JPH07260397A - Heat exchange fan duct for cooling tower and the like - Google Patents

Abstract

PURPOSE:To provide a fan duct having a longer service life, in which a fan is kept from causing duct resonance by designing the duct in such a manner that its natural frequency is different from the frequency of vibrations produced by the fan. CONSTITUTION:A fan duct 3 is arranged on an upper face of a fan deck plate 6, so as to envelope a fan 5 set at an upper section of a heat exchanger 4. In this case, a hollow state enforcement rib as an accommodating section 2 is set with projection with a predetermined space toward longitudinal and lateral direction so as to be in mutual communication. By the way, if a fan 5 is made to rotate with a predetermined rotation number, an agitation frequency generated by a wind pressure coincides with a dominant natural frequency of a fan duct 3, there is a case wherein a resonance phenomenon is generated. In this case, a filler material of sand and the like is inserted and mounted in an inside section of an accommodating section 2. By this, a weight of the fan duct body 1 is increased, and this natural frequency is made to execute a fluctuation. Accordingly, a generation of a resonance phenomenon can be evaded.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a fan duct mainly applied to a heat exchange tower such as a relatively large cooling tower.

[0002]

2. Description of the Related Art Conventionally, as a fan duct for a heat exchange tower of this type, for example, there is a fan duct main body formed by FRP into a cylindrical shape or a divergent cylindrical shape. Such a fan duct can be rotationally driven. It is installed above the heat exchange tower such as a cooling tower so that the fan is installed inside.

[0003]

By the way, when the fan rotates, the fan duct receives the wind pressure.
In this case, the excitation frequency ν by the fan and the predominant natural frequency
When f '(peak value of natural frequency f) matches, a resonance phenomenon occurs, that is, a phenomenon in which the amplitude of the fan duct is amplified to about several times.

When the amplitude of the fan duct is amplified in this way, noise is generated and not only the deterioration of peripheral members such as the fan duct and the connecting fittings is promoted, but also the fan comes into contact with the inner peripheral surface of the fan duct. There is also a possibility that the fan duct may be damaged and the durability life of the fan duct is significantly reduced.

Therefore, the present invention has been made in order to solve the above-mentioned problems of the prior art, and prevents the occurrence of a resonance phenomenon by the fan and suppresses the vibration of the fan duct, thereby shortening the durable life thereof. The challenge is to reduce

[0006]

That is, in order to solve the above problems, the present invention is a fan duct for a heat exchange tower such as a cooling tower in which a fan 5 is installed.
Is a fan duct for a heat exchange tower such as a cooling tower, which is set so as not to match the vibration frequency ν of the fan 5.

Further, the natural frequency f may be changed so that the predominant natural frequency f'of the fan duct can be changed at any time. As a means for changing the natural frequency f, for example, the accommodating portion 2 is provided in the fan duct main body 1. There is a means for providing the filling 7 with the filling 7 or attaching the weight 8 which is not continuous in the circumferential direction to the fan duct main body 1.

[0008]

The fan duct having the above-mentioned structure is installed above the heat exchange tower so that the fan 5 for discharging the air inside the heat exchange tower to the outside is installed.
Since the excitation frequency ν generated by the rotation of the fan 5 and the predominant natural frequency f ′ of the fan duct are set so as not to match, no resonance phenomenon occurs.

When the natural frequency f of the fan duct is changed so that the natural frequency f'of the fan duct can be changed at any time, the natural frequency f can be changed as appropriate to obtain the dominant natural frequency f '. If the vibration frequency and the vibration frequency ν do not match, the occurrence of the resonance phenomenon can be avoided.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a fan duct according to the present invention will be described below with reference to the drawings.

In FIG. 1, reference numeral 1 denotes a divergent cylindrical fan duct body made of FRP, and the outer peripheral surface thereof is shown in FIG.
As shown in FIG. 5, hollow reinforcing ribs as the storage portion 2 are provided so as to project at predetermined intervals in the vertical and circumferential directions, and the reinforcing ribs communicate with each other. An opening 2a is provided at the upper end of the flange surface 1b of the fan duct body 1 so as to communicate with the reinforcing ribs arranged in the vertical direction, and the lower end of the reinforcing rib is closed. The inner diameter of the fan duct body 1 is 4355 mm and the height is 2050 mm.

The fan duct 3 having such a structure is provided with a fan 5 which can be rotationally driven and which is disposed above a heat exchange tower 4 such as a cooling tower as shown in FIG. 3, for example.
Installed on top.

First, when the fan 5 is started and rotated at the required number of revolutions, the vibration frequency ν generated by the wind pressure at that time coincides with the predominant natural frequency f ′ of the fan duct 3 and a resonance phenomenon occurs. It may occur.

In this case, as shown in FIG. 2, sand as the filler 7 is charged from the desired opening 2a of the fan duct body 1 to fill the reinforcing ribs. As a result, the fan duct 3
As a result, the natural frequency f fluctuates.

That is, generally, the natural frequency in the case of a vibration system having one degree of freedom is represented by the general formula f = (k / m) ^1/2 / 2π. Here, k is a spring constant and m is a mass. However, since a complex structure such as the fan duct 3 is a vibration system having multiple degrees of freedom, a general formula expressing its natural frequency f is not clear. However, since the multi-vibration system is still a function of the spring constant k and the mass m like the one-degree-of-freedom vibration system, the spring constant k does not increase due to sand filling, and only the mass m increases. As a result, the natural frequency f fluctuates.

Therefore, the predominant natural frequency of the fan duct 3
Since f ′ and the vibration frequency ν of the fan 5 do not match, the occurrence of the resonance phenomenon is avoided and the vibration of the fan duct 3 is reduced.

However, since the fan duct 3 in the present embodiment is a three-dimensional structure having a complicated shape as shown in FIG. 1, its predominant natural frequency f'is not one, but there are a plurality of it. It is very difficult to determine the amount of sand filled by. Therefore, even if it is possible to shift one dominant natural frequency f ′ by filling sand once, depending on the amount of sand filled, another dominant natural frequency f ′ of the fan duct 3 and the vibration frequency of the fan 5 In some cases, ν may coincide with each other, but in this case, the occurrence of the resonance phenomenon may be avoided by appropriately changing the sand filling amount and the sand filling place again.

When the vibration of the fan duct 3 is reduced, the deterioration of the fan duct 3 and the connecting fittings and the generation of noise are suppressed, and the fan 5 comes into contact with the fan duct 3. Since it can be avoided, it is possible to significantly reduce the deterioration of the durable life.

Further, since the natural frequency f of the fan duct 3 can be easily and arbitrarily changed by filling with sand, the work at the site is not complicated, and in the present embodiment, it is provided in the fan duct body 1. Since the reinforcing rib is configured as the storage portion 2 for the filling 7, the reinforcing rib can be effectively used, and the filling effect of the filling 7 can be improved.

Further, since the fan duct 3 according to this embodiment has a very simple structure, it has an advantage that it can be manufactured easily and at a low cost.

In the above embodiment, the reinforcing ribs provided on the outer peripheral surface of the fan duct 3 are configured as the storage portion 2 in order to change the natural frequency f of the fan duct 3.
Is not limited to this, and may be provided on the inner peripheral surface of the fan duct body 1, for example, regardless of its shape or position. The point is that the storage unit 2 may be configured so that the filling 7 can be filled.

As the filler 7, in addition to the sand of the above embodiment, for example, a granular material such as stones or metal pieces, a powder material, or a fluid such as water may be used. The type of the filling material 7 is not limited as long as the mass m can be increased.

Further, the filling 7 may be gradually filled while observing the vibration of the fan duct 3, but in consideration of the case of overfilling, for example, the housing 2 of the fan duct main body 1 is taken into consideration. A hole through which the filling material 7 can be discharged,
If this is closed with a lid that can be opened and closed, the filling amount of the filling 7 can be adjusted as necessary, which is convenient in terms of work.

As a means for changing the natural frequency f of the fan duct 3, a weight 8 made of a metal plate, a bolt, a metal ball, wood or the like is directly or indirectly attached to the peripheral surface of the fan duct 3 through a string or the like. You may take the means to do. FIG. 4 shows an example of this, in which a metal plate is used as the weight 8, and it is detachably fixed to the uppermost stage of the reinforcing ribs in the circumferential direction by bolting through a through hole provided in the central portion. ing.
In this case, a metal plate may be attached to an appropriate portion to change the natural frequency f of the fan duct 3 to avoid the occurrence of resonance phenomenon. Further, the metal plate used as the weight 8 may be provided discontinuously in the circumferential direction of the fan duct 3,
The mounting position is not limited to the position shown in FIG.

However, if a ring-shaped object which is continuous over the entire circumference of the fan duct 3 is used as the weight 8,
Although the mass m of the fan duct 3 increases, the spring constant k of the fan duct 3 may increase, and as a result, the natural frequency f of the fan duct 3 cannot be changed. Although the object is not suitable as the weight 8, it can be applied to the case where a part of the weight 8 is cut because the increase of the spring constant k is suppressed.

The applicant of the present application decided to carry out an experiment to confirm the effect of using a metal plate as the weight 8. The fan duct body 1 having an inner diameter D of 4355 mm and a height H of 2050 mm is used, the fan 5 is rotated at a desired rotation speed, and a sensor of a vibration analyzer is attached to a point X in FIG. The natural frequency f was measured. In this case, the vibration frequency ν by the fan 5 was 24.75 Hz. The result is shown in FIG. 5 (a). The vertical axis shows the vibration speed of the fan duct 3 per force applied from the fan 5, and the horizontal axis shows the frequency of the fan duct 3.

In this case, a resonance phenomenon occurs at a certain point,
The amplitude of the fan duct 3 at that time was 590 μm. This is because the frequency at point C, which is one of the predominant natural frequencies f'of the fan duct 3, is 25.25 Hz, which is close to the value of the excitation frequency ν (= 24.75 Hz), and resonance occurs at this point. I think that the.

Next, a plurality of metal plates (with a total weight of 60 kg) are attached on the reinforcing ribs on the upper side of the fan duct 3 as shown in FIG. 4, and the natural frequency f of the fan duct 3 is measured in the same manner. Is shown in FIG. Comparing this with (a) in the figure, it can be seen that the frequency shifts to the lower order (left side), and the natural frequency f of the fan duct 3 is changed by the attachment of the metal plate. As a result, the predominant natural frequency f'at the point C where resonance occurs also moves to the point C'and its value becomes 2
The frequency was 2.75 Hz, and no resonance phenomenon was observed. It is considered that this is because the difference between the predominant natural frequency f ′ at the point C ′ and the excitation frequency ν was enlarged. Incidentally, the amplitude of the fan duct 3 at this time was 250 μm.

That is, by changing the natural frequency f of the fan duct 3 by attaching the weight 8, the predominant natural frequency f
It has been confirmed that the occurrence of the resonance phenomenon can be favorably avoided without making f ′ and the vibration frequency ν by the fan 5 coincide with each other.

Besides, the specific configurations such as the shapes of the fan duct main body 1 and the fan 5 can be arbitrarily changed in design within the scope intended by the present invention.

[0031]

As described above, according to the present invention, since the predominant natural frequency of the fan duct is set so as not to match the vibration frequency of the fan, the occurrence of the resonance phenomenon can be avoided. Noise is not generated as in the past and the deterioration of peripheral members such as the fan duct and the connecting fittings is not promoted, and the special effect that the reduction in the durability life of the fan duct can be significantly reduced compared with the conventional one is obtained. Came to.

Further, if the natural frequency of the fan duct can be changed, it is possible to satisfactorily avoid the occurrence of a resonance phenomenon depending on the situation at the site.

Further, as means for changing the natural frequency, for example, if a means for filling a storage portion provided in the fan duct main body with a filling material or attaching a weight to the fan duct main body is adopted, work on site can be performed. There is also an advantage that a series of change operations can be easily performed without complicating.

Further, since the fan duct according to the present invention has a very simple structure, it has a practical effect that it can be manufactured easily and inexpensively.

[Brief description of drawings]

1 shows an embodiment of the present invention, (a) is a front view of a fan duct, and (b) is a plan view.

2 (a) is a cross-sectional view taken along the line AA of FIG. 1 (a), and (b) is FIG.
(B) BB sectional drawing of FIG.

FIG. 3 is a perspective view including a partial cross section showing a usage state.

FIG. 4 is a front view of a fan duct showing another embodiment.

5 (a) and 5 (b) are graphs showing experimental results of Examples.

[Explanation of symbols]

 1 ... Fan duct main body 2 ... Storage section 5 ... Fan 7 ... Filling material 8 ... Weight

Claims (4)

[Claims] 1. A predominant natural frequency (f ') of a fan duct for a heat exchange tower such as a cooling tower in which a fan (5) is installed.
Is set so as not to match the vibration frequency (ν) of the fan (5), and a fan duct for a heat exchange tower such as a cooling tower. 2. A fan duct for a heat exchange tower such as a cooling tower in which a fan (5) is installed, characterized in that its natural frequency (f) is changeable. Fan duct for heat exchange tower. 3. In a fan duct for a heat exchange tower such as a cooling tower in which the fan (5) is installed, a filler (7) is placed in the fan duct body (1) to change its natural frequency (f). A fan duct for a heat exchange tower such as a cooling tower, which is provided with a storage section (2) filled with. 4. In a fan duct for a heat exchange tower such as a cooling tower in which a fan (5) is installed, the fan duct main body (1) is not continuous in the circumferential direction in order to change its natural frequency (f). Weight
(8) A fan duct for a heat exchange tower such as a cooling tower, characterized in that the fan duct can be attached and detached.