Flue gases from the boiler are supplied to the preheater section through a main supply duct 12 which divides into two branch conduits 14, one for each preheater 10. After having passed through the preheaters 10 the gases are discharged through conduits 16 which merge into a main discharge duct 18.

Air is supplied through two supply ducts 20, one for each preheater 10, and is discharged through discharge ducts 22. For each preheater there is provided a by-pass conduit 24 interconnecting the air supply and discharge ducts 20 and 22. At the branch point on each supply duct 20 is provided a damper 26 by means of which the air quantity by-passed through the by-pass conduit 24 can be controlled.

In the gas supply duct 12 is provided a damper 28 by means of which the distribution of the main gas flow between the two preheaters can be controlled. The damper 28 is actuated by an adjusting device 30 governed by temperature sensing devices 32 located in the gas discharge conduits 16. The signals from the sensing devices 32 bring the adjusting device 30 to move the damper 28 to a position in which the ratio between the gas quantities passing through the branch conduits 14 is such that the gas temperature is the same in both discharge conduits 16.

At a point in the gas discharge duct 18 where the two gas flows from the discharge conduits 16 can be regarded as thoroughly intermingled there is located a further temperature sensing device 34 which controls an adjusting device 36 connected to the two dampers 26 by means of links 38. The adjusting device 36 is adapted to maintain the gas temperature in the duct 18 at or closely above a predetermined minimum value by throttling the by-pass conduits 24 at increasing temperature in the duct 18 and thereby increasing the air quantity passing through the preheaters and by increasing the by-passed air quantity when the temperature in the duct 18 tends to decrease.

From the foregoing description it is apparent that the gas damper 28 serves to maintain the same temperature in both gas discharge conduits 16 and that the air dampers 26 serve to maintain the temperature in the gas discharge duct 18 at a predetermined constant value. However, on account of the inertia of the whole system certain temperature oscillations are inevitable. If the temperatures in the two gas discharge conduits 16 become different the gas damper 28 is actuated to change the relative magnitudes of the two gas flows accordingly. Such temperature oscillations need not necessarily affect the temperature sensed by the sensing device 34 because the mean temperature may remain constant. On the other hand, if the temperature is the same in both gas discharge conduits 16 but increases or decreases the air dampers 26 are actuated while the gas damper 28 is not actuated. Finally, if the temperatures in the conduits 16 changes differently and in such a manner that the mean temperature also changes all dampers 26, 28 are actuated.

It is evident that the temperature sensing device 34 may be replaced by temperature sensing devices in the gas discharge conduits 16 connected to a simple computer device or the like the output signal of the computer being indicative of the mean temperature.

The air may be supplied to the air ducts 20 by separate fans or the ducts 20 may form branches of a larger duct containing a single fan. The fans may be of variable capacity. The air discharge ducts 20 may lead to different parts of the boiler furnace or they may merge into a single larger duct leading to the air distributing means of the boiler.

The temperature sensing devices and the adjusting devices may be of conventional suitable types and the control system as a whole may be designed according to known principles.

Claims

What we claim is:

1. A boiler plant including two rotary regenerative air preheaters connected in parallel in conduits branched off from a main air duct and a main gas duct, characterized by adjustable damper means positioned in the main gas duct upstream of the preheaters for controlling the distribution of gas flow between the two preheaters.

2. A boiler plant as defined in claim 1 in which temperature sensing means are provided in the gas outlet conduit of each preheater and the damper means is adapted to be actuated by an adjusting device controlled by said temperature sensing means in such manner as to keep the temperatures in the gas outlet conduits at the same value.

3. A boiler plant as defined in any of the preceding claims in which each preheater is associated with means for by-passing at least part of the air flow passing through the air branch conduit.

4. A boiler plant as defined in claim 3 in which the by-pass means include adjustable dampers interconnected in such manner as to change the rate of air flow through the preheaters to the same degree in both preheaters.

5. A boiler plant as defined in claim 4 in which an adjusting device for the dampers is controlled by means responsive to the mean temperature of the gas leaving the preheaters whereby to keep said temperature at a desired level.