WO2015133966A1

WO2015133966A1 - A method of and means for optimizing the operating time of a low frequency sound generator

Info

Publication number: WO2015133966A1
Application number: PCT/SE2015/050243
Authority: WO
Inventors: Andreas FREDRIKSSON
Original assignee: Infrafone Ab
Priority date: 2014-03-06
Filing date: 2015-03-04
Publication date: 2015-09-11

Abstract

The invention relates to a method of and means for optimizing the operating time of a low frequency sound generator used for soot cleaning of boilers, comprising a resonator tube (2) and a positive feedback feeder unit (1) connected to supply of compressed air. The operating time should be long enough for effective soot cleaning of the heating surfaces but as short as possible for preventing disturbing noise at the surroundings of the boiler.

Description

A method of and means for optimizing the operating time of a low frequency sound generator

THE FIELD OF THE INVENTION

The present invention refers to a method of optimizing the operating time of a low frequency sound generator according to the pre-characterized portion of claim 1 . The invention also refers to means for optimizing the operating time of a low frequency sound generator according to the pre-characterized portion of claim 3 as well as the use of said method of and said means for optimizing the operating time of a low frequency sound generator for cleaning of boilers, heat exchangers, apparatus for selective catalytic reduction of nitrogen oxides or other apparatus with a gas flow containing dust particles.

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention relates to a low frequency sound generator, particularly a low frequency sound generator of the type described in WO2004/009255. The low frequency sound generator comprises a feeder unit and a resonator tube. The feeder unit comprises a movable piston inside a cylinder. The movable piston performs a reciprocating movement inside the cylinder, which is surrounded by a surge tank connected to a compressed air supply system having an air pressure from one to ten bars. The movable piston regulates a connection opening between the interior of the cylinder and the surge tank.

The dimensions of the resonator tube are chosen in such a way that a standing sound wave may be generated in the resonator tube by air pulses fed to the rear end of the resonator tube by the feeder unit. Preferably, the resonator tube is a quarter wave resonator tube. The standing sound wave in the resonator tube has its maximum sound pressure amplitude at the rear end where the feeder unit is situated. The sound pressure works on the end surface of the movable piston, resulting in a reciprocating movement of the movable piston. The other end of the movable piston is spring-loaded, and the movable piston can move in phase with the variations in sound pressure of the standing sound wave under the condition that the resonance frequency of the oscillating mechanical system is higher than the frequency of the standing sound wave in the resonator tube.

Low frequency sound generators of this type are, for example, used for soot cleaning of heating surfaces and other active surfaces of boilers, heat exchangers, apparatus for selective catalytic reduction of nitrogen oxides and other apparatus with gas containing dust particles. The principle is to expose the heating surfaces to short blasts of low frequency sound during an operating time of some 0.6 seconds with cycle times of some one to five minutes. The aim of the exposure of the heating surfaces is to keep the surfaces free from soot deposits.

The sound pressure amplitude inside the resonator tube at the rear end can be up to 40 000 Pa, resulting in a cyclic varying force having an amplitude of up to 12 000 N. That force can cause a vibration of the low frequency sound generator with a vibration velocity amplitude of up to 120 mm/s.

The amplitude of the sound pressure inside the boiler, the heat exchanger or the apparatus can be up to 600 Pa, which means that the cyclic vibrating force of an outer wall can have an amplitude of up to 6 000 N.

There is a standing sound wave inside the resonator tube of the low frequency sound generator. The positive feedback system of the feeder unit builds up the standing sound wave. Using large low frequency sound generators, a sequence of some ten air pulses, fed by the feeder unit, are needed before the standing wave with full sound pressure amplitude is established, corresponding to a start-up time of about 0.5 seconds.

When the standing sound wave with full sound pressure amplitude inside the resonator tube is established, it takes further some 0.2 seconds until the sound field inside the cavity of the boiler, the heat exchanger or the apparatus has been built up to full sound pressure amplitude. The cyclic varying force acting on the outer walls of said cavity can cause vibrations of the walls and structural sound. These wall vibrations and the structural sound can cause disturbing noise in the surroundings.

It takes some 0.2 seconds from when the full sound pressure amplitude inside the cavity has been built up until the vibrations of the walls of the cavity have reached maximum vibration velocity amplitude.

It takes some 0.2 seconds before a human being is disturbed by noise caused by for example vibrations of the walls of a cavity and/or structural sound.

That entails that the total time from the moment that full sound pressure amplitude inside the cavity is established until the generated noise reaches full disturbing level, is about 0.6 seconds.

The structural mechanical stresses caused by the vibration of the low frequency sound generator as well as the structural mechanical stresses caused by vibrations of the outer walls of the cavity are normally far below the fatigue stress level that the material can endure.

The disturbing noise, however, is often a problem for the personnel working or visiting the surroundings of a boiler, a heat exchanger or an apparatus. The noise level is normally below the threshold level of 85 dB(A) or even 80 dB(A). But the problem is the combination of a disturbing noise having a spectrum of frequencies dominated by high level of low frequency sound and the short cycle time of the disturbing noise giving a feeling that the vibrations can cause fatigue failures of the material exposed to the vibrations.

That entails that the vibrations are normally harmless to as well the material as human beings, but there is a demand of limiting the awareness of the disturbing noise and hence there is a need of a method for minimizing the operating time at full sound pressure. There are also needs of means for minimizing the operating time at full sound pressure. The operating time should normally be maximum 0.6 seconds. This is possible by (1 ) generating the low frequency sound by a positive feedback system resulting in a very pure tone, (2) measuring the sound pressure amplitude at the rear end of the resonator tube, (3) measuring the time at full sound pressure and (4) stopping the low frequency sound generator within a short pre-set time, generally some 0.6 seconds after full sound pressure amplitude is achieved.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method of and means for optimizing the operating time of a low frequency sound generator that overcomes the problems mentioned above.

The object is achieved by a method of optimizing the operating time of a low frequency sound generator defined in claim 1 .

The method is characterized in that the sound pressure amplitude at the rear end of the resonator tube is measured and the operating time of the low frequency sound generator from the moment that the sound pressure amplitude has reached a pre-set value is measured and that the supply of compressed air is closed after a pre-set of said operating time.

It is, as description above, important that the pre-set operating time is short. According to experiences of as well needed time for effective soot cleaning as well as perception time, the operating time at full sound pressure amplitude of the low frequency sound generator should be maximum 0.6 seconds.

The object is also achieved by means for optimizing the operating time of a low frequency sound generator defined in claim 3.

The means are characterized in that the sound pressure amplitude at the rear end of the resonator tube is measured and the operating time of the low frequency sound generator from the moment that the sound pressure amplitude has reached a pre-set value is measured and that the supply of compressed air is closed after a pre-set of said operation time. These means for optimizing the operating time can be installed on existing low frequency sound generators in an easy and cost effective manner.

In one embodiment, the fully open/fully closed valve for the supply of compressed air is a quick ventilation valve with a closing time of less than 0.1 seconds. The advantage of a short closing time is that the exposure time for the noise is short.

In another embodiment, the quick ventilation valve is maneuvered by an electromechanically operated two-port solenoid valve.

The object is also achieved by use of the low frequency sound generator according to any one of claims 3 to 6 for cleaning of boilers, heat exchangers or apparatus.

The advantages of the low frequency sound generator, as well as the preferred embodiments thereof, are apparent from the discussion above with reference to the method of and means for optimizing the operating time of a low frequency sound generator

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now to be explained more closely by means of embodiments, which are disclosed as examples, and with reference to the attached drawings.

Fig. 1 shows schematically a feeder unit and a resonator tube of a low frequency sound generator with the movable piston located at an upper dead centre. Fig. 2 shows schematically a feeder unit and a resonator tube of a low frequency sound generator with the movable piston located at a lower dead centre.

Fig. 3 shows schematically a feeder unit and a resonator tube of a low frequency sound generator with the movable piston located at a seat at the lower part of the cylinder.

Fig. 4 shows schematically a feeder unit and a resonator tube of a low frequency sound generator with a sound pressure transducer installed at the rear end of the resonator tube Fig. 5 shows schematically a side view of the fully open/fully closed valve in open position and a regulating valve

Fig. 6 shows schematically a side view of the fully open/fully closed valve in closed position and a regulating valve

Fig. 7 shows schematically a feeder unit and a resonator tube of a low frequency sound generator with the fully open/fully closed valve in closed position as in

Fig. 6

Fig. 8 shows a graph of measured sound pressure amplitudes at start-up of the low frequency sound generator

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

Fig. 1 shows a feeder unit 1 of a low frequency sound generator mounted on a resonator tube 2. The resonator tube may be a quarter wave resonator tube 2. The feeder unit 1 comprises a movable piston 4 that is arranged to perform a reciprocating movement inside a cylinder 5, between an upper dead centre 3 shown in Fig. 1 , and a lower dead centre 8 shown in Fig. 2. The movable piston 4 is spring- loaded by a spring 6, which is attached to the closed end of the cylinder 5. The cylinder 5 and the movable piston 4 are mounted in a surge tank 7. A compressed air supply system 9, is connected to the surge tank 7. The cylinder 5 has an open end 1 1 facing the resonator tube 2 and communicating therewith. The cylinder 5 is also provided with one or several connection openings 10 in the cylinder wall through which the compressed air supplied to the surge tank 7 can enter into the cylinder 5 and continue into the resonator tube 2. At the upper closed end of the cylinder 5, above the movable piston 4, there is a connection opening 18 to the atmospheric pressure.

When the standing sound wave inside the resonator tube has its maximum sound pressure acting on the movable piston 4, the movable piston 4 is pressed to the upper dead centre 3 as shown in Fig. 1 . At that position, the connection openings 10 in the cylinder 5 are open and an air pulse 12 is fed into the cylinder 5.

When the standing sound wave inside the resonator tube has its minimum sound pressure acting on the movable piston 4, the movable piston 4 is suctioned to its lower dead centre 8 as shown in Fig. 2, and the connection openings 10 are closed. At the end of the cylinder 5, which is not facing the resonator tube 2 and at which end the spring 6 of the movable piston 4 is fitted, there is a second compressed air supply system 14 connected to the cylinder 5. This second compressed air supply system

14 supplies compressed air, regulated by a fully open/fully closed valve 15. The pressure of this second compressed air is one to ten bars, which is much higher than the pressure of the standing sound wave. As long as the fully open/fully closed valve

15 is at the position shown in Fig. 1 and Fig. 2, i.e. the supply from the second compressed air supply system 14 is shut off, the movable piston 4 will move between the upper dead centre 3 and lower dead centre 8 with the frequency of the standing sound wave in the resonator tube 2. Further details regarding the low frequency sound generator are described on pages 2 to 4 of WO 2004/009255, which is hereby included in its entirely by reference.

When the fully open/fully closed valve 15 is at the position shown in Fig. 3 i.e. the second compressed air supply system 14 is in connection to the part of the cylinder 5 above the movable piston 4, the movable piston 4 is pressed down to the seat 19, resulting in that the supply of compressed air supplied to the surge tank 7 cannot enter into the cylinder 5 and the low frequency sound generator cannot operate.

The fully open/fully closed valve 15 may be a quick ventilation valve that can shift from the position shown in Fig. 1 and Fig. 2 to the position shown in Fig. 3 within less than 0.1 seconds.

The quick ventilation valve can preferable be maneuvered by an electromechanically operated two-port solenoid valve.

Fig. 7 shows a sound pressure amplitude measuring transmitter 20 mounted to the rear end of the resonator tube 2. An electrical signal 21 from the sound pressure amplitude measuring transmitter 20 regulates the electromechanically operated two- port solenoid valve 22 and 23, shown in Fig. 5 and Fig. 6, which maneuvers the fully open/fully closed valve 15.

Fig. 5 shows the fully open/fully closed valve 15 in a position open to the atmosphere. Fig.6 shows the fully open/fully closed valve 1 5 in a position closed to the atmosphere.

Fig. 8 shows a graph of sound pressure amplitude measured by the transmitter a function of time at start-up of the low frequency sound generator.

The expression "low frequency sound" as used herein is meant to include sound of a frequency below approximately 38 Hz. A suitable operating frequency would be between approximately 15 and 30 Hz.

The expression "dust particles" as used herein is meant to include soot or any other solid and/or liquid particles present in a gas.

The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims.

Claims

1 . A method of optimizing the operating time of a low frequency sound generator which comprises a resonator tube and a positive feedback feeder connected to a supply of compressed air

characterized in that

the sound pressure amplitude at the rear end of the resonator tube is detected and

the operating time from the moment that the sound pressure amplitude has reached a pre-set value is measured

and

the supply of compressed air is stopped after a pre-set of said operating time.

2. A low frequency sound generator according to claim 1 whereby the pre-set operating time is maximum 0.6 seconds.

3. Means for optimizing the operating time of a low frequency sound generator which comprises a resonator tube and a positive feedback feeder unit connected to a supply of compressed air

characterized in that

the operating time from the moment that the sound pressure amplitude has reached a preset value is measured

and

the supply of compressed air is stopped after a pre-set of said operating time.

4. A low frequency sound generator according to claim 3 whereby the pre-set operating time is maximum 0.6 seconds.

5. A low frequency sound generator according to claim 4 whereby the valve is a quick ventilation valve.

6. A low frequency sound generator according to claims 4 or 5 whereby the quick ventilation valve is maneuvered by a solenoid valve.

7. Use of the low frequency sound generator according to any one of claims 3 to 6 for cleaning of boilers, heat exchangers, apparatus for selective catalytic reduction of nitrogen oxides or other apparatus with gas containing dust particles.