GB2025563A

GB2025563A - Method of removing dust deposits from heat-exchange surfaces

Info

Publication number: GB2025563A
Application number: GB7913393A
Authority: GB
Original assignee: Stal Laval Apparat AB
Current assignee: Stal Laval Apparat AB
Priority date: 1978-04-19
Filing date: 1979-04-18
Publication date: 1980-01-23
Also published as: SE411068B; SE7804421L; DE2914112B2; GB2025563B; IT1118569B; ES479701A1; JPS6227357B2; JPS54140251A; FR2423743B1; FR2423743A1; SE411068C; DE2914112C3; IT7967818A0; DE2914112A1

Abstract

The invention relates to a method of removing dust deposits from heat-exchange surfaces where gas containing dust passes through a pipe or through a chamber (2) containing coolant tubes (3). Sound is generated in known manner from sirens (5) and particulate material (e.g. sand) is fed via a pipe (6) to mix with the gas containing the dust. The "sand" and dust are deposited on the tubes and are removed together by means of the sound generation more easily than would be the case if the "sand" were not being incorporated in the deposit. The grain size of the "sand" should be larger than that of the dust. <IMAGE>

Description

SPECIFICATION Method of removing dust deposits from heat-exchange surfaces The present invention relates to a method of removing dust deposits from heat-exchange surfaces in which a gas entraining dust (e.g.

soot) passes through a pipe or a chamber containing coolant tubes.

The method of the invention permits a gas carrying dust particles for instance exhaust fumes, to be freed from dust for environmental conservation reasons and to prevent deposit build-up, or excessive deposit build-up on a heat-exchange surface, aids in reducing the temperature of the gas to a level suitable for subsequent dust filtering and simultaneously heating a coolant, such as water, circulating in heat-exchange tubes.

A heat-exchange surface contacted by a dust-laden gas on its way through a heatexchanger must in some way be kept clean, or periodically cleaned, from deposits of the dust from the gas.

There are various methods of achieving this dust removal, such as contacting the heat exchange surface with steel balls, blowing the surface with air or steam or cleaning the surface using high-energy sound waves.

When cleaning the surfaces of heat-exchange tubes, the steel ball method has the drawback that there is excessive erosion of the tubes. Cleaning with steam or air blasts requires excessive expenditure of power for the types of dust normally met with and acoustic dust removal is not effective for all types of dust encountered.

The method according to the invention offers a solution of the problems described above and others associated therewith and is characterised in that sound is generated, continuously or intermittently in any suitable manner from one or more sirens or other sound generating means, and is fed to the tubes, and that some solid material in particulate form (e.g. sand) having a larger grain size than that of the dust to be removed is deliverately fed onto the heat-exchange surface with the gas at the latest simultaneously with the sound generation, whereby the particulate material is removed with the dust from the heatexchange surface by means of the acoustic energy and then taken away together from the vicinity of the heat-exchange surface.

Using an acoustic dust removal alone is not efficient enough for removing the fine dust typically found in flue gas but the addition of sand or some other coarser particulate material than the dust to the gas stream causes the dust and sand together to be deposited on the heat-exchange surface tubes so that they can be removed together from the surface by the sound, the particulate material acting as an activator in the dust-removal process. The mixture of dust and particular material is loosened more easily then dust along during the acoustic cleaning operation and the dust and particulate material can, if required, be separated downstream of the cleaned surface.

The particulate material may be supplied and the sound may be generated either continuously or intermittently and at a rate dependent upon the composition of the dust. The particulate material may possibly be supplied shortly before the sound is generated.

The heat-exchange surface can be the outer surface of a tube conveying a coolant, such as water, which is converted to steam, for use in a steam boiler or for other heating purposes, and the dust-laden gas may be the exhaust gases from a furnace.

The sound-generating means may operate at a frequency in the range 300-500 Hz. A typical average grain size for the dust would be 0.1 - 1 m and for the particulate material an average grain size would be of 0.5 - 3 mm diameter, preferably 1 - 2 mm diameter.

The invention will now be further described, by way of example, with reference to the accompanying drawing which shows a heatexchanger designed for use with the method according to the invention.

The heat-exchanger illustrated is for cooling dust-laden gas flowing in at the arrow 1. The gas passes into a chamber (or pipe) 2 in which tubes 3 are located, a coolant such as water flowing through the tubes. The dust from the gas will collect on the outer surfaces of the tubes 3 and cleaned and cooled gas will leave the heat-exchanger at the arrow 4.

Previously it was a problem that the dust became so durably attached to the tubes 3 that it could not be removed satisfactorily by conventional methods.

One or more sirens 5 (or other means of generating sound) having a sound frequency of about 300-500 Hz are arranged in known manner in the wall of the pipe or chamber 2.

These sirens are suitably placed approximately on a level with the tubes 3 and direct acoustic energy onto the tubes. Sand or some other powder or granular material having an average grain size of 0.5-3 mm diameter, preferably 1-2 mm diameter. i.e. of considerably greater size than the average particle size of the dust which would be 0.1 - 1.0 ym, is introduced at 6.

The sand is mixed with the gas and flows through the heat-exchanger to deposit itself with the dust in the gas on the tubes 3. The sand acts as an activator for the dust removal under the influence of the acoustic energy and the acoustic energy may be supplied continuously or intermittently, as may the supply of sand under operating conditions which depend on the type of dust. The sand acts to dislodge the dust from the tubes.

The material loosened from the tube surfaces is carried with the gas to a downstream filter (not shown) which separates the material from the gas. Due to the cooling of the gas, by the tubes 3 the filter can operate at a temperature conducive to lengthening its useful life.

The water heated in the tubes 3 can be converted to steam which can be used for a steam boiler and the gas to be cleaned and cooled in the heat-exchanger can be the exhaust gas from a furnace. The gas cleaned in the heat-exchanger and the filter can be reused for combustion purposes so that its heat content is utilized.

There may be one or more nozzles for blowing in the sand and the gas circulation can be forced by any method known in the field.

One specific example of a method according to the invention is performed as follows: EXAMPLE Gas containing dust from a ferrosilicon furnace is blown in at 1 and sand of a grain size 1-2 mm diameter is blown continuously in at 6. The sand becomes mixed with the gas, and sand and dust are deposited on the surfaces of the tubes 3 while the gas is cooled and the coolant in the tubes 3 is heated. The sand supply is at a rate to give a sand/dust ratio in the deposit of from 1:1 to 1:4 (typically 1:2) expressed by weight.

Sound is generated at intervals from highenergy sirens 5 operating at a frequency of 300 Hz and each at a power output of about 1 30 Decibel A, whereupon the deposit on the outer surfaces of the tubes is loosened and carried away with the gas (at 4) to one or more filters (not shown) where the sand and dust are separated off. The grain size of the sand, frequency and power output of the sound-generators given in the example can be varied within wide limits and will be chosen inter alia with regard to the size of the dust particles to be removed and on the shape and size of the heat-exchange surfaces to be cleaned.

Claims

1. Method of removing dust deposits from a heat-exchange surface in which a gas entraining dust passes through a pipe or a chamber containing coolant tubes characterised in that sound is generated, continuously or intermittently, in any suitable manner from one or more sirens or other sound generating means and is fed to the tubes, and that some solid material in particulate form having a larger grain size than that of the dust to be removed is deliberately fed on to the heatexchange surface with the gas at the latest simultaneously with the sound generation, whereby the particulate material is removed with the dust from the heat-exchange surface by means of the acoustic energy and then taken away together from the vicinity of the heat-exchange surface.

2. A method according to claim 1, in which the particulate material has an average grain size of 0.5-3 mm diameter.

3. A method as claimed in claim 2 in which the average grain size is 1-2 mm diameter.

4. A method as claimed in claim 2 or claim 3 in which the dust has an average particle size of 0.1 - ym.

5. A method as claimed in any preceding claim, in which the acoustic energy has a frequency in the range 300-500 Hz.

6. A method as claimed in any preceding claim, in which the particulate material is sand and is employed in amounts to give a weight ratio of sand/dust on the heat-exchange surface of form 1:1 to 1:4.

7. A method of removing dust deposits from a furnace substantially as hereinbefore specified in the foregoing Example with reference to the accompanying drawing.